Alcohol Reactions: Alkyl Halide formation using hydrohalic acids (HX)

When alcohols are reacted with hydrohalic acids (HBr/HCl/HI...HX), alkyl halides are formed. This reaction proceeds through either the SN1 or SN2 reaction mechanism. Primary alcohols undergo SN2 reaction mechanisms whereas secondary alcohols can undergo either the SN1 or SN2, and tertiary alcohols undergo SN1 mechanisms.

Primary Alcohol

The most variability occurs with secondary alcohols. There are two possible outcomes:

The stereocenter is maintained, but the stereochemistry is inverted.

OR

Hydride shifts can occur which results in the loss of stereocenters.

Secondary Alcohol

Tertiary alcohols are straightforward, similar to primary alcohols. The key difference being that the reaction proceeds through an SN1 reaction mechanism compared to an SN2.

The reaction mechanism is depicted below, showing an SN1 mechanism (SN2 mechanisms are simpler; the halogen attacks the carbon at the exact same time as the H2O leaving, whereas for the SN1, the carbocation is formed as a discreet step):

Mechanism

In the first step, the alcohol-oxygen attacks the hydrogen on the HX molecule with its free electrons.

In the second step, the newly protonated oxygen atom leaves the molecule as H2O, breaking the C-O bond and taking the electrons with it.

In the third step, the positively charged carbocation is attacked by the negatively charged halide (Br- in this example), thereby completing the reaction.

As a reminder, this is the SN1 mechanism. The only difference between the SN1 and the SN2 would be that the Br- attacks the carbon at the exact same time as the H2O molecule leaving.

Practice this reaction using our Reaction Solver!