Haloalkanes and Haloarenes

Question-and-answer flashcards covering classification, nomenclature, bonding, preparation, physical properties, reaction mechanisms, stereochemistry, special reactions, and environmental aspects of haloalkanes, haloarenes and polyhalogen compounds.

What is the difference between a haloalkane and a haloarene?

A haloalkane has a halogen bonded to an sp3-hybridised carbon of an alkyl group, while a haloarene has a halogen bonded to an sp2-hybridised carbon of an aromatic ring.

How are halogenated hydrocarbons classified based on number of halogen atoms?

They are classified as mono-, di- or polyhalogen compounds depending on whether they contain one, two or more halogen atoms, respectively.

Give the general formula for a homologous series of alkyl halides.

CnH2n+1X, where X = F, Cl, Br or I.

Define an allylic halide.

A compound in which the halogen is attached to an sp3 carbon adjacent to a C=C double bond.

Define a benzylic halide.

A compound in which the halogen is attached to an sp3 carbon directly bonded to an aromatic ring.

Define a vinylic halide.

A compound in which the halogen is bonded directly to an sp2 carbon of a C=C double bond.

Define an aryl halide.

A compound where the halogen is bonded directly to the sp2-hybridised carbon of an aromatic ring.

What are geminal dihalides?

Dihalo compounds in which both halogens are attached to the same carbon atom.

What are vicinal dihalides?

Dihalo compounds in which the two halogen atoms are on adjacent carbon atoms.

State Zaitsev’s (Saytzeff’s) rule.

In dehydrohalogenation the preferred alkene is the one with the greater number of alkyl groups attached to the doubly-bonded carbons.

Trend: C–X bond length from C–F to C–I.

Bond length increases: C–F (139 pm) < C–Cl (178 pm) < C–Br (193 pm) < C–I (214 pm).

Trend: C–X bond strength from C–F to C–I.

Bond dissociation enthalpy decreases: C–F > C–Cl > C–Br > C–I.

Why are alkyl halides prepared with SOCl₂ preferred over HCl?

Because SO2 and HCl gases formed escape, giving pure alkyl chloride without further purification.

Write the Swarts reaction in one line.

R–Cl or R–Br + AgF/SbF₃/Hg₂F₂ → R–F (alkyl fluoride).

Describe the Finkelstein reaction.

Alkyl chloride or bromide + NaI (in dry acetone) → Alkyl iodide + NaCl/NaBr precipitate.

What are the three main categories of haloalkane reactions?

(1) Nucleophilic substitution, (2) Elimination, (3) Reactions with metals (e.g., Grignard, Wurtz).

Rate law of an SN2 reaction?

Rate = k[substrate][nucleophile] – bimolecular overall, second order.

Stereochemical outcome of an SN2 reaction on a chiral centre.

Inversion of configuration (Walden inversion).

Rate law of an SN1 reaction?

Rate = k[substrate] – first order, depends only on alkyl halide concentration.

Intermediate formed in an SN1 reaction.

A planar carbocation.

Stereochemical outcome of SN1 on a chiral centre.

Racemisation (mixture of retention and inversion) because nucleophile attacks from both faces of planar carbocation.

Order of reactivity of alkyl halides in SN2.

CH3X > 1° > 2° >> 3° (least steric hindrance reacts fastest).

Order of reactivity of alkyl halides in SN1.

3° > 2° > 1° > CH3X (carbocation stability controls rate).

Define ambident nucleophile and give two examples.

A nucleophile that can attack through two different atoms; examples: CN⁻ (C or N attack) and NO₂⁻ (N or O attack).

What product dominates when haloalkane reacts with KCN vs AgCN?

KCN gives alkyl cyanide (R–C≡N); AgCN gives isocyanide (R–N≡C).

Explain retention of configuration.

Product retains the same spatial arrangement around the stereocentre as the reactant when no bond to that centre is broken during reaction.

Define racemisation.

Formation of an equimolar mixture of two enantiomers resulting in zero optical rotation.

Why are haloalkanes only sparingly soluble in water?

Energy gained from new interactions is insufficient to compensate for breaking strong hydrogen bonds among water molecules.

