Reaction Mechanisms

(i) Addition of HBr to styrene (Markovnikov):
Product: 1-bromo-1-phenylethane (Markovnikov addition via benzylic carbocation).
(ii) Addition of HBr to allylbenzene (Markovnikov):
Product: 3-phenyl-2-bromopropane.
(iii) Addition of HBr to 1-phenylpropene in the presence of peroxides (anti-Markovnikov, radical):
Product: 3-phenyl-1-bromopropane (Br adds to the less substituted end to form a benzylic radical intermediate).
(iv) Wurtz–Fittig coupling (dry ether):
Product: -butylbenzene.
(v) Diazotisation of aniline at 273–278 K:
Product: benzenediazonium halide (X = Cl/Br, as per HX used).

Methylamine ( ) is a weak base. When it reacts with ferric chloride (FeCl ) in the presence of water, it forms a precipitate of hydrated ferric oxide. This occurs due to the basic nature of methylamine, which allows it to react with ferric ions (Fe ). The reaction between methylamine and ferric chloride proceeds as follows: 1. Methylamine ( ) acts as a base and reacts with Fe ions from ferric chloride. 2. The reaction produces a complex between the ferric ion and the amine. 3. The resulting complex leads to the formation of ferric hydroxide, which eventually dehydrates to form hydrated ferric oxide (Fe O ·xH O), which appears as a precipitate. The reaction can be represented as: This reaction is a typical characteristic of amines reacting with ferric salts, where the basicity of the amine induces the precipitation of metal hydroxides or oxides.

Aniline does not undergo the Friedel-Crafts reaction due to the presence of the amino group ( ) attached to the benzene ring. The amino group is an electron-donating group through its lone pair of electrons on nitrogen. This electron-donating effect increases the electron density on the benzene ring, making the ring more reactive towards electrophilic attack. However, the presence of the amino group also reduces the reactivity of the ring towards the Friedel-Crafts alkylation and acylation reactions. In Friedel-Crafts reactions, a Lewis acid such as AlCl is used as a catalyst to generate an electrophile, but the lone pair on nitrogen in aniline can coordinate with the Lewis acid, which deactivates the catalyst and inhibits the reaction. Hence, aniline does not show Friedel-Crafts reactions.

(i) Aldol
An Aldol reaction is a type of organic reaction where an enolate ion (or sometimes an aldehyde or ketone) reacts with a carbonyl compound to form a β-hydroxy aldehyde or ketone, also called an aldol. The reaction occurs due to the nucleophilic attack of the enolate or aldehyde carbonyl carbon. The general form of the aldol reaction is: Example: The reaction between acetaldehyde (CH₃CHO) and acetaldehyde gives 4-hydroxybutanal (a β-hydroxy aldehyde), which is an aldol addition product.

- Haloalkanes: Haloalkanes (R-X) undergo nucleophilic substitution reactions where the halide ion (X) is replaced by a nucleophile. The mechanism involves the attack of a nucleophile (such as OH , Cl , or Br ) on the carbon bonded to the halogen. This happens due to the polar nature of the C-X bond where the carbon is partially positive, making it susceptible to attack by nucleophiles. The typical mechanism for haloalkanes is SN1 or SN2 depending on the substrate.
- Haloarenes: Haloarenes (C6H5-X) undergo electrophilic substitution reactions where the halide is replaced by an electrophile (like NO , SO , or Cl ) on the aromatic ring. The halogen in haloarenes is relatively electron-withdrawing, making the aromatic ring less reactive towards nucleophiles. Instead, the ring undergoes electrophilic substitution where the halogen leaves as a halide ion (X ) and the electrophile takes its place. The reaction often occurs under the influence of catalysts like AlCl or FeCl .

