B.Pharmacy 3rd Semester Pharmaceutical Organic Chemistry II – Theory Important Question Answers

Important Notes B. Pharma 3rd Semester Pharmaceutical Organic Chemistry II – Theory 

Q. resonance in benzene?

Ans. Resonance in benzene is the phenomenon of having two or more equivalent structures that represent the same molecule. Benzene has six carbon atoms arranged in a ring with alternating single and double bonds. However, these bonds are not fixed but rather oscillate between the two possible arrangements. This means that benzene has two resonance structures that are equally valid and contribute to the actual structure of the molecule. The actual structure of benzene is a hybrid of the two resonance structures, where all the carbon-carbon bonds have the same length and strength, which is intermediate between a single and a double bond.

Q. Sponification value?

Ans. Saponification value is a measure of the amount of alkali (such as potassium hydroxide or sodium hydroxide) needed to completely break down a fat or oil into glycerol and fatty acids. It is expressed in milligrams of alkali per gram of fat or oil. 

*Saponification value is useful for determining the average molecular weight and composition of fats and oils. It can also indicate the purity and quality of the sample, as different fats and oils have different saponification values. For example, coconut oil has a high saponification value of 248-265 mg KOH/g, while olive oil has a low saponification value of 184-196 mg KOH/g

Q.Rancidity Of Oils?

Ans. Rancidity of oils is the deterioration of the quality and flavor of oils due to chemical reactions that involve oxygen, water, or microorganisms. The rancidity of oils can be caused by two main processes: oxidation or hydrolysis.

*Rancidity of oils can affect the nutritional value, shelf life, and safety of food products. Rancid oils can lose their essential fatty acids, vitamins, and antioxidants, and can produce harmful compounds that may increase the risk of chronic diseases

Q. Ester Value?

Ans. In pharma organic chemistry, ester value is a parameter that indicates the purity and quality of fats and oils. It is defined as the number of milligrams of potassium hydroxide (KOH) required to saponify the esters present in one gram of the substance. Esters are organic compounds that are formed by the reaction of an acid and an alcohol, with the elimination of water. 

*Ester value = Saponification value - Acid value

Q.Define Huckle's Rule?

Ans. Hückel’s rule is a rule that predicts whether a planar ring-shaped molecule will have aromatic properties or not. Hückel’s rule states that a planar ring molecule will be aromatic if it has 4n + 2 π electrons, where n is a non-negative integer (0, 1, 2, 3, …). 

Q. Richert-Meissl (RM) Value?

Ans. The Richert-Meissl (RM) value is a measure of the amount of volatile fatty acids that can be extracted from a fat or oil by saponification and distillation. Volatile fatty acids are short-chain fatty acids that have low boiling points and are soluble in water. They are mainly found in animal fats, such as Butter, ghee, lard, and tallow.

*The RM value is calculated by saponifying 5 grams of fat or oil with an alkali solution, such as sodium hydroxide or potassium hydroxide, and then distilling the resulting soap solution with water. The distillate contains the volatile fatty acids, which are then neutralized with a standard acid solution, such as 0.1 N hydrochloric acid. The RM value is the number of milliliters of 0.1 N alkali solution, such as 0.1 N sodium hydroxide or potassium hydroxide, that is equivalent to the amount of acid used to neutralize the volatile fatty acids.

*The RM value is an indicator of the quality and purity of fats and oils. It can help to differentiate between animal and vegetable fats, as well as between different types of animal fats. For example, butter has a high RM value (about 26 to 32) because it contains a lot of butyric acid, which is a volatile fatty acid. Vegetable oils have low RM values (less than 1) because they contain mostly long-chain fatty acids that are not volatile. Lard and tallow have intermediate RM values (about 1 to 2) because they contain some volatile fatty acids, such as caproic acid and caprylic acid.

Q. Sulphonation

Ans. Sulphonation is the process of replacing a hydrogen atom of an organic compound with a sulfonic acid (-SO3H) functional group. It can be done by reacting the compound with sulfuric acid, sulfur trioxide, or chlorosulfuric acid. Sulphonation is used to make detergents, dyes, drugs, and other products.

Q.Freidal-Craft Reaction?

Ans. The Friedel-Crafts reaction is a type of electrophilic aromatic substitution that attaches a substituent to an aromatic ring. There are two main types of Friedel-Crafts reactions: alkylation and acylation. Alkylation adds an alkyl group ® to the ring, while acylation adds an acyl group (RCO) to the ring. Both reactions require a Lewis acid catalyst, such as AlCl3 or FeCl3.

Q. What are Phenols? Give Physical properties of it?

Ans . Phenols are organic compounds that have a hydroxyl group (-OH) attached directly to a benzene ring. Some physical properties of phenols are:They are colorless liquids or solids, but they may turn reddish-brown in the air due to oxidation.They have higher boiling points than alcohols of similar molecular weight because they form stronger intermolecular hydrogen bonds.

They are sparingly soluble in water, forming a pink solution, but they are more soluble in organic solvents such as ether, alcohol, and benzene.They have a characteristic smell and taste, and they are corrosive to the skin.

