SolutionsBasicity

Identify the basic groups in the following molecules and write the expected equilibria with water, including the structures of the corresponding conjugate acids:

Ethane and benzene: no lone pairs, not basic

These two compounds do not have any basic group per se, because there is no atom with lone electron pairs that can be donated.

Therefore, an equilibrium with water does not make sense.

In these cases there is at least one basic function, because there are heteroatoms with unshared electrons.

The same compounds with their lone pairs marked

Conjugate acids of each compound, protonated on the most basic atom

An equilibrium with water can be established, and the conjugate acid has a positive charge on a heteroatom that is more electronegative than carbon, which is not a very favourable situation. Therefore the equilibrium lies far towards the base.

Even so, we can study the relative basicity from the resonance forms of the base, when they can be written.

In the positively charged conjugate acid, resonance forms cannot be written because the heteroatom would be surrounded by 10 electrons.

The more the electrons of the starting base are delocalized by resonance, the less available they will be to water (which acts as an acid) and the weaker the base will be.

Order the above compounds by groups from highest to lowest basicity.

2-Chloroethylamine and 3-methylbutylamine, the strongest bases

The most basic will be the aliphatic amines, in which the unshared electron pair of the nitrogen cannot be delocalized by resonance because the nitrogens are bonded to sp³ carbons.

pKb = 3-5

2-Chloroethylamine is expected to be slightly less basic because of the electron-withdrawing inductive effect of Cl (electronegativity), which removes electron density from the nitrogen.

Substituted anilines and their resonance forms, with the explanation of the effect of nitro and methoxy groups on basicity

Next in basicity will be the ANILINES
(pKb = 9-14),
in which the electron pair of the nitrogen is less available than in the previous ones because of resonance with the ring. Electron-withdrawing groups in ortho or para (-M mesomeric effect) further reduce basicity. This is the case of para-nitroaniline. Electron-donating groups (+M mesomeric effect), such as the OMe group, increase basicity because they reduce the delocalization of the nitrogen electron pair.

3-Pentanol, tetrahydrofuran, phenol and 3-methyl-2-butanethiol

Alcohols and ethers are only slightly basic because the oxygen atom is very electronegative and donates its electrons less easily than nitrogen.

Phenols will be less basic than alcohols because the electron density of the oxygen can be delocalized towards the ring.

Thiols will be more basic than alcohols because sulfur is less electronegative than oxygen.

pKb = 15-17

Resonance forms of protonated ketones, carboxylic acids and amides, with the explanation of their relative basicity

The last group of bases will be those with a C=O group. In it, the unshared electrons of the oxygen lie in sp² orbitals, which have more s character and are closer to the nucleus, so they will be less available to an external acid. In any case, protonation of a C=O group by a sufficiently strong acid is very important to increase its reactivity.

pKb = 21