Nucleophilic Substitution

Halogenoalkanes undergo substitution reactions when they react with nucleophiles.

Substitution – this is when a halogen atom in an organic molecule is replaced by a different atom or group.

Nucleophile – a species that donates a lone pair of electrons, such as or .

Understanding the mechanism

In organic chemistry, mechanisms are used to show the step-by-step process of how a reaction occurs.

Nucleophiles are negatively charged or neutral species with a lone pair of electrons. They attack carbon atoms that are slightly positive (δ⁺) due to the polar nature of the carbon–halogen bond. The halogen is more electronegative than carbon, pulling electrons towards itself and leaving the carbon slightly electron-deficient – making it open to attack by a nucleophile.

We use curly arrows to represent the movement of a pair of electrons. These always start from either a lone pair or the middle of a bond.

The speed of substitution depends heavily on how easy it is to break the C–X bond.

• Weaker bonds are easier to break – so they react faster.

• Stronger bonds are harder to break – so substitution is slower.

Bond enthalpies of C–X bonds :

So, iodoalkanes (containing iodine) react the fastest, and fluoroalkanes are the least reactive due to the very strong C–F bond.

Comparing the Rate of Hydrolysis in Halogenoalkanes

Hydrolysis is the breaking of a chemical bond through reaction with water. In the case of halogenoalkanes, the C–X bond is broken and replaced with an –OH group:

General reaction:

(where or )

Why water?

Although water is a weak nucleophile, it can slowly attack the carbon bonded to the halogen in a nucleophilic substitution reaction, especially under warm conditions. The lone pair on the oxygen atom in water is attracted to the slightly positive carbon atom.

Tracking the reaction – using silver nitrate

To compare how quickly different halogenoalkanes undergo hydrolysis, we use aqueous silver nitrate as a test reagent.

• When the halogenoalkane is hydrolysed, the halide ion is released.

• These halide ions react with ions from silver nitrate to form a silver halide precipitate.

E.g.

(yellow precipitate)

The silver halide forms only once the halide ion has left, so the speed at which the precipitate appears tells us the rate of hydrolysis.

How to interpret the results

• The faster the precipitate forms, the faster the substitution reaction is.

• This tells us how reactive the halogenoalkane is.

• Reactivity depends on the strength of the carbon–halogen bond (C–X):

  • Weaker bonds break more easily – faster reaction.

  • Stronger bonds are more difficult to break – slower reaction.

Trend in bond strengths and precipitate colour

This matches the bond enthalpies so forms fastest and forms last. As a result, iodoalkanes hydrolyse fastest, while chloroalkanes hydrolyse slowest.

Nucleophilic Substitution with Aqueous Hydroxide Ions

Functional Group Change:

Halogenoalkane → Alcohol

Reagents and Conditions:

• Reagent: Sodium hydroxide () or potassium hydroxide ()

• Conditions: Warm, in aqueous solution

• Mechanism: Nucleophilic substitution

• Type of reagent: Nucleophile () – donates a lone pair of electrons

Alternative Mechanism: (Tertiary Halogenoalkanes)

Tertiary halogenoalkanes, such as 2-bromo-2-methylpropane, undergo nucleophilic substitution via a two-step mechanism known as (substitution, nucleophilic, unimolecular).

Why ?

• Tertiary carbocations are stable due to the three methyl groups surrounding the positive carbon. These groups push electron density towards the carbocation via the inductive effect, helping to stabilise it.

• Steric hindrance: The bulky groups make it difficult for OH⁻ to directly attack the carbon in a one-step () mechanism, so the reaction proceeds via the carbocation instead.

Nucleophilic Substitution with Cyanide Ions ()

Functional Group Change:

Halogenoalkane → Nitrile

Reagents and Conditions:

• Reagent: Potassium cyanide ()

• Solvent: Ethanol/water mixture

• Conditions: Heat under reflux

• Mechanism: Nucleophilic substitution

• Type of reagent: Nucleophile ()

Naming Nitriles:

• The C in the –CN group counts as part of the main chain.

• Start numbering from the nitrile carbon.

Example:

• → propanenitrile (3 carbons in total)

Not “propanitrile” – the e stays!

Nucleophilic Substitution with Ammonia ()

Functional Group Change:

Halogenoalkane → Primary Amine

Reagents and Conditions:

• Reagent: Ammonia () in ethanol

• Conditions: Heated under pressure in a sealed tube

• Mechanism: Nucleophilic substitution

• Type of reagent: Nucleophile ()

Naming Amines:

• Ethylamine refers to a 2-carbon chain with an group.

• Or use IUPAC: ethanamine

Minimising Further Substitution:

Ethylamine, like ammonia, has a lone pair and can act as a nucleophile – potentially forming a secondary amine if it reacts again.

(diethylamine)

To limit this, use excess ammonia. This ensures ammonia outcompetes the amine in further substitution.