3.3.3 Halogenooalkanes

Halogenoalkane

An alkane with at least one halogen atom in place of a hydrogen atom

Polarity of halogenoalkanes

Given that halogens are generally more electronegative than carbon, most carbon-halogen bonds are polar

Electrophile

An electron deficient (and usually positively charged) species which is attracted to regions of high electron density

The δ+ carbon (in the halogenoalkane) doesn’t have enough electrons, hence it is classed as an electrophile

Nucleophile

A species that forms a bond with an electrophile by donating a pair of electrons

Nucleophilic substitution

A reaction mechanism where a nucleophile substitutes for an atom (or group of atoms) in a molecule

Nucleophilic substitution of halogenoalkanes

In a nucleophilic substitution reaction, a nucleophile attacks a polar molecule, kicks out a functional group and settles itself in its place

Reaction of halogenoalkanes with hydroxides

Bromoethane can be changed to ethanol in a nucleophilic substitution reaction, where warm aqueous sodium or potassium hydroxide is used

Nitriles

Nitriles have a CN group, the carbon atom and the nitrogen atom are held together by a triple bond

Making nitriles from halogenoalkanes

Reacting bromoethane with potassium cyanide under reflux will produce propanenitrile and potassium bromide

Amines

An amine has the structure R₃N, the R groups can be hydrogens or another group

In amines, the nitrogen always has a lone pair

Making amines from halogenoalkanes

If you warm a halogenoalkane with excess ethanolic ammonia (ammonia dissolved in ethanol) in a sealed tube, the ammonia swaps places with the halogen to form an amine

Reactivity of halogenoalkanes

The carbon-halogen bond enthalpy decides reactivity, the C-F bond is the strongest, hence it has the highest bond enthalpy, thus fluoroalkanes undergo nucleophilic substitution reactions more slowly than other halogenoalkanes

Elimination reactions

A reaction mechanism in which a molecule loses atoms or groups of atoms

Halogen elimination from a halogenoalkane

If you warm a halogenoalkane with hydroxide ions dissolved in ethanol instead of water, an elimination reaction occurs where you end up with an alkene