Nucleophilic Substitution Reactions

Nucleophilic substitution reactions are reactions in which a nucleophile attacks a molecule and replaces one if its functional groups.

Exploring nucleophilic substitution reactions

Here, we are going to focus specifically on nucleophilic substitution reactions involving halogenoalkanes. Let's break the term nucleophilic substitution down a little. First, substitution reactions:

Next, let's look at nucleophiles.

By looking at the term nucleophile, we can form a picture of what they actually are; -phile comes from the Greek word philos, which means to love, and nuclei are positively charged areas. Therefore, nucleophiles must ‘love’ positive regions - they are attracted towards them.

Nucleophiles all contain a lone pair of electrons on a negative or δ- atom. You'll remember that the delta symbol, δ, represents a partial charge.

Examples of nucleophiles include:

• The cyanide ion,
• The hydroxide ion,
• Ammonia,

What's the difference between nucleophilic and electrophilic substitution reactions?

Nucleophilic substitution reactions involve attack by a nucleophile. Electrophilic substitution reactions, on the other hand, involve attack by an electrophile.

Examples of electrophiles include:

• Hydrogen halides,
• 

How do halogenoalkanes react in substitution reactions?

We know that halogenoalkanes are polar molecules (explore Halogenoalkanes to refresh your memory). Because the halogen atom is a lot more electronegative than the carbon atom in the C-X bond, it attracts the shared pair of electrons towards itself. You know that electrons are negatively charged. This makes the halogen atom partially negatively charged and leaves the carbon partially positively charged. Nucleophiles, which we now know ‘love’ positive regions, can attack this carbon atom.

The general mechanism of nucleophilic substitution

Nucleophilic substitution of halogenoalkanes follows a similar mechanism. The nucleophile attacks the δ+ carbon in the C-X bond, which replaces the halogen atom. In the example mechanism below, we represent the nucleophile with and the halogen with .

The general mechanism for the nucleophilic substitution of halogenoalkanes. Anna Brewer, StudySmarter Originals

Here, the lone pair of electrons on the nucleophile is attracted towards the δ+ carbon in the C-X bond. This causes the C-X bond to break, and the bonded pair of electrons then moves to the halogen, forming a negative halide ion. The curly arrow shows the movement of electrons.

The overall equation is as follows:

This type of mechanism is known as an mechanism. The S stands for substitution, the N stands for nucleophilic, and the 2 shows that the initial reaction step involves two species: the halogenoalkane and the nucleophile.

If the halogenoalkane is a tertiary halogenoalkane, it will react using a different mechanism. This is known as an mechanism and happens because all the alkyl groups surrounding the C-X carbon atom prevent the nucleophile from attacking the carbon. As you can probably guess, the reaction involves just one species in the initial step. Most exam boards won’t expect you to know this, but it is interesting to understand.

The halogenoalkane initially ionises to form a carbocation - an ion with a positive charge on one of its carbon atoms - and a negative halide ion. The carbocation can then react with the nucleophile, which adds on to the molecule. An example is shown below using 2-bromomethylpropane.

The general mechanism for the reaction between a nucleophile and a tertiary halogenoalkane. Anna Brewer, StudySmarter Originals

Stereochemical aspects nucleophilic substitution

Above, we saw how nucleophilic substitution can have an or mechanism. These two different mechanisms produce products with different stereochemical aspects:

• mechanisms produce just one product. The bonds in this product are inverted compared to the bonds in the original reactant molecule.
• mechanisms produce two enantiomers. Enantiomers are stereoisomers with the same structural formulae but different arrangements of atoms around a central carbon. These two enantiomers are produced in a 50:50 mixture known as racemic mixture, or a racemate.

Reactivity of halogenoalkanes in nucleophilic substitution

The halogen, or halide ion, is known as the leaving group.

Some halogens are much more able to act as a leaving group than others. This means that they react much more readily in nucleophilic substitution reactions. Interestingly, their ability to act in this way increases as you move down the periodic table.

For example, fluoroalkanes with C-F bonds do not undergo nucleophilic substitution whereas iodoalkanes with weak C-I bonds react rapidly with nucleophiles. This is because iodine is a much larger atom than fluorine. Its valence electrons are a lot further away from its nucleus and the C-I bond is longer than the C-F bond. This means the bond has a much lower enthalpy and requires less energy to break. The bond is more reactive. Bond lengths increase as you move down the group in the periodic table. Therefore, the reactivity of halogenoalkanes in nucleophilic substitution increases as you move down the group.

For more information on halogenoalkanes and their reactivity, see Halogenoalkanes.

Iodine is a much larger atom than fluorine. This means that when it bonds with carbon, it forms a longer bond with a weaker enthalpy. commons.wikimedia.org

Examples of nucleophilic substitution

Halogenoalkanes can react with the hydroxide ion, cyanide ion and ammonia molecule in nucleophilic substitution reactions. The mechanisms are all similar to the general mechanism we learnt earlier. Shown below, it can be used to create alcohols and nitriles.

The general mechanism of a nucleophilic substitution reaction. Anna Brewer, StudySmarter Originals

Nucleophilic substitution with the hydroxide ion

Halogenoalkanes react with aqueous sodium or potassium hydroxide to form an alcohol and a halide ion. Alcohols have the hydroxyl functional group and are represented by the general formula . The potassium/sodium ion acts as a spectator ion and is not shown in the mechanism.

