Haloalkanes and Haloarenes

Haloalkanes (alkyl halides) are formed when one or more hydrogens of an aliphatic hydrocarbon are replaced by a halogen, giving the general formula $\text{R–X}$ (where X = F, Cl, Br, I). The defining feature is the polar C–X bond: the halogen is more electronegative than carbon, so carbon carries a partial positive charge ($\delta+$) and becomes the site that nucleophiles attack. This single idea drives almost all of the chapter's reactions, so it is worth fixing firmly in mind.

These compounds are classified in three useful ways. By the carbon bearing the halogen, they are primary ($1^\circ$), secondary ($2^\circ$) or tertiary ($3^\circ$) — a distinction that decides which substitution mechanism dominates. By the type of carbon, they are alkyl/benzylic/allylic (X on an $sp^3$ carbon) or vinylic/aryl (X on an $sp^2$ carbon, which are far less reactive). And by the number of halogens, they are mono-, di- or polyhalogen compounds. NEET frequently asks you to label a structure as $1^\circ$, $2^\circ$ or $3^\circ$.

The most important preparations are from alcohols, since alcohols are cheap and abundant. Alcohols react with concentrated halogen acids ($\text{HX}$, often with a $\text{ZnCl}_2$ catalyst — the basis of the Lucas test), with phosphorus halides ($\text{PCl}_3$, $\text{PCl}_5$, or red P + $\text{Br}_2$/$\text{I}_2$), and best of all with thionyl chloride ($\text{SOCl}_2$), because the by-products ($\text{SO}_2$ and $\text{HCl}$) are gases that escape, leaving a pure alkyl chloride. Other routes are free-radical halogenation of alkanes (gives mixtures), and addition of $\text{HX}$ or halogens to alkenes.

Two named halide-exchange reactions are NEET favourites. The Finkelstein reaction converts an alkyl chloride or bromide into the iodide using $\text{NaI}$ in dry acetone (NaCl/NaBr precipitate out, driving the reaction). The Swarts reaction makes alkyl fluorides by heating an alkyl chloride/bromide with a metal fluoride such as $\text{AgF}$ or $\text{Hg}_2\text{F}_2$. Physically, haloalkanes are denser than water, are insoluble in water but soluble in organic solvents, and their boiling points rise with molecular size (R–I $\gt$ R–Br $\gt$ R–Cl $\gt$ R–F for the same R) and fall with branching.

Because the carbon of a C–X bond is electron-poor, the signature reaction of haloalkanes is nucleophilic substitution: a nucleophile (an electron-rich species such as $\text{OH}^-$, $\text{CN}^-$, $\text{NH}_3$) replaces the halide, which leaves as $\text{X}^-$. Two distinct mechanisms operate, and telling them apart is one of the most heavily tested NEET skills.

The SN2 mechanism (substitution, nucleophilic, bimolecular) happens in a single step: the nucleophile attacks the carbon from the side opposite the leaving group while the halide departs, passing through a five-coordinate transition state. Its rate depends on both the halide and the nucleophile (second order), and because attack is from the back, the configuration is turned inside-out — inversion (the Walden inversion). Steric crowding slows it down, so the reactivity order is $1^\circ \gt 2^\circ \gt 3^\circ$ (methyl fastest). Strong nucleophiles and polar aprotic solvents favour SN2.

The SN1 mechanism (substitution, nucleophilic, unimolecular) happens in two steps: first the C–X bond breaks to give a flat carbocation (the slow, rate-determining step), then the nucleophile adds to it. The rate depends only on the halide (first order). Because the carbocation is planar, the nucleophile can attack either face, giving a mixture of both configurations — racemisation. Stability of the carbocation controls the rate, so the order is reversed: $3^\circ \gt 2^\circ \gt 1^\circ$. Tertiary halides and polar protic solvents (which stabilise the carbocation) favour SN1.

Competing with substitution is elimination ($\beta$-elimination, or dehydrohalogenation): a strong base removes a hydrogen from the carbon next to the C–X carbon (the $\beta$-carbon) while the halide leaves, forming a C=C double bond (an alkene). When more than one alkene can form, Saytzeff's rule applies — the major product is the more substituted, more stable alkene. Substitution and elimination always compete; strong, bulky bases, higher temperature and tertiary halides push the reaction towards elimination, while good nucleophiles in mild conditions favour substitution. A linked idea NEET often probes is optical isomerism: a carbon with four different groups is chiral, and SN2's inversion versus SN1's racemisation is a direct test of the mechanism.

