Which of the following is an example of vic-dihalide? (i) Dichloromethane (ii) 1,2-dichloroethane (iii) Ethyli

NCERT Exemplar · Q11

Q. Which of the following is an example of vic-dihalide?

(i) Dichloromethane

(ii) 1,2-dichloroethane

(iii) Ethylidene chloride

(iv) Allyl chloride

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Concept understanding — Carboxylic Acid Halogenation

Carboxylic Acid Halogenation: The Hell–Volhard–Zelinsky Reaction

Imagine you have a carboxylic acid — say, propanoic acid ( $\mathrm{CH_3CH_2COOH}$ ). You want to replace one of the hydrogen atoms on the carbon chain with a halogen (like bromine or chlorine). But here's the catch: the carboxylic acid group ( $-\mathrm{COOH}$ ) is already quite reactive. If you just add bromine directly, nothing useful happens — the $\alpha$ -carbon (the carbon right next to the $-\mathrm{COOH}$ group) is not reactive enough to attack bromine on its own.

The trick is to activate the $\alpha$ -carbon first. This is exactly what the Hell–Volhard–Zelinsky (HVZ) reaction does.

The Intuition

The $-\mathrm{COOH}$ group is electron-withdrawing. That makes the $\alpha$ -carbon slightly positive (electrophilic), but not enough to react with a halogen directly. To make it work, we convert the acid into an acyl halide (like $\mathrm{RCOBr}$ ) using $\mathrm{PBr_3}$ or $\mathrm{PCl_3}$ . The acyl halide is even more electron-withdrawing, which makes the $\alpha$ -hydrogen more acidic — it can be removed by a base (like a catalytic amount of $\mathrm{PBr_3}$ or $\mathrm{Br_2}$ itself) to form an enol or enolate intermediate. This enol then attacks a halogen molecule, giving an $\alpha$ -haloacyl halide. Finally, water hydrolyses it back to the $\alpha$ -halo carboxylic acid.

In short: activate → enolize → halogenate → hydrolyse.

The Precise Statement

\mathrm{RCH_2COOH \xrightarrow[1.\ \mathrm{Br_2,\ PBr_3}]{2.\ \mathrm{H_2O}} RCHBrCOOH}

The reaction is regioselective: halogenation occurs exclusively at the $\alpha$ -carbon (the carbon adjacent to the $-\mathrm{COOH}$ group). No other position on the chain is halogenated.

Step-by-Step Mechanism

Formation of acyl bromide The carboxylic acid reacts with $\mathrm{PBr_3}$ (or $\mathrm{PCl_3}$ ) to form an acyl bromide:

\mathrm{RCH_2COOH + PBr_3 \rightarrow RCH_2COBr + H_3PO_3}

Enolization A catalytic amount of $\mathrm{PBr_3}$ or $\mathrm{Br_2}$ acts as a Lewis acid, making the $\alpha$ -hydrogen more acidic. A base (often $\mathrm{Br}^-$ from the reaction) abstracts this hydrogen, forming an enol:

\mathrm{RCH_2COBr \rightleftharpoons RCH=C(OH)Br}

Halogenation The enol attacks a $\mathrm{Br_2}$ molecule, giving the $\alpha$ -bromoacyl bromide:

\mathrm{RCH=C(OH)Br + Br_2 \rightarrow RCHBrCOBr + HBr}

Hydrolysis Water hydrolyses the acyl bromide back to the carboxylic acid:

\mathrm{RCHBrCOBr + H_2O \rightarrow RCHBrCOOH + HBr}

Note

The $\mathrm{PBr_3}$ is catalytic — it is regenerated in the hydrolysis step. Only a small amount is needed.

Why This Matters

The $\alpha$ -halo carboxylic acid is a versatile intermediate. You can:

Substitute the halogen with $\mathrm{OH}$ to get $\alpha$ -hydroxy acids (like lactic acid).
Substitute with $\mathrm{NH_3}$ to get $\alpha$ -amino acids (the building blocks of proteins). …

Why this formula?

Carboxylic Acid Halogenation — The Hell-Volhard-Zelinsky (HVZ) Reaction

Let's start with the core reaction and then unpack why it works the way it does.

