How to Read an Organic Reaction: Bonds Broken and Bonds Formed

Almost every classification question in organic chemistry can be started with two counts: how many species go in, and how many come out.

Start from the species count

Two reactants that collapse into one product describe an addition. One reactant that splits into two products describes an elimination. Two reactants that trade groups and give two products describe a substitution. One reactant that gives one product with the same formula but a different arrangement describes a rearrangement.

The species count alone settles most textbook cases. It is also the only step that requires nothing more than the balanced structures in front of you.

Then follow the sigma and pi bonds

The species count is the first pass; the second pass follows the bonds on the carbon skeleton.

• Addition consumes a $\pi$ bond and creates two $\sigma$ bonds. Propene reacting with $\mathrm{Br_2}$ gives 1,2-dibromopropane: the C=C $\pi$ bond is gone and two C–Br $\sigma$ bonds take its place.
• Elimination does the reverse. It removes two $\sigma$ bonds from adjacent atoms and creates a $\pi$ bond. Cyclohexanol treated with acid loses water and forms the C=C bond of cyclohexene.
• Substitution keeps the bond count the same and swaps one group for another. Hydrolysis of methyl propanoate replaces the $\mathrm{OCH_3}$ group with $\mathrm{OH}$; the carbonyl carbon still carries four bonds before and after.
• Rearrangement keeps both the species count and the bond count and changes the connectivity. The keto–enol interconversion of a simple ketone turns one constitutional isomer into another without changing the formula.

Work one unfamiliar example

Take the acid-catalysed hydration of an alkene, $\mathrm{CH_3CH{=}CH_2 + H_2O \rightarrow CH_3CH(OH)CH_3}$. Two species give one product and the alkene $\pi$ bond has been consumed, so the label is addition; the specific variant is hydration. Nothing in that labelling step needed the mechanism, only the two structures.

Now read the same equation backwards, $\mathrm{CH_3CH(OH)CH_3 \rightarrow CH_3CH{=}CH_2 + H_2O}$. One species gives two, and a $\pi$ bond appears: the label is elimination. Addition and elimination are the same process read in opposite directions, which is why they are taught as a pair.

Keep the two classification systems apart

Structural labels — addition, elimination, substitution, rearrangement — answer the question "what changed". Mechanistic labels — polar or radical — answer the question "how did the electrons move". The two systems are independent. A substitution can run through a one-step polar attack or through a stepwise route involving a carbocation, and both routes still carry the same structural label.

Mixing the systems is the most common source of confusion. Calling a reaction "an SN2" describes its mechanism; calling it "a substitution" describes its structure. Both statements can be true at once, and neither replaces the other.

A checklist that survives new problems

1. Count the species going in and the species coming out.
2. Compare the $\sigma$-bond count on the carbon skeleton.
3. Note whether a $\pi$ bond appeared or disappeared.
4. Only then assign the label addition, elimination, substitution, or rearrangement.
5. Treat anything that looks like a single step as a possible addition-then-elimination sequence, and label the overall change you were actually given.

Where the labels stop being useful

Some reactions genuinely belong to more than one category depending on how the question is scoped, and a few — combustion, for example — are not usefully described by any of the four labels at all. The classification is a reading aid, not a law. Its value is that it forces you to look at bond changes before you start drawing arrows, which is exactly the order in which the rest of organic chemistry expects you to work.