Q. Define Iodine Number (Iodine Value) ?

Ans:- The iodine number (or iodine value) is a measure of the degree of unsaturation of a substance, such as an oil, fat, or wax. It is defined as the mass of iodine, in grams, that is consumed by 100 grams of the substance. The higher the iodine number, the more unsaturated fatty acids are present in the substance. For example, the iodine numbers of linseed oil, olive oil, and coconut oil are approximately 175–201, 77–91, and 8–9.5 respectively.

Q. Define Acetyl Value ? Give Its Significance ?

Ans. The acetyl value is a measure of the free hydroxyl groups in a substance, such as a fat or oil, as determined by acetylation. It is defined as the number of milligrams of potassium hydroxide required to neutralize the acetic acid formed by hydrolysis of one gram of the acetylated substance. The significance of the acetyl value is that it can be used to measure the hydroxy acids in lipids, which are responsible for rancidity and instability of fats and oils.

Q. Hydrogenation Reaction Of Oils?

Ans. Hydrogenation of oils is the process of adding hydrogen atoms to unsaturated hydrocarbons (which have double or triple bonds) to produce saturated hydrocarbons (which have only single bonds). The hydrogenation reaction occurs in the presence of a catalyst, such as nickel. The hydrogenation process is widely used in industry, and it is used to make vegetable ghee or vanaspati ghee from vegetable oils. Vegetable oils are liquid at room temperature, while vegetable ghee is solid or semi-solid at room temperature. Hydrogenation also increases the melting point and shelf life of the oils.

Q. Baeyer's Strain theory And Its Limitations?

Ans. Baeyer’s strain theory is a theory proposed by Adolf von Baeyer in 1872 to explain the relative stability of different cycloalkanes, which are aliphatic compounds that contain a ring of carbon atoms. The theory is based on the following assumptions:

All cycloalkanes are planar, meaning they lie on a flat plane.

The normal bond angle for carbon atoms in a tetrahedral geometry is 109.5°.

Deviation from the normal bond angle causes angle strain, which reduces the stability of the molecule.

According to the theory, the smaller the ring size, the greater the angle strain. For example, 

cyclopropane has a bond angle of 60°, which is much smaller than 109.5°, so it has a high angle strain and low stability. Cyclobutane has a bond angle of 90°, which is also smaller than 109.5°, so it has a lower angle strain and higher stability than cyclopropane. Cyclopentane has a bond angle of 108°, which is close to 109.5°, so it has a very low angle strain and high stability. Cyclohexane has a bond angle of 120°, which is larger than 109.5°, so it has a negative angle strain and low stability.

However, Baeyer’s strain theory has some limitations, such as:

It cannot explain the effect of angle strain in larger ring systems, such as cycloheptane or cyclooctane, which are more stable than expected.

It predicts that cyclopentane should be more stable than cyclohexane, but in reality, cyclohexane is more stable than cyclopentane.

It assumes that all cycloalkanes are planar, but in fact, they can adopt non-planar conformations to reduce angle strain and torsional strain (caused by eclipsing interactions between adjacent bonds). For example, cyclohexane can adopt a chair conformation, which has no angle strain and no torsional strain.

Q. Sachse Mohr's theory?

Ans. Sachse Mohr’s theory is a theory that explains the stability of cyclohexane and higher cycloalkanes, which are aliphatic compounds that contain a ring of six or more carbon atoms. The theory was proposed by Hermann Sachse in 1890 and Ernst Mohr in 1918. The theory is based on the following assumptions:

All carbons of the ring are not planar, meaning they do not lie on a flat plane.

The normal bond angle for carbon atoms in a tetrahedral geometry is 109.5°.

The ring can adopt non-planar conformations, such as chair, boat, twist-boat, etc., to avoid angle strain and torsional strain.

According to the theory, the most stable conformation of cyclohexane is the chair conformation, which has no angle strain and no torsional strain. The chair conformation can interconvert into another chair conformation through a half-chair intermediate. The theory also predicts the existence of cis and trans isomers of decalin (a bicyclic compound with two fused cyclohexane rings), which were confirmed experimentally by Walter Hückel in 1925.

Q. What is Addition reaction ?

Ans. An addition reaction is a type of chemical reaction in which two or more molecules combine to form a larger one (the adduct). Addition reactions are common for unsaturated organic compounds, which have carbon-carbon double or triple bonds, and for compounds with carbon-oxygen or carbonnitrogen double bonds. There are different types of addition reactions, depending on the mechanism and the nature of the reactants. Some examples are:

Electrophilic addition: A reaction in which an electrophile (an electron-deficient species) attacks an unsaturated bond and forms a carbocation, which then reacts with a nucleophile (an electron-rich species) to form the adduct. For example, the addition of hydrogen bromide to an alkene.

Nucleophilic addition: A reaction in which a nucleophile attacks a polarized unsaturated bond and forms a new bond with the electropositive atom, while the electronegative atom leaves as a leaving group. For example, the addition of water to an aldehyde or ketone.

Free-radical addition: A reaction in which a free radical (an unpaired electron species) attacks an unsaturated bond and forms a new radical, which then reacts with another molecule to form the adduct. For example, the addition of hydrogen to an alkene in the presence of a radical initiaton.

