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Topic: Acidity of alcohols  (Read 49969 times)

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Offline Nemesisof99

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Acidity of alcohols
« on: October 11, 2009, 09:43:56 AM »
The acidic character of alcohols is due to polarity of -OH group. An alkyl group intensifies the negative charge at the oxygen. This should increase the polarity of the -OH bond..correct?

Is the following idea correct?
Greater the polarity of the -OH group, greater is the acidity of an alcohol.

Offline Arctic-Nation

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Re: Acidity of alcohols
« Reply #1 on: October 11, 2009, 10:03:20 AM »
You've got to be careful with a term like polarity, especially when talking about single bonds, and I've always preferred to explain acidity differences between similar molecules in terms of electron density.

In short, an alkyl group is a weak electron donor, increasing electron density on whatever it is attached to. An increase in electron density leads to a strenthening of neighbouring bonds. Disregarding any other possible influences, the O-H bond of an alcohol becomes stronger, and thus less acidic, when electron density is increased. This explains why methanol is more acidic than ethanol, and why tert-butanol is a very weak acid for an alcohol (and thus makes a strong base).

In terms of polarity, an increase of electronic charge to be shared between two atoms will decrease the polarity. In other words, as polarity is the separation of electric charge between two atoms, anything that adds more electronic density to the bond and thus decreases polarity will increase bond strength. Your statement that a greater polarity leads to higher acidity is correct, but the reasoning behind it is faulty.

Offline Nemesisof99

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Re: Acidity of alcohols
« Reply #2 on: October 11, 2009, 11:29:24 AM »
So, the tert-butoxide ion is more basic than ethoxide ion.

In Williamson's ether synthesis, in order to prepare tert. butyl ethyl ether, only one route can be followed. ie reacting tert. butyl alcohol with ethyl chloride. If it's done the other way, the ethoxide ion formed is too basic and abstracts hydrogen from tert. butyl chloride leading to elimination.

If the tert. butoxide ion is more basic then won't there be elimination to form ethene in the above reaction?

Offline Arctic-Nation

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Re: Acidity of alcohols
« Reply #3 on: October 11, 2009, 05:43:24 PM »
That's a good question, but there's only one straight answer: it depends. Substitution and elimination reactions often compete with each other, and in many cases a mixture of both reaction products will be formed. Whether a certain pathway will be preferred over any other depends on factors such as leaving group, nucleophile/base, steric crowding, stabilization of intermediates and the solvent everything takes place in.

That being said, tert-butoxide might pose problems, as due to its bulkiness it makes a pretty lousy nucleophile in combination with everything but the least hindered electrophiles. It reacts to form ethers with halomethanes, but in the case of chloroethane you'll probably get a mixture of ether and ethylene gas.

But in the end, industrial preparation of tert-butyl compounds usually involves isobutene, which avoids these pesky problems. ;)

Offline sri

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Re: Acidity of alcohols
« Reply #4 on: April 26, 2010, 01:12:12 PM »
I don't understand how does polarity of O-H bond decrease with increase in electronic charge over oxygen atom. instead if oxygen is having an increase of electronic charge over itself and hydrogen is already positive the polarity should increase and hydrogen atom should be easily be able to leave.

Offline MrTeo

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Re: Acidity of alcohols
« Reply #5 on: April 26, 2010, 01:32:09 PM »
instead if oxygen is having an increase of electronic charge over itself [...]

From what I know and what I've understood reading the previous posts I'd say that the electron density increases not only on the oxygen atom but in the whole bond, thus making the H at the end less acidic.
The way of the superior man may be compared to what takes place in traveling, when to go to a distance we must first traverse the space that is near, and in ascending a height, when we must begin from the lower ground. (Confucius)

Offline nikium

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Re: Acidity of alcohols
« Reply #6 on: May 30, 2010, 06:32:19 PM »
I agree with everything that's been said. There's one thing I would like to emphasize. You cannot possibly determine the acidity of a single compound given its structure. You can however determine relative acidities. Say, the acidity of methanol compared to ethanol.

Second, whenever you approach an acid/base problem, ask yourself "what is an acid?" Acids are proton donors. If the acidity of some alcohol is in question, write down the equilibrium of the compound in question acting like an acid:


It is easy to see now that the strength of the acid can be determined by how much the equilibrium lies to the right. That is, how wiling is it to give up a proton and thus act as an acid. This, of course can be determined by the stability of the R-O(-) species.

As an example, lets compare the acid strength of water, Methanol, and tert-Butanol:



The best acid is the species that is most stable as a conjugate base (specie with the negative charge). Recall what stabilizes a negative charge. Negative charge is rich in electron density, so something electron-withdrawing will take away some of the negative charge and thus stabilize the structure. We know that alkyl groups donate electron density, thus they exasperate the problem. tert-butoxide is worse off then methanol since it has more electron donating carbons. Tert-butanol is the worse acid, followed by methanol. Water is the best acid since the hydrogen in the hydroxide doesn't have any electron-donating tendencies.

Other considerations:
1. The atom on which the negative charge is planted can be a factor in determining acid/base strength. Atoms that are more electronegative on the periodic table are better at "handing" the negative charge. Thus a negative charge on chlorine is more stable than a negative charge on nitrogen.

2. If you can draw resonance structures for the structure that bears the negative charge. This delocalization, this spreading out of negative character stabilizes the structure. Recall that a molecule for which you can draw multiple resonance structures for is actually the average of all of those structures. The molecule doesn't switch back and forth between structures, it is all of those structures all at once. So if you can draw three different structures for which the negative charge is on three different atoms, each atom essentially bear (1/3) of the negative charge. This is better than one atom bearing all of the negative charge. This is the argument for why carboxylic acids are better acids than alcohols:


"paradox" is only a conflict between reality and your feeling of what reality "ought to be."
---- Richard Feynman

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