Alcohols can be converted to alkyl halides by reacting with HX (X = Cl, Br, I) acids. The reaction works for 1°, 2°, and 3° alcohols:

Let’s now understand how this happens.

We have seen many times in S_N1 and E1 reactions that alcohols can serve as weak nucleophiles and weak bases when reacted with alkyl halides:

The first reaction of going from an alkyl halide to an alcohol is the reversed reaction of converting alcohols to alkyl halides, and you might have a good question to ask here – how do you control the outcome of the reaction?

And the answer is that it is all about the concentration of the reactants. If you need to convert the alcohol to an alkyl halide, you’d add a lot of acid, and if you need to prepare an alcohol from the alkyl halide you add a lot of water or hydroxide, depending on what substrate you are using.

Aside from this, the equilibrium can be shifted towards the products by removing one of the products, for example, by distillation.

In this post, we will focus on the conversion of alcohols to alkyl halides, as the opposite reaction(s) are covered in the S_N1 and S_N2 reactions.

What is the Mechanism of converting alcohols to alkyl halides?

We are working with a substitution – the OH group is replaced with a halogen. Therefore, regardless of what alcohol is used (1^o, 2^o, or 3^o), the principle of this substitution is to convert the OH into a good leaving group and kick it out by the halide ion (S_N2) or the nucleophile may attack after the OH departs as a leaving group, which would correspond to an S_N1 process:

For both mechanisms, the first step is the protonation of the alcohol to create the good leaving group H₂O, after which the nucleophilic substitution occurs.

There are some differences in the mechanism of the substitution depending on the type of alcohol. Let’s discuss them separately.

Reaction of HX acids with Methyl and Primary Alcohols

Methyl and primary alcohols are converted to alkyl halides via S_N2. The I^– and Br^– are good nucleophiles and attack the carbon, kicking out the ⁺OH₂ in the form of neutral water molecule.

HCl works fine as well; however, it is not as strong an acid, and the chloride ion is not a great nucleophile in these conditions. Thus, ZnCl₂is sometimes added as a catalyst to speed up the reaction.

The Reaction of HX Acids with Secondary Alcohols

Secondary alcohols can undergo S_N2 and S_N1 reactions:

The problem with the S_N1 mechanism is the possible rearrangements for certain alcohols, as well as the racemization if the carbon atom is a chiral center:

At this point, we can see the two main disadvantages of converting alcohols to alkyl halides using hydrogen halides:

1) The strong acidic conditions are often not suitable for organic molecules.

2) The S_N1 mechanism lacks the stereochemical control, and rearrangements add another liability as the regiochemical outcome of the reaction.

For this, there are alternative methods such as the use of SOCl₂, PBr_3, and converting alcohols to sulfonyl esters like mesylates and tosylates.

Both methods are covered in separate posts, as there is too much to talk about in one article:

SOCl₂ and PBr₃ for Conversion of Alcohols to Alkyl Halides

Mesylates and Tosylates as Good Leaving Groups

Reaction of HX acids with Tertiary Alcohols

Tertiary alcohols work the best for acid-catalyzed conversion to alkyl halides. The reaction goes by an S_N1 mechanism, but the carbocation is “very” stable and there is usually no problem of a rearrangement: