alevels2k17
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which of the following reacts with hydrogen cyanide to make a racemic mixture
A methanal
B ethanal
C propanone
D pentan-3-one

isnt the condition to form optical isomers that the chiral center has FOUR different groups attached to it????????? in planar molecules only three different groups are attached to it so that means they cant form optical isomers right?? what is this question
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charco
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(Original post by pondsteps)
which of the following reacts with hydrogen cyanide to make a racemic mixture
A methanal
B ethanal
C propanone
D pentan-3-one

isnt the condition to form optical isomers that the chiral center has FOUR different groups attached to it????????? in planar molecules only three different groups are attached to it so that means they cant form optical isomers right?? what is this question
The optical isomers are formed by the reaction. The idea here is that the planar molecule can be attacked from either side of the plane, but only one of them makes an optically active molecule, i.e. one with four different substituents on the carbon atom.
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alevels2k17
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so the molecule must be planar in order to be attacked and hence form an optically active isomer??? if the intermediate was tetrahedral wont it form an optical isomer? (eg.the intermediate in nucleophilic addition of CN to aldehyde)
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on_martin
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It would be ethanal.

The CN-H bond breaks and the CN group attacks the carbonyl carbon. At the same time, the C=O bond donates electrons to the Oxygen forming an O- which bond with the H+ that was originally part of the HCN molecule.

You'll be left with the optically active isomer 2hydroxypropanenitirle as the original carbonyl carbon now has a CN group, a CH3 group, and OH group and a H.
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Pigster
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(Original post by pondsteps)
so the molecule must be planar in order to be attacked and hence form an optically active isomer??? if the intermediate was tetrahedral wont it form an optical isomer? (eg.the intermediate in nucleophilic addition of CN to aldehyde)
I guess the unstated comparison is to SN2 in which you get inversion of stereochemistry (for example see here: https://www.youtube.com/watch?v=3mdiyiDPiPI)

For this reaction - addition of CN- to a carbonyl, the nucleophile can attack from the top or bottom of the planar molecule forming a tetrahedral intermediate with equal chances of forming, therefore forming a racemate.
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alevels2k17
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(Original post by Pigster)
I guess the unstated comparison is to SN2 in which you get inversion of stereochemistry (for example see here: https://www.youtube.com/watch?v=3mdiyiDPiPI)

For this reaction - addition of CN- to a carbonyl, the nucleophile can attack from the top or bottom of the planar molecule forming a tetrahedral intermediate with equal chances of forming, therefore forming a racemate.
thanks a lot
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