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Introduction to Organic Chemistry: Question 8

Syllabus 13.2

Multiple choice AS 1 mark

Reaction mechanisms in organic chemistry are often described as electrophilic, nucleophilic or free-radical, depending on the species that attacks the substrate.

Which statement correctly defines the terms nucleophile and electrophile?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: What a nucleophile does

A nucleophile (“nucleus-loving”) is attracted to a region of positive or partial positive charge. It possesses at least one lone pair of electrons, which it donates to form a new covalent bond with an electron-deficient atom. A nucleophile can be a negative ion (for example a hydroxide ion, OH−\text{OH}^-) or an electrically neutral molecule with a lone pair (for example ammonia, NH3\text{NH}_3, or water, H2O\text{H}_2\text{O}).

Step 2: What an electrophile does

An electrophile (“electron-loving”) is attracted to a region of negative charge or high electron density, such as a C=C double bond or a lone pair. It is electron-deficient (it has either a full or partial positive charge, or simply an empty orbital available) and it accepts a pair of electrons donated by a nucleophile to form a new covalent bond. An electrophile can be a positive ion (for example H+\text{H}^+) or a neutral molecule that becomes polarised so that one atom is short of electron density.

Step 3: Match this to option A

Option A states exactly this pairing: nucleophile = electron-pair donor, electrophile = electron-pair acceptor. This is the correct definition.

Why the other options are wrong

  • B swaps the two roles: it describes the nucleophile as accepting electrons and the electrophile as donating them, which is the reverse of the correct definitions.
  • C is too restrictive: many important nucleophiles are electrically neutral molecules with a lone pair (such as NH3\text{NH}_3 or H2O\text{H}_2\text{O}), not just negative ions; likewise, not every electrophile need be a full positive ion. It only needs to be electron-deficient at the reacting atom.
  • D describes a free radical (a neutral species with a single unpaired electron, produced by homolytic fission), not a nucleophile or electrophile. Nucleophiles and electrophiles react by donating or accepting a full pair of electrons, not by pairing up single unpaired electrons.

Final answer

A, a nucleophile donates a lone pair of electrons to form a new bond; an electrophile accepts a pair of electrons to form a new bond.