Topic 10 · page 1 of 27

Seven types of reaction

Every organic reaction in this topic can be labelled as addition, elimination, substitution, oxidation, reduction, hydrolysis or polymerisation. One reaction can carry two labels, and the question tells you which to give.

10 min read10.1

Seven labels 10.1

You are not asked to memorise hundreds of reactions. You are asked to say what kind of change is happening. Topic 4 gave you five labels. This topic adds two more, elimination and hydrolysis, and extends oxidation and reduction to include electrons.

Type What happens Example
Addition two molecules join to make one product; a C=C is used up CHX2=CHX2+BrX2→CHX2BrCHX2Br\ce{CH2=CH2 + Br2 -> CH2BrCH2Br}
Elimination a small molecule (H₂O or HBr) is removed from one molecule and a C=C forms CHX3CHX2Br→CHX2=CHX2+HBr\ce{CH3CH2Br -> CH2=CH2 + HBr}
Substitution an atom or group is replaced by another CHX3CHX2Br+OHX−→CHX3CHX2OH+BrX−\ce{CH3CH2Br + OH- -> CH3CH2OH + Br-}
Oxidation gain of oxygen or loss of hydrogen by an organic molecule; loss of electrons; oxidation number rises CHX3CHX2OH+[O]→CHX3CHO+HX2O\ce{CH3CH2OH + [O] -> CH3CHO + H2O}
Reduction gain of hydrogen or loss of oxygen by an organic molecule; gain of electrons; oxidation number falls CuX2++2 eX−→Cu\ce{Cu^2+ + 2e- -> Cu}
Hydrolysis a bond is broken by water or aqueous alkali CHX3CHX2Br+HX2O→CHX3CHX2OH+HBr\ce{CH3CH2Br + H2O -> CH3CH2OH + HBr}
Polymerisation many monomer molecules join into one long chain n CHX2=CHX2→(−CHX2−CHX2−)Xn\ce{n CH2=CH2 -> (-CH2-CH2-)_n}

Addition and elimination are opposites. Addition removes a C=C and joins two molecules into one. Elimination makes a C=C and gives off a small molecule.

How to decide 10.1

MethodClassifying a reaction

  1. Count the products and compare them with the reactants. One product from two reactants points to addition (or polymerisation if very many units join).
  2. If a small molecule is lost and a C=C appears, it is elimination.
  3. If one group is swapped for another and a different small species leaves, it is substitution.
  4. Look for oxygen or hydrogen being gained or lost, or for electrons. Gain of O or loss of H, or loss of electrons, is oxidation. The reverse is reduction.
  5. Ask whether water (or aqueous alkali) breaks a bond. If it does, add hydrolysis.

Worked exampleClassify CH₃CH₂CH₂Br + KOH(aq) → CH₃CH₂CH₂OH + KBr

  1. There are two products, so it is not addition.
  2. No C=C forms, so it is not elimination.
  3. The Br is replaced by OH, so it is substitution.
  4. The reagent is aqueous alkali and the C–Br bond is broken by it, so it is also hydrolysis.
  5. Answer: substitution and hydrolysis.

The "Unit 1" reactions you have already met also fit these labels:

Check yourself

Classify each reaction as addition, elimination, substitution, oxidation, reduction, hydrolysis or polymerisation. Name all that apply.

  1. CHX3CH(OH)CHX3→CHX3CH=CHX2+HX2O\ce{CH3CH(OH)CH3 -> CH3CH=CH2 + H2O}
  2. CHX3CHX2CHX2OH+[O]→CHX3CHX2CHO+HX2O\ce{CH3CH2CH2OH + [O] -> CH3CH2CHO + H2O}
  3. CHX3CH=CHX2+HX2→CHX3CHX2CHX3\ce{CH3CH=CH2 + H2 -> CH3CH2CH3}
  4. CHX3CHX2Br+KCN→CHX3CHX2CN+KBr\ce{CH3CH2Br + KCN -> CH3CH2CN + KBr}
  5. Mg(s)+CuX2+(aq)→MgX2+(aq)+Cu(s)\ce{Mg(s) + Cu^2+(aq) -> Mg^2+(aq) + Cu(s)}
  6. Chloroethene molecules join to make poly(chloroethene). State two labels.

Answers cover these until you have tried the questions

  1. Elimination. A small molecule, water, is removed and a C=C forms.
  2. Oxidation. The alcohol loses hydrogen (the aldehyde has two fewer H atoms), and [O] is the oxidising agent.
  3. Addition and reduction. Two molecules make one product, and the organic molecule gains hydrogen.
  4. Substitution. The Br is replaced by CN. (It is not hydrolysis: no water is involved.)
  5. Oxidation (Mg loses electrons) and reduction (Cu²⁺ gains electrons): a redox reaction.
  6. Polymerisation and addition (the monomers join by opening the C=C, with no other product).