Assertion: Aromatic 1° amines can be prepared by Gabriel Phthalimide Synthesis. Reason: Aryl halides undergo n

NCERT Exemplar · Q76

Q. Assertion: Aromatic 1° amines can be prepared by Gabriel Phthalimide Synthesis. Reason: Aryl halides undergo nucleophilic substitution with anion formed by phthalimide.

(i) Both assertion and reason are wrong.

(ii) Both assertion and reason are correct statements but reason is not correct explanation of assertion.

(iii) Assertion is correct statement but reason is wrong statement.

(iv) Both assertion and reason are correct statements and reason is correct explanation of assertion.

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Concept understanding — Gabriel Phthalimide Limitation

The Gabriel Phthalimide Limitation – First Principles

Imagine you want to make a primary amine ( $R-NH_2$ ) from an alkyl halide ( $R-X$ ). The obvious idea is to just let ammonia ( $NH_3$ ) attack the halide. But ammonia is a nucleophile and also a base — it will keep reacting. The product amine is even more nucleophilic than ammonia, so it attacks another alkyl halide molecule, giving a secondary amine ( $R_2NH$ ), then tertiary ( $R_3N$ ), and finally a quaternary ammonium salt ( $R_4N^+$ ). You end up with a messy mixture.

The Gabriel synthesis was invented to solve this: it gives only the primary amine, cleanly. But it has a hard limit.

The Intuition: Why the Gabriel Method Works (and Where It Breaks)

The trick is to use phthalimide — a molecule with two carbonyl groups flanking an $N-H$ bond. The $N-H$ is acidic enough to be deprotonated by a mild base (like $KOH$ or $K_2CO_3$ ), giving a phthalimide anion. This anion is a great nucleophile but a terrible base — it won't deprotonate the alkyl halide or cause elimination. It attacks the alkyl halide in an $S_N2$ reaction, forming an $N$ -alkylphthalimide.

Then you hydrolyse (or use hydrazine) to break the two amide bonds, releasing the primary amine and regenerating phthalic acid.

Note

The key limitation is $S_N2$ reactivity. The phthalimide anion is bulky and not very nucleophilic. It can only attack primary alkyl halides (or very reactive secondary ones like allyl/benzyl halides). Tertiary halides? They undergo elimination instead of substitution. Secondary halides? Very slow, often give poor yields.

The Precise Statement of the Limitation

Gabriel phthalimide synthesis fails for alkyl halides that are sterically hindered or prone to elimination. Specifically:

Tertiary alkyl halides ( $R_3C-X$ ) do not react — they undergo $E2$ elimination instead of $S_N2$ substitution.

Secondary alkyl halides ( $R_2CH-X$ ) react very slowly, if at all, and yields are poor.

Aryl halides (like chlorobenzene) do not react because $S_N2$ on an $sp^2$ carbon is impossible.

Alkyl halides with bulky groups near the reaction centre (neopentyl, etc.) also fail.

Watch out

A common mistake: students think the limitation is about the hydrolysis step. No — the limitation is entirely in the alkylation step. The phthalimide anion simply cannot force an $S_N2$ reaction on a hindered carbon.

Why This Matters for Exams

You'll be asked to identify which alkyl halides cannot be used in the Gabriel synthesis. The answer is always: tertiary halides, most secondary halides, and aryl halides. For example:

Alkyl Halide	Works?	Reason
$CH_3CH_2CH_2Br$	Yes	Primary, unhindered
$(CH_3)_2CHBr$	Poor	Secondary, slow $S_N2$
$(CH_3)_3CBr$	No	Tertiary — elimination dominates
$C_6H_5Br$	No	Aryl — $S_N2$ impossible on $sp^2$ carbon
$CH_2=CHCH_2Br$	Yes	Allylic — very reactive $S_N2$

Important

The Gabriel phthalimide synthesis is a reliable method only for preparing primary amines from primary alkyl halides (or very reactive secondary ones). For tertiary amines or hindered substrates, you need alternative methods (like reduction of nitriles or amides).

The Deeper Reason (For the Curious) …

Why this formula?

Gabriel Phthalimide Limitation — Why It Exists

The Gabriel phthalimide synthesis is a classic method to prepare primary amines ( $\text{R-NH}_2$ ) from alkyl halides. However, it has a critical limitation: it fails with secondary and tertiary alkyl halides (and also with aryl halides). Let's understand why this happens — the reasoning is rooted in reaction mechanism and steric hindrance.

