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Chapter 18: Q27P (page 952)

In the mechanism for acetal hydrolysis shown, the ring oxygen atom was protonated first, the ring was cleaved, and then the methoxy group was lost. The mechanism could also be written to show the methoxy oxygen protonating and cleaving first, followed by ring cleavage. Draw the alternative mechanism.

Short Answer

Expert verified

Oxygen of methoxy group gets protonated in the presence of acidic medium due to which oxygen acquires positive charge and we know that, positive charge on an electronegative atom is not stable, thus, methoxy group will leave as leaving group and carbocation is formed which is stable due to mesomeric effect by the neighbouring oxygen. The, water acts as a nucleophile and attacks at the carbocation.

Protonation of methoxy group

Step by step solution

01

Protonation of methoxy oxygen:

Oxygen of methoxy group gets protonated in the presence of acidic medium due to which oxygen acquires positive charge and we know that, positive charge on an electronegative atom is not stable, thus, methoxy group will leave as leaving group and carbocation is formed which is stable due to mesomeric effect by the neighbouring oxygen. The, water acts as a nucleophile and attacks at the carbocation.

Protonation of methoxy group

02

Formation of hemiacetal and ring cleavage:

In the next step, formation of hemiacetal takes place when positive charge on oxygen is neutralized. Then, oxygen of hemiacetal gets protonated and acquires positive charge, then ring cleavage or ring opening occurs in order to neutralize positive charge on oxygen. After hydrolysis, we get our required product.


Formation of hemiacetal and ring cleavage

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Most popular questions from this chapter

(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.

For each compound,

  1. Name the functional group,
  2. Show what compounds result from complete hydrolysis.

2,4鈥�Dinitrophenylhydrazineis frequently used for making derivatives of ketones and aldehydes because the products (2,4鈥�dinitrophenylhydrazones, called 2,4鈥�DNP derivatives) are even more likely than the phenlyhydrazones to be solids with sharp melting points. Propose a mechanism for the reaction of acetone with 2,4鈥�dinitrophenylhydrazinein a mildly acidic solution.

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.

Show how Wittig reactions might be used to synthesize the following compounds. In each case, start with an alkyl halide and a ketone or an aldehyde.

(a)

(b)

(c)


(d)
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