Nucleophilic Substitution (DP IB Chemistry): Revision Note

Nucleophilic substitution

What is a nucleophile?

• A nucleophile is an electron-rich species that can donate a pair of electrons

  • The term 'nucleophile' means 'nucleus/positive charge loving'

  • Nucleophiles are attracted to positively charged species

• Nucleophilic refers to reactions that involve a nucleophile

• Some species make better nucleophiles than others

• For example, comparing hydroxide ions and water molecules:

  • A hydroxide ion (OH⁻) carries a full negative charge

  • The oxygen in H₂O has only a partial negative charge

  • This makes OH⁻:

    • More reactive

    • More likely to donate its lone pair quickly

    • A better nucleophile

Examples of neutral and charged nucleophiles

• Here are some common examples of nucleophiles:

  • Neutral nucleophiles:

    • H₂O (water)

    • NH₃ (ammonia)

    • ROH (alcohols)

    • RNH₂ (amines)

  • Charged nucleophiles:

    • OH^- (hydroxide ion)

    • Cl^- (chloride ion)

    • CN^- (cyanide ion)

    • R^- (carbanions)

Nucleophilic substitution overview

• 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

Nucleophilic substitution diagram

• Halogenoalkanes undergo nucleophilic substitution reactions due to the polar C-X bond

Partial positive C atom and partial negative X atom

Equations for nucleophilic substitution

Hydrolysis of halogenoalkanes 

• The nucleophile in this reaction is the hydroxide ion, OH^– 

• 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 because they form relatively weak bonds with carbon

  • Their high electronegativity pulls electron density away from carbon

  • This makes the carbon more δ⁺ and susceptible to nucleophilic attack

• The rate of this reaction depends on the halogen: 

  • The stronger the C–X bond, the slower the reaction

  • Bond enthalpy trend: C–F > C–Cl > C–Br > C–I

  • Fluoroalkanes are unreactive, iodoalkanes react very quickly

Nucleophilic substitution mechanism of bromoethane with a hydroxide ion 

Neutral nucleophiles

• When the nucleophile is neutral, e.g. H₂O:

  • The initial product is positive

  • It then deprotonates, losing H⁺ to form a neutral product

CH₃CH₂Cl + H₂O → CH₃CH₂OH + :H⁺

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