TBR General Chemistry Kinetics Passage IX Question

[Mces97]

---

Members don't see this ad.

I am confused about question 62. The question states if in one reaction the R group is methyl (CH3), what is observed when the methyl is replaced by ethyl (CH2-CH3)?
I thought the answer should have been b. - The reaction rate decreases with the ethyl because steric hindrance of the leaving group has increased. The answer however is A- the reaction rate increases with ethyl because steric hindrance of the leaving group has increased. I am probably forgetting some rule in organic chemistry, as I took it two years ago, and have not started reviewing it just yet. Can someone explain why as steric hindrance is increased, this makes for a better leaving group, thus increasing the reaction rate.

 

[Morsetlis]

Steric hindrance decreases stability of the whole molecule.

Removing the sterically hindering group increases the stability of the dissociated two molecules (i.e. increasing the stability of the molecule-without-the-sterically-hindering group).

That is, it is easier to carry a small item than a bulky item. With a bulky item, you want to put it down/away from you to maintain stability.

That said, ethyl is not extremely sterically hindering... but I guess it's be more than methyl.

 

[Mces97]

Thanks for the quick reply. I think I understand your explanation. If the molecule is less stable, the rate would increase. What I am still confused on though is if ethyl is more sterically hindering than methyl, wouldn't it be harder for a reaction to occur since electrons from one element/molecule would have a harder time getting to the more sterically hindered molecule, thus making it leave?

 

[Morsetlis]

Thanks for the quick reply. I think I understand your explanation. If the molecule is less stable, the rate would increase. What I am still confused on though is if ethyl is more sterically hindering than methyl, wouldn't it be harder for a reaction to occur since electrons from one element/molecule would have a harder time getting to the more sterically hindered molecule, thus making it leave?

Steric hindrance is electronegative repulsion from the electron density of one (part of a) molecule to another. There's already a force pushing them apart. If the reaction goes forward, then it is favorable to dissociate and break the bond. Of course, ethyl groups don't spontaneously dissociate: a lot of times bond energies are stronger than steric hindrance.

In your example, it would seem that breaking the bond is favorable. The factors that would go into that are either myriad or involve a single -deltaG, depending on the level of details.

 

[BerkReviewTeach]

Whether steric hindrance speeds a reaction up or slows it down depends on the reaction. For a reaction that depends on molecules coming together in the transition state, steric hindrance will slow a reaction down. This is seen with reactions like the SN2 and Diels Alder. For reactions where a leaving group first leaves to form an intermediate, steric hindrance helps facilitate the leaving group leaving and speeds up the reaction. This can be seen in reactions like the SN1.

In this particular question, the larger R-group makes the leaving group more crowded, which forces it off the reactant faster.

 

[Mces97]

Thank you for clearing this up Berkeley. If I still remembered most of the reactions in Organic Chemistry I would have realized, but you made me recall some stuff with the explanation. On a brighter note I just finished Gen Chem. Now off to Physics.