Q. 21

[w-huang36150]

21. The 1HNMR of compound X was performed with CDCl3 as a solvent and 8 signals were observed. A second 1HNMR of compound X was performed after the addition of D20 to the 1HNMR solution, and seven signals were observed. The vanishing of the 1HNMR signal is an indication that X contains which of the following?

1. A carboxylic acid [x]
2. An acidic hydrogen [x]
3. An alkene [x]
4. An aldehyde

What is the difference between a carboxylic acid and an acidic hydrogen? I thought carboxylic acid has an acidic hydrogen.

[admin]

Yes, you are correct that a carboxylic acid has an acidic hydrogen. However, from the question, all that we can conclude is that compound X lost a proton and is therefore an acid. Do we know what kind of acid it is? Carboxylic acids are the most common organic acid but there are many other options: sulfonic acids are actually more acidic than carboxylic acids, then there are weaker acids with functional groups like -OH, -SH, -enol, etc.

In other words, answer choice B is the best answer because it is the one thing that we can be sure of.

[dnpgr16513]

I don't understand why this answer isn't D. Aldehydes only have 1 type of proton, so in a reaction, the observance of the disappearance of a hydrogen wouldn't signify that it was an aldehyde?

Thanks.

[calena7178147]

Answer B is correct. Aldehydes do not all have only one type of protons. In particular when the carbon chain is long you have also protons attached to the carbon. Aldehydes have the following structure of RCOH. In addition, an acidic proton is lost with D2O replacement as stated and so B makes the most sense.