Bond Strength and Substitution Confusion

[justadream]

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TBR Orgo Example 1.4 (page 11 in early 2012 edition).

The example basically compares 2 bonds that are BOTH sp3-carbon to sp2 carbon.

However, the two sp3-hybridized carbons differ in their substitution.

One of the sp3 carbons is less substituted (bonded to hydrogens) while the other is more substituted (bonded to 2 methyl groups). As a result, the bond (from the sp2-carbon) to the less substituted sp3-carbon is apparently stronger than the bond (from the same sp2-carbon) to the more substituted sp3-carbon.

Can someone explain why this is?

 

[sillyjoe]

I don't have the image in front of me but I would guess that it is due to steric hinderance. The large alkyl groups make that R group C-C bond more unstable/repulsive and thus longer than the less substituted R group C-C bond.

 

[Chrisz]

I think I have explained this question somewhere on studentdoctor.net earlier, but I will explain this question again. Why it is easier to break a more substituted bond than less substituted bond? let me ask you a question. What you going to do to break a bond? Add energy. How much energy to add? This depends on the final product after the bond is broken. The higher the energy the final product the higher energy required to break the bond. Lets break the bond homolytically, which means to form radicals. The more substituted radicals are more stable compared to less substituted radicals. What does that mean to you? the more substituted radical would be lower in energy than less substituted. Therefore, it is easier to break a more substituted bond. Put in other words, more substituted would be lower in strength

 

[sillyjoe]

I think I have explained this question somewhere on studentdoctor.net earlier, but I will explain this question again. Why it is easier to break a more substituted bond than less substituted bond? let me ask you a question. What you going to do to break a bond? Add energy. How much energy to add? This depends on the final product after the bond is broken. The higher the energy the final product the higher energy required to break the bond. Lets break the bond homolytically, which means to form radicals. The more substituted radicals are more stable compared to less substituted radicals. What does that mean to you? the more substituted radical would be lower in energy than less substituted. Therefore, it is easier to break a more substituted bond. Put in other words, more substituted would be lower in strength

This is a better answer than mine.

 

[Chrisz]

This is a better answer than mine.

Thanks for your compliment🙂