Cycloaddition: A reaction in which two or more unsaturated molecules react to form a cyclic adduct. For example, the Diels-Alder reaction between a diene and a dienophile

Q. Explain Inductive effect ?

Ans. Inductive effect is a phenomenon in which the electron density of a covalent bond is shifted towards the more electronegative atom, creating a permanent dipole. The inductive effect can be transmitted through a chain of atoms connected by sigma bonds, resulting in different degrees of polarity. The inductive effect can be classified into two types:

-I effect (negative inductive effect): This occurs when an electron-withdrawing group (such as a halogen or a nitro group) is attached to a molecule, and pulls electrons away from the rest of the molecule, making it more positive. For example, in CH3-CH2-Cl, the chlorine atom has a -I effect on the carbon atoms.

+I effect (positive inductive effect): This occurs when an electron-donating group (such as an alkyl group or an amino group) is attached to a molecule, and pushes electrons towards the rest of the molecule, making it more negative. For example, in CH3-CH2-NH2, the amino group has a +I effect on the carbon atoms.

The inductive effect can affect the stability, acidity and basicity of molecules, depending on the charge and the groups present. For example, a molecule with a positive charge will be more stable if it has a +I  group than a -I group, because the +I group will reduce the positive charge. Similarly, a molecule with a negative charge will be more stable if it has a -I group than a +I group, because the -I group will reduce the negative charge.

Q. Define aromaticity.

Ans. Aromaticity is a property of some cyclic molecules that have delocalized electrons in pi bonds and are more stable than other arrangements with the same atoms. Some examples of aromatic compounds are benzene, naphthalene, and pyridine

There are some rules or conditions for a compound to be aromatic, such as:

The molecule must be cyclic (a ring of atoms)

The molecule must be planar (all atoms in the ring lie in the same plane)

The molecule must be fully conjugated (p orbitals at every atom in the ring)

The molecule must have 4n+2 pi electrons (n = 0 or any positive integer)

Q. Give The Difference between Friedal Craft's Alkylation & acylation ?

Ans. Friedel Craft’s alkylation and acylation are both types of electrophilic aromatic substitution reactions that add a functional group to an aromatic ring. The main difference is that alkylation adds an alkyl group ® while acylation adds an acyl group (RC=O). Some advantages of acylation over alkylation 

are:

Better control over the reaction products

No chances of rearrangement of the electrophile

Easier removal of the acyl group if desired

The mechanism of both reactions involves the formation of an electrophilic species by the reaction of a Lewis acid (such as AlCl3) with an alkyl or acyl halide. The electrophile then attacks the aromatic ring, forming a cyclohexadienyl cation as an intermediate, which then loses a proton to restore the aromaticity. Some examples of these reactions are:

Friedel Craft’s alkylation: CH3Cl + AlCl3 + C6H6 -> CH3-C6H5 + HCl + AlCl4-

Friedel Craft’s acylation: CH3COCl + AlCl3 + C6H6 -> CH3CO-C6H5 + HCl + AlCl4-

Q. On reaction with any NaOH, phenol forms salt whereas alcohol does not. Why?

Ans. Phenol is acidic enough to react with NaOH and form sodium phenoxide, while alcohol is neutral and does not react with NaOH. This is because the negative charge on the oxygen atom of phenoxide can be delocalized by resonance to the aromatic ring, making it more stable.

Q. Define the iodine value of an oil or fats. What does it indicates ?

Ans. The iodine value of an oil or fat is the mass of iodine in grams that is consumed by 100 grams of the substance. It indicates the degree of unsaturation in the oil or fat, which means the number of double bonds present in the fatty acids. The higher the iodine value, the more unsaturated the oil or fat is. The degree of unsaturation in an oil or fat affects its physical and chemical properties, such as melting point, stability and reactivity. Unsaturated oils and fats tend to be liquid at room temperature, while saturated fats tend to be solid. Unsaturated oils and fats can react with bromine water and decolorize it, while saturated fats do not. This is a way to test the level of unsaturation in an oil or fat. 

Q.Define R-M Value ?

Ans. Reichert value (also Reichert-Meissl number), which is a measure of how much volatile fatty acid can be extracted from a fat or oil through saponification.

Q. How Will you Convert benzene to anthracene ?

Ans. synthesis of anthracene from benzene. According to one source1, the steps are:

Treat benzene with phthalic anhydride in the presence of aluminum chloride to form o-benzoylbenzoic acid.

Heat o-benzoylbenzoic acid with concentrated sulfuric acid to give 9,10-anthraquinone.

Distill 9,10-anthraquinone with zinc dust to yield anthracene.

The mechanism of each step in the synthesis :

The first step is a Friedel-Crafts acylation, where benzene reacts with phthalic anhydride in the presence of aluminum chloride as a Lewis acid catalyst. The mechanism involves the formation of an acylium ion from phthalic anhydride and aluminum chloride, which then attacks the benzene ring as an electrophile. The resulting intermediate loses a proton to regenerate the catalyst and form o-benzoylbenzoic acid.The second step is a dehydration, where o-benzoylbenzoic acid is heated with concentrated sulfuric acid as a dehydrating agent. The mechanism involves the loss of a water molecule from the carboxylic acid group, followed by a rearrangement of the carbonyl group to form 9,10-anthraquinone.The third step is a reduction, where 9,10-anthraquinone is distilled with zinc dust as a reducing agent. The mechanism involves the transfer of two electrons from zinc to the carbonyl groups of 9,10-anthraquinone, followed by the elimination of two protons to form anthracene.

