Resonance and acids

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Tinisha 1G
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Joined: Fri Sep 28, 2018 12:17 am

Resonance and acids

Postby Tinisha 1G » Thu Dec 06, 2018 10:09 am

If a molecule has resonance, does that mean they are a stronger or weaker acid?

Posts: 34
Joined: Mon Oct 01, 2018 12:16 am

Re: Resonance and acids

Postby 905109118 » Thu Dec 06, 2018 2:12 pm

A molecule is said to have resonance when its structure cannot be adequately described by a single Lewis structure. Since a weaker base has a stronger conjugate acid, a compound whose conjugate base enjoys resonance stabilization will be more acidic.

David S
Posts: 54
Joined: Fri Sep 28, 2018 12:15 am

Re: Resonance and acids

Postby David S » Thu Dec 06, 2018 3:20 pm

Resonance definitely plays a role as a form of charge delocalization.

To explain, I'll use the fact that when an acid has proton transfer reaction with water, it produces a conjugate base (can be anion like ClO4- or neutral molecule like NH3). If the conjugate base is strong, then a majority of the conjugate base produced will readily react with hydronium ions produced, and reverse to a large extent the proton transfer reaction. The net effect of this is that most of the original acid will remain intact and few hydronium ions will be produced. Thus the original acid would be a weak one. Conversely, if the conjugate base is weak, it will not likely react with most the hydronium ions produced; thus much of the original acid will be deprotonated, making it a strong acid.

That being said, how can we determine whether the conjugate base is strong or weak? Well, a base's job is to accept protons, which entails donating a lone pair to protons. Thus, conjugate bases that can more readily donate e- pair will be stronger bases, with the original acid consequently being weaker.

So how do we determine if a conjugate base is good at donating e-? Conjugate bases with easily accessible e- pair associated with electronegative, electron rich atoms (i.e. electronegative atoms in molecule with negative formal charge) will be more likely to lose such e- pairs to protons in solution.

If there is resonance in the conjugate base, that means that some e- pairs that could've been concentrated on electronegative, electron rich atoms, will be delocalized and thus less accessible. This will result in a diminished ability of the conjugate base to donate e- pairs -> weaker conjugate base -> stronger original acid.

For an example, acetic acid CH3COOH is stronger acid than ethanol, CH3CH2OH, because the conjugate base anion for acetic acid has resonance within the carboxylate (COO-) group that delocalizes a lone pair and makes it harder for proton to attach back onto one of the O atoms to regenerate acetic acid. In contrast, the conjugate base for ethanol has an oxygen atom with three lone pairs (FC=-1, the oxygen is electron rich), making it easier for a proton to form coordinate covalent bond with the oxygen to remake ethanol. Acetic acid ends up producing more hydronium in solution than ethanol as a result.

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