SN1 reaction question?

[cheepcheep]

My textbook says that a SN1 reaction results in a loss of optical activity (which could or could not be due to racemization of the solution?). However, there is a higher chance of inversion than retention. If that is so, where do SN1 reactions result in a loss of optical activity (since the solution would not be a racemic mixture)?

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[cheepcheep]

Also, why does inversion in SN2 reactions not change optical activity at all? If the position in which the nucleophile in the SN1 reaction can change optical activity in that they cancel each other out (racemic mixture), how come the position in which the nucleophile bonds to the substrate in the SN2 reaction does not do anything to optical activity?


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Um ignore this haha ^ I guess I read the textbook wrong? I'm attempting to self study OChem and I think I've aged 10 years doing so.

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Actually, when it says that "Optical activity is retained in SN2 reactions," does it mean that optical activity is still DISPLAYED, or if the substrate was +, it'll still remain +?

 

[Mornhavon]

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[N1st]

In SN1 the intermediate is planar, and can be attacked from either side. This results in a roughly equal distribution of products (R/S) that cancel each other's optical activity out.

In SN2, the original reactant is optically active, then it just changes from R to S or vice versa due to the inversion, and is still optically active.

Your reaction can have a mixture of reaction types...you could have an SN1 reaction with a little bit of SN2 character so one product is favored (major) compared to another (minor), so then your solution will still be optically active.

Technically in SN2 it doesn't change from R to S (necessary). The relative configuration is what changes i.e. it's as if you switched from R to S and then replaced the leaving group with the nucleophile (which might result in R or S, depending on the priority of the nucleophile). But otherwise, yes that's right.

 

[cheepcheep]

In SN1 the intermediate is planar, and can be attacked from either side. This results in a roughly equal distribution of products (R/S) that cancel each other's optical activity out.

In SN2, the original reactant is optically active, then it just changes from R to S or vice versa due to the inversion, and is still optically active.

Your reaction can have a mixture of reaction types...you could have an SN1 reaction with a little bit of SN2 character so one product is favored (major) compared to another (minor), so then your solution will still be optically active.

Ahh I see, thank you! I just read somewhere that because the leaving group kind of blocks that side of the compound, there is a higher chance of inversion than retention in SN1 reactions. But I guess it isn't big enough to make a difference?

 

[Perpetually]

Like previous poster have mentioned, the inherent nature of the SN1 reaction is the planar, Sp2 carbocation intermediate. This intermediate structure enables the incoming nucleophile to approach from either above or below the plane of the carbocation; thus resulting in loss of optical activity in our product as a racemic mixture forms.

The only possible instance where an SN1 reaction results in an optically active product, is when the nucelophile does not have the same propensity for approaching the carbocation from both sides.

This is really rare and likely beyond the scope of the MCAT.

Just associate SN1 with an optically inactive product and you should be fine.

 

[Mornhavon]

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[Testosterone]

When the leaving group blocks, that is when you start to get an SN2 character in the reaction, causing a non-racemic product ratio.

SN1= leaving group leaves before the attack

SN2= leaving group is pushed off during attack

Better leaving group= More SN1
Worse leaving group = More SN2

Substitution reactions both require good leaving groups. What differentiates SN1 from SN2 is the amount of steric hinderance of the electrophile, the strength and amount of steric hinderance of the nucleophile and the type of solvent more so than the properties of the leaving group.