Question

Which of the following compounds are chiral? Draw them, and label the chirality centers. (a) 2,4 -Dimethylheptane (b) 5 -Ethyl-3,3-dimethylheptane (c) cis-1,4-Dichlorocyclohexane

Short answer

Only 5-Ethyl-3,3-dimethylheptane is chiral.

Step-by-step solution

Understand Chirality

A molecule is chiral if it has a non-superimposable mirror image. Typically, this involves having at least one carbon atom (chirality center) bonded to four different groups.

Analyze 2,4-Dimethylheptane

Begin by drawing the structure. The molecule is a heptane chain with methyl groups on the second and fourth carbons. Check each carbon for four distinct substituents. None of the carbons in this molecule have four distinct groups, so it is achiral.

Analyze 5-Ethyl-3,3-dimethylheptane

Draw the structure: start with a heptane chain, then add methyl groups on the third carbon and an ethyl group on the fifth. Check for chirality centers. The fifth carbon is attached to four different groups (hydrogen, ethyl, the main chain, and one end of the chain) and therefore is a chirality center. This compound is chiral.

Analyze cis-1,4-Dichlorocyclohexane

Draw the cyclohexane structure with chlorine atoms at the 1st and 4th positions. In the cis isomer, both chlorines are on the same side of the ring, leading to a symmetrical structure. This molecule is achiral because it doesn't have chirality centers; the plane of symmetry makes it superimposable with its mirror image.

Key concepts

Chirality Centers

Chirality centers are essential to understanding chiral molecules in organic chemistry. A chirality center, often a carbon atom, is bonded to four different groups. This structure allows it to have non-superimposable mirror images, which makes the molecule chiral. A simple way to identify a chirality center is to look for carbon atoms connected to four unique substituents. If any carbon in a molecule meets this criterion, it's a chirality center. When analyzing molecules, always check each carbon atom to see if it meets the requirements for chirality. This simple step is crucial in identifying which molecules can be chiral. In 5-Ethyl-3,3-dimethylheptane, the fifth carbon is a chirality center. It connects to four different types of groups, highlighting its potential for chirality.

Chiral Compounds

Chiral compounds are fascinating in chemistry because they have non-superimposable mirror images called enantiomers. These structures occur when a molecule has at least one chirality center. Enantiomers hold significance because, despite having the same molecular formula, they have different properties, particularly in how they interact with polarized light. This interaction can affect reactions in biological systems. When identifying if a compound is chiral, find and label any chirality centers. In structural formulas, this helps visualize non-superimposable mirror images effectively. For example, 5-Ethyl-3,3-dimethylheptane is a chiral compound because it contains a chirality center. This presence confirms that it has enantiomers and is, therefore, chiral.

Structural Analysis

Structural analysis is a technique used to determine the chirality of molecules. It involves drawing the structure of the compound to understand better the arrangement of its atoms and any present symmetry. For structural analysis:

• Draw the complete molecular structure.
• Identify any chirality centers by looking for asymmetric carbons.
• Look for planes of symmetry, as they indicate achirality.

By drawing compounds like 2,4-Dimethylheptane and cis-1,4-Dichlorocyclohexane, it's clear why these molecules are achiral. 2,4-Dimethylheptane lacks chirality centers, as no carbon is bound to four differing groups. Meanwhile, in cis-1,4-Dichlorocyclohexane, a plane of symmetry negates chirality. Through careful structural analysis, students can determine whether molecules are chiral or achiral effectively.