- Potassium cyanide (KCN) reacts with haloalkanes (R-X) in a nucleophilic substitution reaction, where the cyanide ion (CN ) attacks the electrophilic carbon in the C-X bond, displacing the halide ion (X ) and forming an alkyl cyanide (R-CN). The product is a simple alkyl cyanide.
- Silver cyanide (AgCN), on the other hand, forms an isocyanide (R-NC) when it reacts with haloalkanes. The difference arises because in the presence of silver, the cyanide ion exists as a complex with Ag , making it more likely to attack the carbon from the nitrogen side, leading to the formation of an isocyanide (also known as isonitrile). In this reaction, the CN group attaches to the carbon in a different orientation than in the case of KCN.

The rate of reaction (velocity of reaction) is defined as the change in concentration of the reactant or product per unit time. For this case, the change in concentration of A is given as:
The time interval is 10 minutes, or 600 seconds. The velocity of the reaction is calculated as:
Thus, the velocity of the reaction is .

(x) Order of a reaction and molecularity:
- Order of a reaction refers to the sum of the exponents of the concentration terms in the rate law. It indicates how the rate of reaction is affected by the concentration of the reactants. For example, in a reaction with the rate law , the order of the reaction is , where and are the orders with respect to reactants A and B, respectively.
- Molecularity refers to the number of reacting species (atoms, molecules, or ions) that collide in an elementary step of the reaction. Molecularity is an integer and can only be 1 (unimolecular), 2 (bimolecular), or 3 (termolecular) for most elementary reactions. Unlike order, molecularity is determined from the balanced equation of the elementary reaction, not from the experimental data.
Key Difference: The order of a reaction is determined experimentally, while molecularity is determined from the balanced equation of an elementary reaction.
(y) Average and instantaneous velocity:
- Average velocity is defined as the total displacement (change in position) divided by the total time taken. In terms of reaction rate, it can be defined as the total change in concentration of reactants or products divided by the total time interval.
- Instantaneous velocity is the velocity of an object at a specific instant in time. In terms of reaction rate, it refers to the rate of change of concentration at a particular moment, and it can be calculated by taking the derivative of the concentration with respect to time.
Key Difference: Average velocity is the total change over a time interval, while instantaneous velocity refers to the rate of change at a specific point in time.

- Potassium cyanide (KCN) reacts with haloalkanes (R-X) in a nucleophilic substitution reaction, where the cyanide ion (CN ) attacks the electrophilic carbon in the C-X bond, displacing the halide ion (X ) and forming an alkyl cyanide (R-CN). The product is a simple alkyl cyanide.
- Silver cyanide (AgCN), on the other hand, forms an isocyanide (R-NC) when it reacts with haloalkanes. The difference arises because in the presence of silver, the cyanide ion exists as a complex with Ag , making it more likely to attack the carbon from the nitrogen side, leading to the formation of an isocyanide (also known as isonitrile). In this reaction, the CN group attaches to the carbon in a different orientation than in the case of KCN.

- Haloalkanes: Haloalkanes (R-X) undergo nucleophilic substitution reactions where the halide ion (X) is replaced by a nucleophile. The mechanism involves the attack of a nucleophile (such as OH , Cl , or Br ) on the carbon bonded to the halogen. This happens due to the polar nature of the C-X bond where the carbon is partially positive, making it susceptible to attack by nucleophiles. The typical mechanism for haloalkanes is SN1 or SN2 depending on the substrate.
- Haloarenes: Haloarenes (C6H5-X) undergo electrophilic substitution reactions where the halide is replaced by an electrophile (like NO , SO , or Cl ) on the aromatic ring. The halogen in haloarenes is relatively electron-withdrawing, making the aromatic ring less reactive towards nucleophiles. Instead, the ring undergoes electrophilic substitution where the halogen leaves as a halide ion (X ) and the electrophile takes its place. The reaction often occurs under the influence of catalysts like AlCl or FeCl .

(i) Concentration:
According to the law of mass action, the rate of a chemical reaction is directly proportional to the concentration of reactants. Higher concentration more molecules per unit volume greater frequency of effective collisions higher reaction rate. For example, doubling the concentration of reactant in a first-order reaction doubles the rate.
(ii) Temperature:
Raising temperature increases the kinetic energy of molecules. This leads to: - More frequent collisions, and - A higher fraction of molecules possessing energy greater than the activation energy.
As per the Arrhenius equation, , the rate constant increases exponentially with temperature. Roughly, a rise in temperature nearly doubles the rate of many reactions.
(iii) Catalyst:
A catalyst provides an alternate reaction pathway with lower activation energy ( ). This does not affect the equilibrium position but significantly increases the rate at which equilibrium is achieved. Both homogeneous and heterogeneous catalysts enhance the velocity by lowering the energy barrier for effective collisions.

