Haloalkanes and Haloarenes
Class 12 · Chemistry
4 ideas in Class 12 Chemistry, explained in plain words, with games and quizzes to practise.
Types, Names and the Carbon-Halogen Bond
Part of: Families, Names and Preparation
Organic halides are grouped by the carbon that holds the halogen: a side-chain saturated carbon gives alkyl and aralkyl halides, a ring carbon gives aryl halides, and carbons near a double bond give allylic (on a saturated carbon) and vinylic (on a double-bond carbon) halides. They are further sorted by halogen count and by how crowded the halogen-bearing carbon is: primary, secondary or tertiary.
Why it matters. The family fixes the chemistry: vinylic and aryl halides resist substitution while allylic and benzylic ones react briskly. Correct names and structures are the entry ticket to every reaction in this chapter.
Key words
- Alkyl halide
- : A halogen joined to a saturated side-chain carbon.
- Aryl halide
- : A halogen joined directly to a carbon of a benzene ring.
- Allylic halide
- : A halogen on a saturated carbon sitting next to a double bond.
- Vinylic halide
- : A halogen joined straight to a double-bond carbon.
- Geminal dihalide
- : Two halogens carried on the same carbon atom.
- Vicinal dihalide
- : Two halogens carried on neighbouring carbon atoms.
- Primary alkyl halide
- : Halogen on a carbon attached to only one other carbon.
Try it in two minutes
- Locate the carbon carrying the halogen.
- Note whether it is saturated or part of a ring or double bond.
- Name the parent chain and number the halogen position.
Try at home. Draw five halide structures and pile them into alkyl, aryl, allylic and vinylic groups.
Making Haloalkanes and Haloarenes
Part of: Families, Names and Preparation
Hydroxy compounds give alkyl halides with hydrogen halides, phosphorus halides or thionyl chloride; tertiary hydroxy compounds react fastest. Alkanes add halogen by sunlight-driven radical routes, alkenes add HX by the Markovnikov pattern, and chlorides swap to iodides with sodium iodide in dry acetone. Aryl halides come from direct ring halogenation with a Lewis acid carrier, or from diazonium salts through the Sandmeyer change.
Why it matters. Choosing the right maker decides purity and yield: thionyl chloride leaves only gases behind, sulphuric acid ruins iodide preparations, and Sandmeyer opens the door to aryl halides that direct routes cannot reach.
Key words
- Thionyl chloride method
- : A hydroxy compound plus SOCl2 giving alkyl chloride with gaseous byproducts.
- Finkelstein reaction
- : Alkyl chloride or bromide swapped to iodide using NaI in dry acetone.
- Sandmeyer reaction
- : Diazonium salt turned into aryl halide with cuprous halide.
- Markovnikov addition
- : HX adds so the halogen lands on the more crowded double-bond carbon.
- Lucas test
- : A hydroxy compound plus HCl and zinc chloride sorting tertiary, secondary and primary by speed.
Try it in two minutes
- Start from the given hydroxy compound, hydrocarbon or diazonium salt.
- Match the reagent to the halogen wanted.
- Check reactivity order and side products.
Try at home. Write the full preparation route for bromobenzene from aniline in three steps.
Substitution, Elimination and Grignard Reagents
Part of: Reactions and Special Topics
Haloalkanes swap their halogen for OH, CN, NH2 and similar groups by two routes: SN1 goes in two steps through a flat carbocation and suits tertiary halides, while SN2 strikes once from the back and suits methyl and primary halides. Strong bases in hot ethanol instead pull off HX to make alkenes, with the more substituted alkene winning. Alkyl halides also join dry magnesium in dry ether to form Grignard reagents, prized carbon-chain builders.
Why it matters. Rate orders, solvent choice and Saytzeff outcomes are favourite board questions, and nearly every later synthesis leans on substitution or Grignard steps.
Formulas
- SN1 rate law: rate = k[RX]
- SN2 rate law: rate = k[RX][nucleophile]
- Saytzeff rule: the more substituted alkene is the major product
- Walden inversion: SN2 attack from the back flips the configuration
Key words
- Nucleophile
- : An electron-rich group that seeks a positive carbon centre.
- SN1 reaction
- : Two-step substitution through a carbocation, rate set by the halide alone.
- SN2 reaction
- : One-step backside substitution that inverts the carbon arrangement.
- Carbocation
- : A carbon with only six outer electrons and a positive charge.
- Saytzeff rule
- : Elimination favours the more substituted, stabler alkene.
- Walden inversion
- : The flip of arrangement at carbon during backside attack.
- Grignard reagent
- : An alkyl magnesium halide made in dry ether for building chains.
Try it in two minutes
- Check the halide degree: tertiary hints SN1, primary hints SN2.
- Note the solvent: water-like favours SN1, dry acetone-like favours SN2.
- For elimination, build the most substituted alkene.
Try at home. Predict major products when 2-bromobutane meets aqueous KOH versus hot alcoholic KOH.
Haloarenes, Chirality and Everyday Polyhalides
Part of: Reactions and Special Topics
Aryl halides shrug off nucleophilic attack because electron delocalisation lends the C-X link partial double-bond character and the ring carbon resists backside entry; instead they undergo electrophilic substitution guided ortho and para by the halogen. Molecules with a carbon holding four different groups are chiral and twist plane-polarised light, with mirror twins called enantiomers and an equal mix called a racemic mixture. Polyhalides like chloroform and DDT show how useful and how risky halogen compounds can be.
Why it matters. Chirality decides how medicines and sugars behave in the body, and the DDT story shows why persistence matters as much as potency.
Key words
- Chirality
- : A shape that cannot overlap its own mirror image.
- Enantiomers
- : A mirror-image pair of non-overlapping isomers.
- Racemic mixture
- : An equal mix of both enantiomers with zero net rotation.
- Optical activity
- : The power to rotate plane-polarised light left or right.
- Electrophilic substitution
- : Ring attack where an electron-seeker replaces a ring hydrogen.
- DDT
- : A long-lasting chlorinated insecticide now restricted in many countries.
Try it in two minutes
- Test the holder carbon for four different groups.
- Pair each structure with its mirror image.
- Judge preparation and use claims against persistence and safety.
Try at home. Build 2-butanol and its mirror image with clay balls and check they never overlap.
What your child will practise
- Haloarenes, Chirality and Everyday Polyhalides: Desert Quick Fire (Quick Fire)
- Making Haloalkanes and Haloarenes: Space Puzzle (Clue Puzzle)
- Making Haloalkanes and Haloarenes: Space Quick Fire (Quick Fire)
- Substitution, Elimination and Grignard Reagents: Ocean Quick Fire (Quick Fire)
- Types, Names and the Carbon-Halogen Bond: Jungle Puzzle (Clue Puzzle)
- Quiz: Haloarenes, Chirality and Everyday Polyhalides (8 questions)
- Quiz: Making Haloalkanes and Haloarenes (8 questions)
- Quiz: Substitution, Elimination and Grignard Reagents (8 questions)
- Quiz: Types, Names and the Carbon-Halogen Bond (8 questions)