Question

Histidine has 3 nitrogens. arrange the indicated nitrogens in decreasing order of basicity (1 = most basic, 3 = least basic). if there are fewer than 3 nitrogens in the molecule, leave that answer space blank. if two nitrogens have the same basicity, give them the same number. histidine

Answer 1

Nitrogen 7 = 1 ---- Most Basic

Nitrogen 1 = 2

Nitrogen 3 = 3 ------ Least Basic

Explanation:

The basicity of amino acids is as a result f the unshared pair of electrons in the Nitrogen atoms.

Part of the rule of basicity of organic compounds(amino acids) posits that:

The more unstable an electron pair is, the more basic it is.

From the diagram attached below;

Nitrogen 7 posses and unstable electron due to the lone pair of electron in the NH₃⁺ which renders it most basic .

Nitrogen 1 and 3 are in ring form thereby they are less basic But nitrogen 1 is more basic compared to nitrogen 3 because lone pair on Nitrogen 1 is not involved in resonance unlike Nitrogen 3 that is resonance stabilized (i.e alternating by a resonance effect).

Another phenomenon to deduce the basicity of Histidine is the effect of hybridization.

Basicity of Organic Compounds increases with decreasing s character.

Therefore, the trend of basicity is as follows: sp³ > sp² > sp

Because in sp; the lone pairs are most stable, hence least basic

And in sp³; the lone pairs are least stable; hence most basic.

Therefore; From the diagram attached below:

Nitrogen 7 = 1 ---- Most Basic

Nitrogen 1 = 2

Nitrogen 3 = 3 ------ Least Basic