The Cannizzaro reaction is a redox (disproportionation) reaction that occurs when an aldehyde without an -hydrogen atom (such as formaldehyde, benzaldehyde) is treated with a concentrated alkali.
In this reaction:
- One molecule of the aldehyde is oxidized to a carboxylate ion, and
- Another molecule of the same aldehyde is reduced to a primary alcohol.
General reaction:

Example with formaldehyde:

Thus, the Cannizzaro reaction simultaneously produces an alcohol and a carboxylate salt.

Step 1: Recall the requirement.
The question asks for the preparation of primary amines from alkyl halides without changing the number of carbon atoms in the chain. This means the reaction must directly substitute the halogen with an –NH group, giving a pure primary amine.
Step 2: Analyze each option.
- This involves the conversion of amides to amines using Br and NaOH. However, it reduces the carbon chain length by one (–CO group is removed). Hence, it is not suitable here.
- In this method, phthalimide reacts with alkyl halide to form an N-alkyl phthalimide, which on hydrolysis gives a pure primary amine. This method introduces the –NH group without changing the carbon skeleton. This is the correct choice.
- This is used to prepare aryl halides from aromatic amines via diazonium salts. It is not a method to prepare primary amines.
- :} \) Direct reaction of alkyl halides with NH gives amines but also produces secondary and tertiary amines due to multiple substitutions. Hence, it is not a selective method for pure primary amines.
Step 3: Final Answer.
Thus, the best method is Gabriel’s phthalimide synthesis.

Step 1: Reaction Mechanism Overview
The reaction of diethyl ether with HI involves the cleavage of the ether bond, resulting in the formation of ethyl iodide and ethanol. The reaction proceeds via a nucleophilic substitution mechanism (S 2).
Step 2: Mechanism
1. The protonation of the ether oxygen occurs due to the acidic nature of HI: 2. The protonated ether is now more electrophilic and undergoes cleavage, with the iodide ion ( ) attacking one of the ethyl groups, leading to the formation of ethyl iodide: Final Answer: The reaction of diethyl ether with HI produces ethyl iodide and ethanol via an S 2 mechanism. Correct Answer: Ethyl iodide and ethanol are formed.

Step 1: Reaction Overview
Dehydration of alcohols typically occurs in the presence of a strong acid, such as sulfuric acid ( ), resulting in the elimination of a water molecule to form an alkene.
Step 2: Mechanism
1. Protonation of the alcohol's hydroxyl group to form a better leaving group: 2. The protonated alcohol undergoes loss of water to form a carbocation: 3. The carbocation undergoes a rearrangement (if necessary) to form the most stable carbocation. 4. The carbocation then undergoes elimination of a proton ( ) to form the alkene: Final Answer: The dehydration of alcohols proceeds via the formation of a carbocation and elimination of a proton to form an alkene. Correct Answer: The mechanism involves protonation, carbocation formation, and proton elimination.

Esterification of carboxylic acids is typically carried out by reaction with an alcohol in the presence of an acid catalyst (usually concentrated sulfuric acid). This reaction forms an ester and water. The mechanism involves the following steps: 1. Protonation of the carboxyl group of the carboxylic acid to make it more electrophilic. 2. Nucleophilic attack of the alcohol’s hydroxyl group on the electrophilic carbonyl carbon of the carboxylic acid. 3. Formation of a tetrahedral intermediate, followed by proton transfer. 4. Loss of a proton from the tetrahedral intermediate to form the ester and water. (B) Hell-Volhard-Zelinsky Reaction: The Hell-Volhard-Zelinsky (HVZ) reaction is a halogenation of α-position of carboxylic acids. In this reaction, a carboxylic acid reacts with a halogen (such as bromine) in the presence of phosphorus tribromide (PBr₃) to form an α-halogenated carboxylic acid. The mechanism involves the formation of an acyl halide intermediate, which reacts with halogen to introduce a halogen at the α-position. Final Answer: The esterification of carboxylic acids involves a reaction with alcohols to form esters and water, with sulfuric acid acting as a catalyst. The Hell-Volhard-Zelinsky reaction involves the halogenation of the α-position of carboxylic acids using bromine and PBr₃. Correct Answer: The mechanisms for esterification and Hell-Volhard-Zelinsky reaction are provided above.

