Question

Terminal alkynes are certainly unsymmetrical, yet in these cases hydration is a useful process. In principle, either ketones or aldehydes could be formed, but in practice only ketones are produced. Write mechanisms for both ketone and aldehyde formation and explain why the product is always the ketone. You may write open vinyl carbocations in your answer for simplicity, but be aware that the real intermediates may be more complex cyclic species.

Short answer

Ketones are favored over aldehydes due to higher stability provided by electron-donating alkyl groups.

Step-by-step solution

Understand Terminal Alkynes

Terminal alkynes have the general structure RC≡CH, where R is any substituent group. These compounds can undergo hydration, which involves adding water to form new products.

Mechanism for Ketone Formation

The first step in the mechanism for ketone formation involves protonation of the alkyne to form a vinyl carbocation. Water then attacks this carbocation to form an enol intermediate. The enol tautomerizes to form a ketone.

Mechanism for Aldehyde Formation

Though less common, an alternative path involves formation of an aldehyde. After protonation of the terminal alkyne, water molecules attack differently, forming a different intermediate that rearranges to yield an aldehyde.

Tautomerization

Both the enol intermediate leading to ketones and the other intermediates can undergo tautomerization, which is a process where a proton shifts position, reconfiguring the molecule without breaking the carbon skeleton.

Why Ketones are Favored

Ketones are the main products because they are typically more stable than aldehydes. This stability can be attributed to the electron-donating effects of the alkyl groups attached to the carbonyl carbon in ketones, which stabilizes the molecule.

Key concepts

vinyl carbocation

A vinyl carbocation is formed during the hydration of terminal alkynes. This happens when a protonation step adds an extra proton to the alkyne, creating a positively charged carbon atom, hence a 'carbocation.'
The vinyl carbocation stage is crucial because it determines what will happen next in the reaction mechanism. Water molecules will attack this carbocation, leading to further steps in the formation of either ketones or aldehydes.
The nature of vinyl carbocations allows us to consider them as intermediates even if they might be more complex in reality. Understanding this intermediate stage helps make sense of subsequent chemical transformations.

enol tautomerization

Enol tautomerization is a key step in converting enols to more stable compounds like ketones or aldehydes.
Initially, water reacts with the vinyl carbocation to form an enol intermediate. An enol contains both a double bond (alkene) and an alcohol group (OH).
This enol is not very stable, so a process called tautomerization happens. During tautomerization, a proton (hydrogen atom) moves, reconfiguring the molecule into a more stable form.
This rearrangement doesn't break the carbon skeleton, but it changes the position of the atoms, converting the enol into a more stable ketone or aldehyde. Enol tautomerization is a fundamental concept in many organic reactions where enols are intermediates.

ketone stability

Ketones are typically more stable than aldehydes, which is why they are generally the products in the hydration of terminal alkynes.
Ketone stability primarily comes from the electron-donating effects of alkyl groups attached to the carbonyl carbon. These alkyl groups help to stabilize the carbonyl group by dispersing the charge density.
In other words, the additional groups (identified as R-groups) next to the carbonyl in ketones act in a way that distributes and reduces stress within the molecule. This stabilizing effect makes ketones energetically more favorable, leading to their dominance as the final product in these reactions.

aldehyde formation

Aldehydes can indeed form from the hydration of terminal alkynes, but this happens less frequently compared to ketone formation.
After forming a vinyl carbocation and reacting with water, the intermediates can rearrange differently to yield an aldehyde. However, the pathway to form an aldehyde is less energetically favorable.
Aldehydes only have a single alkyl group next to the carbonyl carbon, making them less stable compared to ketones. In addition, aldehydes can be more reactive due to the presence of a hydrogen atom bonded directly to the carbonyl carbon.
This lesser stability explains why, despite being possible, aldehyde formation is not the dominant pathway in the hydration of terminal alkynes.