1. The Key Reaction Steps (Brief Recap)

The synthesis proceeds in two main steps:

Formation of potassium phthalimide Phthalimide ( $\text{C}_6\text{H}_4(\text{CO})_2\text{NH}$ ) is treated with alcoholic KOH to give the potassium salt:

\text{C}_6\text{H}_4(\text{CO})_2\text{NH} + \text{KOH} \rightarrow \text{C}_6\text{H}_4(\text{CO})_2\text{N}^- \text{K}^+ + \text{H}_2\text{O}

N-alkylation (the critical step) The phthalimide anion acts as a nucleophile and attacks an alkyl halide ( $\text{R-X}$ ) via an $\text{S}_\text{N}2$ mechanism:

\text{C}_6\text{H}_4(\text{CO})_2\text{N}^- + \text{R-X} \rightarrow \text{C}_6\text{H}_4(\text{CO})_2\text{N-R} + \text{X}^-

Hydrolysis to release the primary amine.

2. Why the Limitation Exists — The $\text{S}_\text{N}2$ Bottleneck

The key formula that governs the success of this reaction is the rate law for $\text{S}_\text{N}2$ :

\text{Rate} = k[\text{Nucleophile}][\text{Alkyl halide}]

For the Gabriel synthesis, the nucleophile is the phthalimide anion — a bulky, planar, and resonance-stabilized species. This has two consequences:

A. Steric Hindrance at the Electrophilic Carbon

In an $\text{S}_\text{N}2$ reaction, the nucleophile must approach the backside of the carbon bearing the leaving group.
Primary alkyl halides ( $\text{RCH}_2\text{X}$ ) have a small, unhindered carbon — the nucleophile can easily attack.
Secondary alkyl halides ( $\text{R}_2\text{CHX}$ ) have moderate steric hindrance — the bulky phthalimide anion struggles to approach.
Tertiary alkyl halides ( $\text{R}_3\text{CX}$ ) are severely hindered — the backside is blocked by three alkyl groups. The $\text{S}_\text{N}2$ transition state is impossible to achieve.

B. The $\text{S}_\text{N}2$ Transition State Geometry

The $\text{S}_\text{N}2$ transition state requires a linear arrangement of nucleophile, carbon, and leaving group:

\text{Nu}^- \cdots \text{C} \cdots \text{X}

For the phthalimide anion, this linear approach is sterically impossible when the carbon is tertiary (and difficult for secondary). The bulky phthalimide group cannot fit into the crowded transition state.

3. What Happens Instead? — Elimination Dominates

When a secondary or tertiary alkyl halide is used, the strongly basic phthalimide anion does not perform $\text{S}_\text{N}2$ — it instead acts as a base and promotes $\text{E}2$ elimination:

\text{R}_3\text{C-X} + \text{Phth}^- \rightarrow \text{Alkene} + \text{H-Phth} + \text{X}^-

This is because: …

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Gabriel Phthalimide Synthesis fails for aromatic primary amines because aryl halides do not undergo nucleophilic substitution with the phthalimide anion under normal conditions. The assertion is wrong, the reason is also wrong — so both are incorrect.

The Concept First

Gabriel Phthalimide Synthesis is a classic method to prepare aliphatic primary amines cleanly, without over-alkylation. The trick: phthalimide has an acidic N–H (pKa ~8.3) because the two carbonyl groups pull electron density away. Deprotonate it with a base like KOH, and you get a strong nucleophile — the phthalimide anion. This anion attacks an alkyl halide via SN2, giving an N-alkylphthalimide. Then hydrolysis (or hydrazinolysis) liberates the primary amine.

The key limitation: the alkyl halide must be a good SN2 substrate. Aryl halides (like chlorobenzene) are terrible at SN2 — the carbon-halogen bond has partial double-bond character, and the aromatic ring sterically blocks backside attack. So the reaction simply does not work for aromatic amines.

Now let's examine the assertion and reason.

Step-by-Step Reasoning

1. The assertion says: "Aromatic 1° amines can be prepared by Gabriel Phthalimide Synthesis."

This is false. Gabriel synthesis works only for aliphatic primary amines. To make an aromatic primary amine like aniline, you'd need to reduce nitrobenzene (or use other methods). The phthalimide anion cannot displace a halide from an aryl halide under normal conditions.

