Orgo Question

Don't quote me on this since I have yet to get to this point in my review, but from what I remember, the aprotic solvent is bulky, and the positive dipole is blocked by steric hinderence. Let me know if this doesn't help, and I will find it in my book.

Adam

The positive ends are not strong enough. Example...
Acetone is a polar aprotic solvent. The acetone will encage the cation but the CH3 ends of the acetone are not strong enough to attract the anion.

The anion is the nucleophile.

edit to add: I think you might be talking about polar protic solvents..

aamartin81 said:

the aprotic solvent is bulky, and the positive dipole is blocked by steric hinderence.

Click to expand...

This is correct. The atom with the partially positive charge on it (the carbonyl carbon in the acetone example that the previous poster gave) is hindered, while the negative end of the dipole "sticks out" into the solvent. Even though carbonyls, being sp2-hybridized, are flat, remember that the dipole is a vector, so it points in a certain direction. That's why, in this case, the bulkiness of the carbonyl substituents makes a difference to the positive end of the dipole but not the negative end.

Can anyone tell me how I can post a diagram on my message? When I click the little image icon above the message, it asks me to enter the text to be formatted. So do I need to upload the image to a website first or something?

To tie this in to relevant MCAT stuff, you can look at it in terms of the differences in Sn1 and Sn2 rxn's.

The logic goes like this:
Since Sn1 rxn's are a two step process, involving the formation of the carbocation, anything that can stabilize the carbocation is in high demand.

So, the choice is to use a polar protic solvent. It can sneak right in there and encage the large carbocation with it's partial negative charge. Since it's protic, and H atoms are very small, it can get into a perhaps highly steric carbocation.

But, in an Sn2 rxn, since it's a once step process in which the nucleophile is a major participant, anything that messes with the Nu: will not be good for the rxn.

A polar PROTIC solvent, which would have a partial negative charge (on the N, O, or F), but also a partial positive charge on the H atom, would tend to obstruct/encage the Nu: since the partial positive charges on the H atoms of the protic solvent would encage the Nu: that is negative or at least has some lone electron pairs to interact with.

Thus, Sn2 rxn's favor a polar aprotic solvent, that doesn't have a partial positive charge on an easily accessible, small, H atom. Ofcourse, a polar aprotic solvent with some nice, large steric groups would shelter the partial positive on the carbon from interacting with the Nu:

**FYI, I just got done reading Kaplan online material. I really hope this helps. It helped me just to write it, frankly, since it's been SO long since I had this stuff.

too add:
Common polar protic solvents are OH groups...water and low-molecular-weight alcohols.