Haloarenes are aromatic compounds in which a halogen is bonded directly to the carbon of a benzene ring (for example chlorobenzene). Although they look similar to haloalkanes, their chemistry is strikingly different: they are extremely unreactive towards nucleophilic substitution. Explaining why is one of the most reliable NEET questions in this chapter.

There are four reasons. First, resonance: the lone pairs on the halogen delocalise into the ring, giving the C–X bond partial double-bond character, so it is shorter, stronger and harder to break. Second, the ring carbon is $sp^2$ hybridised (more s-character) so it holds the bonding electrons closer, again strengthening the bond. Third, the electron-rich ring repels the approaching nucleophile. Fourth, an SN1 route would require a phenyl cation, which is highly unstable. Together these make ordinary substitution almost impossible.

Haloarenes are prepared by electrophilic halogenation of arenes (benzene + $\text{Cl}_2$/$\text{Br}_2$ with a Lewis-acid catalyst such as $\text{FeCl}_3$ or $\text{FeBr}_3$), and from diazonium salts — a versatile route in which an aromatic amine is diazotised and the $-\text{N}_2^+$ group is replaced by a halogen. The Sandmeyer and Gattermann reactions introduce Cl or Br (using $\text{CuX}$ or Cu/HX), while the Balz–Schiemann reaction is used for fluorine.

Their reactions therefore fall into two groups. Nucleophilic aromatic substitution happens only under drastic conditions — for example chlorobenzene with $\text{NaOH}$ at about 623 K and 300 atm gives phenol (the Dow process) — and is greatly eased if strong electron-withdrawing groups (such as $-\text{NO}_2$) sit at the ortho/para positions, because they stabilise the negative intermediate. Electrophilic substitution on the ring still occurs (nitration, halogenation, Friedel–Crafts); here the halogen behaves as an ortho/para-directing but deactivating group — directing through resonance yet slowing the ring through its electron-withdrawing inductive effect. This split personality of the halogen is a classic NEET point that links back to the directing-effects topic.

Haloalkanes react with several metals to make important reagents. The most useful is the Grignard reagent, $\text{R–MgX}$, formed when a halide reacts with magnesium turnings in dry ether. The Mg–C bond is highly polar with carbon $\delta-$, so the reagent behaves like a carbanion and is extremely reactive — it adds to carbonyl compounds and $\text{CO}_2$ to build larger molecules, which makes it a key tool in organic synthesis. Crucially, even traces of water (or any compound with an acidic H) destroy it, simply giving back the alkane; this is why everything must be scrupulously dry.

Other metal reactions extend the carbon skeleton. The Wurtz reaction couples two alkyl halides with sodium in dry ether to give a symmetrical alkane. Its aromatic relatives are the Wurtz–Fittig reaction (an alkyl halide + an aryl halide + Na, giving an alkyl-substituted benzene) and the Fittig reaction (two aryl halides + Na, giving a biaryl such as biphenyl). NEET often gives the reagents and asks for the product, or vice versa.

Polyhalogen compounds — molecules with more than one halogen — appear repeatedly in NEET because of their uses and environmental impact. Dichloromethane ($\text{CH}_2\text{Cl}_2$) is a common solvent and paint remover. Chloroform ($\text{CHCl}_3$) was an early anaesthetic; it is slowly oxidised by air and light to the poisonous gas phosgene ($\text{COCl}_2$), so it is stored in dark, completely filled bottles with a little ethanol added to destroy any phosgene. Carbon tetrachloride ($\text{CCl}_4$) was used as a fire extinguisher (pyrene) and solvent, but it harms the liver and depletes ozone. Iodoform ($\text{CHI}_3$) was used as an antiseptic (its action is due to the iodine it liberates).

Two classes carry the heaviest environmental warnings. Freons (chlorofluorocarbons, CFCs, e.g. Freon-12, $\text{CCl}_2\text{F}_2$) were ideal refrigerants and aerosol propellants, but in the stratosphere UV light releases chlorine radicals that catalytically destroy ozone, thinning the protective ozone layer — so they are being phased out. DDT (p,p'-dichlorodiphenyltrichloroethane), the first major synthetic insecticide, is chemically very stable and therefore not biodegradable; it persists in the environment and bio-accumulates up food chains, harming birds and fish, which is why its use is now banned or tightly restricted in most countries. These real-world links make the topic high-yield and easy to remember.