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A level chem question about zwitterions

I was wondering why A,B,C in the question below are not valid answers even though they all have the amino acid functional group
image_2023-05-01_105215916.png
(edited 11 months ago)
Original post by FM1/FP1
I was wondering why A,B,C in the question below are not valid answers even though they all have the amino acid functional group
image_2023-05-01_105215916.png


Hi,

For A: 2 amine groups have gained a hydrogen, but only one hydrogen has been lost from the COOH group. Overall, there are more hydrogens gained than lost, so this answer is incorrect.

For B: 2 hydrogens lost from COOH groups, but only one hydrogen gained on the amine group. There are more hydrogens lost than gained, so it can't be this answer.

For C: Zwitterions are when the COOH group has donated a hydrogen to the amine group - for this structure, a hydrogen has been donated to the alcohol group, which does not meet the definition of a zwitterion, so this structure is incorrect.

For D: this answer is correct, as the number of hydrogens gained and lost adds up to 0, and there has only be hydrogens gained/lost between the amine and COOH group.

Hope this helps! :smile:
(edited 11 months ago)
Reply 2
Original post by FM1/FP1
I was wondering why A,B,C in the question below are not valid answers even though they all have the amino acid functional group
image_2023-05-01_105215916.png

A zwitterion needs to have an overall charge of 0, which eliminates A and B.

And for C, you would expect the NH2 group to be protonated (to [NH3]+)
https://chemguide.co.uk/organicprops/aminoacids/acidbase.html
Reply 3
Original post by Biolover2020
Hi,

For A: 2 amine groups have gained a hydrogen, but only one hydrogen has been lost from the COOH group. Overall, there are more hydrogens gained than lost, so this answer is incorrect.

For B: 2 hydrogens lost from COOH groups, but only one hydrogen gained on the amine group. There are more hydrogens lost than gained, so it can't be this answer.

For C: Zwitterions are when the COOH group has donated a hydrogen to the amine group - for this structure, a hydrogen has been donated to the alcohol group, which does not meet the definition of a zwitterion, so this structure is incorrect.

For D: this answer is correct, as the number of hydrogens gained and lost adds up to 0, and there has only be hydrogens gained/lost between the amine and COOH group.

Hope this helps! :smile:


Original post by bl0bf1sh
A zwitterion needs to have an overall charge of 0, which eliminates A and B.

And for C, you would expect the NH2 group to be protonated (to [NH3]+)
https://chemguide.co.uk/organicprops/aminoacids/acidbase.html


Thanks guys!

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