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What Is Orgo? Classify Addition, Elimination, and Substitution Reactions

Orgo is the campus shorthand for organic chemistry. Classify three textbook reactions as addition, elimination, or substitution by counting the species and tracking sigma and pi bond changes.

Question

Classify each of the following reactions as an addition, elimination, substitution, or rearrangement:

(a) CH3Br+KOH→CH3OH+KBr\mathrm{CH_3Br + KOH \rightarrow CH_3OH + KBr}

(b) CH3CH2Br→H2C=CH2+HBr\mathrm{CH_3CH_2Br \rightarrow H_2C{=}CH_2 + HBr}

(c) H2C=CH2+H2→CH3CH3\mathrm{H_2C{=}CH_2 + H_2 \rightarrow CH_3CH_3}

Answer

Classify each reaction by counting the species and tracking what happens to the bonds: (a) substitution, (b) elimination, (c) addition.

(a) CH3Br+KOH→CH3OH+KBr\mathrm{CH_3Br + KOH \rightarrow CH_3OH + KBr} — substitution

Two reactants give two products. The oxygen of hydroxide forms a bond to carbon while bromide leaves, so Br\mathrm{Br} has been replaced by OH\mathrm{OH} on the organic skeleton. That carbon still carries four bonds before and after the swap. Two species in, two species out, with one group traded for another, is the substitution signature.

(b) CH3CH2Br→H2C=CH2+HBr\mathrm{CH_3CH_2Br \rightarrow H_2C{=}CH_2 + HBr} — elimination

One reactant gives two products. A C–Br bond and a C–H bond on the neighbouring carbon are both removed, and the electron pairs left behind become the C=C π\pi bond of ethene. Because the departing fragments leave as separate molecules while the skeleton loses two σ\sigma bonds and gains a π\pi bond, this is an elimination.

(c) H2C=CH2+H2→CH3CH3\mathrm{H_2C{=}CH_2 + H_2 \rightarrow CH_3CH_3} — addition

Two reactants give one product and no atom is left over. The π\pi bond of ethene is consumed, and two new C–H σ\sigma bonds take its place. Two species in, one species out is the addition signature.

The decision table

ReactionSpecies in →\rightarrow outBond change on the organic skeletonClass
(a)2 →\rightarrow 2C–Br replaced by C–OH; σ\sigma count unchangedsubstitution
(b)1 →\rightarrow 2C–H and C–Br removed; a C=C π\pi bond appearselimination
(c)2 →\rightarrow 1C=C π\pi bond removed; two C–H σ\sigma bonds appearaddition

Three reaction types compared by their bond changes: substitution swaps one group for another on the same carbon, elimination removes two sigma bonds and creates a pi bond between the same two carbons, and addition consumes a pi bond and creates two sigma bonds.

Why the classification stops there

Those four labels describe structural change only. They say nothing about how the bonds actually break — whether the electron pair departs with bromide or with hydroxide, whether the pathway is polar or radical, or how fast either reaction runs. Those are mechanistic questions, and one structural label can cover several mechanisms.

One common trap: some reactions look like a substitution but are an addition followed by an elimination. When a problem supplies only the starting and final structures, classify by the overall change you were actually given.

Evidence boundary

This page treats OpenStax Organic Chemistry Problem 6-1 as a complete representative exercise for the course shorthand “orgo.” Orgo is an informal name for organic chemistry, and the page does not claim the search phrase corresponds to one specific original exam item. The three equations and their order are taken from that textbook problem without adding or removing any part. Classification here follows structural change only; no mechanism, rate, yield, or stereochemical conclusion is drawn.

Sources

These references support the concepts and methods used in the explanation above.

What Is Orgo? Classify Addition, Elimination, and Substitution Reactions | Verla