'A' Level Chemistry Problem Analysis: Organic Reactions (Electrophiles)

QN: Which of the following are species of electrophiles?

Thought process:

Electrophiles are formal positive or partial positive charged species that invite attack by nucleophiles (species that have at least one available lone pair for donation). As for diatomic molecules like Br2, there can be instantaneous or induced dipoles that cause a shift in electron density between the atoms, resulting in a delta-positive or delta-negative, the delta-positive functioning as an electrophile to be attacked by nucleophiles such as the pi-bond of an alkene.

Note that electrophiles must have energetically accessible orbitals for attack by nucleophiles. For instance, NH4+ is not an electrophile because being in Period 2, nitrogen does not have empty d orbitals to expand its octet, thus it cannot be attacked by nucleophiles, even if it has a positive formal charge on the nitrogen.

That being said, there can be instances where resonance will result in energetically accessible orbitals being freed up in an electrophile to enable nucleophilic attack. For instance, draw the Kekule structure of NO2+. Initially, the main resonance contributor (ie. the main Kekule structure) is O=N+=O, two double bonded oxygens with a central positive formal charged nitrogen. Although N is in Period 2 and cannot expand its octet, when the nucleophile (eg. pi-electrons of benzene ring) attacks the N, one of the pi-bonds between N and O will shift over to become a lone pair on O, resulting in a negative formal charge on that O. The final NO2 attached to the benzene ring (for instance, if this was an electrophilic aromatic substitution reaction), has 2 formal charges which cancel out : a +ve formal charge on N (because it only has 4 valence electrons from 4 bond pairs, but it is in group V; but note that it certainly has a stable octet), and a -ve formal charge on one of the Os.

Br2, NO2+, (CH3)C+ are hence functional electrophiles as explained above. AlCl3 functions as an Lewis acid by accepting an electron pair from a nucleophile (since the Al does not yet have a stable octet in AlCl3, and is willing to accept an electron pair from a Cl2 molecule, forming -AlCl4 and a Cl+ electrophile). The Kekule structure of HNO3 has a negative formal charge on the O, and a positive formal charge on the N. However, HNO3 or nitric acid, functions as an acid (proton donor) or a base (proton acceptor, in the case of reacting with sulfuric acid in the nitrating mixture), an also as a strong oxidizing agent (eg. with Bart Simpson adding ethanol to nitric acid), but not so much as either a nucleophile or an electrophile (note that it has both a positive and a negative formal charge side by side on the N and O, which repels incoming electrophiles and nucleophiles respectively. Moreover, if HNO3 were to behave as an electrophile, and a nucleophile successfully attacks the positive formal charged N, to avoid violating the octet rule, one of the pi-bonds with oxygen will have to become a lone pair on the oxygen, resulting in a species with too many negative formal charges to be stable; bearing in mind that being a strong acid, the acidic proton would also dissociate resulting in yet another negative formal charge for yet another O in the species, which is too unstable).

As for Na+, it has neither the inclination nor tendency to accept electron pairs - Na desperately wants to be oxidized to Na+ (energetically stable octet), which will resist efforts to be reduced back to Na.

A carbocation has 3 bond pairs and 0 lone pairs, it certainly functions well as an electrophile. And the 3 methyl groups are electron donating by induction, stabilizing somewhat the carbocation allowing it to exist long enough as an electrophile species.


The above content is contributed by Mr Heng, owner and 'A' Level Chemistry tutor at Bedok Funland JC. He also goes by the handle UltimaOnline on various online popular homework forums.


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