2. The reason says: "Aryl halides undergo nucleophilic substitution with anion formed by phthalimide."

This is also false. Aryl halides do undergo nucleophilic substitution, but only under extreme conditions (high temperature, strong electron-withdrawing groups, or via benzyne mechanism). The phthalimide anion is not a strong enough nucleophile to force this, and typical Gabriel conditions (reflux in ethanol or DMF) are far too mild. So the reason is factually incorrect. …

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Here’s a breakdown of the common mistakes students make on this specific assertion-reason question, along with how to avoid each.

Mistake 1: Assuming Gabriel Phthalimide Synthesis works for all 1° amines

The mistake: Students often memorize that Gabriel Phthalimide Synthesis is a general method for preparing primary amines and forget the critical limitation — it only works for aliphatic primary amines, not aromatic ones.

Why it’s wrong: The reaction involves an alkyl halide ( $R-X$ ) reacting with the phthalimide anion. Aryl halides ( $Ar-X$ ) do not undergo nucleophilic substitution under these conditions because the carbon-halogen bond in aryl halides has partial double-bond character and is much less reactive.

How to avoid: Always mentally add the word “aliphatic” when recalling the scope of Gabriel Phthalimide Synthesis. Make a clear rule:

✓ Alkyl halides → works
✗ Aryl halides → does not work

Mistake 2: Confusing the reactivity of aryl halides in nucleophilic substitution

The mistake: Students think that because phthalimide anion is a strong nucleophile, it can displace halide from an aromatic ring just like it does from an alkyl halide.

Why it’s wrong: Aryl halides do not undergo simple $S_N2$ reactions due to steric hindrance and the $sp^2$ hybridized carbon. They require extreme conditions (high temperature, strong electron-withdrawing groups) for nucleophilic aromatic substitution — conditions not present in Gabriel synthesis.

How to avoid: Remember the key difference:

Alkyl halides → $S_N2$ possible
Aryl halides → $S_N2$ impossible under normal conditions

Mistake 3: Misreading the “Reason” statement

The mistake: Students see “nucleophilic substitution” in the Reason and assume it’s correct because they know phthalimide anion acts as a nucleophile.

Why it’s wrong: The Reason says “Aryl halides undergo nucleophilic substitution with anion formed by phthalimide.” This is factually incorrect — aryl halides do not undergo this reaction. The statement is wrong, not just incomplete.

How to avoid: Read the Reason statement literally and check if it matches known facts. If it says “aryl halides undergo” — ask yourself: Do they really? If not, mark the Reason as wrong.

Mistake 4: Choosing option (B) — “Both correct but Reason not correct explanation” …

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Q.Assertion: Aromatic 1° amines can be prepared by Gabriel Phthalimide Synthesis.
Reason: Aryl halides undergo nucleophilic substitution with anion formed by phthalimide.

Concept understanding — Gabriel Phthalimide Limitation

The Gabriel Phthalimide Limitation – First Principles

The Intuition: Why the Gabriel Method Works (and Where It Breaks)

The Precise Statement of the Limitation

Why This Matters for Exams

The Deeper Reason (For the Curious) …

Why this formula?

Gabriel Phthalimide Limitation — Why It Exists

1. The Key Reaction Steps (Brief Recap)

2. Why the Limitation Exists — The $\text{S}_\text{N}2$ Bottleneck

A. Steric Hindrance at the Electrophilic Carbon

B. The $\text{S}_\text{N}2$ Transition State Geometry

3. What Happens Instead? — Elimination Dominates

The Concept First

Step-by-Step Reasoning

Concept: Gabriel Phthalimide Synthesis — Scope and Limitation

Method: Reaction Feasibility Analysis

Mistake 1: Assuming Gabriel Phthalimide Synthesis works for all 1° amines

Mistake 2: Confusing the reactivity of aryl halides in nucleophilic substitution

Mistake 3: Misreading the “Reason” statement

Mistake 4: Choosing option (B) — “Both correct but Reason not correct explanation” …

Q.Assertion: Aromatic 1° amines can be prepared by Gabriel Phthalimide Synthesis. Reason: Aryl halides undergo nucleophilic substitution with anion formed by phthalimide.

The Concept First

Step-by-Step Reasoning

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Q.Assertion: Aromatic 1° amines can be prepared by Gabriel Phthalimide Synthesis.
Reason: Aryl halides undergo nucleophilic substitution with anion formed by phthalimide.

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