Which one would it be?

Chemistry Level 2

Consider the catalytic hydrogenation of the following compound :

IMG 20170215 233602.jpg

Which double bond will get reduced?

Number 1 Number 4 Number 2 Number 3

1 solution

Sumanth R Hegde
Feb 15, 2017

Ethylene group is more prone to catalytic hydrogenation than other double bonds. Hence the answer is $\color{#D61F06}{Number 2}$

Don't you think that the ethylene one is in conjugation and hence more stable?

Aniket Sanghi - 4 years, 3 months ago

When hydrogenation takes place. , the lesser the steric hindrance, the better. Hence the highly substituted doubt bond in the ring is ruled out. Among the conjugated ones, ethylene group is most reactive . Although I don't have exact values of enthalpy changes, I am pretty certain that even though there is conjugation, hydrogenation takes place at this site because the main factor for catalytic hydrogenation is the steric factor and in this case the ethylene group seems to be the most suitable choice

Sumanth R Hegde - 4 years, 3 months ago

Even i had this confusion about conjugation but I Noticed that the alkene 1 is Extremely stable due to hyperconjugation. Further Hydrogenation proceeds by Adsorption phenomenon on metal surface sites and hence i dont think intermediate stability should be considered . Moreover we are not destroying the conjugation completely as two double bonds will still be conjugated . Althought for exact answer enthalpy will be required but i do think answer given is correct

Prakhar Bindal - 4 years, 3 months ago

what about anti-aromaticity

A Former Brilliant Member - 4 years, 3 months ago

@A Former Brilliant Member – Abe kaun anti aromatic hai?

Prakhar Bindal - 4 years, 3 months ago

@Prakhar Bindal – mera upper wala comment padh , i meant non-aromaticity.

A Former Brilliant Member - 4 years, 3 months ago

@A Former Brilliant Member – Abe bhai wo ring planar he nhi h antiaromatic kaise hogi+ Wo non aromatic hai agar tu hydrogenation karaega to bhi non aromatic he rahegi

Prakhar Bindal - 4 years, 3 months ago

@Prakhar Bindal – bhai mai non aromatic hi to kah ra hun

A Former Brilliant Member - 4 years, 3 months ago

@A Former Brilliant Member – To farak kya pada? kuch bhi nhi to wo factor lena he kyo

Prakhar Bindal - 4 years, 3 months ago

Hey hydrogenation by using H2/Pt makes use of a four membered cyclic ring.If we consider 1 ring formation is unstable here on account of repulsion with the ch2 groups.Similarly one can eliminate 4 between 2 and 3 ring formation is more stable in case of 3 as the ch2 are far apart.So it's more prone to hydrogenation Am I wrong??

Spandan Senapati - 4 years ago

i think the answer must be 1 @Aniket Sanghi pointed out the number 2 is in conjugtion , moreover if you study the mechnism of catalytic hydrogention it involves homolytic fission and the addition of h-atom, the stability of free radical is increasing in the order 3 degree>2 degree >1 degree , the intermediate is more stable in the hydrogention of number 1 , and it is non-aromatic too , making it more unstable and then number 1 i destroyed, it's non-aromaticity will be destroyed , it ill be more stable then now i have told u my point let me hear yours too :)

A Former Brilliant Member - 4 years, 3 months ago

correct me if i'm wrong !

A Former Brilliant Member - 4 years, 3 months ago

Catalytic hydrogenation using metal catalysts takes place by the formation of transition states , not reactive intermediates

Sumanth R Hegde - 4 years, 3 months ago

even then the ans be 2

A Former Brilliant Member - 4 years, 3 months ago

Which one would it be?

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