IBChemistryHLRevision NotesWhat are the Mechanisms of Chemical Change?Electron Pair Sharing ReactionsNucleophilic Substitution

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Nucleophilic Substitution (HL IB Chemistry)

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Philippa

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Philippa

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Chemistry

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

Diagram to show how the polarity arises in a C-X bond

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

Diagram to show nucleophilic substitution involving water

Nucleophilic substitution reactions with neutral nucleophiles involves deprotonation

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