Random Ochem questions

[tyjacobs]

1) Given this picture: what is the most acidic proton?

Proton e has inductive stabilization from the carbonyl but proton A has resonance stabilization from the conjugated system. What is more important when determining acidity: inductive stabilization, or resonance? Why?


2) Which of the following 5 structures has the most stable resonance form?

First, is the question equivalent to: which is the most stable carbocation? If so, how does having a benzene ring attached to a carbocation stabilize it? I understand that structures 1, 2, 4, and 5 are all allylic carbocations but don't necessarily understand why 5 would be more stable than 1, 2 and 4.


3) relationship of physical and chemical properties between stereoisomers:

- enantiomers have identical and physical properties except for their ability to rotate plane polarized light?
- cis/trans isomers have different physical and chemical properties?
- how do the physical and chemical properties of 2 diastereomers relate?


thanks

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[FeralisExtremum]

1) Resonance tends to take priority over induction because it allows for greater stabilization of the subsequently created negative charge from deprotonation than induction alone.

2) You shouldn't necessarily treat this the same way as most stable carbocation. A benzene ring can stabilize a carbocation via resonance - the positive charge ends up getting distributed across multiple atoms rather than being focused on just one. The reason why 5 is more stable than 1, 2, and 4 (equivalent resonance structures) is because 1, 2, and 4 are not actually aromatic. Only in 5 is the aromaticity of the ring maintained, which makes it most stable.

To make sure you've got the concept of aromaticity down, see if you can explain why 1, 2, and 4 are not aromatic structures despite conjugation being maintained throughout the ring.

3) Correct about enantiomers, cis/trans isomers can have different physical properties (e.g. melting and boiling points), diastereomers have different physical properties.

 

[bodmon]

1) Resonance tends to take priority over induction because it allows for greater stabilization of the subsequently created negative charge from deprotonation than induction alone.

2) You shouldn't necessarily treat this the same way as most stable carbocation. A benzene ring can stabilize a carbocation via resonance - the positive charge ends up getting distributed across multiple atoms rather than being focused on just one. The reason why 5 is more stable than 1, 2, and 4 (equivalent resonance structures) is because 1, 2, and 4 are not actually aromatic. Only in 5 is the aromaticity of the ring maintained, which makes it most stable.

To make sure you've got the concept of aromaticity down, see if you can explain why 1, 2, and 4 are not aromatic structures despite conjugation being maintained throughout the ring.

3) Correct about enantiomers, cis/trans isomers can have different physical properties (e.g. melting and boiling points), diastereomers have different physical properties.

I understand your explanation for 2), but I also remember learning that carbocations are most stable when they are bonded with other electron donating groups. Why wouldn't Structure 3 be stable? since the carbon would be a tertiary carbon

 

[FeralisExtremum]

I understand your explanation for 2), but I also remember learning that carbocations are most stable when they are bonded with other electron donating groups. Why wouldn't Structure 3 be stable? since the carbon would be a tertiary carbon

The cation in structure 3 would be stable, in theory, but it doesn't actually exist. You wouldn't have a carbocation on a carbon with 4 bonds.

 

[bodmon]

The cation in structure 3 would be stable, in theory, but it doesn't actually exist. You wouldn't have a carbocation on a carbon with 4 bonds.

thank you for clearing that up!