Question

Rank the following compounds from easiest to hardest at removing a proton from its methyl substituent:

Short answer

Pyridine is an aromatic compound containing nitrogen in the ring. Due to the presence of this nitrogen, which exerts negative inductive effect, it facilitates the easy nucleophilic attack on the ring.The electron-withdrawing nitrogen and the ability to delocalize the negative charge it causes the hydrogens on carbons to get attached at the 2- and 4-positions of the pyridine ring.

Pyridine is less reactive as compared to benzene in electrophilic aromatic substitution reaction, but in nucleophilic substitution reaction pyridine shows greater extent of reactivity as compared to the benzene.

Step-by-step solution

Step-by-Step Solution  Step 1: Pyridine ring

Pyridine is an aromatic compound containing nitrogen in the ring. Due to the presence of this nitrogen, which exerts negative inductive effect, it facilitates the easy nucleophilic attack on the ring.The electron-withdrawing nitrogen and the ability to delocalize the negative charge it causes the hydrogens on carbons to get attached at the 2- and 4-positions of the pyridine ring.

Pyridine is less reactive as compared to benzene in electrophilic aromatic substitution reaction, but in nucleophilic substitution reaction pyridine shows greater extent of reactivity as compared to the benzene.

Ranking of given compounds

Order of the given compounds from easiest to hardest at removing a proton from its methyl substituent is shown below:

As we know, the positively charged nitrogen accepts the delocalized electrons more rapidly as compared to the neutral nitrogen. So, it is easiest to remove a proton from the methyl group of 1-ethyl-4-methylpyridin-1-ium, because in this compound electrons are left behind when the proton is removed from its methyl group, which can be delocalized on to the positively charged nitrogen atom.

It is easier to remove a proton from 4-methylpyridine than 3-methylpyridine. Because in 4-methyl pyridine, the left behind electrons after removal of protoncan be delocalized on electronegative nitrogen atom. Whereas, in 3-methylpyridine the electrons can be delocalized only on carbons not on a nitrogen atom.