Q. Basicity of Amines?

Ans. Amines are basic compounds that have a nitrogen atom with a lone pair of electrons. Amines can react with acids to form ammonium salts. The basicity of amines depends on several factors, such as:The electronic properties of the substituents on the nitrogen atom. Alkyl groups increase the basicity by donating electrons, while aryl groups decrease the basicity by withdrawing electrons.The degree of solvation of the protonated amine. More solvation stabilizes the positive charge and increases the basicity. Steric hindrance by bulky groups reduces the solvation and decreases the basicity.The hybridization of the nitrogen atom. sp3-hybridized nitrogen is more basic than sp2-hybridized nitrogen, which is more basic than sp-hybridized nitrogen. This is because the s-character of the orbital decreases as the hybridization changes from sp to sp3, and the s-orbital is more electronegative than the p-orbital.

Q. what is diazotization ? Discuss the synthetic uses of diazonium salt in details . 

Ans. Diazotization is the process of converting a primary aromatic amine into its corresponding diazonium salt. Diazonium salts are compounds that have a nitrogen-nitrogen double bond with a positive charge on one nitrogen and an anion on the other. Diazonium salts are useful in synthesis because they can react with various nucleophiles to form different products, such as:Azo dyes, by coupling with phenols or aromatic amines. Chlorobenzene, by reacting with copper chloride. Iodobenzene, by reacting with potassium iodide. Nitrobenzene, by reacting with nitrous acid.Phenol, by reacting with water.

Q. Discuss different chemical reaction of benzoic acid ?

Ans. Benzoic acid is an organic compound that has a carboxyl group attached to a benzene ring. Some of the chemical reactions of benzoic acid are:

Formation of salt, by reacting with a base like sodium hydroxide to produce sodium benzoate Formation of ester, by reacting with an alcohol in the presence of sulfuric acid to produce benzoate ester

Formation of acid halide, by reacting with thionyl chloride to produce benzoyl chloride

Sulfonation, by reacting with sulfuric acid and fuming sulfuric acid to produce sulfonic acid derivatives

Nitration, by reacting with nitric acid and sulfuric acid to produce nitrobenzoic acid

Halogenation, by reacting with halogens and iron catalysts to produce halobenzoic acid

Decarboxylation, by heating with soda lime to produce benzene

Reduction, by reacting with lithium aluminium hydride to produce benzyl alcohol

Q. Define Acid & Iodine Value ?

Ans. Acid value is a measure of the amount of free fatty acids in a fat or oil. It is defined as the mass of potassium hydroxide (KOH) in milligrams that is required to neutralize one gram of the fat or oil. Iodine value is a measure of the degree of unsaturation in a fat or oil. It is defined as the mass of iodine in grams that is consumed by 100 grams of the fat or oil. The higher the iodine value, the more double bonds are present in the fat or oil

Some factors that affect the acid value of a fat or oil are:

Hydrolysis, which is the breakdown of triglycerides into fatty acids and glycerol by water. This can increase the acid value as more free fatty acids are formed.

Oxidation, which is the reaction of triglycerides with oxygen or other oxidizing agents. This can also increase the acid value as more free fatty acids are formed.

Fatty acid composition, which is the type and number of fatty acids in the triglycerides. Different fatty acids have different susceptibility to hydrolysis and oxidation. Generally, unsaturated fatty acids are more prone to oxidation than saturated ones

Q. Define Sachse Mohr Theory And Its Limitations ?

Ans. Sachse-Mohr theory is a theory that explains the stability and structure of cycloalkanes, which are organic compounds that contain a ring of carbon atoms. The theory proposes that the ring carbons are not forced into one plane, as assumed by Baeyer, but can adopt different conformations that minimize the strain in the ring. For example, cyclohexane can exist in a chair conformation, where the ring carbons alternate between two parallel planes.

Some limitations of Sachse-Mohr theory are:

The theory assumes that the material is homogeneous and isotropic, which may not be true for some real-world materials.

The theory does not account for the effects of molecular interactions, such as hydrogen bonding or van der Waals forces, that can affect the stability and shape of the ring.

The theory does not explain the existence of other conformations, such as boat or twist-boat, that are also observed for some cycloalkanes

Q. Write Esterfication Reaction ?

Ans. Esterification is a chemical reaction that forms an ester from an alcohol and a carboxylic acid, usually in the presence of an acid catalyst. The general equation for this reaction is:

RCOOH + R’OH ⇌ RCOOR’ + H2O

where R and R’ can be the same or different alkyl groups.

For example, ethanoic acid and ethanol can react to form ethyl ethanoate and water:

CH3COOH + CH3CH2OH ⇌ CH3COOCH2CH3 + H2O

Here is the mechanism of this reaction, which involves the formation of a carbocation intermediate.

Q. Define Angle Stain ?

Ans. Angle strain is the increase in potential energy of a molecule due to bond angles deviating from the ideal values. Angle strain typically affects cyclic molecules, which lack the flexibility of acyclic molecules. Angle strain destabilizes a molecule, as manifested in higher reactivity and elevated heat of combustion.For example, cyclopropane has a bond angle of 60° between the carbons, which is much smaller than the ideal bond angle of 109.5° for sp3 hybridized carbons. This causes angle strain in cyclopropane.

