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Chapter 18: Q15P (page 942)

Propose a mechanism for each cyanohydrin synthesis just shown.

Short Answer

Expert verified

The conjugate base of hydrogen cyanide (HCN) is cyanide ion (CN-) . Cyanide ion is considered to be a strong base as well as a strong nucleophile also. Cynaide ion can attack carbonyl compounds (aldehydes and ketones) to form an additional product which is more commonly known as the cyanohydrins

Step by step solution

01

Cyanohydrins

The conjugate base of hydrogen cyanide (HCN) is cyanide ion (CN-) . Cyanide ion is considered to be a strong base as well as a strong nucleophile also. Cynaide ion can attack carbonyl compounds (aldehydes and ketones) to form an additional product which is more commonly known as the cyanohydrins

02

Mechansim for the synthesis of each cyanohydrin

The formation of cyanohydrin is a type of base-catalyzed addition to a carbonyl group where a strong nucleophile (CN-) adds in the first step to give an alkoxide which on protonation in the second step forms the cyanohydrin.

(propanol) (propanol cyanohydrin)

(butane-2-one) (butane-2-one cyanohydrin)

(di-tert-butylketone)

Ketones hindered by the presence of large alkyl groups usually react very slowly with hydrogen cyanide and give poor yields of cyanohydrins. Poor yield is due to steric effects.

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Most popular questions from this chapter

Question. (a) Simple aminoacetals hydrolyze quickly and easily in dilute acid. Propose a mechanism for hydrolysis of the following aminoacetal:

(b) The nucleosides that make up DNA have heterocyclic rings linked to deoxyribose by an aminoacetal functional group. Point out the aminoacetal linkages in deoxycytidine and deoxyadenosine.

(c) The stability of our genetic code depends on the stability of DNA. We are fortunate that the aminoacetal linkages of DNA are not easily cleaved. Show why your mechanism for part (a) does not work so well with deoxycytidine and deoxyadenosine.

Question. An unknown compound gives a molecular ion of m/z 70 in the mass spectrum. It reacts with semicarbazide hydrochloride to give a crystalline derivative, but it gives a negative Tollens test. The NMR and IR spectra follow. Propose a structure for this compound, and give peak assignments to account for the absorptions in spectra. Explain why the signal atin the IR spectrum appears at an unusual frequency.

Show how you would accomplish the following syntheses.

(a).benzene→n - butylbenzene

(b).benzonitrile→propiophenone

(c).benzene→p - methoxybenzaldehyde

(d).Ph(CH2)4-OH→tetralone

Question. The mass spectrum of unknown compound A shows a molecular ion at m/z 116 and prominent peaks at m/z 87 and m/z 101. Its UV spectrum shows no maximum above 200 nm. The IR and NMR spectra of A follow. When A is washed with dilute aqueous acid, extracted into dichloromethane , and the solvent evaporated, it gives a product B. B shows a strong carbonyl signal at 1715 cm-1in the IR spectrum and a weak maximum at 274nm(E =16) in the UV spectrum. The mass spectrum of B shows a molecular ion of m/z 72. Determine the structures of A and B, and show the fragmentation to account for the peaks at m/z 87 and 101.

Two structures of the sugar fructose are shown next. The cyclic structure predominates in aqueous solution.

(a). Number the carbon atoms in the cyclic structure. What is the fuctional group at C2 in the cyclic form?

(b). Propose a mechanism for the cyclization, assuming a trace of acid is present.

Fructose fructose

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