Meso compounds

[cloak25]

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which of the following compounds does NOT show any optical rotation of planar light?

A. 2R, 3R dibromobutane
B. 2R, 3S dibromobutane
C. 2R, 3R dibromohexane
D. 2R, 3S dibromohexane

answer : B

Molecules must have R, S chirality to be meso??

 

[gettheleadout]

which of the following compounds does NOT show any optical rotation of planar light?

A. 2R, 3R dibromobutane
B. 2R, 3S dibromobutane
C. 2R, 3R dibromohexane
D. 2R, 3S dibromohexane

answer : B

Molecules must have R, S chirality to be meso??

The configurations of the stereocenters must be opposite in order to cancel each other out and prevent any rotation of the light. Why isn't D right as well?

 

[MedPR]

The configurations of the stereocenters must be opposite in order to cancel each other out and prevent any rotation of the light. Why isn't D right as well?

No plane of symmetry in a 2,3 substituted hexane.

 

[cloak25]

The configurations of the stereocenters must be opposite in order to cancel each other out and prevent any rotation of the light. Why isn't D right as well?

did you misread D as a dibromocyclohexane instead of dibromohexane?

 

[HoltMD]

The configurations of the stereocenters must be opposite in order to cancel each other out and prevent any rotation of the light. Why isn't D right as well?

Due to lack of symmetry.

 

[MedPR]

did you misread D as a dibromocyclohexane instead of dibromohexane?

It wouldn't matter. 2R,3S-dibromocyclohexane does not exist.

 

[cloak25]

It wouldn't matter. 2R,3S-dibromocyclohexane does not exist.

but if it did exist, it would be meso correct?

 

[MedPR]

but if it did exist, it would be meso correct?

If it was written 1R,2S-dibromocyclohexane, then yes it would be meso. Alternatively it could be written cis-1,2-dibromocyclohexane and that is also meso. trans-1,2 is not meso.

 

[gettheleadout]

Sorry guys, I totally forgot that a plane of symmetry must exist so the stereocenters can cancel properly. Meso compounds were one of the few things I didn't feel like we covered well in Organic I. Thanks for the clarification everyone!

 

[SaintJude]

Mhh, I also made a terrible mistake. I misread all the compound choices as cyclical just b/c I usually consider whether cyclical compounds area meso. Fallacy of the familiar. I need to really make sure I stay attentive to the problems. Good question.

which of the following compounds does NOT show any optical rotation of planar light?

A. 2R, 3R dibromobutane
B. 2R, 3S dibromobutane
C. 2R, 3R dibromohexane
D. 2R, 3S dibromohexane

answer : B

The configurations of the stereocenters must be opposite in order to cancel each other out and prevent any rotation of the light.

 

[chiddler]

i'm not seeing it. It looks like the RR has a line of symmetry, not the RS.

 

[MedPR]

i'm not seeing it. It looks like the RR has a line of symmetry, not the RS.

What. RR has them both on the same side (coming out of the page) while RS has one going in and one coming out.

 

[chiddler]

yes yes. that's right.

i need to review meso compounds.