Question

When (cis)-1-bromo-2 methylcyclohexane is treated with methanol and heat, four different products are formedtwo by substitution and two by elimination.

Short answer

The four products formed when (cis)-1-bromo-2-methylcyclohexane is treated with methanol and heat are as follows: 1. Substitution product with methyl group (axial) and -OCH3 group (axial) in cis configuration. 2. Substitution product with methyl group (equatorial) and -OCH3 group (equatorial) in cis configuration. 3. Elimination product with double bond between carbon 1 and 2, and -OCH3 group attached to carbon 1 in cis configuration (beta elimination). 4. Elimination product with double bond between carbon 2 and 3, and -OCH3 group attached to carbon 1 in cis configuration (beta-2 elimination).

Step-by-step solution

Identify the starting compound

First, it is essential to know the structure of (cis)-1-bromo-2-methylcyclohexane, which is a six-membered cyclic compound with a bromine atom at first carbon and a methyl group at the second position, both in the cis configuration.

Identify the reagent and conditions

The reagent used is methanol, which can act as a nucleophile in substitution reactions or a base in elimination reactions. The presence of heat can also favor elimination reactions over substitution reactions.

Substitution reaction

In a substitution reaction, the nucleophile, methanol (CH3OH), will replace the leaving group, bromide ion (Br-) from the (cis)-1-bromo-2-methylcyclohexane, leading to the formation of two substitution products, one with the methyl group and -OCH3 group in the axial positions and the other with the methyl group and -OCH3 group in equatorial positions. Product 1: Methyl group (axial) and -OCH3 group (axial) on carbons 1 and 2 of cyclohexane ring in the cis configuration. Product 2: Methyl group (equatorial) and -OCH3 group (equatorial) on carbons 1 and 2 of cyclohexane ring in the cis configuration.

Elimination reaction

In an elimination reaction, the base (methanol) will deprotonate one of the beta hydrogens, leading to the formation of a double bond and the expulsion of the bromide ion from the carbon atom. There are two possible sites for this elimination: from the carbon next to the bromine group (beta elimination) or the carbon next to the methyl group (beta-2 elimination). Product 3: Beta elimination: The double bond is formed between carbon 1 and carbon 2, and -OCH3 group is attached to carbon 1 in cis-configuration. Product 4: Beta-2 elimination: The double bond is formed between carbon 2 and carbon 3, and -OCH3 group is attached to carbon 1 in cis-configuration. In conclusion, when (cis)-1-bromo-2-methylcyclohexane is treated with methanol and heat, four different products are formed, two through substitution reactions and two through elimination reactions.

Key concepts

Substitution Reactions

Substitution reactions are a fundamental concept in organic chemistry where one functional group in a molecule is replaced by another group. In the context of the reaction with (cis)-1-bromo-2-methylcyclohexane, methanol acts as a nucleophile, targeting the bromine atom, which is a good leaving group. During the substitution process, there are two possible conformations for the methyl and methoxy (-OCH3) groups: either both are positioned axially or both are equatorially placed on the cyclohexane ring.

• Axial position: This leads to increased steric hindrance due to less space around the axial position, making some reactions less favorable.
• Equatorial position: This is usually more stable because these groups have more room, reducing steric hindrance.

Methanol replaces the bromide ion, resulting in two distinct substitution products. Each product's stability can be influenced by these positional differences, with equatorial positions often being more thermodynamically favored due to minimized steric clashes.

Elimination Reactions

Elimination reactions involve the removal of a functional group and a hydrogen atom, leading to the formation of a double bond. In the scenario of (cis)-1-bromo-2-methylcyclohexane, elimination occurs when methanol acts as a base, removing a beta hydrogen atom adjacent to the bromine, which results in the formation of a double bond between carbon atoms. There are two pathways for elimination:

• Beta elimination: Takes place between the carbon bearing the bromine and its adjacent carbon, leading to the formation of a double bond between these two.
• Beta-2 elimination: Involves the adjacent carbon of the methyl group, forming a double bond further along the ring.

These reactions yield two possible products, similar to the substitution process, both influenced by the position of the newly formed double bond. The nature of the base and the reaction conditions, such as heat, significantly affect which elimination path is favored.

Stereochemistry

Stereochemistry plays a crucial role in determining the products of reactions involving chiral centers or cis/trans configurations, like in the case of (cis)-1-bromo-2-methylcyclohexane. This compound features groups arranged in a specific spatial configuration, with the bromine and methyl group both in the cis orientation. This stereochemical arrangement influences both substitution and elimination reactions, as the positioning of substituents affects reaction pathways and product stability. For substitution reactions:

• The cis configuration means that products formed will reflect this spatial arrangement, impacting their reactivities and potential steric interactions.

For elimination reactions:

• Stereochemistry determines which hydrogen atoms are accessible for removal, influencing the positioning of the resultant double bond.

Understanding the stereochemical implications aids in predicting the outcome of these organic reactions and ensures accurate depiction of product structures.