# substitution on bond strength and bond energy

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#### loveapplejuice789

##### New Member
For question 4, A is the correct answer because "a less stable reactant yields a greater amount of heat upon reaction, so the pi-bond must be weakest in an unsubstituted alkene (making it the least stable of the alkenes)".

Applying the same logic, I thought 7 would be D because unsubstituted carbon would be least stable (weakest), but the solution manual said B. Can anyone explain this to me? Thank you!

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I think you’re confusing the impact of substitution on a double bond versus single bond. Also, for problem 7, we’re talking about halogens (Cl and I), so think in terms of what substitutions do with respect to the electronegativity and effective radii of the halogens.

I think you’re confusing the impact of substitution on a double bond versus single bond. Also, for problem 7, we’re talking about halogens (Cl and I), so think in terms of what substitutions do with respect to the electronegativity and effective radii of the halogens.

hi there - thanks for the response. can you elaborate further what differences substitution would have on single bond versus double bond?

For 7, Cl is smaller than I, so the bond length of Cl is shorter than I, which makes Cl to form stronger bond. This eliminates A and C. But I still can't decide on B or D based on the impact of substitution. Thank you.

We know for a double bond, substitution is better because the pi electrons can delocalize more freely (remember that carbon is weakly electronegative). Similarly, when there’s there’s a halogen (which is strongly electronegative), you don’t want any electronegative substituents because you want most of the electrons to be attracted to the halogen.