Halogenoalkanes: substitution, elimination, hydrolysis

Last time

Before we start: last time, in Free-radical substitution, cracking and alkene oxidation, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

A carbon with a positive charge to attack

Take a C–Br bond. Bromine is more electronegative than carbon, so the carbon is slightly positive. An electron-rich particle, a nucleophile, with a lone pair, is attracted to it. It can replace the bromine. Or, with a strong base and heat, it can pull off a hydrogen and eject the bromine, forming an alkene.

Classification

A primary halogenoalkane has the halogen on a carbon bonded to one other carbon. A secondary has it on a carbon bonded to two other carbons. A tertiary, three. 2-chlorobutane is secondary, because the C–Cl carbon is bonded to two other carbon atoms. A nucleophile is an electron-pair donor. Examples: hydroxide, cyanide, ammonia, water.

Nucleophilic substitution reactions

Hydrolysis: warm with aqueous sodium hydroxide. C two H five Br plus NaOH goes to C two H five OH plus NaBr. Cyanide: heat under reflux with ethanolic potassium cyanide. C two H five Br plus NaCN goes to C two H five CN plus NaBr. It adds a carbon atom. Ammonia: heat with excess ethanolic ammonia in a sealed tube, to make an amine. An alkoxide: sodium propan-2-oxide with 2-bromopropane gives an ether.

The SN2 mechanism

Primary halogenoalkanes react by SN2: S for substitution, N for nucleophilic, 2 for two species in the rate-determining step. The nucleophile attacks the delta plus carbon from the side opposite the halogen, with a lone pair. At the same time the C–X bond breaks heterolytically. There is a transition state with partial bonds, not an intermediate. The curly arrows: lone pair to carbon, C–X bond to the halogen. The rate depends on both the halogenoalkane and the nucleophile.