18. Aromaticity

Each of the following heterocycles includes one or more nitrogen atoms. Classify each nitrogen atom as strongly basic or weakly basic, according to the availability of its lone pair of electrons.

(d)

(e)

Each of the following heterocycles includes one or more nitrogen atoms. Classify each nitrogen atom as strongly basic or weakly basic, according to the availability of its lone pair of electrons.

(a)

(b)

(c)

Explain why, when the imidazole ring of histidine is protonated, the double-bonded nitrogen is the nitrogen that accepts the proton.

Why is the pK_a of the arginine side chain greater than the pK_a of the lysine side chain?

Histidine is an important catalytic residue found at the active sites of many enzymes. In many cases, histidine appears to remove protons or to transfer protons from one location to another.

(a) Show which nitrogen atom of the histidine heterocycle is basic and which is not.

(b) Use resonance forms to show why the protonated form of histidine is a particularly stable cation.

(c) Show the structure that results when histidine accepts a proton on the basic nitrogen of the heterocycle and then is deprotonated on the other heterocyclic nitrogen. Explain how histidine might function as a pipeline to transfer protons between sites within an enzyme and its substrate.

Ciprofloxacin is a member of the fluoroquinolone class of antibiotics.

(b) Which nitrogen atoms are basic?

Ciprofloxacin is a member of the fluoroquinolone class of antibiotics.

a. Which of its rings are aromatic?

Some of the following compounds show aromatic properties, and others do not.

1. Predict which ones are likely to be aromatic, and explain why they are aromatic.

2. Predict which nitrogen atoms are more basic than water and which are less basic.

(d)

Some of the following compounds show aromatic properties, and others do not.

1. Predict which ones are likely to be aromatic, and explain why they are aromatic.

2. Predict which nitrogen atoms are more basic than water and which are less basic.

(e)

Some of the following compounds show aromatic properties, and others do not.

1. Predict which ones are likely to be aromatic, and explain why they are aromatic.

2. Predict which nitrogen atoms are more basic than water and which are less basic.

(f)

Some of the following compounds show aromatic properties, and others do not.

1. Predict which ones are likely to be aromatic, and explain why they are aromatic.

2. Predict which nitrogen atoms are more basic than water and which are less basic.

(m)