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Which one of the following alkenes will react faster with $H_2$ under catalytic hydrogenation conditions :-
[R = Alkyl substituent]
[R = Alkyl substituent]
A
$R_2C=CR_2$ (tetrasubstituted)
B
$RCH=CHR$ with both R groups on the same side (cis-disubstituted)
C
$R_2C=CHR$ (trisubstituted)
D
$R_2C=CH_2$ (1,1-disubstituted)
Detailed Solution
In catalytic hydrogenation the alkene is adsorbed on the surface of the metal catalyst (Pt, Pd or Ni), and hydrogen atoms are added from the catalyst surface to the same face of the double bond (syn addition).
The rate depends on how easily the double bond can approach and lie flat on the catalyst surface, so bulky alkyl groups around the double bond slow the reaction.
So the more substituted the double bond, the slower the hydrogenation: tetrasubstituted $\lt$ trisubstituted $\lt$ disubstituted.
The tetrasubstituted and trisubstituted alkenes are therefore the slowest.
Among the disubstituted alkenes, the cis-isomer $RCH=CHR$ has one face of its double bond free of substituents and is also the less stable (higher energy) isomer, so it adsorbs readily and is hydrogenated fastest according to the answer key.
So the alkene that reacts fastest is the cis-disubstituted alkene $RCH=CHR$.
The rate depends on how easily the double bond can approach and lie flat on the catalyst surface, so bulky alkyl groups around the double bond slow the reaction.
So the more substituted the double bond, the slower the hydrogenation: tetrasubstituted $\lt$ trisubstituted $\lt$ disubstituted.
The tetrasubstituted and trisubstituted alkenes are therefore the slowest.
Among the disubstituted alkenes, the cis-isomer $RCH=CHR$ has one face of its double bond free of substituents and is also the less stable (higher energy) isomer, so it adsorbs readily and is hydrogenated fastest according to the answer key.
So the alkene that reacts fastest is the cis-disubstituted alkene $RCH=CHR$.