Substitution versus Addition: MDCAT Chemistry notes
Substitution versus Addition for MDCAT: why alkanes substitute, alkenes and alkynes add, benzene substitutes, and the reactivity order of hydrocarbons.
Two ways a hydrocarbon can react
In a substitution reaction one atom or group is replaced by another, and the number of groups on the carbon stays the same. In an addition reaction two atoms or groups are added across a multiple bond, which is then reduced (a double bond becomes single, a triple bond becomes double and then single). Which one a hydrocarbon undergoes depends on whether it has $\pi$ electrons available.
Alkanes: substitution only
Alkanes are saturated: each carbon is sp$^3$ and uses four strong $\sigma$ bonds. There is no $\pi$ bond, so nothing can add. Alkanes are therefore fairly unreactive ("paraffins", little affinity). Under suitable conditions they undergo:
- Free radical substitution, e.g. halogenation in diffused sunlight: $\mathrm{CH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + HCl}$, continuing to $\mathrm{CH_2Cl_2}$, $\mathrm{CHCl_3}$ and $\mathrm{CCl_4}$.
- Combustion to $\mathrm{CO_2}$ and $\mathrm{H_2O}$, and controlled oxidation.
- Cracking (pyrolysis) at high temperature.
Alkanes cannot undergo addition. Hydrogenation is itself an addition of $\mathrm{H_2}$ to a multiple bond, so an alkane cannot be hydrogenated either; hydrogenation is the way alkanes are made from alkenes and alkynes.
Alkenes and alkynes: electrophilic addition
The $\pi$ bond of an alkene lies above and below the plane of the molecule. Its electrons are loosely held and exposed, so they attract electrophiles. Typical additions are $\mathrm{H_2}$ (Ni catalyst), halogens, hydrogen halides, water (acid catalysed) and cold concentrated $\mathrm{H_2SO_4}$. An alkyne has two $\pi$ bonds and can add two molecules of reagent, e.g. ethyne + 2 $\mathrm{Br_2}$ gives 1,1,2,2-tetrabromoethane.
Why alkenes are more reactive than alkynes
Alkynes have more $\pi$ electrons, yet they are less reactive than alkenes towards electrophilic addition. The sp carbons are more electronegative and hold the $\pi$ electrons more tightly, and the carbocation formed from an alkyne (a vinyl cation) is less stable than the one formed from an alkene. Hence the textbook order:
$$\text{Alkenes} > \text{Alkynes} > \text{Alkanes}$$
Benzene: unsaturated but prefers substitution
Benzene is unsaturated, but its six $\pi$ electrons are delocalized over the ring. Addition would destroy this stable aromatic system, so benzene undergoes electrophilic substitution (nitration, halogenation, sulphonation, Friedel–Crafts) and adds only under drastic conditions.
| Class | Bonding | Main reaction type | Example |
|---|---|---|---|
| Alkane | Only $\sigma$ bonds, sp$^3$ | Free radical substitution | Chlorination of methane |
| Alkene | One $\pi$ bond, sp$^2$ | Electrophilic addition | Ethene + $\mathrm{Br_2}$ → 1,2-dibromoethane |
| Alkyne | Two $\pi$ bonds, sp | Electrophilic addition (two steps) | Ethyne + 2$\mathrm{H_2}$ → ethane |
| Benzene | Delocalized $\pi$ system | Electrophilic substitution | Nitration to nitrobenzene |
Common MDCAT traps
- The order is alkene > alkyne > alkane, not alkyne > alkene. More $\pi$ bonds does not mean more reactive.
- Read the direction of the signs: "Alkene < Alkyne < Alkane" is the reverse of the correct order.
- Alkanes can burn and substitute; the reaction they cannot undergo is addition.
- Benzene is unsaturated but reacts mainly by substitution, not addition.
Quick revision
- Saturated compounds substitute; unsaturated compounds add.
- Halogenation of alkanes is a free radical chain reaction needing light or heat.
- Alkenes and alkynes react with electrophiles because of their exposed $\pi$ electrons.
- An alkyne can add two molecules of $\mathrm{H_2}$, $\mathrm{X_2}$ or HX.
- Reactivity towards electrophilic addition: alkenes > alkynes.