Q. Give Electrophillic substitution reactions of benzene ?

Ans. Electrophilic substitution reactions of benzene are reactions in which an electrophile attacks the benzene ring and replaces one of the hydrogen atoms with itself. The benzene ring is preserved in these reactions, but its reactivity is reduced due to the loss of electron density.Benzene is a planar molecule which has delocalized electrons above and below the plane of the ring. Being electron-rich, it is highly attractive to electron-deficient species i.e., electrophiles.Some examples of electrophilic substitution reactions of benzene are:

Nitration: Benzene reacts with nitric acid and sulfuric acid to form nitrobenzene. The electrophile is the nitronium ion (NO2+), which is generated by the protonation of nitric acid by sulfuric acid.

Sulfonation: Benzene reacts with sulfur trioxide and sulfuric acid to form benzenesulfonic acid. The electrophile is the sulfur trioxide molecule (SO3), which is activated by sulfuric acid.

Halogenation: Benzene reacts with halogens (X2) in the presence of a Lewis acid catalyst (such as FeX3 or AlX3) to form halobenzene2. The electrophile is the halonium ion (X+), which is formed by the interaction of the halogen and the Lewis acid.

Friedel-Crafts alkylation: Benzene reacts with alkyl halides (R-X) in the presence of a Lewis acid catalyst (such as AlCl3 or FeCl3) to form alkylbenzene2. The electrophile is the carbocation (R+), which is generated by the dissociation of the alkyl halide by the Lewis acid.

Friedel-Crafts acylation: Benzene reacts with acyl halides (R-CO-X) or acid anhydrides (R-CO-O-CO-R) in the presence of a Lewis acid catalyst (such as AlCl3 or FeCl3) to form acylbenzene. The electrophile is the acylium ion (R-CO+), which is formed by the cleavage of the acyl halide or the acid anhydride by the Lewis acid.

The general mechanism of these reactions involves three steps:

Generation of electrophile: The electrophile is formed by the action of an acid or a catalyst on the reagent.

Formation of arenium ion: The electrophile attacks the benzene ring and forms a positively charged intermediate called an arenium ion or a sigma complex, which has a non-aromatic structure.

Regeneration of benzene ring: A hydrogen atom attached to the carbon that bears the positive charge is removed by a base, usually water or a halide ion, and restores the aromaticity of the benzene ring

Q. Write a despective note on analytical constant of fats and oils. 

Ans. Analytical constants of fats and oils are numerical values that indicate the physical and chemical properties of fats and oils, such as their purity, composition, stability, and quality. They are useful for identifying, classifying, and evaluating fats and oils for various purposes, such as food, cosmetics, pharmaceuticals, and biodiesel.

Some of the common analytical constants of fats and oils are:

Acid value: It is the number of milligrams of potassium hydroxide (KOH) required to neutralize the free fatty acids present in one gram of fat or oil. It measures the acidity and freshness of fats and oils. Higher acid value indicates more free fatty acids, which are harmful to human health and cause rancidity.

Saponification value: It is the number of milligrams of KOH required to saponify (convert into soap) one gram of fat or oil. It measures the average molecular weight and chain length of the fatty acids in fats and oils. Lower saponification value indicates higher molecular weight and longer chain length, which affect the melting point, hardness, and solubility of fats and oils.

Ester value: It is the difference between the saponification value and the acid value of a fat or oil1. It measures the amount of esterified fatty acids in fats and oils. Higher ester value indicates more esters, which are the main components of fats and oils.

Iodine value: It is the number of grams of iodine that can be added to 100 grams of fat or oil under specified conditions. It measures the degree of unsaturation (number of double bonds) of the fatty acids in fats and oils. Higher iodine value indicates more unsaturation, which affects the stability, reactivity, and nutritional value of fats and oils.

Acetyl value: It is the number of milligrams of KOH required to neutralize the acetic acid released by one gram of fat or oil after hydrolysis with acetic anhydride. It measures the amount of hydroxyl groups in the fatty acids in fats and oils. Higher acetyl value indicates more hydroxyl groups, which are found in some special fats and oils, such as castor oil and ricinoleic acid.

Reichert-Meissl (RM) value: It is the number of milliliters of 0.1 normal KOH required to neutralize the volatile fatty acids distilled from five grams of fat or oil after saponification1. It measures the amount of short-chain fatty acids (with less than 10 carbon atoms) in fats and oils. Higher RM value indicates more short-chain fatty acids, which are characteristic of some animal fats, such as butter and lard

Q. Give detailed notes on naphthalene and anthracene.

Ans. Naphthalene and anthracene are both polycyclic aromatic hydrocarbons (PAHs), which are organic compounds that consist of two or more fused benzene rings1. They are widely found in coal tar, petroleum, and other fossil fuels, and are also formed by the incomplete combustion of organic matter. They have various applications in industry, medicine, and agriculture, but they are also considered as environmental pollutants and potential carcinogens.

Some of the main differences between naphthalene and anthracene are:

Structure: Naphthalene has two benzene rings that share two carbon atoms, forming a fused bicyclic structure. Anthracene has three benzene rings that share four carbon atoms, forming a fused tricyclic structure.

