Nucleophilic Substitution Halogenoalkanes

Starter Try and write definitions for these key words. Halogenoalkane Nucleophile Electrophile Electronegativity

What do Nucleophiles do? These species are electron pair donors They will donate a pair of electrons to something that doesn’t have enough electrons (areas of low electron density) Common examples include, cyanide ions, ammonia and hydroxide ions and even water.

Nucleophilic substitution Nucleophilic substitution definition: An electron rich species (nucleophile) attacks an electron deficient atom to remove another atom or group of atoms in a molecule. e.g. CH3Cl + OH-  CH3OH + Cl-

Nucleophilic substitution hydroxide ion with bromoethane mechanism CH3CH2Br + OH- (aqueous) CH3CH2OH + Br- ethanol cyanide ion with iodoethane mechanism CH3CH2I (ethanol) + CN-(aq) CH3CH2CN + I- propanenitrile ammonia with bromoethane mechanism CH3CH2Br + NH3 2 CH3CH2NH2 + NH4+Br- aminoethane

hydroxide ion with bromoethane Nucleophilic substitution mechanism hydroxide ion with bromoethane CH3 H Br C CH3 H OH C + - Br - - OH ethanol reaction equation

cyanide ion with iodoethane Nucleophilic substitution mechanism cyanide ion with iodoethane CH3 H I C CH3 H CN C + - I - CN - propanenitrile reaction equation

ammonia with bromoethane Nucleophilic substitution mechanism ammonia with bromoethane CH3 H Br C H CH3 NH2 C + Br - + - NH3 NH3 CH3 H NH2 C H NH3+Br - aminoethane reaction equation

Knowledge Check Draw mechanisms for the following reactions: Chloromethane + Sodium Hydroxide  Methanol + Sodium chloride 1-iodo-2-chloroethane + Hydrogen cyanide  1-cyano-2-chloroethane + Hydrogen iodide 1-chloro-2-bromo-3-iodopropane + ammonia  1-amino-2-bromo-3-iodopropane + ammonium chloride

Preparation of Alcohols Nucleophilic substitution reactions are used to prepare alcohols from halogenoalkanes. E.g butan-1-ol from 1-bromobutane. The OH- nucleophile is provided by aqueous sodium hydroxide. The reaction is very slow. To increase the rate of the reaction, the reactants are heated. The reaction is done under reflux to prevent loss of liquids through evaporation.

Reflux Apparatus Upon heating, the liquids evaporate and escape form the flask, however the condenser cools these vapours, turning them back into a liquid which drips back into the reaction vessel. The reaction can continue for as long as necessary with no loss of material.

Nucleophilic substitution When hydroxide ions (e.g. NaOH) or water is used the reaction is again nucleophilic substitution but can also be called hydrolysis. Hydrolysis involves a chemical reaction where water or hydroxide causes the breaking of a bond in a molecule.

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