Gabriel Synthesis and Strecker

[Dochopeful13]

Can someone explain why Gabriel Synthesis is SN2 and not SN1. In addition Kaplan says that Gabriel is SN2 and produces a racemic mixture of products. I was under the impression that only SN1 has retention of products/racemix mixture? Lastly, is the Strecker synthesis formation of an I-mine SN1?
@aldol16

Thanks!

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[Jacquejam1234]

Can someone explain why Gabriel Synthesis is SN2 and not SN1. In addition Kaplan says that Gabriel is SN2 and produces a racemic mixture of products. I was under the impression that only SN1 has retention of products/racemix mixture? Lastly, is the Strecker synthesis formation of an I-mine SN1?
@aldol16

Thanks!

SN1 is racemic and inversion and SN2 is only inversion. Yes the formation of an Imime is nucleophillic substitution reaction. This sounds like a TBR question I’ve seen.

@BerkReviewTeach

Is the best one to ask though.

 

[Dochopeful13]

SN1 is racemic and inversion and SN2 is only inversion. Yes the formation of an Imime is nucleophillic substitution reaction. This sounds like a TBR question I’ve seen.

@BerkReviewTeach

Is the best one to ask though.

Thank you. @BerkReviewTeach do you have any wisdom you can share?

 

[PositivePhotoresist]

First of all, remember that Sn1 isn't technically completely racemic due to possibly because of an interfering ion effect. There's at least Sn2 step in the Gabriel Synthesis (the one that's in every Gabriel Synthesis mechanism is the one where the deprotonated phthalimide attacks the primary or secondary substrate with a good leaving group). Ok. If the stereochemistry of the substrate is relevant, it'll get inverted here. I'm assuming that you continue on past the Gabriel Synthesis by incorporating a decarboxylation step to form an amino acid. Remember that decarboxylation initially forms the enol product which then rapidly converts to the keto form. When the enol product is formed, the double-bond is on an sp2 carbon. Because of this, the hydrogen can be attached to either side of this trigonal planar carbon. This is why we get a racemic mixture. It doesn't matter what happens in the SN2 step anymore because of this. Now, watch out for other stereocenters on the molecule because you could also form diastereomers since the other sterocenters can be fixed onto a sp3 carbon.

 

[PositivePhotoresist]

And the formation of an imine through the Strecker occurs through essentially an acyl substitution.