Diastereomers and optical activity

[saltyload]

Do diastereomers have optical activity? For example a compound that is SS, RR, or RS, SR?

For example, 2, 3-dichlorobutane. If this compound seen on a fischer projection with both Cl on the same side.

C
C-Cl
C-Cl
C

I know this configuration is SR. Does the S and R cancel each other out and make it optically inactive?

Must a compound with two chiral centers be SS or RR to be optically active?

---

Members don't see this ad.

 

[Rabolisk]

Your question does not make any sense.

Nevertheless, any compound that is chiral is optically active. 2,3-dichlorobutane contains 2 chiral carbons, so it appears to be optically active. However, 2R,3S-dichlorobutane and 2S,3R-dichlorobutane have an internal plane of symmetry, and are one equivalent meso compound. It is not optically active, although its two diastereomers (RR and SS) are.

 

[saltyload]

Your question does not make any sense.

Nevertheless, any compound that is chiral is optically active. 2,3-dichlorobutane contains 2 chiral carbons, so it appears to be optically active. However, 2R,3S-dichlorobutane and 2S,3R-dichlorobutane have an internal plane of symmetry, and are one equivalent meso compound. It is not optically active, although its two diastereomers (RR and SS) are.

So if the compound were 2-bromo-3-chlorobutane, a 2R,3S and 2R,3R would both be optically active because there is no plane of symmetry?

I think my initial confusion of a compound with 2 chiral centers having RS configuration will cause R and S to cancel each other out because carbon 2 and 3 both have chlorine.

Anyone have some input on this? Trying to make sure I'm on the right track.

 

[orgohacks]

So if the compound were 2-bromo-3-chlorobutane, a 2R,3S and 2R,3R would both be optically active because there is no plane of symmetry?

I think my initial confusion of a compound with 2 chiral centers having RS configuration will cause R and S to cancel each other out because carbon 2 and 3 both have chlorine.

Anyone have some input on this? Trying to make sure I'm on the right track.

If the molecule has a mirror plane it will not be chiral and thus will not have optical activity.

In your first example, 2,3-dichlorobutane, the 2(S),3(R) isomer is meso (as is the 2(R),3(S) isomer). Therefore it cannot rotate plane polarized light, as it is not a chiral molecule.
The only stereoisomers in this case that can rotate plane polarized light are the 2(S),3(S) and the 2(R),3(R) isomers.

In your second example, 2-bromo-3-chlorobutane, the 2(S),3(R) isomer is *not* meso, as the molecule does *not* have a mirror plane. Actually all 4 stereoisomers can rotate plane polarized light in this case.

 

[WhatMD88]

If the molecule has a mirror plane it will not be chiral and thus will not have optical activity.

In your first example, 2,3-dichlorobutane, the 2(S),3(R) isomer is meso (as is the 2(R),3(S) isomer). Therefore it cannot rotate plane polarized light, as it is not a chiral molecule.
The only stereoisomers in this case that can rotate plane polarized light are the 2(S),3(S) and the 2(R),3(R) isomers.

In your second example, 2-bromo-3-chlorobutane, the 2(S),3(R) isomer is *not* meso, as the molecule does *not* have a mirror plane. Actually all 4 stereoisomers can rotate plane polarized light in this case.

I was going through the EK orgo and one of the questions stumped me. I know that racemic mixture of enantiomers and mesocompounds don't rotate polarized light. But what about racemic mixtures of DIASTEROMERS? (ie I'm referring to questions #32 and 33 in the 30min lecture 2 exam)

 

[GreenRabbit]

I was going through the EK orgo and one of the questions stumped me. I know that racemic mixture of enantiomers and mesocompounds don't rotate polarized light. But what about racemic mixtures of DIASTEROMERS? (ie I'm referring to questions #32 and 33 in the 30min lecture 2 exam)

1. Yes, racemic mixtures of enantiomers don't rotate light.
2. There is no such thing as a racemic mixture of mesocompounds.
3. There is no such thing as a racemic mixture of diastereomers.

In diastereomers, both can rotate light, one can rotate light, or neither can rotate light. Even when both rotate light, unlike enantiomers they do not rotate it in equal magnitude in opposite directions.