Question

Suggest the identities of the major products in the acid-catalysed dehydrations of (a) butan-2-ol, (b) 2methylbutan-1-ol, (c) pentan-I-ol.

Short answer

(a) But-2-ene (cis and trans), (b) 2-Methylbut-2-ene, (c) 1-Pentene.

Step-by-step solution

Understanding Dehydration Reaction

In an acid-catalyzed dehydration reaction, an alcohol is converted to an alkene by the loss of a water molecule. It generally follows the E1 or E2 mechanism, where the alcohol is first protonated and then loses water, forming a carbocation intermediate, followed by loss of a proton to form an alkene.

Dehydrate Butan-2-ol

Butan-2-ol will first undergo protonation to form a good leaving group, which is water, and then lose this to form a secondary carbocation at C-2. This carbocation then rearranges to form the more stable secondary carbocation, if necessary. The double bond primarily forms between C-2 and C-3, resulting in but-2-ene. However, since the alkene can exist as geometrical isomers, both cis-but-2-ene and trans-but-2-ene can form.

Dehydrate 2-methylbutan-1-ol

2-Methylbutan-1-ol protonates and loses water to form a secondary carbocation at C-1. There is a possibility of carbocation rearrangement to form a more stable tertiary carbocation. The major product would form by deprotonation leading to a double bond at C-2 and C-3, resulting in 2-methylbut-2-ene as the major product.

Dehydrate Pentan-1-ol

Pentan-1-ol will be protonated, leading to the loss of water and formation of a primary carbocation, which is not stable and will rearrange to a more stable secondary carbocation at C-2. Deprotonation will lead to 1-pentene as the major product, forming mainly an alkene between C-1 and C-2.

Key concepts

Alcohol Conversion to Alkene

Alcohols can be transformed into alkenes through a process known as acid-catalyzed dehydration. This is an interesting reaction because it provides a pathway for making alkenes, which are important starting materials for many chemical syntheses. The term "dehydration" refers to the removal of a water molecule (_2O) from the alcohol. During this process, the alcohol is first protonated by the acid catalyst, typically sulfuric or phosphoric acid, making the -OH group a better leaving group.

Once protonated, the molecule loses the water molecule, leaving behind a carbocation intermediate in most cases. From here, the variety of possible reactions broadens due to carbocation stability and potential rearrangement. The subsequent step is losing a proton to form a double bond, yielding the corresponding alkene.

• Dehydration conditions often involve heating to assist the reaction.
• Typically, the reaction order follows Zaitsev's rule, which predicts that the more substituted alkene will be formed predominantly.

The acid-catalyzed dehydration is a powerful tool in both academic and industrial chemistry due to its simplicity and utility in generating diverse alkenes.

Carbocation Intermediate

Carbocations are positively charged ions that have a significant role in many chemical reactions, including acid-catalyzed dehydration. In these reactions, after the alcohol is protonated and the water molecule leaves, the atom that loses water ends up being positively charged, thus forming a carbocation.

Carbocations are crucial because their stability determines the course of the dehydration reaction. General guidelines for carbocation stability are as follows:

• Tertiary carbocations are more stable than secondary and primary due to hyperconjugation and the inductive effect.
• It is possible for carbocations to rearrange for increased stability, often through a shift of hydrogen or alkyl groups within the molecule.

In acid-catalyzed dehydration, a stable carbocation intermediate can form a more substituted and stable alkene by eliminating a proton. Such rearrangements provide the predominant alkene product through easier points of attack for the outgoing proton, leading to the formation of the alkene.

E1 and E2 Mechanisms

The formation of alkenes from alcohols proceeds through either an E1 or E2 mechanism, depending on the reaction conditions and substrates involved. Both mechanisms are pathways of elimination reactions, which result in the formation of a carbon-carbon double bond.

The E1 mechanism is a "two-step" reaction often associated with the acid-catalyzed dehydration of secondary or tertiary alcohols. This mechanism involves:

• Protonation of the alcohol to form a better leaving group, which is water.
• Formation of a carbocation intermediate as the water leaves.
• Subsequent deprotonation to form the double bond in the alkene.

The E1 route typically includes carbocation rearrangements owing to intermediates.

Conversely, the E2 mechanism occurs in a "concerted" step, where proton loss and _2O removal happen simultaneously. This is more usual in primary alcohols or when strong bases are involved, which can promote the single-step process. Choice between E1 and E2 pathways depends on the condition such as the type of alcohol and the presence of a strong base or strong acid.