why in cis and trans isomers the double bond cannot be broken once and again joined to convert them from cis to trans is this type of bond breaking not allowed or is this allowed?
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It can.$\mathstrut$ – Ivan Neretin Apr 18 '20 at 06:52
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If it could not, you would not see. – Poutnik Apr 18 '20 at 08:12
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The double bond consists of a sigma and pi bond, which is formed between two p orbitals. When the molecule has enough energy, obtained by random collisions in solution, the pi bond can break leaving just the single sigma bond. At this point should the double bond reform it is as if nothing has happened, however, if there is a slight rotation of the end group when only the single bond exists (close to the transition state) then cis-trans conversion can take place and the pi bond reforms after 180 degree rotation of one half of the molecule.
(Recall that a molecule's bonds vibrate and groups have have bending/ rotational motion also. The actual motion during isomerisation is more complex involving a stretching of the double bond first, which weakens it as the transition state is approached, and then oscillatory motion around the transition state.)
porphyrin
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The breaking of double bond requires a sufficently large energy which may also dissociate the compound also first breaking double bond and then again reforming through a reduction reaction is not feasible...pls comment if this helps you
Sarthsham
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