Q.(a) Write reasons for the following :

The Intuition: Why Some Groups "Point" the Next Attack

Imagine you're trying to add a second substituent to a benzene ring that already has one group attached. The ring already has six hydrogens, but they aren't all equal anymore — the first group has changed the electron density at different positions. Some positions become more "attractive" to an incoming electrophile (a positive or electron-seeking species), while others become less attractive.

Ortho-para directing groups are substituents that make the next electrophile prefer to attack the positions next to the group (ortho, positions 2 and 6) or directly opposite it (para, position 4), rather than the meta position (position 3 and 5).

The Precise Statement

The Mechanism: How They Work

The key lies in the stability of the intermediate carbocation (the arenium ion / sigma complex) formed during the attack.

When an electrophile attacks benzene, the ring temporarily loses its aromaticity and becomes a positively charged carbocation. This intermediate is stabilised if the positive charge can be delocalised onto the substituent. Ortho-para directing groups are able to donate electron density into the ring, either through:

1. Resonance effect (most important): The group has lone pairs or pi electrons that can be pushed into the ring, creating extra resonance structures where the positive charge is on the substituent (which is more stable).
2. Inductive effect: The group is electron-donating through sigma bonds (e.g., alkyl groups like methyl).

Let's see what happens when an electrophile attacks the ortho position of aniline (NH₂ group):

The Two Categories of Ortho-Para Directors

Hofmann Bromamide Degradation

Imagine you have an amide — a molecule with a carbonyl group ($\ce{-CO-}$) attached to a nitrogen. You want to turn it into a primary amine, but you also want to chop off one carbon from the chain. That is exactly what the Hofmann bromamide reaction does: it shortens the carbon skeleton by one carbon and gives you an amine.

The Intuition

The reaction uses bromine ($\ce{Br2}$) in the presence of a strong alkali (like $\ce{NaOH}$ or $\ce{KOH}$). The alkali first deprotonates the amide nitrogen, making it a strong nucleophile. This nucleophile attacks bromine, forming an N-bromoamide. Under the strongly basic conditions, this intermediate loses a bromide ion and undergoes a rearrangement — the alkyl group attached to the carbonyl carbon migrates from carbon to nitrogen. The result is an isocyanate intermediate ($\ce{R-N=C=O}$). Finally, the isocyanate is hydrolysed by the aqueous alkali to give a primary amine and carbon dioxide.

The net effect: the carbonyl carbon is lost as $\ce{CO2}$, and the alkyl group ends up attached to the nitrogen.

The Precise Statement

Hofmann bromamide degradation (also called Hofmann rearrangement) is the conversion of a primary amide to a primary amine with one fewer carbon atom, using bromine and an aqueous alkali (usually $\ce{NaOH}$ or $\ce{KOH}$).

The general reaction is:

Or, in a more compact form:

Step-by-Step Mechanism

1. Deprotonation: The amide nitrogen is deprotonated by the strong base, forming an amide anion.

2. Bromination: The amide anion attacks bromine, forming an N-bromoamide.

3. Second deprotonation: The N-bromoamide is deprotonated again by the base.

4. Rearrangement: The alkyl group migrates from the carbonyl carbon to the nitrogen, with simultaneous loss of bromide ion. This forms an isocyanate.

5. Hydrolysis: The isocyanate reacts with water to form a carbamic acid, which spontaneously decarboxylates (loses $\ce{CO2}$) to give the primary amine.

Key Points for Exams

• Starting material: Primary amide ($\ce{R-CONH2}$) only. Secondary or tertiary amides do not undergo this reaction.
• Reagents: $\ce{Br2}$ and $\ce{NaOH}$ (or $\ce{KOH}$). Sometimes $\ce{Cl2}$ can be used instead of $\ce{Br2}$, but bromine is more common.
• Product: Primary amine with one fewer carbon.
• By-products: $\ce{NaBr}$, $\ce{Na2CO3}$, $\ce{H2O}$ (or $\ce{CO2}$ if written in the simplified form). …