Properties of Alkyl Halides (Halo Alkanes) & Aryl Halides (Halo Arenes)

The properties of alkyl halides come from the polar C-X bond: the carbon carries a partial positive charge, so nucleophiles attack it and the halide leaves. Alkyl halides therefore undergo nucleophilic substitution (${\text{S}}_{\text{N}}1$ or ${\text{S}}_{\text{N}}2$), elimination to alkenes, and reactions with metals such as Na and Mg. Aryl halides are far less reactive because their C-X bond has partial double-bond character. The physical and chemical properties of alkyl halides and haloarenes are among the most tested areas of this chapter in JEE Main and NEET.

1. Nature of the C-X Bond

A halogen is more electronegative than carbon, so the C-X bond is polar: the carbon carries a partial positive charge ($\delta +$) and the halogen a partial negative charge ($\delta -$). As the halogen gets larger from F to I, the bond gets longer and weaker.

2. Physical Properties

2.1 Haloalkanes

• Boiling point and halogen: for the same alkyl group, RI > RBr > RCl > RF. A larger, heavier halogen gives stronger van der Waals forces.
• Boiling point and size: for the same halogen, the boiling point rises with molecular mass. $\mathrm{C}{\mathrm{H}}_3\mathrm{Cl}$ boils lower than $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{Cl}$.
• Boiling point and number of halogens: $\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{l}}_2$ (313 K) < $\mathrm{CHC}{\mathrm{l}}_3$ (334 K) < $\mathrm{CC}{\mathrm{l}}_4$ (350 K).
• Boiling point and branching: among isomers, more branching gives a more compact, nearly spherical molecule with less surface contact, so the boiling point falls.
• Solubility: only very slightly soluble in water, because the energy released by new attractions to water is less than the energy needed to break water's hydrogen bonds. They dissolve readily in organic solvents.
• Density: bromo, iodo and polychloro compounds are denser than water. Density rises with the number and atomic mass of the halogen atoms.

The straight-chain isomer boils highest and the most branched (tertiary) isomer lowest.

2.2 Haloarenes

• Boiling point: iodobenzene > bromobenzene > chlorobenzene.
• Solubility: insoluble in water, acids and bases, because they cannot form hydrogen bonds with water. They are soluble in organic solvents.
• Density: all are heavier than water; iodo > bromo > chloro.
• Melting points of dichlorobenzenes: the three isomers boil at nearly the same temperature, but the para isomer melts 70-100 K higher than the ortho and meta isomers.

3. Chemical Properties: Overview

Haloalkanes are a highly reactive class of aliphatic compounds because of the polar C-X bond. Their reactions involve breaking the C-X bond, so the weaker the bond, the faster the reaction. For a given alkyl group:

The chemical reactions of haloalkanes are of four types:

1. Nucleophilic substitution reactions
2. Elimination (dehydrohalogenation) reactions
3. Reactions with metals
4. Reduction reactions

4. Nucleophilic Substitution Reactions

A nucleophile (an electron-rich species) attacks the electron-deficient carbon and the halide ion leaves:

The reaction can follow either of two mechanisms, ${\text{S}}_{\text{N}}1$ or ${\text{S}}_{\text{N}}2$.

4.1 The ${\text{S}}_{\text{N}}1$ mechanism (unimolecular)

The rate depends only on the concentration of the alkyl halide: rate $=k[\mathrm{RX}]$. The reaction takes place in two steps:

1. Slow step: the alkyl halide ionises into a carbocation and a halide ion. This step decides the rate.
2. Fast step: the carbocation immediately combines with the nucleophile.

Since the slow step forms a carbocation, reactivity follows carbocation stability: $3^{\circ }>2^{\circ }>1^{\circ }$. Allylic and benzylic halides are also very reactive by ${\text{S}}_{\text{N}}1$, because their carbocations are stabilised by resonance.

4.2 The ${\text{S}}_{\text{N}}2$ mechanism (bimolecular)

The rate depends on the concentrations of both the alkyl halide and the nucleophile: rate $=k[\mathrm{RX}][\mathrm{N}{\mathrm{u}}^{-}]$. The nucleophile attacks from the side opposite the halogen (backside attack). The C-Nu bond forms while the C-X bond breaks, in a single step, through a transition state in which carbon is partly bonded to both.

The three other groups on carbon flip over like an umbrella in the wind, so the configuration is inverted (Walden inversion). Bulky groups block the backside attack, so the order of reactivity is $\mathrm{C}{\mathrm{H}}_3\mathrm{X}>1^{\circ }>2^{\circ }>3^{\circ }$.

