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Grade 12th passOrganic Chemistry

Please sir explain this Q.I am posting it 4th time.Please sir help me with this Q as I completely dependent on askiitian. This is a Q of heat of hydrogenation. I have confusion in str.1 and 4 .Both have 8 alpha H and 1st str.is additionally stabilized by resonance also,while 4th is not .Then how 4th str. Have highest heat of hydrogenation or least stability as it has 8alpha H which is greater then 2 and 3.(Correct ans.is option 3.This Q is from MS Chouhan .So the answer must be correct.)Please help me out with this question.

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Profile image of Kaushki
7 Years agoGrade 12th pass
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1 Answer

Profile image of Mahima Kanawat
ApprovedApproved Tutor Answer7 Years ago
Dear student 
For stability we would first check resonance, the Hyperconjugation. 
Due to resonance 1,3> 2,4
Now Hyperconjugation
1 has 8 alpha, 3 has 5 alpha.. So, 1 > 3
In 2 option, the two double bonds have 2 alpha H in common, therefore these H are more active H.. Tautomerization will occur at these two alpha, the double bond will change position and resonance will occur.
In 4  there can be tautomerization, but resonance will not occur after tautomerization.
So, 2>4
Finally 1>3>2>4
Therefore heat of hydrogenation 4>2>3>1
WITH REGARDS 
MAHIMA 
ASKIITIANS FORUM EXPERT 

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