In the case of carboxylic group, why is decanoic acid (the number of carbon : 10) is stronger than octanoic acid (8) in acidity ?
I think I can figure out why roughly. But, I know I do not know as much as I explain why by organic chemistry.
I'd very appreciate it if you'd help me to unerstand why.
Dear Jason,
as a matter of principle the acidity of a carboxylic acid will decrease with increasing length of alkyl chain.The reason is the positive inductive effect of any alkyl group which is the stronger the longer the alkyl group is (if it is nonbranched). This +I-effect weakens the possibility to release a protoin from the COOH group.
The effect can be seen best at the begin of the homolgous series.
formic acid pka 3.77
acetic acid pKa 4.76
propanoic acid 4.87
With any "staircase" on this stairs it fades out, that means the contribution of yet another CH2 group becomes marginal, and the whole lowering of acidity by chain length an asymptotic business.
If then two very similar values are "mixed" this may be due to different authors or sources for these values but cannot be due to chemical structures. Chemically, a C8 acid should be a little bit stronger than a C10 acid.
Dear Jason Ju,
Do you mean the formulas are CH3(CH2)8COOH (decanoic acid) and CH3(CH2)6COOH (octanoic acid)?
If this is true, I think that their acidity is not significant different (pKa around 4.9).
so please mention the formula of your molecules as well, because in organic chemistry if any electron withdrawing group or electron donor group attached on your parent molecules such as decanoic acid will influence the acidity.
Thank you.
Dear Jason,
Decanoic acid pKa is 4.90
Octanoic acid pKa = 4.895
Alkyl group is electron donating group, therefore you expect that the acidity of decanoic acid will be a little weaker (higher pKa) than octanoic acid since decyl group is stronger electron donating group than octyl. And actually this is the case as seen from the pKa values depicted above. The small difference in the pKa values is due to the small difference in the number of carbons of the carbolic acids; C8 for octanoic vs. C10 for the decanoic (difference in only two carbons).
Alky group generally destabilizes the negative ion on the carboxylate group, thus decreasing the acidity of the carboxylic acid.
Summary: the slight difference in acidity between both carboxylic acids is due to the inductive effect of the alkyl group which donates electrons.
Hoping this will be helpful,
Rafik
Dear Jason,
as a matter of principle the acidity of a carboxylic acid will decrease with increasing length of alkyl chain.The reason is the positive inductive effect of any alkyl group which is the stronger the longer the alkyl group is (if it is nonbranched). This +I-effect weakens the possibility to release a protoin from the COOH group.
The effect can be seen best at the begin of the homolgous series.
formic acid pka 3.77
acetic acid pKa 4.76
propanoic acid 4.87
With any "staircase" on this stairs it fades out, that means the contribution of yet another CH2 group becomes marginal, and the whole lowering of acidity by chain length an asymptotic business.
If then two very similar values are "mixed" this may be due to different authors or sources for these values but cannot be due to chemical structures. Chemically, a C8 acid should be a little bit stronger than a C10 acid.
I would like to agree with what Sicker has said.
In fact, the acidity of carboxylic acids stems out of the stability of the carboxylate anion. It is plausible that a long alkyl chain is going to destabilise the anion, which in turn is likely to decrease the acidity of the corresponding carboxylic acid.
Dear all,
I would like you to read my first answer which depicts the pKa values of decanoic and octanoic acids. It is very clear from the values that octanoic acid is much acidic than decanoic acid (4.895 vs, 4.9) because of the inductive effect of the alkyl electron donating group.
Regarding Sicker's answer "
With any "staircase" on this stairs it fades out, that means the contribution of yet another CH2 group becomes marginal, and the whole lowering of acidity by chain length an asymptotic business.
If then two very similar values are "mixed" this may be due to different authors or sources for these values but cannot be due to chemical structures. Chemically, a C8 acid should be a little bit stronger than a C10 acid".
There is no mix in the values. The values actually reflect what it should be. The values that I listed are the official values known for these two acids.
"Hoping that I clarified the point.
Rafik
I think a very import point is missing in this discussion. In fact, the acidity constat of long chain fatty acids in water is not really constant. Short chain carboxylic acids, such as acetic and propionic acid, are well soluble in aqueous solutions, and, in most conditions at which titrations are performed, do not interact with each other.
Long chain fatty acids, however, are either insoluble in acidic water or form emulsions. Except for high pH where self-assembled aggregates are found. In such conditions, the carboxylic headgroups strongly interact with each other, and an increasingly larger electrostatic repulsion has to be overcome with increasing degree of ionization. This results in an increasing pKa value with pH.
To make a long story short, the pKa value depends on the experimental conditions, and, most notably, on degree of ionization and therefore on pH. This is the case also for polyacids like PAA.
- Badges
- Science topic
- Similar topics
- Chemistry
- Organic Chemistry