Question

The \(\mathrm{H}_{2} \mathrm{SO}_{4}\) -catalyzed addition of water to 2 -methylpropene could give two isomeric alcohols. (a) Write their condensed structures. (b) Actually, only one forms. It is not possible to oxidize this alcohol to an aldehyde or to a ketone having four carbons. Which alcohol, therefore, is the product? (c) Write the structures of the two possible carbocations. (d) One of the two carbocations of part \((\mathrm{c})\) is more stable than the other. Which one is it, and how can you tell?

Short answer

The condensed structures of the alcohols are (CH3)3COH and CH3CH(OH)CH(CH3)2. The product is 2-methyl-2-propanol because it can't be oxidized to a four-carbon aldehyde or ketone. The carbocations are (CH3)3C+ and CH3CH+CH(CH3)2, and the tertiary (CH3)3C+ carbocation is more stable.

Step-by-step solution

- Write Condensed Structures of Isomeric Alcohols

In an acid-catalyzed hydration of 2-methylpropene, the addition of water (H2O) across the double bond can occur in two ways, producing two different alcohols. One addition leads to the formation of 2-methyl-2-propanol (tert-butanol), with the OH group attaching to the already substituted carbon. The second addition leads to the formation of 2-methyl-1-propanol (isobutyl alcohol), with the OH group attaching to the less substituted carbon. The condensed structures for these are (CH3)3COH for 2-methyl-2-propanol and CH3CH(OH)CH(CH3)2 for 2-methyl-1-propanol.

- Determine Which Alcohol is Formed

To determine which alcohol forms, we look at the possible oxidation products. Since the alcohol cannot be oxidized to a four-carbon aldehyde or ketone, it means that the alcohol must not have a hydrogen atom that can be removed from the carbon bearing the OH group. In this case, the alcohol formed must be 2-methyl-2-propanol since its tertiary carbon (where the OH is attached) does not have any hydrogen to be removed to form a carbonyl group (aldehyde or ketone).

- Write the Structures of Carbocations

The two possible carbocations resulting from the protonation of 2-methylpropene are: a tertiary carbocation where the positive charge is on the tertiary carbon (more substituted) - (CH3)3C+, and a primary carbocation where the positive charge is on the primary carbon (less substituted) - CH3CH+CH(CH3)2. These carbocations are intermediates in the acid-catalyzed hydration reaction.

- Identify the More Stable Carbocation

The tertiary carbocation (CH3)3C+ is more stable than the primary carbocation CH3CH+CH(CH3)2. This is because tertiary carbocations are stabilized by the inductive effect and hyperconjugation from three adjacent methyl groups, which spread out the positive charge more effectively. The primary carbocation lacks such stabilizing effects and is, therefore, less stable.

Key concepts

Isomeric Alcohols

Isomeric alcohols are compounds that have the same molecular formula but differ in the structure or position of the hydroxyl (OH) group attached to the carbon skeleton. In the acid-catalyzed hydration of 2-methylpropene, we encounter such isomers. When water is added to the double bond of 2-methylpropene, two isomeric alcohols can be formed: 2-methyl-2-propanol and 2-methyl-1-propanol. These isomers differ in which carbon the OH group attaches to after the double bond is broken by the addition of water.

Understanding the formation of different alcohol isomers is crucial because their chemical properties can vary significantly. In a classroom setting, it's helpful to visualize the molecular structures and consider functional group placement to fully appreciate how isomers behave differently in chemical reactions. For instance, the position of the OH group affects the alcohol's ability to be oxidized into other compounds, such as aldehydes or ketones.

Carbocation Stability

Carbocation stability is pivotal in organic chemistry because it greatly influences the outcome of reactions such as hydrations, eliminations, and rearrangements. A carbocation is a positively charged carbon ion, and its stability is affected by several factors, including the number of alkyl groups attached to the positively charged carbon.

In the context of acid-catalyzed hydration, the more substituted carbocation usually forms because it is more stable. Tertiary carbocations, like the (CH3)3C+ formed during the hydration of 2-methylpropene, are stabilized by the inductive effect and hyperconjugation from adjacent alkyl groups. Primary carbocations lack this stabilization, making them less likely to occur as reaction intermediates. It is this concept of carbocation stability that predicts the major product in such reactions. Students should internalize that the most stable intermediate leads to the predominant product in reactions involving carbocations.

Oxidation of Alcohols

Oxidation of alcohols is a transformation that's fundamental in organic synthesis, where the hydroxyl group of an alcohol is converted into a carbonyl group, forming aldehydes, ketones, or carboxylic acids. The type of alcohol—primary, secondary, or tertiary—dictates the possible oxidation products.

Primary alcohols can be oxidized to aldehydes and then to carboxylic acids, while secondary alcohols can be transformed into ketones. Tertiary alcohols, like 2-methyl-2-propanol formed from the hydration of 2-methylpropene, resist oxidation because there are no hydrogen atoms bonded to the carbon with the hydroxyl group that can be removed.

Understanding the differing oxidization potentials helps students predict the outcomes of reactions involving alcohols, and knowing these potential products is vital for proposing mechanisms and understanding organic reaction pathways.