2 quick organic chem questions

[Mantis Toboggin]

Hey guys, was hoping for some help here. I took ochem this spring but I'm a bit rusty so here it is:

1.)

(Sorry for the tacky paint image... didn't know what this molecule's name is)

Anywho, I was asked what kind of molecule this is. Ester immediately popped into my mind-- and it was right-- but what confused me is why this molecule can't be considered an ether too. R O R is an ester, so wouldn't it count here?

Or is that designation not given when an aforementioned R is a carbonyl carbon?

2.) I was brushing up on the definition of being conjugated and came across this issue.

An amide is evidently conjugated. So from what I understand when nitrogen is part of, say, a ring system where it is double bonded on one side, with a single bond on the other, and has a lone pair, it is sp2 (given there is another atom that is also Sp2 part of that ring) and thus conjugated with that atom.

So my question is: that lone pair which normally makes the amine basic-- is it NOT going to react as a lewis base because it helps to make the molecule conjugated?

By extension then: if nitrogen had 2 single bonds to 2 carbons, and 1 bond to a hydrogen with a lone pair-- would it then be basic (as donating its electrons would make it conjugated)? If so, then is it the electronegativity of the carbonyl carbon that makes amides not as basic as they should be?



Now my question for the amide part: Because the nitrogen is sp3, would it donate its lone pair to become conjugated with the carbonyl carbon? If so, why are amides


3.) Which is more polar, an amide or alcohol? I'd appreciate an explanation on this if possible.

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[MD Odyssey]

1) As you mentioned, there are two ester functional groups present. The reason it is an ester and not an ether is because we assign a priority in terms of reactivity:

COOH- > -COO- > -CO- > -COH > R-O-R

I've never had a really satisfactory answer on this from first principles, I've just learned to follow the convention. In case you were wondering, that compound is 2,5-dimethyl-1,3-dioxane-4,6-dione.

2) First of all, I would recommend against trying to memorize specific conditions under which a particular system is conjugated. If you understand how conjugation works, you should be able to predict whether a system is conjugated or not. Based on some of your statements, I suspect that you might not have a really clear handle on how to tell if a system is conjugated.

Now, to your question of basicity, think stability of the lone pair. The more stable the charge is, the less likely it will be to react with a Lewis acid. Since conjugation is a contributing factor to charge stability, a lone pair in a conjugated system like you describe is going to be more stable and less reactive than if the p-orbital orbital the electrons are in is not delocalized.

3) Bond polarity is determined by the difference in electronegativity between the bonding atoms. To simply matters, I compared acetamide and ethanol.

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For acetamide, there are two net dipoles to add together, the dipole from the carbonyl and the one from the C-NH2 complex. If you look at the C-NH2 complex, you can see that there will be some cancellation of the dipoles, since a C-N is about as polar as an N-H bond. But, since these are vectors that we have to add, the cancellation isn't going to be complete, so the net dipole is going to point from carbon to nitrogen in the direction of the C-N bond. The carbonyl bond has only partial double-bond characteristics, but it remains a polar bond, just not as polar as a C=O bond would be expected to be. Still, the net dipole on this molecule is pretty huge.

Now, for ethanol. Ethanol is clearly polar - the vector sum of the C-O bond dipole and the O-H dipole is obviously non-negative. But, this is the only thing which gives ethanol a dipole moment. There isn't anything else.

Comparing the two, it seems difficult to conclude that the net dipole moment of ethanol is larger than for acetamide. Not only is the carbon-oxygen dipole larger, but there is also an additional C-N bond pulling the negative charge away. You can see this is in the relative boiling points of the two. Ethanol boils at 77 °C while acetamide boils at 222 °C. Of course, there is more to the difference in boiling points than just bond polarity, but a significant part of the increase is due to the difference in bond polarity.

Hope this helps - draw out a picture for the two compounds and I think you'll see what's going on.
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