Deduce Optical Activity of Meso Compound

Let's compare these 2 compounds (A and B) and deduce which is optically inactive.

meso compound optical activity which is inactive

Each compound contains 2 chiral carbons.

A chiral carbon is a saturated sp3 hybridised carbon bonded to 4 different groups.

The mirror images of a compound with chiral carbon are non-superimposable hence they are isomers of each other.

We call these optical isomers or enantiomers.

Check out this video lesson to learn more about optical isomerism and other types of stereoisomerism.

What is interesting about compounds A and B is each chiral carbon is bonded to the same 4 groups:

- bromine
- hydrogen
- ethyl group CH2CH3
- the other chiral carbon

This means some of the optical isomers will have mirror symmetry which will cancel its optical activity.

In order to visualise and compare the chiral carbons better, let's apply the following transformations.

1. Convert ethyl group CH2CH3 to Et

The long carbon chain is distracting and since both carbons in ethyl group are non-chiral, we can just replace CH2CH3 with Et without affecting the stereochemistry of the compound.

meso compound optical activity convert ethyl group

2. Rotate compound slightly

It will be easier to compare the compounds if the C-C bond for both chiral carbons is horizontal.

So let's rotate the compound slightly.

meso compound optical activity rotate compound

3. Rotate C-C bond by 180 degrees

We want both ethyl groups to point down to find the mirror plane easily so let's rotate the C-C bond by 180 degrees.

Notice the orientation of Br group will change due to this rotation:

- Br pointing towards you before rotation will be pointing away from you after rotation
- Br pointing away from you before rotation will be pointing towards you after rotation

meso compound optical activity rotate carbon carbon bond

4. Find mirror plane

We can now compare the 2 compounds.

meso compound optical activity find mirror plane

Compound A has no mirror plane so it will still be asymmetrical and optically active.

Compound B has a mirror plane between both chiral carbons.

The optical activities of both chiral carbons will cancel out.

Therefore compound B will be optically inactive.

Compound B is an example of a meso compound, where chiral carbons are present but the compound is optically inactive due to a mirror plane within the compound.

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

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