Question

A naive graduate student attempted the preparation of \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CDBrCH}_{3}\) from \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CDOHCH}_{3}\) by heating the deuterio-alcohol with \(\mathrm{HBr}\) and \(\mathrm{H}_{2} \mathrm{SO}_{4}\). He obtained a product having the correct boiling point, but a careful examination of the spectral properties by his research director showed that the product was a mixture of \(\mathrm{CH}_{3} \mathrm{CHDCHBrCH}_{3}\) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CDBrCH}_{3}\). What happened?

Short answer

The graduate student obtained a mixture of CH3CHDCHBrCH3 and CH3CH2CDBrCH3 because the carbocation intermediate formed during the reaction was prone to rearrangement via hydride shift, resulting in the formation of a more stable carbocation. This then reacted with the bromide ion, leading to the formation of both the expected product and the rearranged product.

Step-by-step solution

Identify the given compounds

We have the following compounds: 1. CH3CH2CDOHCH3 - starting compound (deuterio-alcohol) 2. CH3CH2CDBrCH3 - expected product 3. CH3CHDCHBrCH3 - actual product obtained (part of the mixture) 4. HBr and H2SO4 - reactants used

Identify the initial reaction

The reaction that the student attempts is the substitution of the -OD group in the starting compound (CH3CH2CDOHCH3) with a -Br group from the HBr/H2SO4 reagents.

Formation of a carbocation intermediate

As the reaction proceeds, a carbocation intermediate is formed. This occurs due to the protonation of the deuterio-alcohol by the strong acid H2SO4, followed by the loss of the deuterium (D)+ ion: \[CH3CH2CDOHCH3 + H^+ \rightarrow CH3CH2CD(OH2)CH3^+\] \[CH3CH2CD(OH2)CH3^+ \rightarrow CH3CH2C^+CH3 + D^+\]

Reaction with HBr

The carbocation intermediate then reacts with the bromide ion (Br-) from HBr, which leads to the formation of two products: \[CH3CH2C^+CH3 + Br^- \rightarrow CH3CH2CDBrCH3\] \[CH3CH2C^+CH3 \rightarrow CH3CH^+CHBrCH3\] The formation of the second carbocation can arise from the hydride shift in the carbocation, which occurs to achieve a more stable carbocation intermediate (as it forms a more substituted, tertiary carbocation).

Formation of both CH3CHDCHBrCH3 and CH3CH2CDBrCH3

Since the carbocation intermediate can rearrange via hydride shift, both CH3CHDCHBrCH3 and CH3CH2CDBrCH3 products can be obtained.

Concluding the explanation

The graduate student obtained a mixture of CH3CHDCHBrCH3 and CH3CH2CDBrCH3 because the carbocation intermediate formed during the reaction was prone to rearrangement. This resulted in the formation of a more stable carbocation, which then reacted with the bromide ion. Therefore, both the expected product and the rearranged product were formed.

Key concepts

Organic Chemistry

Organic chemistry is the branch of chemistry that studies the structure, properties, and reactions of organic compounds, which are made up of carbon. In this exercise, we dealt with an organic reaction involving a deuterio-alcohol and a substitution process with hydrogen bromide (HBr) and sulfuric acid (H2SO4). Here, the focus is on understanding how carbon frameworks change during reactions.

• Carbon atoms form the backbone of organic molecules.
• Reactions in organic chemistry often involve changes to these structures, like substitutions or rearrangements.
• The movement of atoms within a molecule during a reaction is crucial to understand the outcome.

In this exercise, the reaction mechanism leads to the formation of a carbocation, showcasing core principles of organic chemistry such as rearrangement and the stability of intermediates.

Reaction Mechanism

A reaction mechanism describes the step-by-step sequence of elementary reactions by which overall chemical change occurs. For the process outlined in the exercise, the mechanism involves a substitution reaction where a deuterium-containing alcohol reacts with HBr. The key pathway involves carbocation formation and rearrangement.

• Here, the reaction begins with protonation of the alcohol by sulfuric acid, leading to the formation of a carbocation.
• This reactive intermediate can lead sometimes to rearrangement, a move that increases stability.
• Finally, the capture of this cation by a bromide ion completes the substitution.

Understanding the reaction mechanism elucidates why the expected and unexpected products both form.

Substitution Reaction

A substitution reaction involves replacing an atom or a group of atoms in a molecule with another atom or group. The student's reaction attempted to substitute a hydroxyl group (-OD) with a bromide (-Br) from HBr. This reaction is facilitated by the strong acid, H2SO4, which helps in protonating the -OD group.

• The key step in substitution is the formation of a reactive intermediate - a carbocation.
• The carbocation formed is then attacked by a nucleophile, in this case, the bromide ion.
• Successful substitution yields the brominated product, though carbocation rearrangements may alter the expected outcome.

Understanding substitution reactions is pivotal as they are one of the most common types of reactions in organic chemistry. These reactions underpin the synthesis of countless compounds in the lab and industry.

Deuterium Chemistry

Deuterium chemistry is an area dealing with reactions of compounds containing deuterium, a heavy isotope of hydrogen. Deuterium is used in labeling studies because its presence and effects can help track chemical processes.

• In our exercise, a deuterio-alcohol is being converted in a reaction while preserving deuterium at specific positions.
• This characteristic aids in studying mechanisms, especially carbocations and rearrangements.
• Deuterium substitution can subtly alter reaction pathways due to isotope effects, as seen where the student found an unexpected product formation.

Understanding these effects is key to interpreting the results when deuterium is involved, making it a valuable tool in organic chemistry.