Chemistry: Class 12 — Chapter 1: Haloalkanes and Haloarenes

Compounds are classified depending on whether they contain one, two, or more halogen atoms in their structures:

• Monohaloalkanes / Monohaloarenes: Contain one halogen atom (e.g., $C_2H_5X$).
• Dihaloalkanes / Dihaloarenes: Contain two halogen atoms (e.g., $CH_2X-CH_2X$).
• Trihaloalkanes / Trihaloarenes: Contain three halogen atoms.

• Alkyl halides or haloalkanes (R—X): The halogen atom is bonded to an alkyl group. They form a homologous series represented by $C_nH_{2n+1}X$. They are further classified into:
  • Primary ($1^\circ$): Halogen attached to a primary carbon.
  • Secondary ($2^\circ$): Halogen attached to a secondary carbon.
  • Tertiary ($3^\circ$): Halogen attached to a tertiary carbon.
• Allylic halides: Halogen atom is bonded to an $sp^3$-hybridised carbon atom adjacent to a carbon-carbon double bond ($C=C$).
• Benzylic halides: Halogen atom is bonded to an $sp^3$-hybridised carbon atom attached to an aromatic ring.

• Vinylic halides: Halogen atom is bonded to an $sp^2$-hybridised carbon atom of a carbon-carbon double bond ($C=C$).
• Aryl halides: Halogen atom is directly bonded to the $sp^2$-hybridised carbon atom of an aromatic ring.

• Common Names: Derived by naming the alkyl group followed by the halide name (e.g., n-propyl bromide).
• IUPAC Names: Named as halosubstituted hydrocarbons (e.g., 1-Bromopropane).
• Dihaloalkanes: Classified as:
  • Geminal halides (gem-dihalides): Both halogen atoms are on the same carbon atom (alkylidene halides).
  • Vicinal halides (vic-dihalides): Halogen atoms are on adjacent carbon atoms (alkylene dihalides).

Halogen atoms are more electronegative than carbon, making the carbon-halogen bond highly polarized. The carbon atom bears a partial positive charge ($\delta+$) while the halogen atom bears a partial negative charge ($\delta-$).

• Bond Length: Increases down the group from C—F to C—I as the size of the halogen increases ($C-F < C-Cl < C-Br < C-I$).
• Bond Enthalpy: Decreases down the group as bond length increases.
• Dipole Moment: Highly influenced by electronegativity and bond length.

The hydroxyl group of an alcohol is replaced by halogen using various reagents:

• By reaction with concentrated halogen acids in the presence of a catalyst ($ZnCl_2$): $$R-OH + HCl \xrightarrow{ZnCl_2} R-Cl + H_2O$$
• By reaction with phosphorus halides: $$3R-OH + PX_3 \rightarrow 3R-X + H_3PO_3 \quad (X = Cl, Br)$$ $$R-OH + PCl_5 \rightarrow R-Cl + POCl_3 + HCl$$
• By reaction with thionyl chloride (Thionyl chloride is preferred because the other products, $SO_2$ and $HCl$, are escapable gases, yielding pure alkyl halides): $$R-OH + SOCl_2 \rightarrow R-Cl + SO_2(g) + HCl(g)$$

• By Free Radical Halogenation of Alkanes: Chlorination or bromination in the presence of UV light or heat yields a complex mixture of isomeric mono- and polyhaloalkanes.
• From Alkenes:
  • Addition of hydrogen halides: Alkenes react with $HX$ ($HCl, HBr, HI$) to form alkyl halides. Unsymmetrical alkenes follow Markovnikov's rule. $$CH_3-CH=CH_2 + H-I \rightarrow CH_3-CH(I)-CH_3 \quad (Major)$$
  • Addition of halogens: Addition of bromine in $CCl_4$ to an alkene results in the discharge of the reddish-brown color of bromine, serving as an important test for unsaturation. $$CH_2=CH_2 + Br_2 \xrightarrow{CCl_4} BrCH_2-CH_2Br \quad (vic\text{-Dibromide})$$

• Finkelstein Reaction: Alkyl iodides are prepared by reacting alkyl chlorides/bromides with sodium iodide ($NaI$) in dry acetone. $$R-X + NaI \xrightarrow{\text{dry acetone}} R-I + NaX \quad (X = Cl, