AS Level Chemistry · An introduction to AS Level organic chemistry. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.
Organic reactions are described with two labels. The first says what happens overall: addition (two molecules join to make one), substitution (one atom or group replaces another) or elimination (a small molecule is removed, leaving a double bond).
The second label says what kind of species attacks. A free radical has an unpaired electron. A nucleophile donates a pair of electrons to an electron-poor atom. An electrophile accepts a pair of electrons from an electron-rich region.
Put the two labels together to name the mechanism, for example nucleophilic substitution. In mechanisms a curly arrow shows a pair of electrons moving. It must start at a bond or a lone pair.
Students often swap nucleophile and electrophile. A nucleophile is electron-rich and donates a pair of electrons; an electrophile is electron-poor and accepts a pair.
Answer: Electrophilic addition.
Try each one, then open the answer.
Answer: A. In homolytic fission each atom keeps one electron of the shared pair, giving two Cl• radicals. Heterolytic fission gives ions, with both electrons going to one atom.
Answer: A. In heterolytic fission the bond breaks unevenly: both electrons go to one atom (usually the more electronegative), forming ions such as a carbocation and Br−. Even splitting into radicals is homolytic fission.
Answer: A. Two molecules join together with the loss of a small molecule (water), which is a condensation reaction. The reverse, breaking the ester with water, is hydrolysis.
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