Question

You know that alcohols are poor leaving groups. A protonated alcohol is a good leaving group and is an intermediate in the E1 dehydration reaction of alcohols. However, we cannot use a protonated alcohol in an E2 reaction. Explain why.

Short answer

Protonated alcohols cannot be used in an E2 reaction because E2 requires strong basic conditions, which are incompatible with the acidic conditions needed to protonate the alcohol.

Step-by-step solution

- Understanding Leaving Groups

A leaving group is an atom or group of atoms that detaches from a molecule during a reaction. In the context of alcohols, the hydroxyl group (-OH) is a poor leaving group because it is not stable when it leaves. However, when an alcohol is protonated, its hydroxyl group becomes water (H2O), which is a better leaving group.

- E1 Dehydration Reaction Mechanism

In an E1 dehydration reaction, the alcohol is first protonated to form a better leaving group (water). The reaction typically occurs in acidic conditions. The protonated alcohol then loses a water molecule, forming a carbocation intermediate, which subsequently undergoes deprotonation to form an alkene.

- E2 Reaction Mechanism

An E2 reaction involves the simultaneous removal of a hydrogen atom and a leaving group from adjacent carbon atoms in a single step, leading to the formation of a double bond (alkene). This reaction is typically facilitated by a strong base.

- Protonated Alcohol in E2 Reaction

A protonated alcohol cannot be used in an E2 reaction because the strong acidic conditions needed to protonate the alcohol will not be compatible with the strong basic conditions required for an E2 mechanism. Additionally, the formation of a protonated alcohol favors the E1 mechanism because it assists in forming a good leaving group (water) and the subsequent carbocation formation, which is specific to the E1 pathway.

- Summary

In summary, the conditions required for the E1 and E2 reactions are incompatible. E1 requires acidic conditions, while E2 requires basic conditions. Protonated alcohols favor E1 due to the formation of a good leaving group and the carbocation intermediate, while E2 relies on the presence of a strong base in basic conditions.

Key concepts

alcohol protonation

Alcohols are compounds containing a hydroxyl group (-OH). These are generally poor leaving groups because the hydroxyl group is not stable when it leaves the molecule. However, in acidic conditions, an alcohol can be protonated.
When an alcohol is protonated, the hydroxyl group becomes water (H2O), which is a much better leaving group.
In chemical terms, protonation means adding a proton (H+) to the molecule.
This transforms the -OH group into a positively charged water molecule which can easily leave the parent molecule.

E1 dehydration reaction

The E1 dehydration reaction is a type of elimination reaction where water is removed. This reaction typically takes place under acidic conditions.
Here’s how it works:

• The alcohol is first protonated to form a protonated alcohol.
• The protonated alcohol then loses a water molecule, forming a carbocation intermediate.
• The carbocation intermediate then loses a proton to form an alkene.

This mechanism occurs in two steps. First, the leaving group departs, creating the carbocation intermediate. Second, the intermediate loses a proton to form the double bond.

E2 reaction mechanism

Unlike the E1 mechanism, the E2 reaction occurs in a single step. Here are the key points to understand:

• It involves the simultaneous removal of a hydrogen atom and the leaving group from adjacent carbon atoms.
• A strong base is required to abstract the hydrogen atom.
• The result is a double bond (alkene) formation.

Because it occurs in one step, there is no formation of a carbocation intermediate. The strong base's presence is crucial because it facilitates the removal of the hydrogen atom.

carbocation intermediate

The carbocation intermediate is a critical concept in the E1 dehydration reaction.
A carbocation is a positively charged carbon atom that has only six electrons in its valence shell, making it highly reactive and unstable.
This intermediate is formed after the protonated alcohol loses its water molecule.
The formation of a carbocation intermediate is essential for the reaction to proceed as it allows the molecule to undergo deprotonation to form the final alkene.
Without this intermediate step, the elimination reaction cannot proceed via the E1 mechanism.