Hello. I have two unrelated ochem questions. I'm taking the MCAT before sitting through an ochem course (it works better for me logistically) so I am having to teach MCAT ochem to myself. I appologize if these are basic questions, but hear me out.
1) Why are alcohol groups electron donating? Oxygen is electronegative, so I would suspect induction.
2) I don't get the process of alkylation by alkyl halides. I'm specifically referring to a problem covering acetoacetic ester synthesis in which an alpha hydrogen is removed by a base and is replaced by an R group that was previously attached to a halide (both the base and the R-X were present in this step). After the hydrogen is removed from the alpha carbon, doesn't that carbon assume a positive charge? An R-X molecule will have a negative partial charge on the X, making it a nucleophile, so why doesn't the X attack the alpha carbon? Or does it? Edit: To clarify, the product contains the alpha carbon with one H and the R attached to it.
good questions:
1) Hydroxyl groups are only electron donating when they are directly attached to an sp2 (or sp, but this never happens) hybridized atom. The reason for this is that the oxygen can donate a lone pair into the sp2 hybridized atom (resonance) to stabilize it. Thus, electron donation by hydroxyl groups is a resonance effect. This is seen most commonly when a hydroxyl group is directly attached to a carbocation, or in phenol derivatives (hydroxyl group directly attached to benzene ring). A carbocation is sp2 hybridized, and benzene ring carbons are also sp2 hybridized. The same thing is seen with neutral amines. However, if the hydroxyl group is not directly attached to an sp2 hybridized atom, it functions as an electron withdrawing group, since it can't dump its electrons to an adjacent atom, but has high electronegativity.
2) Before I go through this, I have to go through a quick review of hydrogen and some related topics since you seemed to use them interchangeably in your question.
Hydrogen is an
element and consists of one proton and one electron, so it has no net charge. Since it only has one electron, it's not paired, so hydrogen is a
radical.
Hydride is an
ion and consists of one proton and two electrons, so it has a net charge of
-1. It is not a radical, it has a lone pair of electrons. A
proton is also an
ion, and since it's just a proton, it has a net charge of
+1.
Another thing: Bronsted acid/base reactions involve transfer of
protons, not hydrogen radicals or hydride ions.
Ok, nowwwwww, let's look at the question:
Start with acetoacetate, add base. I just added a base, so that means that I'm going to pull off a
proton. So what happens when I remove a proton? Well, I get a carbanion at the alpha carbon (it was neutral before, then I removed a proton so I should have a negative charge. Put another way, the proton I removed was once part of a C-H bond, and both of the electrons in the C-H bond are now part of a lone pair on the carbon). The carbanion is stabilized as an enolate, but effectively, an enolate is just a carbon nucleophile at the alpha carbon. So your deprotonated aceotacetate is a nucleophile, not an electrophile. So now you add your alkyl halide, R-X, and the acetoacetate nucleophile attacks the alkyl halide just like any other nucleophile would, displacing X- as a leaving group.
When in doubt, think about it like this: You've added a base. In a normal acid/base reaction, when you add a base to an acid, the base gets protonated so its charge goes up by 1 (becomes more positive/less negative), and the acid gets deprotonated so its charge goes down by 1 (becomes less positive/more negative).
