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Which compound would you expect to have the smallest heat of hydrogenation:3,4-dimethyl-2-hexene, 2,3-dimethyl-2-hexene, or 4,5-dimethyl-2-hexene?

Answer 1

Addison Armstrong

The compound with the smallest heat of hydration is 2,3-dimethyl-2-hexene.

The structure of 3,4-dimethyl-2-hexene (A) is CH₃CH₂CH(CH₃)C(CH₃)=CHCH₃.

The structure of 2,3-dimethyl-2-hexene (B) is CH₃CH₂CH₂C(CH₃)=C(CH₃)₂.

The structure of 4,5-dimethyl-2-hexene (C) is CH₃CH(CH₃)CH(CH₃)CH=CHCH₃.

If you draw the structures, you find that A is a trisubstituted alkene, B is tetrasubstituted, and C is disubstituted.

The order of stability of substituted alkenes is

So the most substituted alkene (B) is at the lowest energy level.

The dimethylhexane hydrogenation products should all be at about the same energy level.

The heat of hydrogenation diagram would be like that below.

So B has the lowest heat of hydrogenation.

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Answer 2

Nicholas Antonuccio

2,3-dimethyl-2-hexene

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.