[physstudent1]

Alcohols are bad leaving groups but if they are protonated they can become leaving groups correct?  I'm wondering when should I predict that an alcohol will be protonated and when should I know that an alcohol will not be protonated and will not be a LG.  The only thing I could come up with so far is that if an alcohol is reacting with a strong base it will be protonated is this correct are there any other conditions that will protonate an alcohol? thanks a lot.

[nj_bartel]

I don't know much about the specific subject matter, but it would seem to me alcohols would be protonated by strong acids... not strong bases.  The C-O bonding electrons could then attack the positively charged oxygen to leave as neutral H₂O.  So any acid with a pKa lower than the pKa of RO-H would be able to protonate the alcohol.

Haven't learned my S_N reactions, but this seems logical to me.  Take it for what it's worth.

[macman104]

nj, well done, you are pretty much dead on!  If the alcohol is in a medium where it has a higher pKa, then it will abstract the proton and become a better neutral H₂.  Good reasoning!

[azmanam]

The C-O bonding electrons could then attack the positively charged oxygen

Not exactly sure what you're saying here, but I think the language could be tightened up a bit.

If you're talking about the mechanism by which the alcohol becomes protonated, then a lone pair on the oxygen belonging to the alcohol moves to attack a proton on the acid.  The lone pair on the oxygen forms a sigma bond to the proton, and the electrons in the proton-acid bond move on to the acid to form the conjugate base.  The oxygen of the alcohol is now sigma bonded to two protons and a carbon, and thus has a positive charge.

I think you're talking about the subsequent substitution reaction, though.  Here, the electrons in the carbon-oxygen bond of the C-O⁺H₂ group can heterolytically cleave and both electrons can move on to the oxygen atom.  This leaves a carbocation and neutral water (I think this is what you were saying).  This is the rate determining step of the S_N1 reaction.  It's not an 'attack,' just a heterolytic cleavage.

But bottom line, both of you are right.  To protonate an alcohol, you need an acid with a lower pk_a than the protonated alcohol.  A strong base will deprotonate an alcohol, making it an even worse leaving group (O^2-) than it already is (O^-)

[nj_bartel]

[quoteThis leaves a carbocation and neutral water (I think this is what you were saying).  This is the rate determining step of the SN1 reaction.  It's not an 'attack,' just a heterolytic cleavage.
[/quote]

Yes.  Haven't learned this yet, so terminology failure.

[physstudent1]

what i meant to type was strong acid i don't know why I typed strong base, the one thing i don't get is that if the pkA of the acid was lower then the pkA of the protanted alcohol wouldn't that mean the acid is weaker then the protanted alcohol?  I know that HCL can protonate OH- but HCL's pka should be higher than waters shouldn't it?

[physstudent1]

nvm i get it now i was thinking of pkA wrong, the higher the kA the lower the pkA so the stronger the acid the lower the pkA right? 

[azmanam]

the stronger the acid the lower the pkA right? 

yes