What a learner can do afterwards
- Draws the mechanism for hydroxide, for cyanide and for ammonia attacking a haloalkane
- Explains why the carbon carries a partial positive charge in the first place
- Ranks chloro, bromo and iodo compounds by rate and justifies the order using bond enthalpy
- Names the product family for each nucleophile and says why the cyanide route lengthens the carbon chain
1 · Read
A carbon-halogen bond is polar because the halogen pulls the shared electrons toward itself. That leaves the carbon electron poor with a partial positive charge. A nucleophile is an electron rich species that donates a pair of electrons to form a new bond, so it heads for exactly that carbon. Draw two curly arrows: one from the nucleophile's lone pair to the carbon, and one from the carbon-halogen bond onto the halogen, which leaves as a halide ion.
Chloro compounds react slowest, bromo faster, and iodo fastest. The carbon to iodine bond is the weakest and breaks most easily, so bond strength decides the rate. This surprises many learners: the carbon-chlorine bond is the most polar, yet polarity matters less here than how easily the bond breaks.
Three nucleophiles give three product families. Hydroxide replaces the halogen with an OH group and makes an alcohol in one clean swap. Cyanide bonds through its carbon to make a nitrile, which lengthens the carbon chain by one. Ammonia bonds through nitrogen, and losing a proton leaves an amine behind.
The cyanide route is prized in synthesis because it grows the chain. To name any substitution product, find which atom of the nucleophile bonds to carbon: oxygen means alcohol, carbon means nitrile, nitrogen means amine. Rate questions on haloalkanes turn on bond enthalpy, so quote the weakest bond first.
The polar bond invites the nucleophile, and the weakest bond wins the race.
Readings 'Two arrows, one step' and 'Why iodo compounds react fastest' are original to this dataset.
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