Resonance stabilization energy: This is the energy difference between the actual structure of a molecule and the hypothetical structure with localized bonds. It indicates the stability and aromaticity of a molecule. Naphthalene has a resonance stabilization energy per ring slightly less than that of benzene (36 kcal/mol), while anthracene has a resonance stabilization energy per ring much less than that of benzene (28 kcal/mol). This means that naphthalene is more aromatic and stable than anthracene.

Reactivity: Naphthalene and anthracene are both more reactive than benzene in both substitution and addition reactions, because they have more pi electrons that can participate in these reactions. However, anthracene is more reactive than naphthalene in electrophilic aromatic substitution reactions, because the central ring of anthracene is more electron-rich and less hindered than the peripheral rings. On the other hand, naphthalene is more reactive than anthracene in nucleophilic aromatic substitution  reactions, because the alpha position of naphthalene is more activated and less sterically hindered than the beta position of anthracene.

Photocycloaddition: This is a reaction in which two molecules form a cyclic product by absorbing light. 

Naphthalene and anthracene can undergo photocycloaddition with themselves or with each other under certain conditions. However, the photocycloaddition of anthracene is much faster and more efficient than that of naphthalene, because anthracene has a larger conjugated system and a higher excited state energy than naphthalene

Q. Give The Qualitative tests for phenols? 

Ans. Phenols are organic compounds that have a hydroxyl group (-OH) attached to a benzene ring. They have acidic properties and can form complexes with some metal ions. Some of the common qualitative tests for phenols are:

Litmus test: Phenols turn blue litmus paper into red, indicating that they are acidic. Ferric chloride test: Phenols react with freshly prepared ferric chloride solution to give colored complexes, mostly dark colored. The color depends on the number and position of the hydroxyl groups on the benzene ring.

Liebermann’s test: Phenols react with sodium nitrite and concentrated sulfuric acid to give a deep green color, which changes to red when diluted with water. This test is also known as nitrous acid test or nitroso test.

Bromine water test: Phenols react with bromine water to give a white precipitate of 2,4, tribromophenol. This test is also known as the bromination test or Wohl-Ziegler reaction.

Phthalein dye test: Phenols react with phthalic anhydride and concentrated sulfuric acid to give colored 

dyes, such as phenolphthalein (pink), fluorescein (yellow-green), or eosin (red). This test is also known as  phthalic anhydride test or phthalein synthesis

Q. Give a note on resonance on benzene?

Ans. Benzene is a planar molecule that has six carbon atoms arranged in a hexagonal ring, each bonded to a hydrogen atom. The carbon atoms are sp2 hybridized, which means that they have three sigma bonds and one pi bond each. The pi bonds are formed by the overlap of the unhybridized p orbitals of the carbon atoms, which are perpendicular to the plane of the ring.

However, benzene does not have alternating single and double bonds between the carbon atoms, as one might expect from its Lewis structure. Instead, it has a resonance structure, which means that it is a hybrid of two or more possible structures that differ only in the distribution of electrons. The resonance structure of benzene is represented by two equivalent structures with three double bonds each, which can be interconverted by shifting the pi electrons around the ring.

The resonance structure of benzene makes it more stable than any of the individual structures, because it allows the pi electrons to be delocalized or spread over the entire ring. This results in a uniform distribution of electron density and bond length (139 pm) around the ring, which makes the molecule more symmetrical and less reactive. The extra stability of benzene due to resonance is called resonance energy, which is estimated to be about 36 kcal/mol

Q.Discuss hydrolysis and hydrogenation reaction of fats ?

Ans. Fats are lipids that are composed of glycerol and three fatty acids, which are long-chain carboxylic acids. Fatty acids can be saturated or unsaturated, depending on the presence or absence of double bonds in their carbon chains.Hydrolysis is the reaction of fats with water, which breaks them down into glycerol and free fatty acids. Hydrolysis can be catalyzed by acids, bases, or enzymes. When fats are hydrolyzed by alkali, the products are glycerol and the alkali salts of the fatty acids, which are called soaps. This process is called  saponification.

Hydrogenation is the reaction of fats with hydrogen, which adds hydrogen atoms to the double bonds of  the unsaturated fatty acids, making them more saturated. Hydrogenation is usually carried out in the presence of a catalyst, such as nickel. Hydrogenation reduces the number of double bonds and increases  the melting point of fats, making them more solid at room temperature. This process is also called hardening.

Q. write a detail note on coulson and moffitt's modifications ?

Ans. Coulson and Moffitt’s modifications are based on the quantum mechanical calculations and the electron density maps of the cyclopropane ring, which is a three-membered cyclic hydrocarbon. They proposed that the carbon-carbon bonds in cyclopropane are not sp3 hybridized, as assumed by Baeyer’s strain theory, but are bent or banana-shaped, which reduces the angle strain and the torsional strain in the ring. The bent bonds are formed by the overlap of two sp2 orbitals on each carbon atom, which are inclined at an angle of 103° to each other. The remaining sp2 orbital on each carbon atom forms a sigma  bond with a hydrogen atom, and the unhybridized p orbital on each carbon atom is perpendicular to the plane of the ring.

