Question

Draw the two chair conformations of bromocyclohexane. Pick the conformation that will react quicker with potassium tert-butoxide. Show the mechanism of the \(\mathrm{E} 2\) reaction of bromocyclohexane with tert-butoxide.

Short answer

Draw two chair forms of bromocyclohexane. The axial Br conformation reacts quicker via \(\text{E2}\) with tert-butoxide. Show elimination and double bond formation.

Step-by-step solution

- Draw the two chair conformations of bromocyclohexane.

Draw the cyclohexane ring in two different chair conformations. For each conformation, place the bromine atom (Br) at an axial position in one chair form and at an equatorial position in the other.

- Identify the more stable conformation.

Typically, the conformation where the bulky substituent (bromine) is in the equatorial position is more stable due to less steric hindrance.

- Determine the conformation that reacts quicker with potassium tert-butoxide.

Potassium tert-butoxide commonly performs an \(\text{E2}\) elimination. For \(\text{E2}\) reactions, the leaving group (Br) must be in an axial position to allow for the anti-periplanar alignment with a neighboring hydrogen.

- Choose the reactive conformation.

Since the bromine must be axial for an \(\text{E2}\) reaction, the conformation where the bromine is in the axial position is more reactive.

- Show the mechanism of the \(\text{E2}\) reaction.

Draw the cyclohexane ring with bromine in the axial position. Use an arrow-pushing mechanism to illustrate the tert-butoxide attacking a neighboring axial hydrogen (anti to Br), causing the formation of a double bond as bromine leaves.

Key concepts

chair conformations

Cyclohexane is a fundamental structure in organic chemistry. Its flexible nature allows it to adopt different shapes called conformations. The most common and stable conformations are the 'chair conformations'. These are termed so because they resemble a chair when viewed from the side. In chair conformations, carbon atoms alternately point up and down, reducing steric hindrance and allowing for better spatial arrangements of attached groups.
For bromocyclohexane, drawing the two chair conformations involves placing the bromine atom in different positions. In one conformation, the bromine (Br) will be in the axial position (pointing straight up or down), while in the other, it will be in the equatorial position (extending outward around the ring). Understanding these positions is crucial for determining reactivity in chemical reactions.

stereochemistry

Stereochemistry deals with the spatial arrangement of atoms in molecules. In the context of chair conformations of cyclohexane compounds, stereochemistry becomes very important. The placement of substituents (like Br in bromocyclohexane) can significantly impact the molecule's stability and reactivity.
In chair conformations, substituents can occupy either axial or equatorial positions. Generally, bulky groups prefer the equatorial position because it leads to less steric hindrance. Steric hindrance refers to the repulsion between atoms that are too close to each other. Hence, bromocyclohexane is more stable when the bromine is in the equatorial position. However, for some reactions like the E2 elimination, the axial position becomes more crucial.

E2 elimination mechanism

The E2 (bimolecular elimination) mechanism is a type of chemical reaction where a base removes a proton (H) from a carbon atom, and simultaneously, a leaving group (like Br) is ejected from the adjacent carbon atom. This results in the formation of a double bond between the two carbon atoms. This process happens in a single concerted step.
For bromocyclohexane, the E2 reaction with potassium tert-butoxide involves several key points:

• The leaving group (Br) must be in an axial position. This allows for the required anti-periplanar alignment with the hydrogen atom that is going to be removed.
• Potassium tert-butoxide, a strong base, will attack the hydrogen that is anti to the bromine.
• The base abstracts the hydrogen, and electrons form a new double bond as bromine leaves.

This results in the formation of a cyclohexene derivative, showcasing the anti-periplanar requirement for the E2 mechanism.

potassium tert-butoxide

Potassium tert-butoxide (KOt-Bu) is a strong base commonly used in organic chemistry. Its structure includes a potassium ion (K⁺) and a tert-butoxide ion (⁻OtBu), where the tert-butoxide is a bulky base due to its three methyl groups attached to the central carbon.
In the context of E2 reactions, potassium tert-butoxide is particularly effective for several reasons:

• It is a strong base, ensuring a swift removal of protons (H⁺).
• Its bulky nature makes it less likely to participate in nucleophilic substitution reactions (like S_N2), favoring elimination (E2).

When reacting with bromocyclohexane, the bulky tert-butoxide base abstracts a hydrogen atom from the carbon adjacent to the one holding the bromine, leading to the formation of a double bond as bromine is expelled (leaving group). This highlights the efficiency of potassium tert-butoxide in facilitating E2 eliminations, particularly in complex conformational scenarios, such as chair conformations.