Study tip: A spectator ion is an ion that remains in the same form on both sides of the reaction equation. It keeps the same physical state, charge and oxidation state.

If we write out all the ions involved in a reaction, we can see which are spectators. For example, in the reaction between hydrochloric acid and sodium hydroxide, the sodium ions and chloride ions are all spectators - they stay in the same state and aren’t changed in the reaction.

The ions involved in a reaction between hydrochloric acid and sodium hydroxide. Anna Brewer, StudySmarter Originals

Let’s look at the conditions for nucleophilic substitution with hydroxide ions. Halogenoalkanes do not readily mix with water, so ethanol is used as a solvent for the substitution reaction. The mixture is warmed under reflux to increase the rate of reaction:

• Warming the mixture increases the kinetic energy of the molecules. This means that not only do they move faster and so have more collisions, but on average they also have more energy when they collide. This means that they are more likely to meet the activation energy needed for a reaction. There will be a higher proportion of successful collisions and thus a faster reaction. For more on rates of reaction, see Collision Theory and Increasing Rates.
• Reflux is a reaction technique that involves heating the mixture in a sealed vessel. When volatile components in the mixture evaporate, they are trapped in a condenser and cannot escape out of the system, instead condensing back into the container. We can reach higher temperatures and carry the reaction out over a longer period. This increases the opportunity for a successful reaction.

For example, bromoethane reacts with potassium hydroxide to form ethanol and a bromide ion:

Nucleophilic substitution between bromoethane and the hydroxide ion.Anna Brewer, StudySmarter Originals

The bromide ion then reacts with the potassium ion to form potassium bromide. This can be shown by the following overall equation:

Study tip: Remember to use structural formulae when writing equations to show the molecule’s structure and the position of the new functional group.

Another example is the nucleophilic attack of 2-chloro-2-methylpropane by sodium hydroxide, forming 2-methylpropan-2-ol and sodium chloride. To make the mechanism clearer, we've used a simplified version of displayed formula:

Nucleophilic substitution between 2-chloro-2-methylpropane and hydroxide ions. Anna Brewer, StudySmarter Originals

The equation is shown below:

For more information on naming alcohols, see Alcohols.

Study tip: There is another type of reaction involving halogenoalkanes and hydroxide ions. It is called an elimination reaction. In elimination reactions, the nucleophile acts as a base instead of a nucleophile. It produces an alkene, water and a halide ion. The conditions are slightly different too - we use hot and concentrated ethanolic potassium or sodium hydroxide. Explore this in Elimination Reactions.

Substitution with the cyanide ion

Potassium or sodium cyanide react with halogenoalkanes in ethanolic solution to form a nitrile and a halide ion. Nitriles have the functional group , which contains a C≡N triple bond. Once again, the reaction mixture is heated under reflux. This nucleophilic substitution reaction is important industrially as it increases the length of the carbon chain.

For example, chloromethane heated in ethanolic potassium cyanide produces ethanenitrile and a chloride ion:

Nucleophilic substitution between chloromethane and the cyanide ion. Anna Brewer, StudySmarter Originals

Study tip: Look at how the length of the carbon chain has increased from one carbon in chloromethane to two carbons in ethanenitrile. This reaction is important industrially because it increases the molecule’s chain length.

The chloride ion then reacts with potassium to form potassium chloride. The overall equation is as follows:

For more information on nitriles, see Nature and Preparation of Amines.

Substitution with ammonia

The reaction between halogenoalkanes and an excess of ammonia produces a primary amine, a halide ion and an ammonium ion. Amines are ammonia derivatives, where one or more of the hydrogen atoms has been replaced by an alkyl group.

A table comparing ammonia, ammonium and primary amines. Anna Brewer, StudySmarter Originals

We saw above that although ammonia is not a negative ion, it is still a nucleophile. It contains a partially negatively charged atom, , with a lone pair of electrons. When the nitrogen atom donates its lone pair of electrons to the carbon atom, it becomes positively charged. You can see this in the mechanism below. This isn’t great for the molecule - it wants to be neutral, as that’s a lot more stable. To solve this problem, it kicks out a hydrogen atom, but keeps the bonded pair of electrons. The hydrogen atom is now a positive ion. It reacts with a second molecule of ammonia to form a positive ammonium ion. This ion can then react with the bromide ion produced in the substitution reaction, making an ammonium salt.

The reaction is carried out heated in ethanolic solution, in a sealed container under pressure.

For example, bromoethane and ammonia react together to form ethanamine, a bromide ion and an ammonium ion:

Nucleophilic substitution between bromoethane and ammonia. Anna Brewer, StudySmarter Originals

The overall equation is shown below. The ammonium ion reacts with the bromide ion to form an ammonium salt, ammonium bromide:

Another example is the reaction between ammonia and 1-chlorobutane, as shown in the following mechanism. :

Nucleophilic substitution between 1-chlorobutane and ammonia. Anna Brewer, StudySmarter Originals

The organic product is 1-butanamine. The chlorine ion released reacts with the ammonium ion to produce ammonium chloride. The overall equation is shown below:

For further information on amines, see Amines.

Other types of substitution reactions

Other types of substitution reactions include the electrophilic substitution of benzene and other aromatic molecules. You can explore this further in Reactions of Benzene.