Organic Reactions: Addition and Substitution

Introduction

Addition and substitution are two common reaction patterns in carbon chemistry. Ethene can add atoms across C=C, while methane can undergo replacement of an H by a halogen under suitable conditions. The products reveal whether a bond order changed or a group was exchanged.

Core explanation

An addition reaction places parts of a reagent onto atoms that were joined by a multiple bond. With ethene and bromine in a suitable non-aqueous setting, CH₂=CH₂ + Br₂ → BrCH₂CH₂Br. The C=C becomes a C–C single bond; each former double-bond carbon gains one Br. No small molecule leaves in this simple net equation. Hydrogenation is another addition: CH₂=CH₂ + H₂ → CH₃CH₃ with a suitable catalyst.

Substitution replaces one atom or group with another while the carbon skeleton's bond order generally remains unchanged. A simplified methane halogenation example is CH₄ + Cl₂ → CH₃Cl + HCl under suitable light or heat. One C–H bond is replaced by C–Cl, and the displaced H appears in HCl. In real photochemical chlorination, multiple substitutions and product mixtures can occur; the one-step equation represents one selected product.

Compare atom ledgers. Ethene plus Br₂ has C2, H4, Br2 on both sides. Methane plus Cl₂ has C1, H4, Cl2 on both sides when CH₃Cl and HCl are counted together. Merely spotting a halogen in the product is not enough to decide reaction type; examine what bonds changed and whether a byproduct formed.

Saturated alkanes lack a C=C bond, so the elementary alkene addition mechanism cannot be applied to them unchanged. Alkenes can also undergo reactions other than addition under different conditions, and aromatic rings often favour substitution rather than the simple C=C addition expectation. A functional family suggests likely patterns, not a universal rule that overrides reagents and structure.

For aqueous bromine, alkene products can differ from the simple vicinal dibromide drawn for Br₂ in non-aqueous solvent. Bromine water decolourisation supports unsaturation but should not be used to assert a unique product without considering the medium. Naming the solvent and conditions helps prevent overconfident predictions.

Step-by-step reasoning

1. Identify the starting carbon bond order and attached groups. 2. Compare the product's carbon bond order. 3. If multiple-bond order falls and two new attachments appear, classify addition. 4. If one atom/group replaces another with a leaving byproduct, classify substitution. 5. Balance all atoms and check the stated conditions.

Visual explanation

Draw ethene with a bold double line and Br₂ above an arrow; on the right, show one Br attached to each carbon and a single line between them. Below draw methane with one H circled and chlorine taking its place, while that H joins the other Cl as HCl.

Real-world analogy

Addition resembles opening a double-doorway to admit two new visitors, while substitution resembles replacing one occupant with another. Molecules do not literally open doors, but the analogy directs attention to bond-order change versus exchange.

Real-world example

Hydrogenation of unsaturated oils involves addition of H₂ to some carbon–carbon double bonds under catalytic conditions. Halogen substitution of alkanes can supply intermediates for chemical manufacture, although selective control is needed because mixtures may form.

Why?

Why can an alkene undergo addition? The second component of a C=C bond can be reorganised while new bonds form to the two carbons. An alkane has only C–C single bonds and lacks that same unsaturated site.

Common misconception

“A reaction with bromine is always addition.” Bromine can add to an alkene under appropriate conditions, but halogen chemistry can also involve substitution. Look at substrate, conditions and product connectivity.

Worked example

Classify CH₃CH=CH₂ + H₂ → CH₃CH₂CH₃ with catalyst. Propene's C=C changes to C–C, and each double-bond carbon gains H, so this is addition/hydrogenation. Count C3 and H8 on both sides: C₃H₆ + H₂ gives C₃H₈. No H-containing byproduct appears.

Quick check

1. What reaction type is CH₄ + Cl₂ → CH₃Cl + HCl under light? Answer: Substitution; Cl replaces one methane H.

Exam focus

Show the bonds that change and identify any byproduct. State solvent or catalyst if product prediction depends on it. Do not apply a reaction pattern to every compound that happens to contain carbon.

Advanced insight

Organic reaction categories describe net structural change, whereas mechanisms describe electron movement and intermediates. Two reactions both labelled substitution can proceed by very different mechanisms depending on substrate and conditions.

Summary

Addition attaches across a multiple bond and reduces its bond order; substitution replaces an attached atom or group. Ethene bromination and methane chlorination illustrate the contrast, but exact products require conditions.

Practice questions

1. Classify CH₂=CH₂ + H₂ → CH₃CH₃. Answer: Addition, specifically hydrogenation. 2. What byproduct accompanies CH₃Cl in methane chlorination? Answer: HCl. 3. What happens to C=C in simple bromine addition? Answer: It becomes a C–C single bond with a new Br bond on each carbon in the ideal non-aqueous product. 4. Why is bromine-water colour loss not enough to name one exact product? Answer: The aqueous medium can affect the reaction pathway and product mixture.