Electron-pair sharing reactions

Chemistry · Reactivity 3: What are the mechanisms of chemical change?. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A nucleophile is electron-rich and donates a pair of electrons to form a new bond. An electrophile is electron-deficient and accepts a pair. In heterolytic fission a bond breaks with both electrons going to one atom, so ions form, not radicals.

In a halogenoalkane the halogen is more electronegative than carbon, so the carbon is partially positive. A nucleophile attacks this carbon and the halogen leaves as a halide ion. This is nucleophilic substitution.

The C=C bond of an alkene has high electron density, so it attracts electrophiles. In electrophilic addition the double bond becomes a single bond and two new atoms or groups join, one on each carbon.

Rules to remember

Common mistake

Students mix up the two words. A nucleophile is attracted to a positive centre and gives a pair of electrons; an electrophile is attracted to electrons and accepts a pair. Halogenoalkanes are attacked by nucleophiles, alkenes by electrophiles.

Worked example

Propene, CH3CH=CH2, is shaken with bromine. Name the type of reaction, the electrophile and the product.

  1. The C=C bond is electron-rich, so it attracts electrophiles.
  2. The double bond pushes the electrons in the nearby Br–Br bond away, so the nearer bromine atom becomes Brδ+ and acts as the electrophile.
  3. The Br–Br bond breaks heterolytically and one Br atom joins each carbon of the double bond.
  4. Product: CH3CHBrCH2Br, with no double bond left.

Answer: Electrophilic addition; the product is 1,2-dibromopropane.

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