diastereomers, plz help!

[joonkimdds]

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Choose the compound which is capable of existing as diastereomers.
a) ethanol
b) 2-propanol
c) 2-butanol
d) 2-methyl-2-butanol
e) 2,3-butanediol

answer says E even though I don't know if that's correct.
Can someone tell me why answer is E? and why others can't be the answer?

Here is a same type of problem
a)2,3-difluorobutane
b) cyclohexane
c) 1,1-dichlorocyclopropane
d) 2-fluoropropane
e) 2,3- dichloropentane

and the answer says E.

I need an explanation for both problems. Thanks.

 

[joonkimdds]

bump.

 

[will9631]

I'm no chemistry buff, but if I remember this problem type correctly, all they're looking for is the compound with a chirality center (a carbon with four unique substituent groups). then again, I'm visualizing these in my head and I can't seem to eliminate all the incorrect answers, so there's some part of it I'm missing.

 

[rose786]

Usually when I come across a question like this I'll draw out each compound and suddenly I see which one is different from the others. Often I realize a compound is slightly different than what I was imagining in my head. I know this may seem like a really simple solution but try it and see if it helps.

 

[dentstd]

Joon, you've been studying for this exam for too long! Hurry up and graduate from GMU!!!

 

[dentstd]

btw, to answer your question, if you wanna diastereomer, you need a molecule with more than one chiral center. more specifically, 2 or more stereocenters that don't somehow make the compound mesocompounds.

 

[tastybeef]

I think you might have typed up the wrong answer choices from your book.

If "E" is the answer for the first one, then why isn't "A" also a valid answer for the second question? They are essentially analogues of each other with different functional groups.

Anyway, in question 1, E is the only choice with more than 1 stereocenter.
Question 2, I think A and E are both correct answers since they both have more than 1 stereocenter.

 

[dentstd]

I think you might have typed up the wrong answer choices from your book.

If "E" is the answer for the first one, then why isn't "A" also a valid answer for the second question? They are essentially analogues of each other with different functional groups.

Anyway, in question 1, E is the only choice with more than 1 stereocenter.
Question 2, I think A and E are both correct answers since they both have more than 1 stereocenter.

because 2,3 diflurobutane is a mesocompound. (well, at least one of its 2 stereoisomers is). 2 stereocenters isn't the issue. having 2 centers suggest teh potential to be diastereomers, not that the are diastereomers.

 

[tastybeef]

"because 2,3 diflurobutane is a mesocompound. (well, at least one of its 2 stereoisomers is). 2 stereocenters isn't the issue. having 2 centers suggest teh potential to be diastereomers, not that the are diastereomers."

Why must 2,3-difluorobutane a meso compound? You can have RR, SS, and RS (the meso compound). RR and SS are miorror images, but RS and SS are not mirror images of each other nor are they the same, so why aren't they considered diastereomers? Furthermore, this question asks which molecule is capable of existing as diastereomers.

 

[dentstd]

"because 2,3 diflurobutane is a mesocompound. (well, at least one of its 2 stereoisomers is). 2 stereocenters isn't the issue. having 2 centers suggest teh potential to be diastereomers, not that the are diastereomers."

Why must 2,3-difluorobutane a meso compound? You can have RR, SS, and RS (the meso compound). RR and SS are miorror images, but RS and SS are not mirror images of each other nor are they the same, so why aren't they considered diastereomers? Furthermore, this question asks which molecule is capable of existing as diastereomers.

well, that's the mess I was hinting at with the stuff in the parenthesis. it's a big mess of problems that's hard to explain.

The answer is convention. Something like, when there are mulitple ways to view a molecule and it's possible to draw a mesocmpd, the mesocmpd is assumed.

 

[tastybeef]

"Something like, when there are mulitple ways to view a molecule and it's possible to draw a mesocmpd, the mesocmpd is assumed."

You can draw a meso compound for 2,3-butanediol too. So why is that an answer for the first one?