Why does the structure of a carboxylic acid affect its acidity?

[A13235378]

Homework Statement

I saw it in a source that stated that the more branches the carbon chain has, the less acid it will be. An example of this is that 2 methyl propanoic acid (pKa = 4.86) is less acidic than butanoic acid (pKa = 4.82). Another more measurable example is that 2.2 methyl propanoic acid (pKa = 5.05) is less acidic than pentanoic acid (pKa = 4.86). What's the explanation for that?

Relevant Equations

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[epenguin]

For homework help here by the rules, students are supposed to show some effort, which you haven't, before they are given any help. Explanations are given in textbooks of these phenomena in terms of an 'inductive effect' in which for example methyl groups push electrons into the carboxylic acid group O-H bond, stabilising it. although other substituents can withdraw electrons, making the group more acid. You will find systematic explanations in textbooks or other e.g. online sources.

By and large these explanations seem to work but I do have some reservations about their soundness. If the subject is on your syllabus it is a good exercise to find tables of pK_a's that you can easily find online and see if you can explain the various systematic tendencies. In here https://www.engineeringtoolbox.com/...sociation-constant-alcohol-phenol-d_1948.html
for example there are graphs and tables of the pK s of aliphatic carboxylic acids and also alcohols which ought to trend in the same way. See whether you think that all is really explained.

 

[dshoup]

Homework Statement:: I saw it in a source that stated that the more branches the carbon chain has, the less acid it will be. An example of this is that 2 methyl propanoic acid (pKa = 4.86) is less acidic than butanoic acid (pKa = 4.82). Another more measurable example is that 2.2 methyl propanoic acid (pKa = 5.05) is less acidic than pentanoic acid (pKa = 4.86). What's the explanation for that?
Relevant Equations:: None

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The longer the carbon chain on a carboxylic acid the less acidic it becomes. This is because the carbon chain pushes electrons towards the negatively charged carboxyl ion destabilizing it. If a flourine atom is bonded next to the carboxyl ion it becomes more acidic than if a methyl group was attached to the carboxyl ion. This is because the flourine atom is more electronegative than the methyl group and has a stabilizing effect on the negatively charged carboxyl ion. In general the more electronegative the group attached to the carboxyl group is the more acidic the carboxylic acid is(e.g. the smaller the pKa becomes)

 

[Borek]

The longer the carbon chain on a carboxylic acid the less acidic it becomes. This is because the carbon chain pushes electrons towards the negatively charged carboxyl ion destabilizing it. If a flourine atom is bonded next to the carboxyl ion it becomes more acidic than if a methyl group was attached to the carboxyl ion. This is because the flourine atom is more electronegative than the methyl group and has a stabilizing effect on the negatively charged carboxyl ion. In general the more electronegative the group attached to the carboxyl group is the more acidic the carboxylic acid is(e.g. the smaller the pKa becomes)

A lot of wasted words: you haven't addressed the question about why branched carbon chains behave differently than the straight ones.

 

[dshoup]

The longer the carbon chain on a carboxylic acid the less acidic it becomes. This is because the carbon chain pushes electrons towards the negatively charged carboxyl ion destabilizing it. If a flourine atom is bonded next to the carboxyl ion it becomes more acidic than if a methyl group was attached to the carboxyl ion. This is because the flourine atom is more electronegative than the methyl group and has a stabilizing effect on the negatively charged carboxyl ion. In general the more electronegative the group attached to the carboxyl group is the more acidic the carboxylic acid is(e.g. the smaller the pKa becomes)

A branched carbon chain is less electronegative than a straight carbon chain. So a branched carbon chain attached to a carboxyl group will give a weaker carboxylic acid than if a straight carbon chain were attached. For example pentanoic acid is a stronger acid than 2,2 methyl propionic acid.

 

[dshoup]

A lot of wasted words: you haven't addressed the question about why branched carbon chains behave differently than the straight ones.

You should read my reply to my post. They should have been combined. In general a straight carbon chain attached to a carboxyl group gives a stronger acid than a branched chain attached to the carboxyl group. The straight chain is more electronegative than the branched chain. As a result, pentanoic acid is stronger than 2,2 methyl propanoic acid.

 

[chemisttree]

You should read my reply to my post. They should have been combined. In general a straight carbon chain attached to a carboxyl group gives a stronger acid than a branched chain attached to the carboxyl group. The straight chain is more electronegative than the branched chain. As a result, pentanoic acid is stronger than 2,2 methyl propanoic acid.

I think you want to say that the electron donating ability of carbons is diminished by intervening bonds. That is, the electron donating effect of the carbons at positions 4&5 of pentatonic acid is less than the two ‘alpha-substituted’ methyls in 2,2-dimethylpropanoic acid.