AS Chemistry: Halogenoalkanes — Revision Notes

Condensed for the final weeks. For the full explanation, use the Halogenoalkanes study guide.

Classification

Primary — carbon bonded to 1 other carbon. Secondary — 2. Tertiary — 3.

This classification decides the mechanism, so identify it first — it is worth doing as the very first step of any question involving a specific named halogenoalkane, before considering reagents or conditions at all.

Three routes to make a halogenoalkane

Halogenoalkanes are produced from each of the other AS organic classes — worth learning as a connected set rather than three separate facts:

ALKANE  + Cl2/Br2, UV light        -> free-radical substitution
ALKENE  + X2 or HX, room temp      -> electrophilic addition
ALCOHOL + HX(g); or KCl+conc H2SO4/H3PO4;
          or PCl3+heat; or PCl5; or SOCl2  -> substitution

The alcohol route has the most named reagent options — five different ways to swap an –OH for a halogen — while the alkane and alkene routes each have just one.

The two mechanisms

	SN2	SN1
Favoured by	Primary	Tertiary
Steps	One — concerted	Two — via carbocation
Rate depends on (A Level extension — not required at AS)	Both halogenoalkane and nucleophile	Halogenoalkane only
Intermediate	Transition state	Carbocation
Stereochemistry	Inversion of configuration	Racemic mixture (“racemic” is an A Level term — at AS, “equal amounts of both enantiomers” is enough)

Why tertiary favours SN1: three alkyl groups are electron-donating, stabilising the carbocation. Why primary favours SN2: less steric hindrance allows the nucleophile to attack the δ+ carbon.

At AS, this is described qualitatively — SN1 is generally slower for primary halogenoalkanes and faster for tertiary, due to the greater stability of more substituted carbocations. Writing out rate equations, rate constants or identifying the rate-determining step by name is A Level extension content (Topic 26), not required at AS.

Secondary halogenoalkanes proceed by both.

Drawing the mechanisms

• Curly arrows start from a lone pair or a bond, never from an atom.
• SN2: arrow from the nucleophile’s lone pair to the δ+ carbon; simultaneously C–X bond breaks heterolytically, arrow to the halogen.
• SN1: C–X breaks first forming the carbocation, then the nucleophile attacks.
• Show the δ+ and δ− on the C–X bond, and include the lone pair on the nucleophile.

The reactions to know

+ aqueous NaOH, heat under reflux  ->  ALCOHOL
+ ethanolic NaOH, heat             ->  ALKENE   (elimination)
+ ethanolic KCN, heat under reflux ->  NITRILE  (chain lengthens by 1 C)
+ ethanolic NH3, heat in sealed tube -> AMINE
+ aqueous AgNO3 in ethanol         ->  silver halide precipitate

Substitution vs elimination with hydroxide turns entirely on the solvent: aqueous → substitution to an alcohol; ethanolic → elimination to an alkene. That single word in the question decides the product.

Reactivity trend — C–X bond enthalpy

C-F  >  C-Cl  >  C-Br  >  C-I     bond enthalpy DECREASES
                                   reactivity INCREASES

Iodoalkanes react FASTEST (weakest bond, breaks most easily)
Fluoroalkanes are essentially unreactive

The explanation is bond enthalpy, not electronegativity — a common wrong answer, since fluorine is the most polar but the least reactive.

Hydrolysis rate test: warm with aqueous AgNO₃ in ethanol; the precipitate appears fastest for iodide (yellow), then bromide (cream), then chloride (white).

Worked example. Equal amounts of 1-chlorobutane, 1-bromobutane and 1-iodobutane are each warmed with aqueous silver nitrate. Explain the order in which a precipitate appears.

All three are PRIMARY -> all react by SN2, with water as the nucleophile
The single SN2 step includes breaking the C-X bond
Bond strength: C-Cl > C-Br > C-I  ->  weakest bond breaks FASTEST

Order of precipitate appearing (fastest to slowest):
  1-iodobutane   -> AgI, yellow           (fastest)
  1-bromobutane  -> AgBr, cream
  1-chlorobutane -> AgCl, white           (slowest)

Because all three here are primary, the mechanism (SN2) is the same throughout — the difference in rate comes entirely from bond strength, which is exactly why this question tests reactivity trend rather than mechanism choice.

Exam traps

• Explaining reactivity by electronegativity instead of bond enthalpy.
• Confusing aqueous with ethanolic conditions for hydroxide.
• Curly arrows starting from an atom rather than a bond or lone pair.
• Forgetting the lone pair on the nucleophile.
• Saying SN1 is faster because it has two steps — the rate depends on carbocation stability.
• Forgetting that comparing three primary halogenoalkanes’ hydrolysis rates tests bond strength, not mechanism — all three still react by SN2.
• Naming only one of the several reagents (HX(g), KCl/conc. acid, PCl₃, PCl₅, SOCl₂) that convert an alcohol to a halogenoalkane, when a question asks to “give a reagent.”

Self-test

1. Which mechanism does a tertiary halogenoalkane favour, and why?
2. What product forms with hot ethanolic NaOH?
3. Why do iodoalkanes hydrolyse faster than chloroalkanes?
4. Describe, qualitatively, how the rate of SN1 hydrolysis changes from a primary to a tertiary halogenoalkane.
5. Which reagent lengthens the carbon chain by one?
6. Name the three classes of AS organic compound from which a halogenoalkane can be made.
7. Equal amounts of 1-chlorobutane and 1-iodobutane are warmed with aqueous silver nitrate. Which produces a precipitate faster, and why?

Answers: 1. SN1 — the three electron-donating alkyl groups stabilise the carbocation intermediate. 2. An alkene, by elimination. 3. The C–I bond has the lowest bond enthalpy, so it breaks most readily; the trend follows bond strength, not polarity. 4. SN1 is slowest for a primary halogenoalkane and fastest for a tertiary one, because more alkyl groups stabilise the carbocation intermediate more effectively. 5. Ethanolic KCN — the nitrile formed adds one carbon. 6. Alkanes (free-radical substitution with X₂/UV), alkenes (electrophilic addition with X₂ or HX), and alcohols (substitution, e.g. with PCl₅). 7. 1-iodobutane, because the C–I bond is weaker than the C–Cl bond, so it breaks more readily and the substitution is faster, even though both react by the same SN2 mechanism.

For the full mechanism diagrams and production routes, see the Halogenoalkanes study guide.