Which pair of compounds does not form a racemic mixture when the compounds react? But

Which pair of compounds does not form a racemic mixture when the compounds react?
But-2-ene with HCl
Propanal + HCN
Prop-1-ene + HCl
Propanal with HCN

Help please.

I said that a does not form optical isomers at all since no central c atom, b produces equal amounts of isomer so forms a racemic, c does not have 4 different groups, and d does not have 4 different groups either.

The answer is d but I don’t get how?
Thank you

Reply 1

TypicalNerd

Original post by miac0328

Shouldn’t D read as propanone + HCl? - you’ve listed propanal + HCl twice.

Try drawing out the structures of all the substrates (organic reactants) first and then all the products.

A gives CH3CH2CHClCH3

B gives CH3CH2CH(OH)CN

C gives CH3CHClCH3 (and some smaller amounts of CH3CH2CH2Cl)

D gives (CH3)2C(OH)CN

Yeah I agree C and D are both achiral. I missed that initially, hence the edit. Might there have been a typo with option C as that is the only explanation as to why D is the correct answer to the question? (I think it meant pent-1-ene, as propene doesn’t normally have a 1 in it)

Mechanistically, the reactions all require a planar species to be attacked by a nucleophile (Cl^- with the alkenes upon conversion to carbocations and CN^- with carbonyls upon protonation), which means they are equally likely to be attacked from either side. As such, if you have a chiral product, you can expect it to exist as a 50:50 mix of the two enantiomers (a racemic mixture).