Alcohol Reactions: Substitution Reactions: Videos & Practice Problems

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 the context of organic chemistry, substitution reactions involving alcohols are significant for transforming alcohols into alkyl halides. When an alcohol reacts with hydrochloric acid (HCl) or hydrobromic acid (HBr), the hydroxyl group (–OH) of the alcohol is replaced by a halogen atom, either chlorine (Cl) or bromine (Br). This process can be summarized by the general reaction:

R–OH + HX → R–X + H2O

Here, R represents the alkyl group, X can be either Cl or Br, and H2O is water, a byproduct of the reaction. The transformation effectively converts the alcohol into an alkyl halide, where the carbon atom that was previously bonded to the hydroxyl group is now bonded to a halogen. This reaction is crucial in organic synthesis, as alkyl halides serve as important intermediates in various chemical reactions.

To perform this substitution, one would typically use HCl to introduce chlorine or HBr to introduce bromine. Understanding this reaction mechanism is essential for manipulating organic compounds and facilitating further chemical transformations.

Alcohol Reactions Substitution Reactions Example

In the reaction of 1-propanol with hydrobromic acid (HBr), the hydroxyl (–OH) group of the alcohol is converted into a bromine (–Br) atom. This transformation is a classic example of a nucleophilic substitution reaction, where the alcohol is replaced by a halide. The chemical equation for this reaction can be represented as:

CH₃CH₂CH₂OH + HBr → CH₃CH₂CH₂Br + H₂O

As a result, 1-propanol (C₃H₈O) is converted into 1-bromopropane (C₃H₇Br). This reaction illustrates the conversion of an alcohol to an alkyl halide, showcasing the reactivity of alcohols in the presence of strong acids.

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

Answer: (D)

Name the starting alcohol used in the following reaction.

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

Answer: (C)

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.