MHT-CET Chemistry · Halogen Derivatives of Alkanes

Preparation of Alkyl Halides

Alkyl halides come from alcohols (HX, PCl₅, SOCl₂), from alkenes by Markovnikov addition of HX, and from other halides by exchange — Finkelstein for iodides, Swarts for fluorides; the reverse direction, Wurtz and Fittig coupling with sodium, removes the halogen and joins two carbons.

All Halogen Derivatives of Alkanes notes MHT-CET Chemistry strategy

Why this matters

24 PYQs, none HARD. Of the fifteen set from 2021 to 2025, six are the two exchange reactions by name or product (Finkelstein → iodide, Swarts → fluoride, tert-butyl bromide + AgF), six are Wurtz, Fittig and Wurtz–Fittig — which is which, which halide couples to a named alkane, which alkane cannot form — and three are reagent-from-by-products or the alkene that adds HBr to a named product. Names and one Markovnikov rule.

Concept 1 of 3: From Alcohols and Alkenes

An alcohol's OH is swapped for X by HX (tertiary fastest), by PCl₅ (giving HCl and POCl₃) or by SOCl₂ (giving only gases, the cleanest). An alkene adds HX across the double bond with the halogen going to the MORE substituted carbon — Markovnikov — so 1-methylcyclohexene gives 1-bromo-1-methylcyclohexane.

Definition

$\text{R-OH} + \text{HX} \to \text{R-X} + \text{H}_2\text{O}$ ; with HCl needs ZnCl₂ (Lucas); rate 3° > 2° > 1°.
$\text{R-OH} + \text{PCl}_5 \to \text{R-Cl} + \text{HCl} + \text{POCl}_3$ — the by-products HCl + POCl₃ identify PCl₅. $3\text{R-OH} + \text{PCl}_3 \to 3\text{R-Cl} + \text{H}_3\text{PO}_3$ . $\text{R-OH} + \text{SOCl}_2 \to \text{R-Cl} + \text{SO}_2 + \text{HCl}$ (Darzen; gaseous by-products).
Alkene + HX: Markovnikov — H to the carbon with more H, X to the more substituted carbon. 1-Methylcyclohexene + HBr → 1-bromo-1-methylcyclohexane (3° bromide); propene + HBr → 2-bromopropane. Peroxide reverses it for HBr only.
Alkane + X₂ (light): free-radical substitution, a mixture.

Alcohol to halide

\text{R-OH} \xrightarrow{\text{HX / PCl}_5\text{ / SOCl}_2} \text{R-X};\qquad \text{alkene} + \text{HX} \xrightarrow{\text{Markovnikov}} \text{R-X}

Worked example

Name the product of 2-methylpropene with HBr, and identify Q in

\text{R-OH} + \text{Q} \to \text{R-Cl} + \text{SO}_2 + \text{HCl}

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 3rd May Shift 2 · Q84Easy

Example 1 · Halogen Derivatives of Alkanes · Preparation of Alkyl Halides

Identify compound Q in following reaction.

R\text{-}OH + Q \rightarrow R\text{-}Cl + HCl + POCl_3

Adding HBr the wrong way round

Without peroxide the H goes where the hydrogens already are. On 1-methylcyclohexene that puts Br on C1 with the methyl — the tertiary product — not on C2.

Drill 2 more on from alcohols and alkenes

Concept 2 of 3: Finkelstein and Swarts: Swapping the Halogen

Two exchanges with names. Finkelstein: chloride or bromide + NaI in dry acetone gives the iodide, pushed by NaCl or NaBr precipitating out of acetone. Swarts: chloride or bromide + a metal fluoride (AgF, Hg₂F₂, CoF₂, SbF₃) gives the fluoride. Both keep the carbon skeleton — tert-butyl bromide + AgF is 2-fluoro-2-methylpropane.

Definition

Finkelstein: $\text{R-Cl} + \text{NaI} \xrightarrow{\text{dry acetone}} \text{R-I} + \text{NaCl}\downarrow$ — alkyl IODIDES.
Swarts: $\text{R-Br} + \text{AgF} \to \text{R-F} + \text{AgBr}$ — alkyl FLUORIDES (also Hg₂F₂, CoF₂, SbF₃).
tert-Butyl bromide + AgF → 2-fluoro-2-methylpropane, skeleton unchanged.
Fluorination of chloroform gives refrigerant-22, $\text{CHClF}_2$ : $\text{CHCl}_3 \xrightarrow{\text{SbF}_3} \text{CHClF}_2$ .

The two exchanges

\text{Finkelstein: R-X} + \text{NaI} \to \text{R-I};\qquad \text{Swarts: R-X} + \text{AgF} \to \text{R-F}

Worked example

How would you convert 1-chloropropane into 1-iodopropane, and into 1-fluoropropane?

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 9th May Shift 1 · Q86Easy

Example 2 · Halogen Derivatives of Alkanes · Preparation of Alkyl Halides

Which among the following reactions is used for the preparation of alkyl fluorides?

Swapping the two names

Finkelstein — Iodide (NaI). Swarts — Fluoride (AgF). The pairing is asked more often than either reaction, and always with both names as options.

Drill 5 more on finkelstein and swarts: swapping the halogen

Concept 3 of 3: Wurtz, Fittig and Wurtz–Fittig Coupling

Sodium in dry ether pulls the halogens off two molecules and joins the carbons. Two alkyl halides — Wurtz, an alkane with an even carbon count. Two aryl halides — Fittig, a biaryl. One of each — Wurtz–Fittig, an alkylbenzene. A MIXTURE of two alkyl halides gives three alkanes, never one with more carbons than the two combined.

Definition

Wurtz: $2\text{R-X} + 2\text{Na} \xrightarrow{\text{dry ether}} \text{R-R} + 2\text{NaX}$ . tert-Butyl bromide → 2,2,3,3-tetramethylbutane.
Mixture CH₃Br + C₂H₅Br: ethane, propane, butane — NOT pentane.
Fittig: $2\text{C}_6\text{H}_5\text{Cl} + 2\text{Na} \to \text{C}_6\text{H}_5\text{-C}_6\text{H}_5$ (biphenyl).
Wurtz–Fittig: $\text{C}_6\text{H}_5\text{Br} + \text{CH}_3\text{Br} + 2\text{Na} \to \text{C}_6\text{H}_5\text{CH}_3$ (toluene).
Wurtz fails for methyl (no C₂ from one carbon... it gives ethane) and gives poor yields with tertiary halides (elimination competes).

Sodium coupling

\text{R-X} + \text{R'-X} + 2\text{Na} \xrightarrow{\text{dry ether}} \text{R-R'} + 2\text{NaX}

Worked example

Which halide gives 2,3-dimethylbutane by the Wurtz reaction, and what does bromobenzene with ethyl bromide and sodium give?

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2021 · May Shift 1 · Q91Moderate

Example 3 · Halogen Derivatives of Alkanes · Preparation of Alkyl Halides

Which of the following alkyl halides is treated with sodium metal to obtain 2,2,3,3-tetramethylbutane?

Calling the aryl + alkyl coupling 'Fittig'

Fittig needs TWO aryl halides and gives a biaryl. Aryl + alkyl is Wurtz–Fittig. The 2025 papers printed the name as 'Wurtzilite'; the intended reaction is the same.

Drill 5 more on wurtz, fittig and wurtz–fittig coupling

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (3)

From Alcohols and Alkenes
Alcohol to halide
$\text{R-OH} \xrightarrow{\text{HX / PCl}_5\text{ / SOCl}_2} \text{R-X};\qquad \text{alkene} + \text{HX} \xrightarrow{\text{Markovnikov}} \text{R-X}$
Finkelstein and Swarts: Swapping the Halogen
The two exchanges
$\text{Finkelstein: R-X} + \text{NaI} \to \text{R-I};\qquad \text{Swarts: R-X} + \text{AgF} \to \text{R-F}$
Wurtz, Fittig and Wurtz–Fittig Coupling
Sodium coupling
$\text{R-X} + \text{R'-X} + 2\text{Na} \xrightarrow{\text{dry ether}} \text{R-R'} + 2\text{NaX}$

Watch out for (3)

Adding HBr the wrong way round
→ From Alcohols and Alkenes
Swapping the two names
→ Finkelstein and Swarts: Swapping the Halogen
Calling the aryl + alkyl coupling 'Fittig'
→ Wurtz, Fittig and Wurtz–Fittig Coupling

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