4.3 Factors that decide ${\text{S}}_{\text{N}}1$ or ${\text{S}}_{\text{N}}2$

4.4 Stereochemistry of substitution

• ${\text{S}}_{\text{N}}2$ gives inversion. An optically active alkyl halide gives a product with the opposite configuration, as in Figure 2, where (R)-2-bromobutane gives (S)-butan-2-ol.
• ${\text{S}}_{\text{N}}1$ gives racemisation. The carbocation is planar, so the nucleophile can attack from either face with nearly equal chance. An optically active substrate gives a nearly 50:50 mixture of both configurations, which is optically inactive.

4.5 Important nucleophilic substitution reactions

When the alkyl halide is in excess, the amine formed reacts further, replacing the remaining H atoms on nitrogen to give secondary and tertiary amines (and finally a quaternary ammonium salt).

5. Elimination Reactions (Dehydrohalogenation)

When haloalkanes are heated with alcoholic KOH, a hydrogen atom from the $\beta$-carbon and the halogen from the $\alpha$-carbon are removed, and an alkene forms. This is called $\beta$-elimination.

• Order for alkyl groups: tertiary > secondary > primary. Tertiary halides give the most substituted alkenes, which are the most stable and form fastest.
• Order for halogens: RI > RBr > RCl.
• Saytzeff (Zaitsev) rule: when elimination can happen in two directions, the major product is the more substituted alkene (the one with fewer H atoms on the double-bond carbons). 2-Bromobutane gives but-2-ene (80%) and but-1-ene (20%).

6. Reactions with Metals

6.1 Wurtz reaction (sodium)

Haloalkanes react with sodium in dry ether to give alkanes with double the number of carbon atoms:

6.2 Grignard reagents (magnesium)

Haloalkanes react with magnesium in dry ether to form alkylmagnesium halides, called Grignard reagents:

Grignard reagents are organometallic compounds (they contain a carbon-metal bond). The C-Mg bond is highly polar, with carbon carrying a partial negative charge, so they are very reactive. Any proton donor (water, alcohols, amines) converts them to hydrocarbons:

This is why Grignard reagents must be prepared and used under strictly anhydrous conditions.

6.3 Alkyllithium (lithium)

Alkyllithium compounds behave as strong bases and are used like Grignard reagents.

6.4 Tetraethyl lead (sodium-lead alloy)

Tetraethyl lead (TEL) was once used as an anti-knock additive in petrol.

7. Reduction of Haloalkanes

Haloalkanes can be reduced to alkanes in three common ways.

(a) Hydrogen with nickel:

(b) Zinc-copper couple in alcohol, which produces nascent hydrogen:

(c) Lithium aluminium hydride in dry ether, where a hydride ion replaces X (reaction 12 in the table of Section 4.5).

8. Why Aryl Halides Are Less Reactive Than Alkyl Halides

Aryl halides are much less reactive than haloalkanes towards nucleophilic substitution. The main reasons are:

1. Resonance: a lone pair on the halogen is in conjugation with the $\pi$ electrons of the ring. In three of the resonance structures (II, III, IV below) the C-X bond is a double bond, so the real C-X bond has partial double-bond character. It is shorter and stronger than in alkyl halides and is hard to break.
2. Hybridisation: the carbon holding X is ${\text{sp}}^2$ in haloarenes but ${\text{sp}}^3$ in haloalkanes. An ${\text{sp}}^2$ orbital has more s-character, holds electrons closer to the nucleus, and gives a shorter, stronger bond. The C-Cl bond length is 169 pm in chlorobenzene against 177 pm in chloromethane.
3. Lower polarity: the ${\text{sp}}^2$ carbon is more electronegative than an ${\text{sp}}^3$ carbon, so the C-X bond in haloarenes is less polar, and the carbon is a weaker target for nucleophiles.
4. Unstable phenyl cation: an ${\text{S}}_{\text{N}}1$ route would need a phenyl cation, which resonance cannot stabilise.
5. Repulsion: the electron-rich ring repels an approaching electron-rich nucleophile, and the ring blocks backside attack.

9. Reactions of Aryl Halides

9.1 Nucleophilic substitution (drastic conditions)

Replacement by OH: chlorobenzene is heated with aqueous NaOH at 623 K and 300 atm (Dow process). Sodium phenoxide forms, and acidification gives phenol.

Replacement by CN: bromobenzene heated with anhydrous CuCN in pyridine or DMF at 470 K gives cyanobenzene (phenyl cyanide).

Replacement by $\mathrm{N}{\mathrm{H}}_2$: chlorobenzene reacts with aqueous ammonia in the presence of $\mathrm{C}{\mathrm{u}}_2\mathrm{O}$ at high temperature and pressure to give aniline.

9.2 Effect of nitro groups (activated substitution)

An electron-withdrawing group such as $\mathrm{N}{\mathrm{O}}_2$ at the ortho or para position makes substitution much easier. The nucleophile adds first, and the negative charge of the intermediate (Meisenheimer complex) spreads onto the oxygen atoms of the nitro group. A nitro group at the meta position cannot do this.