Q.Give structures of the products you would expect when each of the following alcohol reacts with

The key idea is that the reagent determines the mechanism: primary alcohols follow $S_N2$ with these reagents, while tertiary alcohols follow $S_N1$ (via a carbocation). The product in each case is an alkyl halide.

Step 1 – Butan-1-ol (primary): With $HCl-ZnCl_2$ (Lucas reagent), HBr, or $SOCl_2$, the reaction proceeds via $S_N2$. The $OH$ group is converted to a leaving group (water or a chlorosulfite ester), and the halide attacks from the back. The product is 1-chlorobutane, 1-bromobutane, or 1-chlorobutane respectively.

Step 2 – 2-Methylbutan-2-ol (tertiary): With $HCl-ZnCl_2$ or HBr, the reaction proceeds via $S_N1$. Protonation of $OH$ is followed by loss of water to form a tertiary carbocation, which is then trapped by the halide. The product is 2-chloro-2-methylbutane or 2-bromo-2-methylbutane. With $SOCl_2$, tertiary alcohols also react via $S_N1$ (or an $S_Ni$ mechanism), giving the same chloroalkane.

The reagents

• $HCl$-$ZnCl_2$ (Lucas reagent): converts $-OH$ to $-Cl$. Fast for 3 deg (via a carbocation, $S_N1$), slow for 1 deg ($S_N2$).
• $HBr$: converts $-OH$ to $-Br$ (1 deg by $S_N2$, 3 deg by $S_N1$).
• $SOCl_2$ (thionyl chloride): converts $-OH$ to $-Cl$, with $SO_2$ and $HCl$ escaping as gases -- the cleanest route to the alkyl chloride.

(i) Butan-1-ol -- $CH_3CH_2CH_2CH_2OH$ (primary)

• (a) $HCl$-$ZnCl_2$: $CH_3CH_2CH_2CH_2Cl$ -- 1-chlorobutane
• (b) $HBr$: $CH_3CH_2CH_2CH_2Br$ -- 1-bromobutane
• (c) $SOCl_2$: $CH_3CH_2CH_2CH_2Cl$ -- 1-chlorobutane

(ii) 2-Methylbutan-2-ol -- $CH_3\text{-}C(CH_3)(OH)\text{-}CH_2CH_3$ (tertiary)

• (a) $HCl$-$ZnCl_2$: $CH_3\text{-}CCl(CH_3)\text{-}CH_2CH_3$ -- 2-chloro-2-methylbutane
• (b) $HBr$: $CH_3\text{-}CBr(CH_3)\text{-}CH_2CH_3$ -- 2-bromo-2-methylbutane
• (c) $SOCl_2$: $CH_3\text{-}CCl(CH_3)\text{-}CH_2CH_3$ -- 2-chloro-2-methylbutane

Williamson Ether Synthesis — Concept & Method

Method: Nucleophilic Substitution (SN2) via Alkoxide + Alkyl Halide

Core Concept

Williamson ether synthesis is an SN2 reaction between an alkoxide ion (strong nucleophile) and a primary alkyl halide (electrophile). The alkoxide is prepared by reacting an alcohol with a strong base (e.g., NaH, Na metal).

Steps

1. Deprotonate the alcohol using a strong base to form the alkoxide ion.
2. Choose the alkyl halide — must be primary (or methyl) to avoid elimination.
3. Mix alkoxide + alkyl halide — SN2 attack gives the ether.

⚠️ Key exam point: If the alcohol is tertiary, use the alkoxide from the smaller alcohol and the halide from the tertiary carbon — but this often fails due to elimination. For tertiary alcohols, Williamson is not preferred.

Reaction of Alcohols with (a) HCl–ZnCl₂,

(b) HBr,

(c) SOCl₂

These are not Williamson ether synthesis — they are substitution reactions converting alcohols to alkyl halides.

