O.Chem question

[meant2be10]

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Some of the ochem answer choices will have (+/-), I understand this referes to the direction plane polarized light is rotated, however I don't know when you should chose the an answer with (+/-) over the same answer choice without it. Anyone know a simple way of explaining this?

 

[rickblas]

+ / - means enantiomers I believe so anything that makes enantiomers would be your reasoning for choosing a + / - answer aka think Sn1. Remember Sn1 gives two enantiomers usually in a racemic mixture (optically inactive) both the + and minus enantiomers. I think thats how it works? Anyone else?

 

[meant2be10]

So basically if the carbon being attacked is a stereocenter, then a (+/-) will result?

 

[LaFleur]

Only if it is a sn1 reaction tho.

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[LaFleur]

Sn2 reactions occur with inversion of configuration so they wouldn't produce enantiomers. Elimination reactions would get rid of the steteocenter so again no enantiomers. In a sn1 reaction a carbocation is produced so the nucleophile can attack from both sides, producing a anemic mixture of enantiomers that's not optically active. Since both configurations are optically active (r and s) this means that one will have a + charge and the other will have a - charge, since the overall charge has to cancel out in the anemic mixture. I hope that was clear enough.

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[LaFleur]

and by anemic I meant racemic. I hate t9

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[orgohacks]

Some of the ochem answer choices will have (+/-), I understand this referes to the direction plane polarized light is rotated, however I don't know when you should chose the an answer with (+/-) over the same answer choice without it. Anyone know a simple way of explaining this?

Just to clarify:
the prefix (+)- indicates a single enantiomer which rotates plane polarized light to the right.
the prefix (–)-indicates a single enantiomer which rotates plane polarized light to the left.
the prefix (+/–) indicates a racemic mixture.

It will come up the most during SN1/SN2 questions.
Since the SN2 is stereospecific, if you start with, say, (+)-2-bromobutane and are asked to say what the product will be in the reaction with NaOH, you would choose 2-butanol with the (+) or (-) designation in front of it, indicating a single enantiomer.

If you have an SN1 reaction which uses a pure enantiomer as starting material, your product will have the designation (+/-) indicating a racemic mixture.

If the molecule is achiral, of course, the terminology is unnecessary.

Hope this helps - James