4
$\begingroup$

I would like to know a general procedure for the synthesising cis isomers from their trans counterparts and vice versa.

$\endgroup$
4
$\begingroup$

In general, any setup that gives enough energy to the compound can isomerize it. This can be done by heating it, irradiating it with visible or UV light, or by using a catalyst (reducing the energy required for transition rather than supplying more energy). Excitation of electrons from the $\pi$ to the $\pi^*$ (bonding orbital to anti-bonding orbital) allow free rotation about the $\sigma$ bond. Relaxation back to the $\pi$ orbital locks the configuration. The specific procedures depend on the compound of interest.

FYI, photoisomerization (light-induced isomerization) is the principle on which sight is based.

$\endgroup$
  • $\begingroup$ By the way does photoisomerisation only induce the transition between cis and trans isomers? i.e. can dextrorotatory and levorotatory swapping occur due to photoisomerisation? $\endgroup$ – BH2017 Apr 12 '13 at 9:33
  • 2
    $\begingroup$ @BrentonHorne Changing from cis to trans may change the molecule from + to -, but it will not change the R/S configuration of any stereocenter. The +/- property does not depend on the absolute configuration of the stereocenters in any useful way. $\endgroup$ – Ben Norris Apr 12 '13 at 10:50
0
$\begingroup$

As a particular example of what buckminst is talking about, one common undergraduate organic chemistry laboratory experiment is the one where a tube full of dimethyl maleate is isomerized to dimethyl fumarate, with the help of sunlight and a teeny amount of bromine. Since solid dimethyl fumarate has low solubility in liquid dimethyl maleate, this makes for a nice visual demonstration. See e.g. this and this.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.