(i) Butan-1-ol (primary alcohol)

Structures:

• 1-Chlorobutane: $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{Cl}$
• 1-Bromobutane: $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{Br}$

(ii) 2-Methylbutan-2-ol (tertiary alcohol)

Structures:

• 2-Chloro-2-methylbutane: $\text{CH}_3\text{CH}_2\text{C(Cl)(CH}_3\text{)}_2$
• 2-Bromo-2-methylbutane: $\text{CH}_3\text{CH}_2\text{C(Br)(CH}_3\text{)}_2$

📌 Exam-Ready Summary Table

Key distinction: Primary alcohols follow SN2 (inversion, no rearrangement), tertiary alcohols follow SN1 (racemization, possible rearrangement).

✗ Mistake 1: Confusing Williamson Ether Synthesis with Alcohol Substitution Reactions

The error:

Students see "alcohol" and "HCl–ZnCl₂" and immediately think of Williamson Ether Synthesis (which uses an alkoxide + alkyl halide). But this question is about nucleophilic substitution of the –OH group, not ether formation.

How to avoid:

• Williamson Ether Synthesis requires:

  • An alkoxide ion (RO⁻)
  • A primary alkyl halide (R'–X)
  • No strong acid present

• This question gives:

  • Alcohol + HX or SOCl₂ → substitution product (alkyl halide or sulfite ester)

Key rule: If you see HX or SOCl₂, think substitution, not ether.

✗ Mistake 2: Ignoring Carbocation Stability (for HBr and HCl–ZnCl₂)

The error:

Students treat all alcohols the same — they write the same product for butan-1-ol and 2-methylbutan-2-ol.

Why it's wrong:

• Butan-1-ol (primary alcohol) reacts via SN2 with HBr/HCl–ZnCl₂ → no rearrangement, product is 1-bromobutane / 1-chlorobutane.
• 2-Methylbutan-2-ol (tertiary alcohol) reacts via SN1 → carbocation forms, can rearrange (hydride/methyl shift) to a more stable carbocation.

How to avoid:

• For primary alcohols: SN2 — no rearrangement.
• For tertiary alcohols: SN1 — always check for possible carbocation rearrangements.
• For secondary alcohols: could be SN1 or SN2 depending on conditions — but in this question, only primary and tertiary are given.

Example for (ii) with HBr:

• Initial carbocation: (CH₃)₂C⁺–CH₂–CH₃ (tertiary, stable)
• No rearrangement needed — product is 2-bromo-2-methylbutane directly.

✗ Mistake 3: Forgetting that SOCl₂ inverts stereochemistry (for chiral alcohols)

The error:

Students write the product as R–Cl without considering stereochemistry. But SOCl₂ with pyridine gives inversion via SN2.

How to avoid:

• If the alcohol is chiral (not the case here, but common in exams), SOCl₂ + pyridine → inverted alkyl chloride.
• Without pyridine, it can go through an SNi mechanism (retention) — but for exam purposes, assume inversion with pyridine.

For butan-1-ol:

• Not chiral → no stereochemical issue. Product is 1-chlorobutane.

✗ Mistake 4: Writing wrong product for SOCl₂ with tertiary alcohol

The error:

Students assume SOCl₂ always gives alkyl chloride via SN2 — but tertiary alcohols can't do SN2.

Reality:

• SOCl₂ with tertiary alcohol often gives elimination (alkene) as major product, not substitution.
• In some conditions, substitution occurs via SN1, but elimination is favoured.

How to avoid:

• For tertiary alcohols, SOCl₂ → major product is alkene (E1 elimination).
• For primary alcohols, SOCl₂ → clean SN2 → alkyl chloride.

✓ Summary Table: Correct Products

🧠 Final Exam Tip

Read the reagent list carefully.

If you see HX or SOCl₂, it's not Williamson Ether Synthesis — it's alcohol substitution/elimination.

Always check:

• Primary → SN2 (no rearrangement)
• Tertiary → SN1/E1 (check for rearrangement, elimination often major with SOCl₂)