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Chapter 19: Q11P (page 997)

Propose a mechanism for nitration of pyridine at the 4-position, and show why this orientation is not observed.

Short Answer

Expert verified

This orientation is not observed because electrophilic substitution reaction at 4-position is not stable. The intermediate formed by electrophilic addition at 4-position is less stable than that of 3-position.

Step by step solution

01

Introduction

In above mechanism, addition of an electrophile to pyridine in the 4 position results in the formation of three resonance structures of the intermediate form.

02

Justification

Electronic substitution at 4th position is not stable because 4-position carbon is not the most electron rich C atom in the pyridine ring. Whereas 3-position carbon is the most electron-rich C-atom, therefore pyridine undergoes electrophilic substitution at 3 position rather than 4 position.

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Most popular questions from this chapter

Show how you would use the same sulfonyl chloride as used in the sulfanilamide synthesis to make sulfathiazole and sulfapyridine.

Question. (a) Propose a mechanism for the reaction of 2-bromopyridine with sodium amide to give 2-aminopyridine.

(b) When 3-bromopyridine is used in this reaction, stronger reaction conditions are required and a mixture of 3-aminopyridine and 4-aminopyridine results. Propose a mechanism to explain this curious result.

Which of the amines listed next is resolved into enantiomers? In each case, explain why interconversion of the enantiomers does or does not take place.

(a)Cis-2-methylcyclohexanamine

(b) N-ethyl-N-methylcyclohexanamine

(c) N-methylaziridine

(d) Ethylmethylanilinium iodide

(e) Methylethylpropylisopropylammonium iodide

Macrolide antibiotics all have large rings (macrocycle) in which an ester makes the ring; a cyclic ester is termed a lactone. One example is erythromycin A, first isolated from soil bacteria in the 1950鈥檚. Over time, some pathogenic bacteria have developed resistance to erythromycin by evolving an enzymatic mechanism to cleave the macrocycle at the ketone. To counter this resistance, chemists modified the erythromycin structure to replace the ketone with an amine that the bacteria could not detoxify. This modified antibiotic, azithromycin, trade name Zithromax庐, is one of the most prescribed drugs in the world for respiratory infections.

(a) Identify the lactone group in each structure that merits the classification as macrolides.

(b) Two groups are circled. What type of functional group are they? Explain

(c) Identify the ketone in erythromycin targeted by bacteria as the site for detoxification.

(d) Identify the amine in azithromycin. What type of amine is it?

(e) From what you know about the reactivity of ketones and amines, why was an amine a good choice to be the 鈥渃hemical opposite of a ketone鈥�?

Question. Which of the amines listed next is resolved into enantiomers? In each case, explain why interconversion of the enantiomers does or does not take place.

  1. Cis-2-methylcyclohexanamine
  2. N-ethyl-N-methylcyclohexanamine
  3. N-methylaziridine
  4. Ethylmethylanilinium iodide
  5. Methylethylpropylisopropylammonium iodide
See all solutions

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