Alcohol to Alkyl Halide: Preparation, Reaction & Conversion

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Alcohols cannot typically undergo substitution reactions. In this lesson, we will learn how alcohols are prepared to undergo a substitution reaction and convert into alkyl halides.

Alcohols as a Leaving Group

In order for many types of reactions to occur the carbon chain needs to have a good leaving group. One of the best compounds with a good leaving group is an alkyl halide. An alkyl halide is a carbon chain with a halogen attached to one of the carbon atoms. So, how do we make alkyl halides? One method to make alkyl halides is from alcohol using a substitution (SN2 or SN1) reaction. How does this happen? Alcohols are not good leaving groups because the OH is too small to hold a negative charge on its own. So, how will a substitution reaction occur which will need to kick off the alcohol? This is accomplished by preparing the alcohol with an acid to make it a better leaving group.

Preparing Alcohols

What happens when we add an acid to an alcohol? The oxygen will receive hydrogen from the acid, forming H2 O+. Now, the leaving group would no longer be OH-, but instead, it will be water: H2 O.

Any strong acid can be used to form H2 O+, but it is most effective when we use a halide acid (HX, such as HI, HBr, or HCl), because then the halide is ready for the SN2 or SN1 reaction.

The alcohol is prepared by adding a hydrogen, making water
Alcohol preparation

The Reaction and Converting to Alkyl Halides

Once the alcohol has been converted into a good leaving group we are ready to perform the substitution reaction. So, will a SN2 reaction occur or will a SN1 occur? The answer depends on what type of alcohol we start out with.

Primary Alcohols

If the alcohol was a primary alcohol then a SN2 reaction, or a substitution reaction with one step, will occur. A primary alcohol is an alcohol attached to a carbon atom with 2 hydrogen atoms and only 1 carbon atom, it is when the alcohol is found at the end of the carbon chain. The SN2 reaction needs to occur in these types of alcohols because if the alcohol simply left as in a SN1 reaction the carbon cation would be a primary cation, which is very unstable, so it does not occur. But, with a SN2 reaction, the carbon never actually has a positive charge, because the reaction occurs in a single simultaneous step:

With primary alcohols the reaction occurs in a single step. The halide attacks and the water leaves
SN2 reaction

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