Question

Write a mechanism for the acid-catalyzed cleavage of tert-butyl cyclohexyl ether with trifluoroacetic acid to yield cyclohexanol and 2 -methylpropene.

Short answer

This reaction proceeds through protonation, carbocation formation, rearrangement, and formation of cyclohexanol.

Step-by-step solution

Protonation of Ether

The acid-catalyzed cleavage begins with the protonation of the ether's oxygen by trifluoroacetic acid (TFA), one of the stronger carboxylic acids due to the electronegative trifluoromethyl groups. The ether oxygen shares a lone pair to form a bond with a hydrogen ion from TFA, thus becoming a good leaving group.

Formation of Carbocation

Once the ether oxygen is protonated, it becomes a better leaving group. The bond between the oxygen and the tert-butyl group breaks, resulting in the departure of a water molecule and the formation of a tert-butyl carbocation.

Rearrangement to form 2-Methylpropene

The carbocation then undergoes rearrangement through a hydride shift. This rearrangement stabilizes the positive charge on the carbocation by forming a more substituted (and therefore more stable) alkene, specifically 2-methylpropene.

Formation of Cyclohexanol

Simultaneously, after the departure of the water molecule, the cyclohexyl group that was attached to the ether becomes cyclohexanol. The oxygen retains its bond with the hydrogen, resulting from the initial protonation step, thus converting to an alcohol group.

Key concepts

Ether Cleavage

Ether cleavage is a chemical reaction where an ether compound is broken into two different molecules. This process is basically splitting a molecule at the oxygen to separate the alkyl groups. In the case of tert-butyl cyclohexyl ether, it involves breaking down the ether linkage into an alcohol and an alkene. In an acid-catalyzed scenario, like this one, the reaction typically starts with the protonation of the ether oxygen.
This activated oxygen can then act as a leaving group. The cleavage primarily occurs because the ether's oxygen is made more reactive, thanks to its interaction with an acidic species. In this scenario, trifluoroacetic acid is the catalyst, which significantly increases the reactivity of the reaction.

Trifluoroacetic Acid

Trifluoroacetic acid (TFA) is known for its strong acidity, due largely to its highly electronegative trifluoromethyl groups. This makes it particularly effective in catalyzing the cleavage of ethers.
During the reaction, TFA donates a proton to the ether oxygen, making it more reactive.

• The electron-attracting trifluoromethyl groups increase the effectiveness of the acid by stabilizing the negative charge developed during the reaction.
• This stability makes trifluoroacetic acid a remarkably efficient catalyst for protonating and breaking down molecular structures.

Such interactions mark the importance of TFA in facilitating reactions where strong proton donors are needed, efficiently kicking off mechanisms like ether cleavage.

Carbocation Rearrangement

Carbocations are positively charged ions wherein a carbon atom has only six electrons in its outer shell. During ether cleavage, once the protonated ether loses a water molecule, a carbocation is formed.
This movement is significant because carbocations can rearrange to form more stable structures.

• In the case of tert-butyl cyclohexyl ether, the carbocation rearranges via a hydride shift.
• This shift leads to a more stable carbocation by increasing the substitution level on the carbocation.
• The rearrangement results in the formation of 2-methylpropene, reflecting the stability drive in organic reactions.

This aspect of rearrangement plays a critical role in organic chemistry, indicating how molecules may change in the quest for stability.

Formation of Alcohol

The formation of alcohol in acid-catalyzed ether cleavage involves converting one of the resulting fragments into an alcohol after the ether bond breaks. In this reaction, the departure of the tert-butyl group as a carbocation facilitates the formation of cyclohexanol from the remaining cyclohexyl portion.
The leftover oxygen retains its connection to a hydrogen ion from the initial protonation step. This connection effectively converts the oxygen into an alcohol group.

• The oxygen initially bonded in the ether linkage now attaches firmly into the cyclohexanol group.
• Maintaining the protonated state from the trifluoroacetic acid ensures stability and the successful formation of the alcohol.

This reaction's simplicity demonstrates how acid-catalysis can efficiently result in alcohol formation, a common target in synthetic organic transformations.