The order of a reaction refers to the sum of the exponents of the concentrations of the reactants in the rate law. The order indicates how the rate of reaction is affected by the concentration of the reactants.
1. For the first velocity equation :
The total order of the reaction is the sum of the exponents of the concentrations of the reactants. In this case, the exponents are 1/2 for A and 3/2 for B. Therefore, the total order is:
2. For the second velocity equation :
Similarly, the total order of the reaction is the sum of the exponents of the concentrations of the reactants. Here, the exponents are 3 for A and -1 for B. Therefore, the total order is:
Thus, for both reactions, the total order is 2.

(i) Wurtz Reaction:
The Wurtz reaction is a coupling reaction in which two alkyl halides react with sodium metal in dry ether to form a higher alkane. This reaction proceeds through the formation of free radicals, and the general reaction is as follows: Where: - is an alkyl halide (for example, ethyl chloride ), - is the resulting alkane (for example, ethane ), - is the sodium halide (for example, sodium chloride ). In this reaction, sodium (Na) donates electrons to the alkyl halide, generating free radicals. These free radicals combine to form a new alkane. The Wurtz reaction is primarily used to synthesize symmetrical alkanes.
(ii) Wurtz-Fittig Reaction:
The Wurtz-Fittig reaction is a coupling reaction that involves the reaction of an alkyl halide with an aryl halide in the presence of sodium metal, forming a biphenyl derivative or an alkyl-aromatic compound. It is a combination of the Wurtz reaction and the Fittig reaction. The general reaction is: Where: - is an alkyl halide, - is an aryl halide, - is the resulting alkyl-aromatic compound (for example, toluene, ). This reaction is used to form substituted benzene derivatives. Sodium metal reacts with the halides, resulting in the formation of free radicals that couple to produce the alkyl-aromatic compound.

- Sodium hydrogen sulphite reacts with aldehydes and ketones to form addition compounds, such as bisulfite adducts.
- Phenyl hydrazine reacts with carbonyl compounds (like acetone and benzaldehyde) to form hydrazones.
- Fehling's solution reacts with aldehydes, especially in the presence of a reducing agent.
- Grignard reagents do not react directly with acetone or benzaldehyde, as they are more likely to react with carbonyl compounds to form alcohols upon hydrolysis. Grignard reagents typically do not react with the carbonyl group in the way that sodium bisulfite or hydrazine does.
Therefore, the correct answer is option (D) Grignard reagent.

Dehydration reaction: A dehydration reaction is a chemical reaction in which a molecule of water ( ) is eliminated from the reacting molecule. In the case of alcohols, when heated with a strong acid catalyst (such as conc. H SO or H PO ), alcohols undergo dehydration to form alkenes.
This reaction follows the mechanism of E1 elimination for secondary and tertiary alcohols and E2 elimination for primary alcohols.
Mechanism of Dehydration of Alcohol (E1 for 2 or 3 alcohols): Step 1: Protonation of alcohol.
The lone pair of electrons on the oxygen atom of the alcohol attacks a proton ( ) from the acid catalyst, converting the hydroxyl group into a better leaving group (water). Step 2: Formation of carbocation.
The protonated alcohol loses a water molecule, forming a carbocation intermediate. Step 3: Elimination of proton.
A base (often ) abstracts a -hydrogen atom from the carbon adjacent to the carbocation, leading to the formation of a double bond (alkene). Overall reaction: