Haloalkanes: Nucleophilic Substitution

Haloalkanes are compounds in which one or more hydrogen atoms of an alkane are replaced by halogen atoms. They are important synthetic intermediates — the polar C–X bond makes them highly reactive towards nucleophiles, opening the door to many functional group conversions: alcohols, nitriles, amines, ethers and more. At A-Level you are expected to understand the mechanism and use it to make organic synthesis plans.

This lesson covers the OCR A-Level Chemistry A (H432) specification point 4.2.2 (a)–(b): nucleophilic substitution reactions of haloalkanes with hydroxide, cyanide and ammonia; the SN2 mechanism for primary haloalkanes.

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1. The C–X Bond: Polarity and Reactivity

Halogens are more electronegative than carbon, so the C–X bond is polar:

$\overset{\delta+}{C}\text{—}\overset{\delta-}{X}$Cδ+—Xδ−

Electronegativities (Pauling): C = 2.55, F = 3.98, Cl = 3.16, Br = 2.96, I = 2.66.

The carbon bears a partial positive charge and is therefore susceptible to attack by nucleophiles — species that donate a lone pair of electrons to form a new bond. The halogen leaves as a halide ion (X⁻), taking both bonding electrons with it.

Key Definition — Nucleophile: An electron pair donor. Examples: OH⁻, CN⁻, NH₃, H₂O, RO⁻.

Key Definition — Nucleophilic substitution: A reaction in which a nucleophile replaces a leaving group by donating a lone pair of electrons to an electrophilic carbon.

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2. Nucleophilic Substitution with Hydroxide: Making Alcohols

Heating a haloalkane with aqueous sodium (or potassium) hydroxide produces an alcohol. The hydroxide ion is the nucleophile.

$R\text{-}X + OH^- \rightarrow R\text{-}OH + X^-$R-X+OH−→R-OH+X−

Conditions:

• Aqueous NaOH (or KOH) — not ethanolic.
• Warm under reflux.

Example: 1-Bromobutane → butan-1-ol.

$CH_3CH_2CH_2CH_2Br + OH^- \rightarrow CH_3CH_2CH_2CH_2OH + Br^-$CH3​CH2​CH2​CH2​Br+OH−→CH3​CH2​CH2​CH2​OH+Br−

This reaction is also the basis of the hydrolysis test for identifying haloalkanes: add AgNO₃ in ethanol and water, and the precipitate that forms (white AgCl, cream AgBr, yellow AgI) identifies the halogen (see Lesson 5).

graph LR
  A[Haloalkane R-X] -->|+ OH-<br/>aq NaOH, reflux| B[Alcohol R-OH + X-]

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3. Nucleophilic Substitution with Cyanide: Making Nitriles (Extending the Chain)

Heating a haloalkane with ethanolic potassium cyanide replaces the halogen with a cyano group (–CN):

$R\text{-}X + CN^- \rightarrow R\text{-}CN + X^-$R-X+CN−→R-CN+X−

Conditions:

• KCN dissolved in ethanol (not water — water favours hydroxide substitution because OH⁻ is formed from the equilibrium KCN + H₂O).
• Heat under reflux.

Example: Bromoethane → propanenitrile.

$CH_3CH_2Br + CN^- \rightarrow CH_3CH_2CN + Br^-$CH3​CH2​Br+CN−→CH3​CH2​CN+Br−

Why This Reaction Is Important: Extending the Carbon Chain

Notice that the nitrile adds one carbon atom to the chain — bromoethane (C₂) becomes propanenitrile (C₃). This is one of only a few A-Level reactions that increases the length of the carbon skeleton, which makes it invaluable in synthesis.

Once you have a nitrile, you can:

• Hydrolyse it (dilute HCl, reflux) to the carboxylic acid: R-CN → R-COOH + NH₃.
• Reduce it (H₂, Ni catalyst, or LiAlH₄) to the amine: R-CN → R-CH₂NH₂.

graph LR
  A[Haloalkane R-X<br/>n carbons] -->|+ CN-<br/>ethanol, reflux| B[Nitrile R-CN<br/>n+1 carbons]
  B -->|H2 / Ni or LiAlH4| C[Amine R-CH2-NH2]
  B -->|dilute HCl, reflux| D[Carboxylic acid R-COOH]

Exam tip: If a synthesis question requires you to add a carbon, think KCN in ethanol — it is the most reliable way at A-Level.

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4. Nucleophilic Substitution with Ammonia: Making Amines

Heating a haloalkane with excess ethanolic ammonia in a sealed tube produces a primary amine.

$R\text{-}X + 2\,NH_3 \rightarrow R\text{-}NH_2 + NH_4X$R-X+2NH3​→R-NH2​+NH4​X

Note that two molecules of ammonia are needed: one to act as the nucleophile, the second to neutralise the HX released.

Conditions:

• Concentrated ammonia dissolved in ethanol.
• Excess ammonia (to minimise further substitution).
• Sealed tube (to stop NH₃ escaping) and warmed.

Example: Bromoethane → ethylamine.

$CH_3CH_2Br + 2\,NH_3 \rightarrow CH_3CH_2NH_2 + NH_4Br$CH3​CH2​Br+2NH3​→CH3​CH2​NH2​+NH4​Br

The "Over-alkylation" Problem

The primary amine product is itself a nucleophile (the lone pair on nitrogen), and it can react with another molecule of haloalkane to give a secondary amine, which can react again to give tertiary, and finally give a quaternary ammonium salt:

graph LR
  A[R-X] -->|NH3| B[R-NH2 primary]
  B -->|R-X| C[R2-NH secondary]
  C -->|R-X| D[R3-N tertiary]
  D -->|R-X| E[R4-N+ X- quaternary]

To maximise the primary amine, you use a huge excess of ammonia so the haloalkane is more likely to meet NH₃ than R–NH₂.

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5. The SN2 Mechanism for Primary Haloalkanes

OCR A-Level requires you to be able to draw the mechanism for nucleophilic substitution on primary haloalkanes. This is the SN2 mechanism: Substitution, Nucleophilic, order 2 (bimolecular).

5.1 Key Features of SN2