Aldehydes, Ketones And Carboxylic Acids

- Let's bust some of the common misconceptions from this chapter.

Assuming 1 as the 'Default' Number in Naming ketones and Omitting it while Naming the Compound

When naming aldehydes, the aldehyde functional group is numbered #1, and this number is not included when naming the aldehyde. For example, methanal:
It is possible for students to think that #1 is the default while numbering a functional group. So, when naming ketones, it is possible that they name a ketone without a number, assuming it is at 1 position. This is wrong.
  • Do not assume that the position is #1 for ketone functional group.
  • Secondly, always mention the number of the locant. Do not omit it by thinking that it is at #1 position and so the number is not required (unlike aldehydes).

Carboxylic Acids Have the Highest Boiling Points Among All Functional Groups?

The textbook mentions that "Carboxylic acids are higher boiling liquids than aldehydes, ketones and even alcohols of comparable molecular masses."
Because of this, students generally assume that carboxylic acids have the highest boiling points among all the functional groups. It is easy to think that alcohols generally have reasonably high boiling points. So, if carboxylic acids have higher boiling points than them, then they are leaders of the pack! This is a very common misconception.
  • Carboxylic acids have higher boiling points than hydrocarbons, alcohols, esters, aldehydes and ketones of comparable molecular masses because of hydrogen bonding.
  • But, amides have even higher boiling points because they form more hydrogen bonds than carboxylic acids.
So, if we compare the order of boiling points of molecules of comparable masses, with different functional groups, this would be the order of boiling points:
Amide > Carboxylic Acids > Alcohol > Ketone > Aldehyde > Amine > Ester > Ether > Alkane

In esterification, alcohol loses -OH and carboxylic acid loses -H. Not exactly!

This is one way in which we try to remember an esterification reaction:
It is easy to remember this reaction by drawing a box from -H of the carboxylic acid and -OH of the alcohol, and remember that these are removed to form an ester. But, by doing this, students think that the water molecule is formed from the -H of carboxylic acid and -OH of the alcohol.
But if you see the mechanism, you will see that a more complex process is happening. It's not as simple as -H of carboxylic acid and -OH of alcohol are forming water. Instead, what's really happening here is this:
So, it's the -OH of the carboxylic acid and the -H of the alcohol that is getting lost, and not the other way round.