Nucleophilic Substitution

What is a nucleophile?

A nucleophile is an electron-rich species that can donate a pair of electrons
- ‘Nucleophile’ means ‘nucleus/positive charge loving’ as nucleophiles are attracted to positively charged species
- Nucleophilic refers to reactions that involve a nucleophile

There are various different species which can behave as nucleophiles, and some make better nucleophiles than others

lewis-structure-of-hydroxide-ion-and-water

A hydroxide ion is a better nucleophile as it has a full formal negative charge whereas the oxygen atom in water only carries a partial negative charge

Examples of neutral and charged nucleophiles

Neutral	Charged
H₂O	OH^–
NH₃	Cl^–
ROH	CN^–
RNH₂	R^– (carbanions)

A nucleophilic substitution reaction is one in which a nucleophile attacks a carbon atom which carries a partial positive charge
An atom that has a partial negative charge is replaced by the nucleophile

Equations for Nucleophilic Substitution

Haloalkanes will undergo nucleophilic substitution reactions due to the polar C-X bond (where X is a halogen)

Partial positive C atom and partial negative X atom

Due to large differences in electronegativity between the carbon and halogen atom, the C-X bond is polar

Diagram to show nucleophilic substitution where :Nu^– represents the nucleophile

Diagram to show the general mechanism for nucleophilic substitution

General Mechanism for Nucleophilic Substitution

Hydrolysis of Haloalkanes

The nucleophile in this reaction is the hydroxide, OH^– ion
An aqueous solution of sodium hydroxide (NaOH) or potassium hydroxide (KOH) with ethanol is used
This reaction is very slow at room temperature, so the reaction mixture is warmed
This is an example of a hydrolysis reaction and the product is an alcohol

CH₃CH₂Br + OH^– → CH₃CH₂OH + :Br^–

bromoethane → ethanol

:Br^– is the leaving group
- Halogens make good leaving groups as they form relatively weak bonds with carbon
- Their higher electronegativity also means the bonded electrons are drawn towards the halogen atom making the carbon partially positive, δ+, and susceptible to nucleophilic attack
The rate of this reaction depends on the type of halogen in the haloalkane
The stronger the C–X bond, the slower the rate of the reaction
In terms of bond enthalpy, C–F > C–Cl > C–Br > C–I
Fluoroalkanes do not react at all, but iodoalkanes have a very fast rate of reaction

The nucleophilic substitution mechanisms for the above reactions are as follows:

Nucleophilic substitution mechanism of bromoethane with a hydroxide ion

nucleophilic-substitution-of-bromoethane

Nucleophilic Substitution with OH^–,the bond that forms and the bond that breaks must both involve the carbon atom that is bonded to the leaving group

Neutral nucleophiles

When the nucleophile is neutral, e.g. H₂O, the initial product is positive
The positive product then deprotonates, losing H⁺, and forms a neutral product
- CH₃CH₂Cl + H₂O → CH₃CH₂OH + :H⁺

Diagram to show water acting as a nucleophile forming a positive product which is then deprotonated

Nucleophilic substitution reactions with neutral nucleophiles involves deprotonation

Nucleophilic Substitution (SL IB Chemistry)

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