The isomers have the same chemical formula but different chemical structures of configuration. That’s why the isomers are not the same compounds. The cis-trans isomers are structural isomers. They can be interconnected. So the interconversion of cis-trans isomer is possible and here we will discuss details on this particular topic. So not doing any further delay let’s get started.
Interconversion of cis-trans isomers
The cis and trans isomers can be interconverted. The interconversion of cis-trans isomers could be done by the following methods-
- a)Thermal, b) catalytic, and c) photochemical means.
- Directed fashion.
Now let’s have a little knowledge about the methods mentioned above-
a) Thermal method is not an easy one to interconvert isomers. Overcoming the energy of the rotation barrier is not easy, this method normally gives a mixture of isomers. In some cases, one isomer predominates over the other at equilibrium. For example, stilbene C6H5CH = CHC6H5 ( cis and trans isomers) due to low lying resonance delocalized triplet biradical.
This formed by intersystem crossing; push-pull ethylenes ( where extensive delocalization possible) e.g below virtually almost no C = C double bond, hence single bond rotation, then isomerization
e.g. Maleic and fumaric acid conversion at a high temp.
Another example of thermal isomerization is the following
15,15 – cis-β- Carotene to all-trans isomer
b) Catalyzed equilibration: Catalysts are free radicals and radical generators e.g. oxides of nitrogen, halogen in presence of light that is not absorbed by double bond i.e., not photochemical isomerization, iodine, diphenyldisulfide, etc. Also acids e.g. BF3, H2SO4, alkali metals general mechanism is shown
c) Photochemical Isomerization
The cis-trans isomerization of alkenes occurs under irradiation with light that they absorb. Isomerization takes place through a singlet excited state. Isomerization can also be affected via the triplet excited state. Triplet excited state is attained either by intersystem crossing or by using a photosensitizer that can transfer triplet energy to the compound to be isomerized. A typical sensitizer is C6H5COC6H5 which by irradiation, is excited from ground state Bo to a long-lived singlet Bs* which readily undergoes intersystem crossing to lower-lying triplet Bt* this triplet can be transferred to the molecule to be isomerized.
Directed cis-trans Interconversion
A directed interconversion can be effected in Principle by
- An anti addition followed by syn elimination
- Syn addition followed by anti elimination
- Syn elimination (to an alkyne) followed by anti addition
- Anti elimination followed by syn addition
When elements added are different from elements eliminated then anti-anti e.g. Addition of Cl2 anti; elimination of HCl anti. At the final stage reduction of chloroalkene to an alkene with retention of configuration
Conversion of trans to cis alkene: erythro means the same group on the same side
- A sequence of anti addition followed by apparent syn elimination ( direct anti elimination of BrCl by iodide is so slow that SN2 displacement of Br by I ( with inversion of the configuration takes place) subsequent anti elimination of ICl leads to a cis alkene in high purity; i.e., SN2 inversion plus anti elimination is equivalent to syn elimination
[R’= (CH3)2CH(CH2)4- ; R= -(CH2)9CH3]
[R’= (CH3)2CH(CH2)4- ; R= -(CH2)9CH3]
-An attractive cis-trans interconversion sequence would involve the conversion of the alkene into a 1,2-diol by syn addition followed by an anti elimination – [Hydroxylation of alkenes]
A = KMnO4 or OsO4 or CH3CO2Ag, I2, CH3CO2H-H2O
B = RCO3H followed by hydrolysis or CH3CO2Ag, I2 (dry) or epoxidation followed by hydrolysis
Syn Elimination: Sequence of diol