meso, diastreomers...etc

[joonkimdds]

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Choose the compound which is capable of existing as diastreomers.

a) 2,3-difluorobutane
b) cyclohexane
c) 1,1-dichlorocyclopropane
d) 2-fluoropropane
e) 2,3-dichloropentane




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
I think it says answer was E but I have no way to confirm it yet.

 

[joonkimdds]

bump

 

[sajjy]

Choose the compound which is capable of existing as diastreomers.

a) 2,3-difluorobutane
b) cyclohexane
c) 1,1-dichlorocyclopropane
d) 2-fluoropropane
e) 2,3-dichloropentane




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
I think it says answer was E but I have no way to confirm it yet.

1) A and E
2) E

 

[Imperium]

Don't diastereomers have internal planes of symmetry? E doesn't have it.

 

[Flipper405]

Meso compounds have internal planes of symmetry.

I agree that 1 is A,E and 2 is E.

You just have to look for the compound that has at least two chiral centers. Most of these have one or none.

 

[rose786]

It's easiest to draw them out, then one will jump out at you.

 

[joonkimdds]

Meso compounds have internal planes of symmetry.

I agree that 1 is A,E and 2 is E.

You just have to look for the compound that has at least two chiral centers. Most of these have one or none.

why does it have to have two chiral centers? Can't it just have one?

 

[Flipper405]

Diastereomers don't have to have an internal plane of symmetry... Both A and E in #1 are capable of existing as diastereomers.

Internal planes of symmetry are only a requirement for meso compounds.

You have to have more than one chiral center because otherwise all you have are enantiomers.

 

[Imperium]

Wow, I really need to brush up on this. The mirror images of A,E are nonsuperimposable, so doesn't that mean they are also capable of existing as enantiomers?

 

[sajjy]

Wow, I really need to brush up on this. The mirror images of A,E are nonsuperimposable, so doesn't that mean they are also capable of existing as enantiomers?

Both A and E will occur as diastereoisomers, each of which will have enantiomers.
BUT only A will have meso.

2) E will also form diastereoisomers, enantiomers and meso.

 

[dontwakeme]

Draw it out. If it has more than 1 chiral center, it will have diastereomers. If it has only one chiral center, it will only have an enantiomer, no diastereomers. Compounds with double bonds can also have stereochemistry (E/Z diastereomers). Meso sterioisomers occur when you have two chiral centers with exactly the same substituents but opposite stereochemical configuration. Overall, meso compounds are achiral, despite having chiral centers.

 

[joonkimdds]

Ok so to summarize this.

2 chiral + internal symmetry = can be diastereomer, enantiomer and meso
2 chiral + no internal symmetery = can be diastereomer, and enantiomer
1 chiral = enantiomer

am I right?

 

[joonkimdds]

Draw it out. If it has more than 1 chiral center, it will have diastereomers. If it has only one chiral center, it will only have an enantiomer, no diastereomers. Compounds with double bonds can also have stereochemistry (E/Z diastereomers). Meso sterioisomers occur when you have two chiral centers with exactly the same substituents but opposite stereochemical configuration. Overall, meso compounds are achiral, despite having chiral centers.

sorry for the double post.
But why can't 1 chiral be diastereomer?

For example, 2-butanol can be drawn as
C
|
C-OH
|
C
|
C

or

.....C
.....|
.....C
.....|
OH-C
.....|
.....C
if then, the first image is not a mirror image of second and they are not super posable thus diastereomer.

 

[Flipper405]

I'm not sure about what the stereochemistry is in your drawing, but the second compound is either the same as the first one or it's the mirror image. You really should get a molecular model kit and build this if that's an option... I felt it made things a lot clearer when dealing with stereochemistry.

 

[joonkimdds]

I'm not sure about what the stereochemistry is in your drawing, but the second compound is either the same as the first one or it's the mirror image. You really should get a molecular model kit and build this if that's an option... I felt it made things a lot clearer when dealing with stereochemistry.

Yes, they are the identicaly compound.
But then here is a question for you. (I am not trying to be rude, or tricky)
2,3-butanediol has 2 chiral center.
...CH3
H-C-OH
H-C-OH
...CH3
this is one of them
and mirror image would be
.....CH3
HO-C-H
HO-C-H
.....CH3

the first and second image are mirror image to each other and not superposable.
Is this enantimer or identical?
if I flip it 180 degree, they are the identical compound, aren't they?
if they are identical, why aren't they enantiomer?
if they are enantiomer, why aren't they identical?
if they can be both, can my previous example about 2-butanol also be both identical and diastereomer?


p.s. sorry if I am playing with your brain, but I am seriously having trouble with this. I have molecular model by the way.

 

[dontwakeme]

sorry for the double post.
But why can't 1 chiral be diastereomer?

For example, 2-butanol can be drawn as
C
|
C-OH
|
C
|
C

or

.....C
.....|
.....C
.....|
OH-C
.....|
.....C
if then, the first image is not a mirror image of second and they are not super posable thus diastereomer.

Nope. Don't even think of it as mirror images, it can get confusing.

Very simple rule: To form a compound's enantiomer, merely flip the stereochemical configuration of EVERY chiral center (R to S, S to R). To form a compound's diastereomer, flip 1 or more chiral center configurations, but not all of them. Thus, if you only have one chiral center and you flip it, you got the enantiomer. To form the diastereomer of a compound with a double bond, flip the substituents. No enantiomers for alkenes though.

 

[joonkimdds]

Nope. Don't even think of it as mirror images, it can get confusing.

Very simple rule: To form a compound's enantiomer, merely flip the stereochemical configuration of EVERY chiral center (R to S, S to R). To form a compound's diastereomer, flip 1 or more chiral center configurations, but not all of them. Thus, if you only have one chiral center and you flip it, you got the enantiomer. To form the diastereomer of a compound with a double bond, flip the substituents. No enantiomers for alkenes though.

Ok, I guess I should follow what's so called simple rule to get out of this mess. I will try my best to make all the mirror this mirror that stuff. That's like 200GB ram of my brain already.