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Articles Posted in March 2014 (previously hosted on Blogger)


Effect of the Carbonyl Group on Acidity

posted Oct 31, 2016, 10:57 PM by Grace Ong   [ updated Oct 31, 2016, 11:00 PM ]

The following post was first posted on Blogger on Sunday, 23 March 2014.



When we compare the pKa of carboxylic acids with those of corresponding alcohols, it is clear that carboxylic acids are stronger acids by many times over.

No. of carbon atoms Carboxylic acid pKa* Alcohol pKa*
1 HCO2H 3.77 CH3OH 15.54
2 CH3CO2H 4.76 CH3CH2OH 15.7
3 CH3CH2CO2H 4.87 CH3CH2CH2OH 16
4 CH3CH2CH2CO2H 4.82 CH3CH2CH2CH2OH 16.1
* Quoted pKa values are measured using water as the base.

If we examine the structures of a carboxylic acid and an alcohol, we will find that both are similar in that they each contain a hydroxyl (−OH) group.

structure of carboxylic acid

structure of an alcohol
carboxylic acid alcohol

Since oxygen is more electronegative than hydrogen, there is a shift in electron density toward the O atom in the −OH group. This results in a polar O−H bond, with a partial positive charge on the H atom and a corresponding partial negative charge on the O atom.

polar O-H bond in alcohols and carboxylic acids

If you apply this definition to the O–H bond, you might expect the large dipole moment to facilitate the abstraction of the electrophilic hydroxyl H in both the alcohol and carboxylic acid by the H2O base nucleophile, as illustrated below:

abstraction of proton from carboxylic acid

abstraction of proton from alcohol

And yet, alcohols are such weak acids in water that they are essentially considered neutral. Why does the abstraction of the electrophilic hydroxyl H by the H2O base (nucleophile) not occur as readily in alcohols?

Let’s examine the structures of a carboxylic acid and an alcohol again.

structure of a carboxylic acid

structure of an alcohol
carboxylic acid alcohol

The only structural difference between a carboxylic acid and its corresponding alcohol is the presence of a carbonyl (C=O) group located right next to the –OH group of the former.

Why does the presence of this adjacent C=O group increase the acidity of the hydroxyl H of carboxylic acids so profoundly?

There are two explanations which I come across frequently, and I shall discuss each in turn.

Explanation 1:
The electron-withdrawing carbonyl group increases the withdrawal of electrons away from the H atom in the hydroxyl group. This causes the weakening of the O–H bond which promotes the loss of proton.

I found this explanation in the lecture notes of at least three schools. I don’t know how commonly this is used but truth be told, I have always found this explanation to be rather puzzling for the following reasons:
  • The carbonyl group is electron-withdrawing in that a lone pair of electrons (residing in a p orbital) on the hydroxyl O is delocalised with the π electrons of the carbonyl group.

    resonance structures of an un-ionised carboxylic acid

    Since this delocalisation does not involve the bonding electrons that form the O–H bond, it is unlikely that the O–H bond is weakened. On the contrary, with a bond energy of about 460 kJ mol–1, the O–H bond is a rather strong one.
  • Perhaps the delocalisation of the lone pair of electrons on the hydroxyl O with the π electrons of the carbonyl group has created a larger dipole moment in the O–H group of carboxylic acid molecules, as compared to that of, say, alcohol. (I can't find any data to prove or disprove this.) I have had students suggesting that the O–H bond is weakened as a result of this larger dipole moment.[2]

    This is a misconception, because a covalent bond is not weakened by a greater bond polarity!

    Consider the dipole moments and bond energies of dinitrogen and carbon monoxide.

    substance Dipole moment / D Bond Energy / kJ mol–1
    N2 0 944
    CO 0.122 1077

    Both the polarity and bond strength of the C≡O bond are greater than those of the N≡N bond.[3]

    Note:
    C, O and N are all from the same period, and their atomic radii do not differ significantly. We can thus assume that the extent of N–N and C–O orbital overlap is similar and the difference in bond energies is largely due to the difference in bond polarity.
Explanation 2:
The C=O group allows the carboxylate ion (i.e. conjugate base of carboxylic acid) to be stabilised by resonance.

This is a much more plausible explanation that I would use to explain the effect of the carbonyl group on the acidity of carboxylic acids.

The resonance structures[4] of the carboxylate ion shows that the negative charge is not permanently localised on one oxygen, but is dispersed by delocalisation with the π electrons of the adjacent carbonyl group. The extra stability accorded to the carboxylate ion as a result of resonance promotes the formation this anion. This in turn enhances the acidity of the hydroxyl H in carboxylic acids.

resonance structures of a carboxylate ion

On the other hand, such stabilisation is not available to the alkoxide ion (i.e. conjugate base of alcohol) and its negative charge is localised on one oxygen. Moreover the negative charge is intensified by the electron-donating alkyl group to which the negatively charged oxygen is bonded to. This de-stabilises the alkoxide ion, making it less likely to form.

alcohols are not resonance stabilised

Notes
  1. If you are confused by the different theories of acids and bases, I suggest that you visit Chemguide, which has a concise write-up on the Arrhenius, Bronsted-Lowry, and Lewis theories of acids and bases, and explains the relationship between them.
  2. My take on the possibly more polar O–H bond in carboxylic acids is this: rather than a weakened O–H bond, a larger dipole means a more electrophilic (and acidic) hydroxyl H, which is more susceptible to abstraction by the base nucleophile.
  3. The great Linus Pauling gave two lectures on 'Valence and Molecular Structure' in 1957, during which he explained the relationship between bond polarity and bond strength.
  4. The drawing of resonance structures is not in the H2 Chemistry syllabus. If you are interested in learning how to draw them, this online tutorial, by Prof. Steven A Hardinger, University Distinguished Senior Lecturer in Organic Chemistry, UCLA, has a rather detailed description.

The Trials and Tribulations of a Chemistry Teacher III

posted Oct 25, 2016, 2:14 AM by Grace Ong   [ updated Oct 25, 2016, 2:15 AM ]

The following post was first posted on Blogger on Sunday, 30 March 2014.



Sigh. Sometimes, I wonder what goes on inside a student's head during lessons... Where did all our efforts go?

common mistakes when drawing structural formulae of organic molecules

Notes:
  1. This is an incorrectly drawn structure of the pentavalent transition state of an SN2 mechanism. Besides the reversal of atoms in the structural formula, this student had also forgotten to include a single negative charge for this structure.
  2. The pentavalent transition state, which has five bonds to the central carbon atom, has a trigonal bipyramidal structure. Unlike a reaction intermediate which is a well-defined species with a definite lifetime, a transition state is a theoretical structure used to define a reaction mechanism.

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