Alcohol Reactions: Substitution Reactions - Online Tutor, Practice Problems & Exam Prep

Understanding the transformation of alcohols into alkyl halides through substitution reactions is crucial in organic chemistry. When alcohols react with hydrochloric acid (HCl) or hydrobromic acid (HBr), the hydroxyl (-OH) group is replaced by a halogen (chlorine or bromine, respectively). This process effectively converts an alcohol into an alkyl halide, where the carbon originally bonded to the -OH group now bonds with a halogen atom (Cl or Br). This reaction is a simple yet fundamental example of how functional groups in organic molecules can be modified, leading to significant changes in their chemical properties and reactivity.

Alcohol Reactions: Substitution Reactions

In this video we're going to take a look at alcohol reactions dealing with substitution. Now we're going to say under these substitution reactions, hydrochloric acid or hydrobromic acid react with an alcohol. Here are hydrochloric acid changes, OH into Cl and our hydrobromic acid changes OH into Br.

If we take a look here at this example reaction, we have our alcohol as our starting material and we're reacting it with one of these two acids. X here represents either Cl or Br. All that happens here is that our OH group gets changed into one of the halogens of this acid and the result is we go from having an alcohol to having an alkyl.

Halid Carbon is no longer connected to an OH group, but instead it's going to be connected to a halogen in the form of chlorine or bromine. So just remember that you have an alcohol. Use HCl to change it into Cl or use HBr to change it into Br.

Alcohol Reactions Substitution Reactions Example

Determine the alkyl halid formed in the following reaction. So here we're starting out with our alcohol and this would be 1 propanol. We're going to react with hydrobromic acid.

Remember hydrobromic acid will convert the OH group into ABR. So now it becomes CH3CH2CH2Br. We go from 1 propanol to 1 bromo propane, so here we've gone from an alcohol to an alkyl halid and this would be our final answer.

Name the product formed when 2-pentanol is treated with hydrochloric acid.

• A. 2-pentanoic acid
• B. 3-chloropentane
• C. 3-pentanol
• D. 2-chloropentane

Name the starting alcohol used in the following reaction.

• A. 1-methylcyclohexanol
• B. 3-methylhexanol
• C. 3-methylcyclohexanol
• D. 2-methylcyclohexanaol

Here’s what students ask on this topic:

Why does the required reaction temperature decrease as the alcohol substitution increases?

The required reaction temperature often decreases as alcohol substitution increases due to the increasing stability of the carbocation intermediate formed during the reaction. In organic chemistry, alcohols can undergo substitution reactions where the hydroxyl group (-OH) is replaced by another atom or group of atoms.

Primary alcohols, with the -OH group attached to a carbon with two hydrogens, form less stable carbocations than secondary alcohols, which have the -OH group attached to a carbon with one hydrogen. Tertiary alcohols, with the -OH group on a carbon with no hydrogens, form the most stable carbocations.

Stable carbocations have lower energy and are easier to form, which means the reaction can proceed at a lower temperature. This is because more substituted carbocations are better at dispersing the positive charge over the molecule, often through hyperconjugation or resonance. As a result, as the level of substitution increases from primary to tertiary, the energy barrier for the reaction decreases, allowing the reaction to occur at a lower temperature.