The Reaction in a Nutshell

Carboxylic acids undergo α-halogenation (replacement of an α-hydrogen with a halogen) only under specific conditions:

\ce{R-CH2-COOH + Br2 ->[PBr3 (cat.)] R-CHBr-COOH + HBr}

The key reagents: Br₂ (or Cl₂) + a catalytic amount of PBr₃ (or PCl₃). The product is an α-halo carboxylic acid.

Why Does This Happen? The Step-by-Step Reasoning

1. The Problem: Carboxylic Acids Are Not Enolizable Directly

A carboxylic acid has a carbonyl group ( $\ce{C=O}$ ), but the α-hydrogen is not acidic enough to be removed by a base like $\ce{OH-}$ .
Why? The conjugate base (carboxylate ion, $\ce{RCOO-}$ ) is more stable than an enolate. So enolate formation is disfavoured.

Key insight: We need to activate the carbonyl first.

2. The Solution: Convert to an Acyl Halide (More Electrophilic)

PBr₃ reacts with the carboxylic acid to form an acyl bromide:

\ce{3 R-COOH + PBr3 -> 3 R-COBr + H3PO3}

The acyl bromide has a better leaving group (Br⁻ vs OH⁻) and a more electrophilic carbonyl carbon. This makes enolization easier.

3. Enolization of the Acyl Halide

A small amount of HBr (from the reaction) or Br₂ itself can act as a Lewis acid to polarize the carbonyl.
The α-hydrogen is now removable by a weak base (like Br⁻ or the enol itself), forming an enol:

\ce{R-CH2-COBr <=> R-CH=C(OH)Br}

This enol is nucleophilic at the α-carbon.

4. Halogenation of the Enol

The enol attacks Br₂ (or Cl₂) at the α-position:

\ce{R-CH=C(OH)Br + Br2 -> R-CHBr-C(OH)Br2 -> R-CHBr-COBr + HBr}

The product is an α-bromo acyl bromide.

5. Regeneration of the Acid

The α-bromo acyl bromide reacts with water (or with another molecule of carboxylic acid) to give the α-bromo carboxylic acid:

\ce{R-CHBr-COBr + H2O -> R-CHBr-COOH + HBr}

The HBr produced can re-enter the cycle, making the process catalytic in PBr₃.

The Key Formula(e) — Why They Hold

Overall Stoichiometry

\ce{R-CH2-COOH + Br2 ->[PBr3 (cat.)] R-CHBr-COOH + HBr}

Why this holds:

One Br₂ molecule provides one Br atom for substitution and one for HBr.
The catalyst (PBr₃) is not consumed — it is regenerated in the cycle. …

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Vic-dihalides have two halogen atoms on adjacent carbon atoms. The correct option is (ii) 1,2-dichloroethane, because its two chlorine atoms are on neighbouring carbons — the defining feature of a vicinal dihalide.

The concept: What makes a halide "vicinal"?

The word "vicinal" comes from the Latin vicinus, meaning "neighbouring". In organic chemistry, a vic-dihalide is any compound where two halogen atoms are attached to adjacent carbon atoms — that is, carbons that are directly bonded to each other.

The general structure looks like this:

\ce{R-CHX-CHY-R'} \quad \text{or} \quad \ce{X-CH2-CH2-Y}

where X and Y are halogens (often the same, but not required). The key is the 1,2-relationship — the halogens are on carbons 1 and 2 of a carbon chain.

This is different from a geminal dihalide (both halogens on the same carbon), or a 1,3-dihalide (halogens separated by one carbon), or a vinyl/allyl halide (halogen on a double-bond-adjacent carbon).

Now let's examine each option.

1. Option (i): Dichloromethane

Structure: $\ce{CH2Cl2}$

Both chlorine atoms are attached to the same carbon. This is a geminal dihalide (gem-dihalide), not vicinal. There is only one carbon atom here — no adjacent carbons to speak of.

Watch out

A common mistake is to think "two halogens = vicinal". The halogens must be on different, adjacent carbons. Dichloromethane has two chlorines, but they share one carbon — that's geminal, not vicinal.

2. Option (ii): 1,2-dichloroethane

Structure: $\ce{Cl-CH2-CH2-Cl}$

The two chlorine atoms are on carbon-1 and carbon-2 — which are adjacent carbons. This is the textbook example of a vicinal dihalide. The IUPAC name itself tells you: "1,2-dichloro" means the substituents are on neighbouring positions.

This compound is also called ethylene dichloride (not to be confused with ethylidene chloride — see below).

3. Option (iii): Ethylidene chloride

Structure: $\ce{CH3-CHCl2}$

The two chlorine atoms are both on the same carbon (the second carbon). This is another geminal dihalide. The name "ethylidene" refers to the $\ce{CH3-CH=}$ group, and the two chlorines are on the same carbon of that group. …

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

1. Confusing vic-dihalide with gem-dihalide

The Mistake:

Students often pick Dichloromethane (i) or Ethylidene chloride (iii) because they sound like “dihalides.”

Dichloromethane ( $CH_2Cl_2$ ) has both chlorines on the same carbon → this is a gem-dihalide (geminal).
Ethylidene chloride ( $CH_3CHCl_2$ ) also has both halogens on the same carbon → again, geminal.

Why it’s wrong:

Vicinal means “neighbouring” — the two halogen atoms must be on adjacent carbon atoms.
Geminal means “twin” — both halogens on the same carbon.

How to avoid:

Memorise the root: vic- = adjacent (like “vicinity”).
Draw the structure quickly: if both halogens are on the same C, it’s gem; if on two neighbouring C’s, it’s vic.

2. Misidentifying Allyl Chloride (iv)

The Mistake:

Some students think “allyl” means it must be a dihalide.

Allyl chloride is $CH_2=CH-CH_2Cl$ — it has only one chlorine atom.
A dihalide requires two halogen atoms.

Why it’s wrong:

The question asks for a vic-dihalide, which is a type of dihalide (two halogens).
Allyl chloride is a monohalide, so it cannot be any kind of dihalide.

How to avoid:

Count the number of halogen atoms in the formula before classifying.
If there’s only one halogen, it’s not a dihalide at all.

3. Forgetting the Definition of vic-Dihalide

The Mistake:

Students pick an answer based on name recognition (e.g., “1,2-dichloroethane sounds right but I’m not sure why”).

Why it’s wrong:

Without a clear definition, you can’t distinguish between vicinal, geminal, or other dihalides.

How to avoid:

Definition: A vic-dihalide has two halogen atoms on adjacent carbon atoms.
Example: 1,2-dichloroethane ( $ClCH_2-CH_2Cl$ ) — the two chlorines are on neighbouring carbons.
Write the definition at the top of your rough work before answering.

4. Overlooking the “1,2-” Naming Clue

The Mistake: …

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Q.Which of the following is an example of vic-dihalide?

Concept understanding — Carboxylic Acid Halogenation

Carboxylic Acid Halogenation: The Hell–Volhard–Zelinsky Reaction

The Intuition

The Precise Statement

Step-by-Step Mechanism

Why This Matters

Why this formula?

Carboxylic Acid Halogenation — The Hell-Volhard-Zelinsky (HVZ) Reaction

The Reaction in a Nutshell

Why Does This Happen? The Step-by-Step Reasoning

1. The Problem: Carboxylic Acids Are Not Enolizable Directly

2. The Solution: Convert to an Acyl Halide (More Electrophilic)

3. Enolization of the Acyl Halide

4. Halogenation of the Enol

5. Regeneration of the Acid

The Key Formula(e) — Why They Hold

Overall Stoichiometry

The concept: What makes a halide "vicinal"?

1. Option (i): Dichloromethane

2. Option (ii): 1,2-dichloroethane

3. Option (iii): Ethylidene chloride

Concept: Vicinal Dihalides

Method: Structural Analysis by IUPAC Name

Steps:

Applying the method:

1. Confusing vic-dihalide with gem-dihalide

2. Misidentifying Allyl Chloride (iv)

3. Forgetting the Definition of vic-Dihalide

4. Overlooking the “1,2-” Naming Clue

Q.Which of the following is an example of vic-dihalide?

The concept: What makes a halide "vicinal"?

1. Option (i): Dichloromethane

2. Option (ii): 1,2-dichloroethane

3. Option (iii): Ethylidene chloride

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