Mr Cole Chemistry

Nucleophilic Substitution Questions

Nucleophilic Substitution 1

Halogenoalkanes Worksheet

Understanding Nucleophiles

1. Halogenoalkanes can be attacked by nucleophiles.
a) Define the term nucleophile.

A species that can donate a pair of electrons.

b) Explain why halogenoalkanes are vulnerable to nucleophilic attack.

The C-X bond is polar due to halogens being more electronegative than carbon.

This makes the carbon atom partially positive (δ+), allowing it to accept a pair of electrons from a nucleophile. The C-X bond is also sufficiently weak to break during this process.

c) Give two essential features of a nucleophile.
  • Must have a lone pair of electrons to donate.
  • Does not necessarily need to be negatively charged (can be neutral), but must be electron-rich.
d) Select the species below that are nucleophiles: H2O, NH3, NH4+, H2SO4, CN–.
  • H2O: Yes (albeit a weak one).
  • NH3: Yes.
  • NH4+: No (no lone pair available).
  • H2SO4: Yes (The hydrogens can dissociate to form HSO4– which can act as a nucleophile).
  • CN–: Yes.

Hydrolysis (Reaction with OH–)

2. Bromoethane can be attacked by a nucleophile, such as OH–.
a) State the reagents and conditions for this reaction.

Aqueous sodium (or potassium) hydroxide; heat under reflux.

b) Write an equation for the reaction.
CH3CH2Br + NaOH
→
CH3CH2OH + NaBr
c) Outline the mechanism for the reaction that takes place.
Nucleophilic Substitution Mechanism OH-
d) State the type of bond fission that occurs to the C-Br bond.

Heterolytic bond fission (both electrons from the bond move to the bromine atom).

e) If chloroethane had been used as the reactant instead of bromoethane; state whether the reaction would have been faster or slower and explain your answer.

It would have reacted more slowly.

The C-Cl bond is stronger (has a higher bond enthalpy) than the C-Br bond. This means the activation energy for the reaction is higher, so fewer collisions result in a reaction.

Nitrile Formation

3. 2-chloropropane can be used to make a nitrile via a nucleophilic substitution reaction.
a) State the name of the nitrile that is formed from this reaction and draw the skeletal formula.

2-methylpropanenitrile (also known as methylpropane nitrile)

Structure of 2-methylpropanenitrile
b) State the reactants and the conditions required for this reaction to take place.

NaCN or KCN (Sodium or Potassium Cyanide); ethanolic solution (alcohol solvent, no water); heat under reflux.

c) Outline the mechanism for the reaction.
Mechanism for nitrile formation
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