Q. write a detail note on coulson and moffitt's modifications ?

Ans. Coulson and Moffitt’s modifications are based on the quantum mechanical calculations and the electron density maps of the cyclopropane ring, which is a three-membered cyclic hydrocarbon. They proposed that the carbon-carbon bonds in cyclopropane are not sp3 hybridized, as assumed by Baeyer’s strain theory, but are bent or banana-shaped, which reduces the angle strain and the torsional strain in the ring. The bent bonds are formed by the overlap of two sp2 orbitals on each carbon atom, which are inclined at an angle of 103° to each other. The remaining sp2 orbital on each carbon atom forms a sigma  bond with a hydrogen atom, and the unhybridized p orbital on each carbon atom is perpendicular to the plane of the ring

Q. Give one reaction of cyclopropane ?

Ans. One reaction of cyclopropane is the addition reaction with halogens, such as bromine or chlorine, which leads to the opening of the ring and the formation of 1,3-dihalopropanes. For example:

C3 H6 +Br2 →CH2 BrCH2 CH2 Br

This reaction is not very efficient, as the bromine adds slowly and incompletely to the cyclopropane ring.  A better way to achieve this reaction is to use hydrohalic acids, such as HBr or HCl, which give linear 1-halogenopropanes. For example:

C3 H6 +HBr→CH3 CH2 CH2 Br

This reaction follows Markovnikov’s rule, which states that the hydrogen atom adds to the carbon atom that has more hydrogen atoms already attached to it.

Q. Give reactions for sulfornation of benzene ?

Ans. Sulfonation of benzene is the process of forming benzenesulfonic acid by heating benzene with fuming sulfuric acid (H2SO4 + SO3). This reaction is an example of electrophilic aromatic substitution, where the electrophile is the sulfur trioxide (SO3) molecule. The reaction is reversible, which means that  by adding heat and water to benzenesulfonic acid, the benzene ring can be regenerated. The sulfonic group (-SO3H) can act as a directing group for other substitution reactions, as it activates the ortho and para positions on the benzene ring. The sulfonic group can also be easily removed by hydrolysis, which makes it a useful blocking group for protecting the benzene ring from unwanted reactions.

The general equation for the sulfonation of benzene is:

C6 H6 +SO3 ⇌C6 H5 SO3 H

Q. Give any one reaction for cyclobutane ?

Ans. One reaction of cyclobutane is the addition reaction with hydrogen, which leads to the opening of the ring and the formation of butane. This reaction requires a catalyst, such as nickel or platinum, and high temperature and pressure. The equation for this reaction is:

C4 H8 +H2 →C4 H10 

This reaction is similar to the hydrogenation of cyclopropane, but it is more difficult to achieve because of the larger ring size and lower angle strain.

Q. Explain the mechanism of electrophilic aromatic substitution reactions.

Ans. Electrophilic aromatic substitution (EAS) is a type of reaction in which an electrophile (E+) replaces a hydrogen atom attached to an aromatic ring123. The general mechanism of EAS involves two steps:

The aromatic ring, acting as a nucleophile, attacks the electrophile and forms a carbocation intermediate. This step is slow and rate-determining because it breaks the aromaticity and increases the energy of the system.A base, usually the conjugate base of the electrophile, removes a proton from the carbocation and restores the aromaticity. This step is fast and exothermic, because it lowers the energy of the system.

The general mechanism of EAS can be represented as:

Ar-H+E+→Ar-E+H+

where Ar is the aromatic ring and E is the electrophile.

There are different types of electrophilic aromatic substitution, depending on the nature of the electrophile and the reaction conditions. Some of the most common types are:

Halogenation: The replacement of a hydrogen atom with a halogen atom, such as chlorine or bromine. 

This reaction requires a Lewis acid catalyst, such as iron or aluminum halides, to activate the halogen molecule.

Nitration: The replacement of a hydrogen atom with a nitro group (NO2). This reaction requires a mixture of concentrated nitric and sulfuric acids to generate the nitronium ion (NO2+), which is the electrophile.

Sulfonation: The replacement of a hydrogen atom with a sulfonic acid group (SO3H). This reaction requires fuming sulfuric acid or sulfur trioxide to generate the sulfur trioxide molecule (SO3), which is the electrophile.

Friedel-Crafts alkylation: The replacement of a hydrogen atom with an alkyl group ®. This reaction requires an alkyl halide and a Lewis acid catalyst, such as aluminum chloride, to generate the carbocation (R+), which is the electrophile.

Friedel-Crafts acylation: The replacement of a hydrogen atom with an acyl group (RCO). This reaction requires an acyl halide and a Lewis acid catalyst, such as aluminum chloride, to generate the acylium ion (RCO+), which is the electrophile

Q. Eloborate the synthesisand medical use of Phenanthrene ?

Ans.Phenanthrene is a polycyclic aromatic hydrocarbon (PAH) with formula C 14 H 10, consisting of three fused benzene rings. It can be synthesized by various methods, such as the Barton–Zard synthesis,  the Diels–Alder reaction, the Friedel–Crafts acylation and the Haworth synthesis. Phenanthrene is used to make dyes, plastics and pesticides, explosives and drugs. It has also been used to make bile acids, cholesterol and steroids4. Some phenanthrene derivatives have medicinal uses, such as phenanthrenequinone, which can be used as a fungicide and in the inhibition of some polymerization processes, and 9,10-biphenyldicarboxylic acid, which is used to make polyester and alkyd resin

There are several methods to synthesize phenanthrene in the laboratory, such as:

The Elbs persulfate oxidation of naphthalene, which involves the oxidation of naphthalene with potassium persulfate in sulfuric acid.

