2021 P1 Q14 - Determine Number of Stereoisomers

Let Chemistry Guru, Singapore's top choice for JC Chemistry tuition, guide you through 2021 A Levels H2 Chemistry Paper 1 Question 14.

We are required to determine the number of stereoisomers for compounds Z and Q.

Let's recap the different types of stereoisomerism in A Level Chemistry syllabus.

1. Cis-trans isomerism

Cis-trans isomerism or geometric isomerism applies to alkenes where each of the 2 alkene carbons is bonded to 2 different groups.

The isomer where both bulky groups are on the same side of the alkene is the cis isomer.

The isomer where both bulky groups are on opposite sides of the alkene is the trans isomer.

2. Optical isomerism

Optical isomerism applies to compounds with a chiral carbon that is bonded to 4 different groups.

The mirror images are non-superimposable hence are called optical isomers or enantiomers.

For sketetal formula we can focus on carbons bonded to 3 groups (with a hidden hydrogen) and 4 groups as these 2 types of carbon are usually chiral.

We can ignore other types of carbon such as benzene and alkene carbons since they are not saturated and cannot be bonded to 4 different groups.

Check out this video lesson for a more detailed explanation of stereoisomerism.

We can now look at the compounds and deduce their number of stereoisomers.

Compound Z

cis-trans alkene = 1
chiral carbon = 1
total stereocentres = 2
total stereoisomers = 2² = 4

Take note that cyclopentene will not have cis-trans isomerism as the cyclostructure makes the trans isomer highly unstable due to ring strain.

Compound Q

cis-trans alkene = 0
chiral carbon = 3
total stereocentres = 3
total stereoisomers = 2³ = 8

Therefore the answer to this question is option B.

Topic: Intro to Organic Chemistry, Organic Chemistry, A Level Chemistry, Singapore

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