Question

Which of the following compounds exhibit steroisomerism? (a) 2-methylbutene-1 (b) 3-methylbutyne-1 (c) 3 -methylbutanoic acid (d) 2 -methylbutanoic acid

Short answer

3-methylbutanoic acid and 2-methylbutanoic acid exhibit stereoisomerism.

Step-by-step solution

Understanding Stereoisomerism

Stereoisomerism occurs when compounds have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space. The key types to consider are geometric (cis/trans) and optical isomerism (chiral centers).

Analyzing 2-methylbutene-1

In 2-methylbutene-1, examine the presence of double bonds. A double bond can exhibit cis-trans isomerism if each carbon in the double bond has different substituents. However, in 2-methylbutene-1, terminal double bonds mean cis-trans isomerism is not possible.

Reviewing 3-methylbutyne-1

A triple bond as in 3-methylbutyne-1 cannot support geometric or optical isomerism as linear geometry doesn't allow for variation in spatial arrangements around the bond itself.

Examining 3-methylbutanoic acid

For optical isomerism, look for chiral centers (a carbon bonded to four different groups). In 3-methylbutanoic acid, the second carbon is bonded to four different groups (methyl, hydrogen, carboxyl, and the rest of the chain), making it a chiral center hence showing optical isomerism.

Checking 2-methylbutanoic acid

In 2-methylbutanoic acid, the second carbon is attached to four different substituents, which include a methyl group, hydrogen, a carboxyl group, and the main carbon chain. This configuration creates a chiral center, hence supporting optical isomerism.

Key concepts

Optical isomerism

Optical isomerism is a fascinating type of stereoisomerism. It occurs when molecules are mirror images of each other but cannot be superimposed. Think of your left and right hand; they are mirror images but cannot overlap perfectly. Such molecules are called enantiomers. At the heart of optical isomerism lies the concept of a **chiral center**, which is a carbon atom bonded to four different groups. This creates two non-superimposable mirror images.

Key points about optical isomerism:

• Enantiomers: Pairs of molecules that are mirror images.
• Chirality: It depends on having a chiral center.
• Effect on plane-polarized light: Enantiomers rotate the light in different directions, one isomer clockwise (dextrorotatory) and the other counterclockwise (levorotatory).

For instance, in 3-methylbutanoic acid and 2-methylbutanoic acid, each has a chiral center at the second carbon atom. This makes them capable of showing optical isomerism, through the presence of enantiomers.

Geometric isomerism

Geometric isomerism, or **cis-trans isomerism**, occurs in compounds with restricted rotation, like those with double bonds. In these cases, the spatial arrangement of atoms or groups can be different, leading to distinct isomers. In the case of geometric isomerism:

• **Cis isomers** have substituents positioned on the same side of the double bond.
• **Trans isomers** have substituents on opposite sides.

For a compound to exhibit geometric isomerism, each carbon on the double bond must have two different substituents. Without this condition, such as in 2-methylbutene-1 which has a terminal double bond, geometric isomerism is not possible. This explains why compounds like 3-methylbutyne-1, which contain a triple bond with no potential for varying spatial arrangement, do not exhibit geometric isomerism.

Chiral centers

The presence of a chiral center is crucial for a molecule to exhibit optical isomerism. Simply put, a chiral center is a carbon atom connected to four different atoms or groups, allowing for two non-superimposable arrangements that result in optical activity. Let's boil down the essentials:

• A single chiral center typically leads to two enantiomers.
• The lack of chiral centers means the molecule won't show optical isomerism.
• In some molecules with more than one chiral center, the number of possible stereoisomers increases.

Consider the molecules 3-methylbutanoic acid and 2-methylbutanoic acid: both have a single chiral center allowing them to have enantiomers that are optically active. Identifying chiral centers can be a key step in understanding a molecule's potential for optical isomerism.