The Diels-Alder reaction of benzyne with furan, which produces phenanthrene as a by-product along with dihydrobenzofuran.

The Barton–Zard synthesis of phenanthrene, which involves the reaction of 2-nitrobenzyl bromide with an alkyne, followed by cyclization and aromatization.

The Friedel–Crafts acylation of benzene with phthalic anhydride, followed by dehydrogenation and decarboxylation.

The Bardhan–Sengupta synthesis of phenanthrene, which involves electrophilic aromatic substitution using a tethered cyclohexanol group using diphosphorus pentoxide. The Haworth synthesis of phenanthrene, which involves the reaction of naphthalene with succinic anhydride, followed by dehydration, reduction and decarboxylation. The Pschorr synthesis of phenanthrene, which involves the cyclization of 1,8-dibromonaphthalene with sodium amide.

The Bogert-Cook synthesis of phenanthrene, which involves the reaction of 1-bromo-4 (phenylethynyl)benzene with sodium amide, followed by dehydrohalogenation

Some phenanthrene derivatives that have medicinal uses are:

Phenanthrenequinone, which can be used as a fungicide and in the inhibition of some polymerization processes. 9,10-biphenyldicarboxylic acid, which is used to make polyester and alkyd resin.

Dextromethorphan, which is a synthetic derivative of phenanthrene that has analgesic and antitussive properties. Phenentada, which is a new phenanthrene derivative isolated from Entada abyssinica, a medicinal plant used for the treatment of infections. It has antimicrobial and antioxidant activities.

Q. Synthesis methods of anthracene ?

Ans. Some methods to synthesize anthracene are:

The Haworth synthesis of anthracene, which involves the reaction of benzene with phthalic anhydride, followed by dehydration, reduction and decarboxylation.

The Diels-Alder reaction of 1,4-naphthaquinone with 1,3-butadiene, followed by oxidation and distillation. The coupling reaction of 9-arylanthracene with carbazole-3-yl-aryl derivative, using a metal or a metal compound as a catalyst. The alkylation and reduction of 1,4-hydroxy-anthraquinone, using boric acid as a catalyst and Zn/HAc as  a reducing agent.

Q. Synthesis methods of Napthalene?

Ans. Some methods to synthesize naphthalene are:

The Haworth synthesis of naphthalene, which involves the Friedel-Crafts acylation of benzene with succinic anhydride, followed by Clemmensen reduction, dehydration and dehydrogenation.The distillation of 4-phenyl 3-butenoic acid with concentrated sulfuric acid, followed by reduction with Zn dust.

The extraction of naphthalene from coal tar, by crystallization of the middle oil fraction.

The radical anion salt formation of naphthalene with alkali metals, such as sodium naphthalene.

Q. Write a short note on theory of strainless rings ?

Ans. The theory of strainless rings, also known as the Sachse-Mohr theory, is a theory that explains the stability of higher cycloalkanes (with six or more carbon atoms) by assuming that they are not planar, but have a puckered or folded shape that reduces the angle strain and torsional strain.According to this theory, the carbon atoms in cycloalkanes can adopt different conformations, such as chair, boat, twist-boat, etc., that allow them to have normal tetrahedral angles (109.5°) and staggered bonds. These conformations are interconvertible by ring-flipping or ring-puckering processes.The theory of strainless rings was proposed by Sachse and Mohr in 1918 to overcome the limitations of the Baeyer’s theory, which assumed that all the carbon atoms in cycloalkanes are in the same plane and have a constant bond angle of 120°1. Baeyer’s theory predicted that cyclopentane has the minimum strain and that the strain increases with the ring size, which contradicted the experimental observations.

Q. Difference between sulphonation and saponification ?

Ans. Sulphonation and saponification are two different types of chemical reactions. Sulphonation is the process of introducing a sulfonic acid group (-SO3H) or its salt (-SO3Na, -SO3K, etc.) into an organic molecule by reacting it with sulfuric acid, fuming sulfuric acid, sulfur trioxide, chlorosulfonic acid or other sulfonating agents. Sulphonation is used to make detergents, dyes, drugs and other products.

Saponification is the process of hydrolyzing an ester (usually a fat or oil) with a strong base (usually sodium hydroxide or potassium hydroxide) to produce a carboxylic acid salt (soap) and an alcohol. Saponification is used to make soaps, cosmetics, biodiesel and other products.

Q. Define Polynuclear Hydrocarbon ?

Ans. A polynuclear hydrocarbon is a type of organic compound that consists of two or more fused aromatic rings. These rings share one or more sides and have delocalized electrons that give them stability and aromaticity. A common example of a polynuclear hydrocarbon is naphthalene, which has two fused benzene rings. Polynuclear hydrocarbons are also known as polycyclic aromatic hydrocarbons or PAHs. They are often found in coal, tar, oil, smoke and other sources of incomplete combustion. 

Some PAHs are considered to be pollutants and carcinogens.