The Student Room Group

BP and strength of carboxylic acids

Can somebody please explain to me how chain length affects BP and strength of a carboxylic acid (because I thought increasing chain length would increase BP but apparently the opposite is true)?
And secondly, what other factors affect strenght and the boiling point of a carboxylic acid?

Thanks!
more alkyl groups = more electrons released onto the functional group (more delocalising) = acid is more negative and less stable
Original post by daydream77
more alkyl groups = more electrons released onto the functional group (more delocalising) = acid is more negative and less stable


?? and then what does being less stable imply? stronger acid and lower bp?? sorry im slow
Original post by fluffypoopies
?? and then what does being less stable imply? stronger acid and lower bp?? sorry im slow

as it becomes more negative and less stable, it becomes a weaker acid as acids are electron acceptors and proton donors - it is harder for it to accept electrons if there are more electron releasing alkyl groups.
as for BP, it increases as the chain gets longer because of the hydrogen bonds between molecules and increased london forces as the chain gets longer.
if the carboxylic acid is to behave as an acid then the hydrogen attached to the carboxylate group must be able to leave as a H+ ion. As the length of the alkyl chain increases the ability of the alkyl group to push electrons into the carboxylate group will increases, this will increase the electron density on the oxygen atom and this will result in a stronger O-H bond, meaning that the hydrogen is more tightly held, so I would say that as the alkyl chain length increases the strength of the carboxylic acid will decrease. I would also say that as the chain length increases the molecule will become more "organic in nature" and hence less soluble in water, since acids are solutions of hydrogen ions in water if solubility decreases then I would say that acidity will reduce also!

Also as chain length increases the number of electrons will increase and so will the amount of Van der Waals bonding, so BP will increase.

Quick Reply