Halogenoalkane reaction mechanisms

In a nutshell

Halogenoalkanes are alkanes with a halogen replacing one or more hydrogen atoms. Halogenoalkanes undergo nucleophilic substitution. There are two different types of mechanisms, S$$_N1$$ and S$$_N2$$​. The type of mechanism that they undergo depends on if the halogenoalkane is primary, secondary or tertiary.

Halogenoalkanes

Halogenoalkanes are alkanes with a halogen replacing one or more hydrogen atoms. They are also known as haloalkanes and alkyl halides. There are three types of halogenoalkanes: primary, secondary and tertiary. A halogen is any element from Group $$7$$​. 

Primary ($${1^\circ}$$​) halogenoalkanes consists of one alkyl group being attached to the carbon bonding with the halogen.

Secondary ($${2^\circ}$$) halogenoalkanes consists of two alkyl groups being attached to the carbon bonding with the halogen.​

Tertiary ($${3^\circ}$$) halogenoalkanes consists of three alkyl groups being attached to the carbon bonding with the halogen. 

Nucleophilic substitution 

Halogenoalkanes undergo nucleophilic substitution. Nucleophilic substitution is a mechanism where a nucleophile attacks the carbon-halogen bond and replaces the halogen. The nucleophile tends to be electron rich. It donates electrons at a site that is electron deficient. 

The carbon-halogen bond is polar as the halogen is very electronegative. This means the bonding electrons, in the covalent bond between the carbon and halogen, are more attracted to the electronegative halogen. Therefore, the halogen is delta negative $$(δ−)$$. Due to the bonding electrons being closer to the halogen, the carbon is delta positive $$(δ+)$$. The carbon is an electron deficient site. 

The electron rich nucleophile attacks the polar carbon-halogen bond, donating a pair of electrons to the delta positive carbon and breaks the bond heterolytically (unequally) between the carbon and halogen. The nucleophile forms a new bond with the carbon. 

Nucleophilic substitution has two different types of mechanisms: S$$_N1$$​ and S$$_N2$$​. The type of mechanism that the halogenoalkanes undergoes depends on if the halogenoalkane is primary, secondary or tertiary.

Primary halogenoalkanes undergo a S$$_N2$$​ mechanism. Secondary halogenoalkanes can undergo either a S$$_N1$$​ or S$$_N2$$​ mechanism. Tertiary halogenoalkanes undergo a S$$_N1$$​ mechanism. 

S_N2 reaction

S$$_N2$$​ reactions only have one rate determining step. This mechanism occurs in one step. It has two species in the rate determining step. One of them is a primary halogenoalkane.

Example

The chemical equation for the reaction between chloroethane (primary halogenoalkane) and hydroxide ions is shown below. 

​$$CH_3CH_2Cl +OH^-→CH_3CH_2OH+Cl^-$$

Chloroethane is a primary alkane as the carbon attached to the halogen only bonds to one alkyl group, it undergoes a S$$_N2$$​ reaction.

The hydroxide ion is the nucleophile. The negative hydroxide ion attacks the delta positive carbon, breaking the carbon-chlorine bond heterolytically and forms a new bond with the carbon. 

This mechanism is done in one step because the carbon has two small hydrogen atoms bonded, leaving enough space for the nucleophile to attack. The species in square brackets is the transition state. 

The rate determining step is:

​$$rate=k[CH_3CH_2Cl][OH^-]$$​​

The rate determining step consists of both the reactants, chloroethane and the hydroxide ion. This means the concentration of both of the reactants will affect the rate of the reaction. 

S_N1 reaction 

S$$_N1$$​ reactions has two rate determining steps. This mechanism occurs in two steps. It only has one species in the rate determining step.  

Example

The chemical equation for the reaction between $$2$$​-chloro-$$2$$​-methylpropane (tertiary halogenoalkane) and hydrogen ions is shown below. 

​$$(CH_3)_3CCl+OH^-→(CH_3)_3COH +Cl^-$$

The tertiary halogenoalkane has three alkyl groups attached to the carbon bonded with the halogen. The carbon is surrounded by three bulky alkyl groups. This means there isn't enough space for the nucleophile to attack. Therefore, tertiary halogenoalkanes undergo a S$$_N1$$ reaction.

The first step consists of the carbon and bromine bond breaking. This forms a carbocation, making space for the nucleophile to attack. This is the rate determining step as the nucleophile can't bond with the carbon until the halogen is removed.  

The second step is when the nucleophile, hydroxide ion, bonds with the carbon in the carbocation, forming $$2$$​-methyl-$$2$$​-propanol. This step is relatively fast.

The rate determining step is the first step, it is:   

$$rate=k[(CH_3)_3 CCl]$$​​

The rate determining step only consists of the tertiary halogenoalkane, $$2$$​-chloro-$$2$$​-methylpropane. This means the concentration of the tertiary halogenoalkane only affects the speed of the S$$_N1$$​ reaction.