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

Compounds (c) and (d) exhibit stereoisomerism.

Step-by-step solution

Understanding Stereoisomerism

Stereoisomerism occurs when compounds with the same molecular formula have different spatial arrangements of atoms. It can be identified in alkenes with restricted rotation around double bonds leading to cis and trans isomers, or in chiral molecules with at least one chiral center (a carbon atom bonded to four different groups).

Examine Compound (a): 2-methylbutene-1

2-methylbutene-1 is a compound with a double bond but at the terminal position (between carbon 1 and 2), which means it cannot have stereoisomers due to the presence of terminal carbon bonded to two identical hydrogen atoms.

Examine Compound (b): 3-methylbutyne-1

3-methylbutyne-1 has a triple bond, which does not allow for the existence of geometric cis-trans isomerism due to the linearity of the bond. Therefore, it does not exhibit stereoisomerism.

Examine Compound (c): 3-methylbutanoic acid

3-methylbutanoic acid needs to be checked for chiral centers. The third carbon is bonded to different groups: an ethyl group, a methyl group, a hydrogen atom, and a carboxylic group. This creates a chiral center, leading to the existence of enantiomers.

Examine Compound (d): 2-methylbutanoic acid

In 2-methylbutanoic acid, the second carbon is bonded to four different groups: a methyl group, an ethyl group, a hydrogen atom, and a carboxylic group. This results in a chiral center, allowing for stereoisomerism and the presence of enantiomers.

Identify Compounds with Stereoisomerism

Based on the analysis, compound (c), 3-methylbutanoic acid, and compound (d), 2-methylbutanoic acid, both exhibit chiral centers and thus display stereoisomerism.

Key concepts

Chiral Centers

A chiral center is a central concept in understanding stereoisomerism. It refers to a carbon atom bonded to four distinct substituents. This unique arrangement creates asymmetry, causing non-superimposable mirror images, much like our hands. These configurations in molecules give rise to enantiomers.

Determining a chiral center requires examining each carbon in the compound:

• The carbon atom must be connected to four different groups.
• If two groups are identical, the carbon is not a chiral center.
• Look for compounds with branching or functional groups like carboxylic acids.

A practical example is found in 3-methylbutanoic acid, where one of its carbons connects to a methyl group, hydrogen, an ethyl group, and a carboxylic group. This setup clearly indicates a chiral center.

Does your compound have a carbon connected to four different entities? You could be dealing with stereoisomers!

Enantiomers

Enantiomers are an intriguing subset of stereoisomers. They arise when molecules have at least one chiral center, resulting in non-superimposable mirror images much like left and right hands.

Here’s what to keep in mind about enantiomers:

• They have the same molecular formula and connectivity but differ in spatial arrangement at the chiral center.
• Enantiomers display different optical activities, rotating plane-polarized light in opposite directions.
• The biological activities of enantiomers can differ significantly, making them crucial in pharmaceuticals.

Let's revisit 2-methylbutanoic acid. Its second carbon bonds to groups such as methyl, hydrogen, ethyl, and carboxyl. These groups form a chiral center, yielding two mirror-image configurations or enantiomers with distinct properties.

When analyzing molecules, remember, enantiomers affect properties like smell, taste, and the effectiveness of drugs.

Geometric Isomers

Geometric isomers, often referred to as cis-trans isomers, appear in molecules with double bonds or specific ring structures where rotation is hindered. Unlike enantiomers, geometric isomers arise due to the rigidity of double bonds rather than asymmetric centers.

To spot geometric isomers, consider:

• Look for double bonded carbons, as rotation around a double bond is restricted.
• The two substituents must differ on each of the doubly bonded carbons; identical substituents don’t allow geometric isomerism.
• Cis isomers have substituents on the same side, whereas trans isomers have them on opposite sides.

In the case of 2-methylbutene-1, its double bond is at the terminal position making geometric isomerism impossible. This is because the double bond has identical substituents, making it unfit for cis-trans classification.

Being informed about geometric isomers aids in understanding different physical properties like boiling points and solubility.