Question: Sn2 Inversion

[0938h4gt983]

I know there is typically a inversion of stereochemistry during an Sn2, ie R to S or S to R, but does an Sn2 reaction also change the direction polarized light is rotated?

2006 TPR book says, - in an Sn2 rxn. "If the original substrate is chiral, rotating the plane of polarized light to the right or left, the substituted product is also chiral, but it rotates the plane of polarized light in the opposite direction. That is, Sn2 substitution is accompanies by a complete inversion of configuration.

A few pages on a practice question they state that reacting a halide with an OH in an Sn2 mechanism, you cannot assume that if the light rotated to the right before that it will rotate left afterwards because the substituents are now different.

This contradicts the previous statement?

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[mitchlucker]

I don't know if this answers the question, but I know that R to S/S to R don't always occur. It only happens when the incoming group is of a higher priority (when naming R and S)than the leaving group. If not, it will stay the original configuration (ie- R to R).

 

[0938h4gt983]

Thanks for the response, but that wasn't the answer I am looking for.

I understand that sometimes the confirmation is kept constant which is why I used the term "typically".

What I am wondering is, if the inversion occurs, the light is rotated in the opposite direction even though the substituent is different. ex. a Cl to and OH. In other words, can you assume that a molecule which rotates light to the left before an Sn2 will rotate it to the right after?

 

[orgohacks]

I know there is typically a inversion of stereochemistry during an Sn2, ie R to S or S to R, but does an Sn2 reaction also change the direction polarized light is rotated?

2006 TPR book says, - in an Sn2 rxn. "If the original substrate is chiral, rotating the plane of polarized light to the right or left, the substituted product is also chiral, but it rotates the plane of polarized light in the opposite direction. That is, Sn2 substitution is accompanies by a complete inversion of configuration.

A few pages on a practice question they state that reacting a halide with an OH in an Sn2 mechanism, you cannot assume that if the light rotated to the right before that it will rotate left afterwards because the substituents are now different.

This contradicts the previous statement?

The last paragraph is correct. You cannot assume that if light rotated to the right before that it will rotate left afterwards because the substituents are now different.

You cannot predict direction of optical activity by looking at the structure.

Like mitchlucker said, you also can't assume that R will go to S and vice versa. It will depend on the identities of the nucleophile and leaving group; you'll have to examine the product afresh. hope this helps - James

 

[0938h4gt983]

Thank you sir James.