[Revision] Stereochemistry

Compounds which have different arrangements of atoms in space while having same atoms bonded to each other are said to have

• position isomerism

• functional group isomerism

• chain isomerism

• stereoisomerism

Answer explanation

Explanation: Stereoisomer his contrasts with structural isomers, which share the same molecular formula, but the bond connections or their order differs. By definition, molecules that are stereoisomers of each other represent the same structural isomer.

Which of the following can make difference in optical isomers?

• heat

• temperature

• polarized light

• pressure

Answer explanation

Explanation: An optically active substance is one which can rotate the plane of polarisation of plane polarised light. if you shine a beam of polarised monochromatic light (light of only a single frequency – in other words a single colour) through a solution of an optically active substance, when the light emerges, its plane of polarisation is found to have rotated.

Which of the following terms best describes the following pair of molecules?

• Isomers

• Constitutional isomers

• Configurational isomers

• Geometrical isomers

Answer explanation

Explanation: These molecules have the same molecular formula (C₇H₁₄), making them isomers. However, the molecules differ in their spatial orientations due to a double bond, i.e. cis & trans. Therefore, the molecules can best be described as geometric isomers.

Which of the following is an alkane which can exhibit optical activity?

• Neopentane (2,2-dimethylpropane)

• Isopentane (2-methylbutane)

• 3–Methylpentane

• 3–Methylhexane

Answer explanation

Identify the one which shows E-Z mechanism?

• 3-methylpent-2-ene

• 2-methylpent-2-ene

• Methyl-3-pent-2-ene

• 2,3-methylpentene

Answer explanation

Explanation: In ‘z’ mechanism, the compounds with higher priority will be located opposite to each other of the double bond, in ‘E’ mechanism the compounds with high priority will be located in z corners and hence 3-methylpent-2-ene is the one which shows E-Z mechanism in which the priority group is CH₃ and CH₂CH₃.

Which of the following molecules does not possess enantiomers?

• CH₃CH₂CH₂CHBrCH₃

• CH₃CH₂CBr₂CH₃

• CH₃CHBrCH₂CH₃

• CHBr₂CH₂CHBrCH₃

Answer explanation

Explanation: For being an enantiomer it is necessary that carbon atom has four non-identical substituents around it, making this carbon a chiral center, and as proof of its chirality the molecule has a non-superimposable mirror image. Molecules that are mirror images of each other is enantiomers.