Ochem ACS

[UW09]

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Could you guys explain these.

Which of these tartaric acid isomers will have the same melting point?

(a) (+)-tartaric acid and (-)-tartaric acid only
(b) (+)-tartaric acid and (-)-tartaric acid and racemic (+/-)-tartaric acid only
(c) (+)-tartaric acid and (-)-tartaric acid and meso-tartaric acid only
(d) meso-tartaric acid and racemic (+/-)-tartaric acid

Answer: A (I said b)

How many stereoisomers are there for 1,2-dimethylcyclopropane? (counting enantiomers as 2 two diff. stereoisomers)

Answer: 3 (I said 4 initially since 2 chiral centers...I'm guessing the 2^n doesn't exclude the meso so we have to take that out?)

 

[dentalaee]

Could you guys explain these.

Which of these tartaric acid isomers will have the same melting point?

(a) (+)-tartaric acid and (-)-tartaric acid only
(b) (+)-tartaric acid and (-)-tartaric acid and racemic (+/-)-tartaric acid only
(c) (+)-tartaric acid and (-)-tartaric acid and meso-tartaric acid only
(d) meso-tartaric acid and racemic (+/-)-tartaric acid

Answer: A (I said b)

How many stereoisomers are there for 1,2-dimethylcyclopropane? (counting enantiomers as 2 two diff. stereoisomers)

Answer: 3 (I said 4 initially since 2 chiral centers...I'm guessing the 2^n doesn't exclude the meso so we have to take that out?)

When it comes down to stereoisomers, R and S molecules share exactly the same physical properties except for optical activity.
When a reaction produces racemic mixture, it will have different melting point. You can find this example in almost any text book. They actually use tartaric acid in my book.

For the second question. It will produce enantiomers when two methyl groups are opposite to one another. However, when they are on the same side, it becomes "meso" compound. When they are on the same side, no matter how you rotate it, they are "identical" to one another.

n^2 only applies, when two chiral centers are different from one another.