Q.In which case is a chiral carbon NOT generated by reaction with HCN? a) pentan-3-one, CH3CH2-CO-CH2CH3 (a symmetric ketone with two identical ethyl groups on the carbonyl carbon) b) 3-hydroxypentan-2-one, CH3CH2-CH(OH)-CO-CH3 (an unsymmetric hydroxy-ketone) c) 4-phenylbutan-2-one, CH3-CO-CH2-CH2-C6H5 (an unsymmetric aryl ketone) d) 4-hydroxy-1-phenylbutan-1-one, C6H5-CO-CH2-CH2-CH2-OH (an unsymmetric aryl hydroxy-ketone)

Step 1. HCN addition converts a ketone's planar sp2 carbonyl carbon into a tetrahedral sp3 carbon bearing four groups: -OH, -CN, and the ketone's original two flanking groups.

Step 2. A NEW CHIRAL CENTRE is generated only if those four groups are all DIFFERENT from one another.

Step 3. In pentan-3-one, CH3CH2-CO-CH2CH3, the two groups flanking the carbonyl are BOTH ethyl -- identical -- so after HCN addition the carbon bears -OH, -CN, ethyl, ethyl: only THREE distinct groups, not four, so no chiral centre forms. …

For each option, check whether the two groups flanking the carbonyl carbon are identical (no new chiral centre possible after HCN addition, since two of the four post-addition …

• Assuming every ketone gives a chiral centre with HCN 'because HCN addition always creates a new stereocentre' -- that is only true when the ketone itself is UNSYMMETRICAL; a sym …