Elimination Mechanisms (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about the elimination mechanism of halogenoalkanes.

Elimination and nucleophilic substitution are the same halogenoalkane meeting the same hydroxide ion, and the conditions decide which one you get.

A halogenoalkane heated with sodium hydroxide can give two completely different products. With aqueous sodium hydroxide it gives an alcohol, by substitution. With hot sodium hydroxide dissolved in ethanol it gives an alkene, by elimination. Same reactant, same reagent: the solvent and the temperature decide, because the hydroxide ion can act either as a nucleophile or as a base.

We'll take the elimination reaction and the conditions it needs, then the mechanism itself, and finish with what makes the reaction go one way rather than the other.

An elimination reaction is one in which a small molecule is removed from a larger molecule, forming a multiple bond, usually a carbon-carbon double bond. For a halogenoalkane, the conditions are heating under reflux with concentrated sodium hydroxide dissolved in ethanol.

Under those conditions the carbon to halogen bond breaks heterolytically, the hydroxide ion acts as a base, and a hydrogen atom is removed from a carbon next to the one holding the halogen. A hydrogen halide is eliminated and an alkene is formed. Bromoethane treated this way gives ethene, along with sodium bromide and water. Hydroxide acting as a base, rather than as a nucleophile, is what sends the reaction down the elimination route.

The mechanism is called elimination because a small molecule, the hydrogen halide, is removed from the molecule.

It is a one-step reaction. The base removes a proton, the carbon to hydrogen bond breaks, the carbon to halogen bond breaks at the same time, and the carbon-carbon double bond forms simultaneously. There is no intermediate to draw, because everything happens at once. The same hydroxide ion that would attack the carbon in substitution is here pulling a proton off the neighbouring carbon instead.

The reaction conditions can change the products for the same reactant and the same reagent. In this case it is the solvent and the temperature that determine whether substitution or elimination is favoured.

Hot ethanolic sodium hydroxide gives elimination, and the product is an alkene. Aqueous sodium hydroxide gives nucleophilic substitution, and the product is an alcohol. Nothing about the halogenoalkane has changed and nothing about the reagent has changed. What changes is which of hydroxide's two roles the conditions bring out.

In the elimination mechanism the role of the reagent is both nucleophile and base. It is a nucleophile because the hydroxide ion is an electron-rich species that forms a new covalent bond by donating a pair of electrons to an electron-deficient species. It is a base because it reacts with a hydrogen ion to form water.

Elimination removes a small molecule, a hydrogen halide, from a halogenoalkane and leaves an alkene. It happens in one step, with the carbon to hydrogen bond breaking, the carbon to halogen bond breaking and the carbon-carbon double bond forming all at the same time. The conditions are heating under reflux with concentrated sodium hydroxide in ethanol, not in water. One reagent, two outcomes: hydroxide acting as a base gives the alkene, hydroxide acting as a nucleophile gives the alcohol.