Question

Draw structures from the following names, and determine which compounds are optically active: (a) 3 -bromohexane (b) 3 -chloro-3-methylpentane (c) 1,2 -dibromo-2-methylbutane

Short answer

(a) Optically active, (b) Not optically active, (c) Optically active.

Step-by-step solution

Draw the structure of 3-bromohexane

3-bromohexane has a hexane chain (6 carbon atoms) with a bromo group attached to the third carbon. The structure is as follows: CH3-CH2-CH(Br)-CH2-CH2-CH3

Determine if 3-bromohexane is optically active

Check for a chiral center: The third carbon with the bromine is attached to four different groups (bromo, ethyl, methyl, and hydrogen). This makes 3-bromohexane optically active.

Draw the structure of 3-chloro-3-methylpentane

3-chloro-3-methylpentane has a pentane chain (5 carbon atoms) with a chlorine and a methyl group both attached to the third carbon. The structure is as follows: CH3-CH2-C(Cl)(CH3)-CH2-CH3

Determine if 3-chloro-3-methylpentane is optically active

Check for a chiral center: The third carbon is attached to two identical groups (methyl) and thus does not have four different groups. Therefore, 3-chloro-3-methylpentane is not optically active.

Draw the structure of 1,2-dibromo-2-methylbutane

1,2-dibromo-2-methylbutane has a butane chain (4 carbon atoms) with bromo groups attached to the first and second carbons, and a methyl group also attached to the second carbon. The structure is as follows: CH2(Br)-CH(Br)(CH3)-CH2-CH3

Determine if 1,2-dibromo-2-methylbutane is optically active

Check for a chiral center: The second carbon is attached to four different groups (hydrogen, bromine, methyl, and a butyl chain). Thus, 1,2-dibromo-2-methylbutane is optically active.

Key concepts

chiral centers

A chiral center, also known as a stereogenic center, is a central carbon atom attached to four different groups. Chiral centers are essential in determining whether a molecule is chiral and can exhibit optical activity.
In organic chemistry, identifying chiral centers helps you figure out if a compound can exist in non-superimposable mirror images known as enantiomers. Consider the following:

• Chirality arises from the asymmetrical arrangement around a carbon atom.
• To check for chirality, look for a carbon bonded to four distinct groups.
• Molecules with chiral centers are typically optically active, as they can rotate plane-polarized light.

In the drawn structures from the exercise, we identified chiral centers to determine optical activity. For instance, in 3-bromohexane, the third carbon with a bromine atom, ethyl, methyl, and hydrogen groups is chiral, making the molecule optically active.
Knowing how to identify chiral centers is crucial for understanding stereochemistry and molecular behavior.

organic chemistry structure drawing

Drawing organic compounds correctly is vital to understanding their properties and behaviors. Here’s a brief guide on how to draw structures for the given exercise:

• Start by identifying the longest carbon chain to establish the backbone of the molecule.
Number the carbon atoms in the main chain to determine the correct positions for substituents.
• Add functional groups and substituents to the appropriate carbon atoms according to their names.

For example, drawing 3-bromohexane involves:

• Creating a six-carbon chain (hexane).
• Placing a bromine atom (Br) on the third carbon.
• The final structure: CH3-CH2-CH(Br)-CH2-CH2-CH3.

By accurately drawing the structures, you ensure correct identification of molecular properties, such as optical activity and stereochemistry.

stereochemistry

Stereochemistry focuses on the 3D arrangement of atoms in molecules, impacting their chemical reactions and behaviors. It includes concepts like chirality, optical activity, and isomerism.
Optical isomers, or enantiomers, are one of the key areas in stereochemistry. These are non-superimposable mirror images with different spatial arrangements.

• Enantiomers exhibit distinct interactions with plane-polarized light—one will rotate light in one direction (dextrorotatory), while the other will rotate it in the opposite (levorotatory).
• Compounds with stereoisomers can have significantly different biological activities.

When analyzing stereochemistry for the exercise, we examined chiral centers and the spatial arrangement of groups to determine optical activity. Understanding stereochemistry is vital for predicting how molecules interact in chiral environments, such as biological systems.

chiral molecules identification

Identifying chiral molecules involves checking for chiral centers and understanding their 3D arrangements.
Here are steps for identifying chiral molecules in the given exercise:

• Look for a carbon atom bonded to four different groups.
• Draw the molecular structure to visualize groups around each carbon.

For instance, in 1,2-dibromo-2-methylbutane:

• We have a butane chain with substituents at the first, second, and third carbons.
• The second carbon is bonded to a hydrogen, a bromine, a methyl group, and a butyl chain, making it a chiral center.
• Thus, the molecule is chiral and optically active.

Careful identification of chiral molecules is crucial in organic chemistry to predict their stereochemical behavior and interactions in different environments.