Ch5: E2 stereochemistry

Chapter 5:
Structure and Preparation of Alkenes.
Elimination Reactions

E2 Stereochemistry

E2 reactions occur most rapidly when the H-C bond and C-LG bonds involved are co-planar, most often at 180^o with respect to each other. This is described as an antiperiplanar conformation. This conformation positions the σ bonds that are being broken in the correct alignment to become the π bond.


	The staggered, antiperiplanar alignment is preferred because it aligns the two σbonds that become the π bond.
	The JMOL image to the left allows you to highlight the H that is antiperiplanar to the Br atom and look at this from different view points. Try making it look like the Newman projection on the right above.

Synperiplanar arrangments where the angle between the H-C bond and C-LG is 0^o are also known, usually in systems that are either inflexible rings or intramolecular eliminations.

syn orbital alignment in the E2 reaction

The eclipsed, synperiplanar alignment also aligns the two σ bonds that become the π bond, but is less favourable than the antiperiplanar arrangement.

Newman projection of synperiplanar alignment in the E2 reaction

These alignments are example of a stereoelectronic effect because they involve the specific spatial postioning of the bonds (electrons) in order for the process to occur.

Implications:

trans-

cis-

elimination of 4-bromo-t-butylcyclohexane

The cis- isomer undergoes elimination over 500 times faster than the trans- isomer.

In the cyclic system, in order for the preferred antiperiplanar arrangement favoured by E2 reactions, the C-H and C-LG bonds both need to be axial.

Recall that in chapter 3 that we learnt that the t-butyl group has a strong preference for the equatorial position on cyclohexanes and acts as a "lock". In the trans isomer, this means that the -Br is also equatorial and is therefore anti to C-C bonds, not C-H. Since the cyclohexane is locked it cannot ring flip into the geometry required for the E2 and elimination is slow.
In contrast, in the cis isomer, the -Br is axial and is anti to 2 C-H bonds and the E2 occurs rapidly. Use the JMOL images below to show the antiperiplanar bonds if you need to.