Stereoisomers in Molecules with One Chiral Center


- There are two stereoisomers of 2-bromobutane.

- Let's (for the moment) call them, Isomer A and Isomer B .

- Both of these compounds are 2-bromobutane (i.e. they are linear four carbon molecules with a bromine attached to a "non-end" carbon).

- However, they are NOT the same compound! Notice that the carbon bearing the bromine atom is a chiral center. These two molecules are stereoisomers.

- Examine the structures closely. Rotate each molecule so that the bromine atom is on top and the ethyl group attached to the chiral center is off to the left.

- Notice where the hydrogen attached to the chiral center in each molecule ends up.
In Isomer A, the hydrogen is behind the chiral center (pointing away).

In isomer B, the hydrogen is in front of the chiral center (pointing directly at you).

The two molecules can not be superimposed on top of one another. Thus, these molecules are different.
- These two molecules are stereoisomers, they have the same molecular formula, the same connectivity, but the arrangement of their atoms in three dimensions is different.

- Isomer A and Isomer B are, in fact, mirror images. Rotate one of the structures 180O, in order to see this relationship clearly.

- These two compounds are called enantiomers. They are nonsuperimposable mirror images.

- Enantiomers are one type of stereoisomer. (We will see other types shortly.)

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