Beginning Organic Chemistry (BOC)
8. Isomerism.
b. Sterioisomers.

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Stereoisomers are isomers having exactly the same bond connectivity but with arrangements which differ from one another spacially.

Different spacial arrangements of atoms can give rise to distinct compounds (isomers) in two major ways:

If rotation about a bond is prevented.
If the direct bonding to an atom gives different spacial arrangements.

In the following, links are provided to the relevent parts of my Nomenclature Tutorial. There may be several pages to view.

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1. Rotation about a bond is prevented.

There are a number of situations where rotation about a bond is prevented, or at least strongly hindered, in organic compounds:

An atom-atom double bond (e.g. a C=C) does not allow rotation at room temperature. Nomenclature tutorial page.
A ring of atoms will not allow rotation about one of the ring bonds. Nomenclature tutorial page.
Bulky substituents may prevent the rotation about a bond. Nomenclature tutorial page (atropisomerism)

In these latter two cases the bond could be a single (two-electron) bond which is prevented from its normal rotation by the structure of the rest of the molecule.

(E)-But-2-ene, trans-2-Butene. (Z)-But-2-ene, cis-2-Butene.

2. Different spacial arrangements by direct bonding

In carbon compounds, this can give rise to isomerism because of the tetrahedral arrangement of substituents at a singly bonded carbon atom. It turns out that there are two distinct ways in which four different atoms may be attached in the tetrahedral configuration to a centre atom. Nomenclature tutorial pages. (there are a number of relevent pages here.)

(R)-2-Bromobutane. (S)-2-Bromobutane.

Date created: 2005 06 12.