[Rutherford]

1.I was solving a problem where bromine is added in dark to cis and trans form of 2-methyl-2-butenoic acid. I thought that one pair of enantiomers will be made, as the rotation around a single bond should be allowed here, but in the answer there are 2 pairs of enantiomers. I can't think of why would be the rotation forbidden here and two atropisomers made. Is there a logical explanation for the correct answer (2 pairs)?

2. What is the type of reaction between benzaldehyde and malonic acid in pyridine?

[Dan]

1. Can you clarify this: Do you mean addition of bromine to a mixture of E/Z-2-methyl-2-butenoic acid, or addition of bromine to each isomer separately? Pay attention to mechanism. Hint: bromonium

2. Hint: keto-enol tautomerism

[Rutherford]

1.Bromine is added to a mixture of Z and E isomers. The bromine atoms will be attached to the 2nd and 3rd C atom, but how 4 stereoisomers arise (isn't the rotation around the C-C bond allowed here)?

2.How can benzaldehyde form an enol when the alpha C atoms doesn't have a H attached to it?

[discodermolide]

Dan said bromonium. What is a bromonium species?

The other question: you just don't have benzaldehyde, malonic acid is also present as is pyridine.

[Rutherford]

1. Br⁺ and I know the mechanism. How can Br⁺ help me know if 2 or 4 stereoisomers are made?

2.No idea  .

[discodermolide]

You will get 2 chiral centers, that is 2² isomers = 4 diastereoisomers.
(R)(R), (S)(S), (R)(S) and (S)(R). Two enantiomeric pairs.

[Rutherford]

Right, I was thinking in the wrong direction. Thanks.

[Dan]

2.No idea  .

I gave you a hint: keto-enol tautomerism. Make an enol out of malonic acid.

Enols are nucleophiles, they react with electrophiles. Are there any electrophiles present?