Download Electrophilic addition reactions of acids to alkenes double

Electrophilic addition reactions of acids to alkenes double bonds
http://stream.haverford.edu:8080/ramgen/~cwintner/Lecture15/15-01.smil
On the same reaction manifold, as the SN1 reactions, that we’ve been talking about, and E1, are
addition reactions: electrophilic addition reactions, that is reactions with acids to alkenes, to
double bonds.
And I stressed electrophilic, that is, let use isobutylene as an example.
One wouldn’t expect a nucleophilic addition to such an alkene: it’s electron rich, it doesn’t want
more electrons.
But couldn’t this react with an electrophile, for example a proton.
And…it appears, I think rather a way, that there is a dichotomous possibility here.
There is … this is isobutylene… there is an electron pair, and there is a proton.
And what we were really talking about is the reverse of the loss of the proton from the
carbocation to give the isobutylene. Instead saying: couldn’t we have there an electron pair from
isobutylene reaches out to a proton, an electrophile, makes a bond, so the alkene is now
………...…….. in space, giving its electrons to make a bond, and then we’ll form a cation; an so
what I’ve written is this: where the proton, so over there, where the proton adds to this carbon, CH2
to CH3 , the carbon goes from sp² to sp³, and a cation then is left, the same old carbocation.
On the other hand, going in next reaction, does apparently another possibility, that is: couldn’t we
do it this way? Then we would make a different cation, namely this one.
And suppose then that we were doing this in concentrated HBr, we might think: well we’ll get a
cation and bromine would come along and would make a mixture.
Well, now we won’t! because? Because we won’t get this reaction: this is going up to a much
higher energy. Why? Because this is a primary carbocation! (No! primary carbocation).
So, in fact were we thought we have got a dichotomy: there isn’t . This is what we have going to
get.
And so by this mechanism, you say: if we have isobutylene and we treat it with concentrated HBr,
we obtain t-butyl bromide. By this mechanism. And the proton get here and then bromine gives tbutyl bromide.
If we have isobutylene, and we treat it with diluite acid we get t-butanol. Because, from here, under
those circumstances, water adds, and now we’re on S1 manifold; is this just what we saw before.
Or imagine that we had isobutylene, rather than as we had in other syntheses earlier, isobutanol, in
the same conditions, protic methanol, so… a little few drops of concentrated acid or a few of gas in
methanol, we would go from here to here, and now the methanol, or any other alcohol, will add a
proton that can be lost, and we get the ether.
So entirely analogous to what we’ve been seeing in SN1 reaction, because you can enter that same
manifold, you can enter that same manifold, from isobutylene by simply protonating it and then,
whatever the solvent is, that’s what you’re finally going to see.
But all of these are additions: addition of HBr, addition of HOH, addition of HOCH3 and notice: in
a specific direction, that is a regiospecific reaction…, not stereospecific, but regiospecific, that is it
goes the one way and not the other, goes the way it does because tertiary cation had be formed and
then had precluded the possibility of getting the primary halide, or alcohol, or ether, at this
position, but instead gave us the products that we see: electrophilic additions to alkenes.