Question

Which of the following will most readily be dehydrated in acidic condition? (a) (b) CCCC(C)O (c) CCC(O)C(C)O (d) CC(=O)C(C)CC(C)O

Short answer

Option (c) will be dehydrated most readily.

Step-by-step solution

Understanding Dehydration Reactions

Dehydration in acidic conditions typically involves converting an alcohol into an alkene, where water is lost. The ease of dehydration is influenced by the stability of the carbocation intermediate that forms when the hydroxyl group is protonated and leaves.

Analyzing Option (b)

The structure given for option (b) is 2-pentanol (CCCC(C)O). Upon protonation of the hydroxyl group and loss of water, a secondary carbocation is formed, which is relatively stable but not the most stable variety.

Analyzing Option (c)

For 3-methyl-2-butanol (CCC(O)C(C)O), protonation of either hydroxyl group can lead to a carbocation. Protonating the secondary alcohol at the 2-position would lead to a more stable tertiary carbocation due to its additional alkyl group stabilization.

Analyzing Option (d)

Option (d), which is 3-hydroxy-3-methylpentan-2-one (CC(=O)C(C)CC(C)O), when dehydrated, will lead to a carbocation adjacent to a carbonyl group. While some resonance stabilization is possible, it is not as favorable as a tertiary carbocation.

Comparing Carbocation Stability

The most stable carbocation forms in option (c) due to its tertiary nature, which is known to be more stable than secondary and primary carbocations, or even those adjacent to electron-withdrawing groups like carbonyls. Therefore, option (c) will dehydrate most readily.

Key concepts

Carbocation Stability

In organic chemistry, understanding the stability of carbocations is crucial for predicting the course of many reactions, including dehydration. A carbocation is a positively charged carbon atom that is missing one of its bonds. Depending on the structure around this atom, the carbocation can exhibit different levels of stability.

- **Primary carbocations**: The least stable, occur when the positive charge is on a carbon atom attached to only one other carbon atom. - **Secondary carbocations**: Slightly more stable, these occur when the charge is on a carbon bonded to two other carbons. - **Tertiary carbocations**: The most stable due to steric hindrance and hyperconjugation; these occur when the positive charge is on a carbon bonded to three other carbon atoms.

The stability of carbocations affects a reaction's progress since a more stable carbocation intermediate generally leads to faster and more favorable reactions. As you can see, tertiary carbocations are at the top in terms of stability because the surrounding alkyl groups help to disperse the positive charge around the structure.

Tertiary Carbocation

A tertiary carbocation is a particularly important concept in understanding reactions such as dehydration because of its enhanced stability. When a tertiary carbocation forms, multiple mechanisms work in tandem to stabilize the positive charge:

• **Hyperconjugation**: The overlapping of orbitals in nearby carbon-hydrogen bonds can partially stabilize the positively charged carbon.
• **Inductive effect**: Alkyl groups, being electron-releasing, help stabilize positive charge by pushing electrons towards the positively charged center.
• **Steric hindrance**: Although it may seem counterintuitive, the larger structure of tertiary carbocations means that they can be less reactive overall due to the physical shading of the positive center, lending an inherent stability.

Because of these factors, tertiary carbocations form more readily and are more stable once formed, which is why in option (c) of the exercise, the formation of a tertiary carbocation leads to the most rapid dehydration.

Acid-Catalyzed Dehydration

Dehydration in organic chemistry is the process where an alcohol is converted into an alkene by removing water. When performed under acidic conditions, this process is known as acid-catalyzed dehydration. This method requires:

• **Protonation**: The alcohol's hydroxyl group is protonated by an acid, making it a better leaving group.
• **Carbocation formation**: The loss of water leaves behind a positively charged carbocation—a critical intermediate.
• **Rearrangement**: In certain situations, the already formed carbocation can rearrange to produce a more stable one.
• **Elimination**: Finally, the reaction concludes with the elimination of a proton to form the double bond of the alkene.

The choice of the alcohol substrate, the conditions, and the presence of potential rearrangements all affect the overall result of this reaction. The stability of the intermediate carbocation is a key factor in determining how readily dehydration will occur, which option (c) demonstrates by forming a stable tertiary carbocation.

Alkene Formation

Alkene formation is the ultimate goal in the dehydration of alcohols, especially under acidic conditions. The process through which an alcohol is transformed into an alkene involves several key steps and principles, making understanding their link critical:

- **Elimination reactions**: The dehydration is specifically an E1 mechanism, which involves a two-step process where the rate-determination step is the carbocation formation. - **Regioselectivity**: In cases where multiple alkenes could form, the Zaitsev's Rule generally applies, favoring the formation of the more substituted, more stable alkene. - **Stereochemistry**: If a chiral center is involved, the configuration of the resulting alkene (E or Z) can also influence the reaction outcome.

Thus, the nature of the carbocation, including its stability and capacity to carry out rearrangements, directly affects which alkene product forms most predominantly. For example, in the original exercise, the formation of alkenes from the tertiary carbocation occurs more swiftly than for less stable carbocations.