Density trend among halogen derivatives.

Density increases with greater number or heavier halogens; most bromo, iodo and polychloro derivatives are denser than water.

Why do boiling points of isomeric haloalkanes decrease with branching?

Branching lowers surface area, reducing van der Waals forces.

Grignard reagent general formula.

RMgX (R = alkyl/aryl; X = Cl, Br, I).

Precaution during preparation of Grignard reagents.

Anhydrous conditions are essential because water protonates RMgX, destroying it.

Wurtz reaction equation template.

2 R–X + 2 Na → R–R + 2 NaX (in dry ether).

Why are haloarenes less reactive towards SN?

(i) C–X bond partial double-bond character by resonance; (ii) sp2 carbon more electronegative; (iii) unstable phenyl cation; (iv) nucleophile repelled by electron-rich ring.

Conditions to convert chlorobenzene to phenol.

Aqueous NaOH, 623 K, 300 atm (Dow process).

Effect of –NO₂ at ortho/para on nucleophilic substitution of haloarenes.

Electron-withdrawing nitro stabilises the Meisenheimer carbanion intermediate, greatly increasing reaction rate.

Directive influence of halogen in electrophilic substitution of benzene.

Halogens are ortho-, para-directing but ring-deactivating (–I dominates over +R).

State Fittig reaction.

2 Ar–X + 2 Na → Ar–Ar + 2 NaX (dry ether).

Give two industrial uses of dichloromethane.

Paint remover solvent and aerosol propellant; also used as metal-cleaning solvent.

Hazard of chloroform in presence of air and light.

Oxidises slowly to poisonous phosgene (COCl₂); hence stored in dark, airtight bottles.

Major use of carbon tetrachloride (CCl₄) historically.

Dry-cleaning solvent, fire extinguisher, refrigerant feedstock; now restricted due to toxicity and ozone depletion.

What are Freons?

Chlorofluorocarbon compounds of methane/ethane (e.g., CCl₂F₂) used as refrigerants and aerosol propellants.

Environmental concern with Freons.

Diffuse to stratosphere, release Cl radicals, catalyse ozone depletion.

Full name of DDT.

p,p′-Dichlorodiphenyltrichloroethane.

Why was DDT banned in many countries?

Persistence in environment, bioaccumulation, insect resistance, high toxicity to wildlife.

Define chiral molecule.

A molecule not superimposable on its mirror image; exhibits optical activity.

What structural feature usually imparts chirality in haloalkanes?

An sp3 carbon attached to four different substituents (asymmetric carbon).

Describe Walden inversion.

The umbrella-like flip of configuration occurring in SN2 reactions at a chiral centre, producing the enantiomer.

Which is the better leaving group: Cl⁻ or I⁻? Why?

I⁻; it is larger and stabilises negative charge better, making C–I bond weaker.

Effect of polar protic solvent on SN1 vs SN2.

Polar protic solvents stabilise carbocations and anions, favouring SN1 and slowing SN2 (due to nucleophile solvation).

Effect of bulky nucleophile on reaction pathway.

Favours elimination (E2) over substitution (SN2) due to steric hindrance in backside attack.

Define β-elimination.

Removal of a halogen from the α-carbon and a hydrogen from an adjacent β-carbon to form an alkene.

Identify α and β carbons in 2-bromopropane.

α-carbon: C-2 bearing Br; β-carbons: C-1 and C-3 adjacent to α.

Why does tertiary butyl bromide undergo elimination easily?

Tertiary substrate forms stable carbocation and steric hindrance disfavors SN2, so E1/E2 predominate.

Give two methods to generate alkyl fluorides.

Swarts reaction (R-Cl/R-Br + metallic fluoride) and halogen exchange with Hg₂F₂, AgF, CoF₂ or SbF₃.

Explain why chlorobenzene’s dipole moment is lower than chlorocyclohexane’s.

Resonance in chlorobenzene delocalises electron density, reducing magnitude of the C–Cl dipole vector.

Why are polyhalogen compounds often environmental hazards?

They resist biodegradation, persist, bioaccumulate and may deplete ozone (CFCs) or cause toxicity (DDT, CCl₄).