Determining which molecule is more acidic

The most convenient method for ranking acidic groups is to already know their characteristic pK a values. If you know these values for all of the acidic groups in your molecule, then the group with the lowest pK a contains the most acidic H.

Case closed. If you do not recall pK a values for all of the acidic groups, a few general principles can guide you. Negatively charged acids are rarely acidic.

The formal charge rule applies even more strongly to NH acids. The following chart shows how each group of atoms activates an OH acid pK a values range from 16 to -2 :. However, when acting as acids, only the most acidic proton will participate in the acid-base reaction. Therefore, it is important to be able to identify the most acidic proton in a molecule.

There are three general methods to estimate the acidity of a proton:. Make sure that the proton in question is actually part of the functional group and is not simply attached to atoms that are attached to the functional group. Here is a short list of acidities of common functional groups in organic chemistry Note: that the pKa's are an approximate value for that functional group. The most stable conjugate base will be the strongest acid.

For both ethanol and acetic acid, the hydrogen is bonded with the oxygen atom, so there is no element effect that matters. However, the p K a values and the acidity of ethanol and acetic acid are very different.

What makes a carboxylic acid so much more acidic than an alcohol? As stated before, we begin by considering the stability of the conjugate bases, remembering that a more stable weaker conjugate base corresponds to a stronger acid. For acetate, the conjugate base of acetic acid, two resonance contributors can be drawn and therefore the negative charge can be delocalized shared over two oxygen atoms.

However, no other resonance contributor is available in the ethoxide ion, the conjugate base of ethanol, so the negative charge is localized on the oxygen atom. As we have learned in section 1. The charge delocalization by resonance has a very powerful effect on the reactivity of organic molecules, enough to account for the big difference of over 10 p K a units between ethanol and acetic acid.

The p K a of the OH group in alcohol is about 15, however OH in phenol OH group connected on a benzene ring has a pK a of about 10, which is much stronger in acidity than other alcohols. Explain the difference. The difference can be explained by the resonance effect. There is no resonance effect on the conjugate base of ethanol, as mentioned before.

However, the conjugate base of phenol is stabilized by the resonance effect with four more resonance contributors, and the negative is delocalized on the benzene ring, so the conjugate base of phenol is much more stable and is a weaker base.

Therefore phenol is much more acidic than other alcohols. Recall that in an amide, there is significant double-bond character to the carbon-nitrogen bond, due to a second resonance contributor in which the nitrogen lone pair is part of a p bond. Notice that in this case, we are extending our central statement to say that electron density — in the form of a lone pair — is stabilized by resonance delocalization, even though there is not a negative charge involved.

The lone pair on an amine nitrogen, by contrast, is not part of a delocalized p system, and is very ready to form a bond with any acidic proton that might be nearby. Often it requires some careful thought to predict the most acidic proton on a molecule. Ascorbic acid, also known as Vitamin C, has a pK a of 4. There are four hydroxyl groups on this molecule — which one is the most acidic? If we consider all four possible conjugate bases, we find that there is only one for which we can delocalized the negative charge over two oxygen atoms.

Compare the pK a values of acetic acid and its mono-, di-, and tri-chlorinated derivatives:. The presence of the chlorines clearly increases the acidity of the carboxylic acid group, but the argument here does not have to do with resonance delocalization, because no additional resonance contributors can be drawn for the chlorinated molecules. Rather, the explanation for this phenomenon involves something called the inductive effect. In effect, the chlorine atoms are helping to further spread out the electron density of the conjugate base, which as we know has a stabilizing effect.

In this context, the chlorine substituent is called an electron-withdrawing group. Notice that the pK a -lowering effect of each chlorine atom, while significant, is not as dramatic as the delocalizing resonance effect illustrated by the difference in pK a values between an alcohol and a carboxylic acid.

In general, resonance effects are more powerful than inductive effects. The inductive electron-withdrawing effect of the chlorines takes place through covalent bonds, and its influence decreases markedly with distance — thus a chlorine two carbons away from a carboxylic acid group has a decreased effect compared to a chlorine just one carbon away.