(3)Mandelonitrile. Mandelonitrile (spectra shown below) is made from benzaldehyd
ID: 1083234 • Letter: #
Question
(3)Mandelonitrile. Mandelonitrile (spectra shown below) is made from benzaldehyde and hydrogen cyanide in the presence of a catalyst. catalyst H + HCNMandelonitrile 100 80 60 m/z = 1 33 40 20 0.0 40 120 160 80 m/z QE-300 0.0 CDCI 12 10 (a) Propose a structure for mandelonitrile and justify your structure on the basis of the (b) Will the reaction be catalyzed by H,Of, OH", both, or neither? Write mechanisms for the (c) This reaction works well if a small amount of catalyst is used. Unusually, if spectroscopic data. possible reactions, and explain why the impossible reactions don't work. stoichiometric amounts of the catalyst are used, the reaction fails. Explain why.Explanation / Answer
(a) The compound will be 2-hydroxy-2-phenylacetonitrile. From the 1H NMR spectra (based on the integration hight) around 4 ppm one broad peak for one proton which is attached with oxygen. Around 5.5 ppm one singlate sharp peak blongs to one proton attached with alifatic carbon. Around 7.5 ppm ane multiplate for five proton belongs to all arometic protons. In 13C around 62-65 ppm range the peak belongs to alifatic carbon. Around 120ppm the peak belongs to cyanide carbon. Rest of the peaks in arometic region belongs to arometic region. Out of them last one(around 130 ppm) belongs to quarternary carbon in benzene ring. All of these data again can be confermed by mass as well as IR data.
(b) This reaction is catalysed by catalytic amount of acid.
Here HCN is a weak acid so in the reaction mixture the amount of cyanide ion(which one is the neucleophile) will be very less. Again the catalytic amount acid coordinate with carbonyl oxygen and increase the electrophilicity of the carbonyl carbon which further increase the cyanide attack as well as the rate of the reaction. But nither basic nor nutral solution will be able to increase the carbonyl reactivity. Hence reaction will not proceed.
(c) As HCN is a weak acid so in the reaction mixture the amount of cyanide ion(which one is the neucleophile) will be very less. Now in catalytic amount acid condition , after addition of cyanide the negative charge on the oxygen abstract a proton from another HCN and release a cyanide ion which attack another benzaldehyde. But in presence of stoichiometric amount of catalyst no oxygen ion will be free to abstract the proton from HCN as well as HCN being a weak acid it's dissociation will reduce rapidly which will oppose the reaction.
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.