Explain cyclohexanone is more reactive with
HCN than cyclopentanone.
Answers
when ketones react at the C=O carbon, it usually forms some sort of a quaternary intermediate with four substituents. that means the carbon becomes sp3 hybridised when it was initially sp2 hybridised.
as you might know, the bond angle for sp3 hybridised things is 109.5 degrees and the bond angle for sp2 hybridised stuff is 120 degrees.
now we need to look at the structure of all these cyclo-x-ones. I'm too lazy to draw a structure here, but as you might know, 6-membered rings have the least ring strain, since the carbons in the 6-membered ring can all achieve what is closest to the ideal sp3 bond angle. hence, in going from an sp2 hybridised centre to an sp3 hybridised centre, cyclohexanone becomes more stable because the conformation of that ring favours the sp3 centre.
however, for cyclopentanone and cycloheptanone, these are somewhat further away from the sp3 bond angle and actually favour the sp2 hybridisation since torsional strain increases for them when they go from being sp2 hybridised to sp3 hybridised.
also, if you are confused because you think that cyclohexanone is a flat hexagon on a piece of paper and hence its internal angles should all be 120 which is perfect for sp2, then I strongly recommend you to google about chair/boat conformations and stuff like that.