Question

(a) 4 -chloro- 1 -methylcyclohexane (b) trans 4 -chloro- 1 -methylcyclohexane (c) trans 1 -chloro-4-methylcyclohexane (d) trans 1 -methyl-4-chlorocyclohexane

Short answer

Answer: trans 4-chloro-1-methylcyclohexane

Step-by-step solution

Identifying main functional groups

In order to correctly identify the correct name for the given molecule, first, know the functional groups present. In this case, they are a methyl group (CH3) and a chloro group (Cl). Step 2: Understand the 'trans' prefix

Understanding the 'trans' prefix

The prefix 'trans' refers to the geometric isomerism. In the given exercise, we are dealing with cyclohexane. The 'trans' prefix indicates that the two functional groups (methyl and chloro) are present on the opposite side of the cyclohexane ring, meaning in axial and equatorial positions. Step 3: Correctly number the positions on the cyclohexane ring

Numbering the positions on the cyclohexane ring

Numbering the positions on cyclohexane starts from the first carbon attached to the first functional group and proceeds in such a way to give the lowest numbering to the functional groups. Both the methyl and chloro groups should be present on opposite sides and as close as possible to follow the 'trans' nomenclature. Now let's analyze each given name: (a) 4 -chloro- 1 -methylcyclohexane: This name implies that the Cl group is in the 4th position and the methyl group is in the 1st position, which is a correct numbering, but it doesn't provide information about the axial or equatorial orientation of the functional groups, so it cannot be considered as 'trans'. (b) trans 4 -chloro- 1 -methylcyclohexane: This name implies that the Cl group is in the 4th position and the methyl group is in the 1st position, with both functional groups present on the opposite side, which follows the 'trans' nomenclature. This is the correct name for the given molecule. (c) trans 1 -chloro-4-methylcyclohexane: This name implies that the Cl group is in the 1st position and the methyl group is in the 4th position, with both functional groups present on the opposite side, which also follows the 'trans' nomenclature. However, this is an incorrect numbering because the numbering should start from the carbon attached to the first functional group (CH3). (d) trans 1 -methyl-4-chlorocyclohexane: This name is similar to the previous one, but it has reversed the position of the functional groups chloride and methyl. This is again an incorrect numbering. Thus, the correct IUPAC name for the given molecule is:

Conclusion

trans 4 -chloro- 1 -methylcyclohexane

Key concepts

Functional Groups Identification

Identifying functional groups is a crucial first step in naming any organic compound using IUPAC nomenclature. These groups determine many properties of the molecule. In the context of the given exercise with cyclohexane derivatives, the two functional groups present are:

• Methyl group (CH extsubscript{3}): A very common alkyl substituent, the methyl group contributes a -CH extsubscript{3} moiety to the compound.
• Chloro group (Cl): A halogen atom, chlorine (Cl), replaces one hydrogen atom in the alkane chain, forming chloro derivatives.

To correctly identify these groups, look for these characteristic atoms or moieties in the structure. Once identified, these groups take precedence in deciding the positions and numbering on the cyclohexane ring. This ensures the naming adheres to IUPAC rules, allowing for systematic and predictable compound identification.

Geometric Isomerism

Geometric isomerism, particularly in cyclohexane compounds, is an important concept for understanding molecular geometry. Geometric isomerism refers to the spatial arrangement of atoms around a ring or double bond that cannot be interconverted without breaking chemical bonds. This principle often involves prefixes like 'cis' or 'trans'.

• The 'trans' configuration, as seen in some cyclohexane compounds, indicates that substituents are located on opposite sides of the cyclohexane ring. This typically affects the molecule's stability and how it reacts with other substances.
• In the context of cyclohexane, without rigid double bonds, the terms 'axial' and 'equatorial' positions are sometimes used to understand the three-dimensional positioning of substituents.

Recognizing whether substituents are in trans positions helps predict how the molecule behaves in physical and chemical reactions, facilitating a deeper understanding of its properties and reactivity.

Cyclohexane Numbering

Numbering the carbon atoms in a cyclohexane ring follows specific IUPAC rules to ensure clarity and consistency in nomenclature. When numbering a cyclohexane ring:

• Begin counting from the carbon atom to which the priority group with the lowest possible number is attached. This is important for reducing ambiguity in the compound's name.
• The goal is to assign the lowest numbers to the functional groups while adhering to the 'trans' or 'cis' arrangement if specified in the name.
• In our exercise, the methyl group is on carbon 1, and the chloro group is positioned on carbon 4 to ensure the 'trans' configuration is maintained.

This systematic approach helps avoid confusion and errors in chemical literature by standardizing how structures are identified across various scientific texts and studies.