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Organic Reaction Types: Substitution to Rearrangement

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Every organic reaction belongs to one of four families: substitution (one group swaps for another), addition (two reactants merge across a multiple bond), elimination (a small molecule splits off, forming a multiple bond), or rearrangement (the same atoms reconnect into a new skeleton).

Naming the family is the first step in solving any reaction problem: a double bond plus HBr means addition; an alkyl halide plus alcoholic KOH means elimination.

Key Concepts

  • Substitution: an atom or group is replaced by another. Nucleophilic substitution (SN1, SN2) replaces a leaving group with a nucleophile; electrophilic substitution replaces a hydrogen on a benzene ring with an electrophile such as NO2+ or Br+.
  • Addition: two molecules combine into one, and a pi bond is consumed. Electrophilic addition to alkenes (HBr, Br2, H2O/H+) and nucleophilic addition to carbonyls (HCN, Grignard reagents) are the two main kinds.
  • Elimination: a small molecule such as HBr, H2O, or HCl leaves the substrate and a double bond forms. E1 goes through a carbocation in two steps; E2 removes H and X together in one step.
  • Rearrangement: the molecular formula does not change, but bonds reorganize — typically a hydride or methyl shift moves a carbocation to a more stable position. Example: the neopentyl cation rearranges to the tert-pentyl cation.

Formulas

  • Nucleophilic substitution: R−X + Nu- → R−Nu + X-. Example: CH3Br + OH- → CH3OH + Br-. Atoms balance: 1 C, 3 H, 1 Br, 1 O, and charge −1 on each side.
  • Electrophilic addition: CH2=CH2 + HBr → CH3CH2Br. The pi bond breaks and two new sigma bonds (C−H and C−Br) form; the product has the combined formula C2H5Br.
  • Elimination (dehydrohalogenation): CH3CH2Br + KOH(alc) → CH2=CH2 + KBr + H2O. One molecule of HBr is formally removed.

Worked Example

2-bromobutane reacts with alcoholic KOH. Which alkene is the major product? Count the beta hydrogens.

  1. Structure: CH3−CHBr−CH2−CH3. The alpha carbon (C2) carries Br; the beta carbons are C1 and C3.
  2. C1 (CH3) has 3 hydrogens. Removing one with Br gives CH2=CH−CH2−CH3, 1-butene.
  3. C3 (CH2) has 2 hydrogens. Removing one with Br gives CH3−CH=CH−CH3, 2-butene.
  4. 2-butene is more substituted (two alkyl groups on the double bond versus one in 1-butene), so by Saytzeff's rule it is the major product. Counting 3 vs 2 beta hydrogens shows there are two competing elimination paths, and the more substituted alkene wins.

Common Misconceptions

  • Addition and substitution are the same because atoms get added. Wrong. In addition the pi bond disappears and the product is a single larger molecule; in substitution one group leaves as another enters and the carbon skeleton's unsaturation is unchanged.
  • Rearrangement changes the molecular formula. Wrong. A rearrangement only reconnects the same atoms — the formula before and after is identical, which is how you can spot one.
  • Elimination always needs a strong base. Not always. E1 elimination happens on heating with just a weak base or even the solvent, because the carbocation forms first on its own.

Real-World Applications

  • Petrol production: catalytic cracking (aided by rearrangements of carbocations) converts straight-chain alkanes into branched, high-octane isomers.
  • Alcohol manufacture: ethanol is made industrially by addition of water to ethene across the double bond.
  • Pharmaceuticals: substitution reactions install functional groups (OH, NH2, halogens) onto drug scaffolds with precise control.

Substitution swaps, addition merges, elimination splits off, rearrangement reconnects. Classify first, count atoms next, and the mechanism and major product follow.

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