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Tue 24 Oct, 2006 08:43 am
My original question:How does acidic nature of alcohols vary as we increase the no. of alkyl (R) groups attached to the O-H group?
What I was able to figure out:
As the oxygen atom is more electronegative than hydrogen, the OH bond becomes polarised. As we increase the no. of R groups, the magnitude of +I effect increases too. Consequently, electron density over oxygen increases and the electron pair of O-H bond shifts a little more towards hydrogen. Thus, the polarisation of OH bond weakens. More the no. of R groups, the weaker the polarisation gets.
Follow-up questions: Did I get this right so far? If yes, then please try explaining how does the strength of polarisation determines the acidic nature of alcohols? Like if the bond is more polar, would that increase the chances of breakage of the OH bond? Or would it strenghten the bond and save it from breakage?
Point of confusion: The latter portion of the above section is where I am getting stuck.
Any rectification of my original explantion, any links dealing with the subject will be highly appreciated.
Thank you
Re: Acidic nature of alcohols
spidergal wrote:My original question:How does acidic nature of alcohols vary as we increase the no. of alkyl (R) groups attached to the O-H group?
What I was able to figure out:
Not much. Alcohols don't contain double bonds that might delocalize electrons. Therefore it doesn't make much difference to the H whether there are one, two or many Cs on the other side of the O it's attached to. That said, I would expect a minimal decrease in acidity because there are fewer OH-groups per unit mass. But the effect is barely worth mentioning.
Yes, I understand that. Alcohols are slightly acidic. They don't change the pH of water when mixed into it.
What about the point of confusion?
spidergal wrote:Yes, I understand that. Alcohols are slightly acidic. They don't change the pH of water when mixed into it.
What about the point of confusion?
The answer to the point of confusion is: No, I don't have any links, and my Chemistry schoolbook wouldn't help you; it's in German.
As a rule, covalent bonds are stronger than bonds between ions. So the more polar the bond gets, the more it resembles an ionic bond, the weaker the bond gets, and the more acidic the O-H bond becomes.
The less polarized the OH group is, the less likely it is to dissociate, therefore the less acidic it is. This may actually have some relevance in some industrial processes.
One of the bigger picture items I got from O-chem was that carbon is a slight electron donor. So, the longer an alkyl chain attached to the OH group, the more electrons are pushed toward the relatively electronegative oygen atom, and so, by extension, the more electrons are pushed toward the H atom. Just another way of looking at the same thing and coming up with the same answer. The "carbon as slight electron donor" idea is one that I found useful in muddling through O-chem and biochem.
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