Reaction product chirality AAMC FL #41

[medschoolworries]

Hi,

One of the questions on the full length asks: What factor explains how a single stereoisomer is formed in Reaction 2?

The answer is that the enzyme is chiral and the reactants are not chiral. Does that mean if any of the reactants are chiral (including catalysts) then the product is chiral? What if I had two chiral reactants, what does chiral + chiral = ?

Thank you!

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[BerkReviewTeach]

This is a targeted question focusing on a fact about enzymes. Uncatalyzed reactions with achiral reactants generate either (a) an achiral product (or products) or (b) a racemic mixture that results in no optical activity. However, an enzyme acts as a chiral catalyst that bonds the reactant (substrate) in such a way where the reaction can only occur on one face (side) of the reactant, resulting in exclusively one stereoisomer at that center.

You cannot generalize this being anything chiral in the reactant mixture. For instance, an SN1 reaction can take a chiral electrophile (reactant) and generate a racemic mixture. The combination of chiral + chiral is reaction dependent. If the reactive carbon of the electrophile is a stereocenter, then there are many options from inversion, to racemization, to retention of configuration. You need consider it case by case. Really good examples of the enzyme concept are in the BR organic book 1 (Question #17 of phase 1 and Questions #27 and #38 of phase 2). This concept is covered in detail in the BR organic book if you happen to have it. It's touched on briefly in the general chemistry book.

 

[medschoolworries]

That's really helpful thank you! I only have the old BR organic books that someone gave to me so I don't think the questions numbers quite match up unfortunately.

 

[premedprincess26]

This is a targeted question focusing on a fact about enzymes. Uncatalyzed reactions with achiral reactants generate either (a) an achiral product (or products) or (b) a racemic mixture that results in no optical activity. However, an enzyme acts as a chiral catalyst that bonds the reactant (substrate) in such a way where the reaction can only occur on one face (side) of the reactant, resulting in exclusively one stereoisomer at that center.

You cannot generalize this being anything chiral in the reactant mixture. For instance, an SN1 reaction can take a chiral electrophile (reactant) and generate a racemic mixture. The combination of chiral + chiral is reaction dependent. If the reactive carbon of the electrophile is a stereocenter, then there are many options from inversion, to racemization, to retention of configuration. You need consider it case by case. Really good examples of the enzyme concept are in the BR organic book 1 (Question #17 of phase 1 and Questions #27 and #38 of phase 2). This concept is covered in detail in the BR organic book if you happen to have it. It's touched on briefly in the general chemistry book.

so all enzymes will produce only one stereoisomer?

 

[aldol16]

so all enzymes will produce only one stereoisomer?

Not necessarily in all cases but in a lot of cases, yes. This is because nature has evolved to use stereospecific amino acids and carbohydrates so the enzymes built around this infrastructure must also recognize those specific ones to the exclusion of the opposite stereoisomer(s).

 

[premedprincess26]

Not necessarily in all cases but in a lot of cases, yes. This is because nature has evolved to use stereospecific amino acids and carbohydrates so the enzymes built around this infrastructure must also recognize those specific ones to the exclusion of the opposite stereoisomer(s).

Ok, thank you!