Organic Chemistry: Halogenoalkanes

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Edexcel IAL Chemistry · Organic Chemistry

Halogenoalkanes

🎯 Big Idea: A halogenoalkane is basically an alkane with a "weak spot" — the polar C–X bond — and almost everything in this chapter is just different nucleophiles attacking that weak spot to swap the halogen for something else (OH⁻, CN⁻, NH₃), or ripping off a hydrogen next door to form an alkene instead.

📋 Summary — What This Chapter Covers

Halogenoalkanes are named by adding fluoro-/chloro-/bromo-/iodo- prefixes to the parent alkane name, with position numbers and alphabetical ordering of substituents.
They're classified as primary, secondary, or tertiary based on how many carbon (alkyl) groups are attached to the carbon bearing the halogen.
The C–X bond is polar (Cδ+ — Xδ−) because halogens are more electronegative than carbon — this is *why* nucleophiles attack.
Nucleophilic substitution reactions swap the halogen (X) for: OH⁻ (→ alcohol), CN⁻ (→ nitrile, extends chain by 1 carbon), or NH₃ (→ primary amine).
Elimination reactions (with ethanolic NaOH + heat) remove HX to form an alkene instead of substituting.
The mechanism for substitution always follows the same logic: nucleophile's lone pair attacks the δ+ carbon, the C–X bond breaks heterolytically, X⁻ leaves.
Reactivity trend: C–I is the weakest bond (breaks fastest) → C–F is the strongest bond (barely reacts at all). This is tested experimentally using silver nitrate precipitation.

1. Naming & Classifying Halogenoalkanes

Naming Rules

Think of a halogenoalkane's name as built in three layers, applied in this order:

Start from the parent alkane — count the longest carbon chain and name it as you would an alkane (methane, ethane, propane...).
Add the halogen prefix — fluoro-, chloro-, bromo-, or iodo- — with a number showing which carbon it's attached to. Number the chain so the substituent gets the lowest possible number.
If there's more than one substituent, list them alphabetically (bromo before chloro, chloro before fluoro, etc.) — not in the order they appear on the chain.

Worked Examples

CH₃CH₂CH₂Br → 1-bromopropane

CH₃CH(Br)CH₃ → 2-bromopropane

CH₃CH(Cl)CH₂Br → 1-bromo-2-chloropropane

Notice in the last example: bromo is listed first because "b" comes before "c" alphabetically — even though the bromine is on carbon 1 and the chlorine is on carbon 2. Alphabetical order for listing, chain numbering for position — don't mix these two jobs up.

Primary, Secondary, Tertiary

This classification is all about the carbon directly attached to the halogen (call it the "halogen-bearing carbon"). Ask: how many other carbon groups (alkyl groups) is that carbon also attached to?

H CH₃ CH₃ | | | H — C — X H — C — X H₃C — C — X | | | CH₃ CH₃ CH₃ PRIMARY SECONDARY TERTIARY (1 alkyl group (2 alkyl groups (3 alkyl groups attached) attached) attached)

total that one specific carbon

⚠️ Common Mistake carbon-containing groups two

Practice Question 1.1

Classify (CH₃)₂CHCH₂Br as primary, secondary, or tertiary, and explain why.

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