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

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In summary, the carbon chain's ability to donate electrons is diminished by intervening bonds. This is why 2,2-dimethylpropanoic acid is less acidic than pentatonic acid, which has a carbons at positions 4 and 5 that donate electrons.
  • #1
A13235378
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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?
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  • #2
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 pKa'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.
 
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  • #3
A13235378 said:
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)
 
  • #4
dshoup said:
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.
 
  • #5
dshoup said:
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.
 
  • #6
Borek said:
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.
 
  • #7
dshoup said:
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.
 

What is the definition of acidity of carboxylic acids?

The acidity of carboxylic acids refers to their ability to donate a proton (H+) in a chemical reaction, making them acidic.

What factors affect the acidity of carboxylic acids?

The factors that affect the acidity of carboxylic acids include the strength of the carboxylic acid, the stability of the conjugate base, and the presence of electron-withdrawing or electron-donating groups on the molecule.

How is the acidity of carboxylic acids measured?

The acidity of carboxylic acids is measured using a scale called pKa, which represents the negative logarithm of the acid dissociation constant (Ka). The lower the pKa value, the more acidic the carboxylic acid is.

What is the relationship between the structure of carboxylic acids and their acidity?

The structure of carboxylic acids plays a significant role in determining their acidity. Carboxylic acids with more stable conjugate bases, such as those with resonance structures or electron-withdrawing groups, tend to be more acidic.

How do carboxylic acids behave in water?

Carboxylic acids are generally soluble in water due to their polar nature. In water, they can dissociate into their conjugate base (carboxylate ion) and a hydronium ion (H3O+), resulting in an acidic solution.

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