Why is pyrrole such a weak base

In general, a nitrogen atom with three bonds and a lone pair of electrons can potentially act as a proton-acceptor a base - but basicity is reduced if the lone pair electrons are stabilized somehow. We already know that amines are basic, and that the pKa for a protonated amine is in the neighborhood of We also know that, due to resonance with the carbonyl bond, amide nitrogens are not basic in fact they are very slightly acidic, with a pKa around Next, let's consider the basicity of some other nitrogen-containing functional groups.

Aniline, the amine analog of phenol, is substantially less basic than an amine. We can use the same reasoning that we used when comparing the acidity of a phenol to that of an alcohol. In aniline, the lone pair on the nitrogen atom is stabilized by resonance with the aromatic p system, making it less available for bonding and thus less basic.

With anilines just as with phenols, the resonance effect of the aromatic ring can be accentuated by the addition of an electron-withdrawing group, and diminished by the addition of an electron-donating group. Which of the two compounds below is expected to be more basic? Use resonance drawings to explain your reasoning. Recall that an imine functional group is characterized by an sp2-hybridized nitrogen double-bonded to a carbon.

The lower basicity of imines compared to amines can be explained in the following way:. Recall from section 2. In the aromatic ring of pyrrole, the nitrogen lone pair electrons are part of the aromatic sextet, and are therefore much less available for forming a new bonding to a proton.

Below is a summary of the five common bonding arrangements for nitrogen and their relative basicity:. The strong acids are hydrochloric acid , nitric acid , sulfuric acid , hydrobromic acid , hydroiodic acid , perchloric acid , and chloric acid. The only weak acid formed by the reaction between hydrogen and a halogen is hydrofluoric acid HF.

Salts of weak acids or bases can affect the acidity or basicity of their aqueous solutions. There are very few strong bases see Table A good base is usually a good nucleophile. So, strong bases — substances with negatively charged O, N, and C atoms — are strong nucleophiles. Some strong bases are poor nucleophiles because of steric hindrance.

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