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Isomerism

This question is about stereoisomerism / chiral isomerism / optical isomerism (all the same thing)

I know that by definition stereoisomerism works only when we have four different groups around a carbon centre BUT
why?

Why is this an stereoisomer: CH3CHClBr

But not

CH3CH2Br or CH3CHBr2

thanks

Reply 1

Kingbradley6

Original post by jsmith6131

CH3CHClBr has four different groups around the chiral carbon, which are:

1) CH3
2) H
3) Br
4) Cl

But when you look at the other two, they only have three different groups.

CH3CH2Br:

1) CH3
2) H
3) Br
4) H

As you can see group 2 and 4 are the same.

The same is true for CH3CHBr2, only it has two bromine groups.

Reply 2

username459260

I can see that but I don't understand why the one with four different groups can be chiral and not the others

Reply 3

Kingbradley6

Something is chiral when it has four different groups attached to one of the carbons. The first compound has four totally unique groups thus it is chiral.

The other two compounds have four groups, however there is only three unique groups. Two of the four are identical groups (H & H or Br & Br), thus it is not chiral.

If you don't get it try to understand what it means for a compound to be chiral: that is, it does not super-impose on its mirror image. The groups with only three unique groups would be able to super-impose over each other as in its tetrahedral structure, if you rotate it, eventually they would become identical in space. On the other hand a chiral compound never becomes identical (due to the tetrahedral shape). It may be better to imagine it looking online at a 3D model or constructing a model out of MolyMod. A 2D image doesn't show this particularly well.

(edited 13 years ago)

Reply 4

charco

Study Forum Helper

Original post by jsmith6131

I can see that but I don't understand why the one with four different groups can be chiral and not the others

Chiral means that it has a non-superimposible mirror image. Tetrahedral (with four different groups) substances have this possibility.

If you can't see this (and it isn't easy) then you should try to build a molecule using molymods and its stereoisomer and try to superimpose them.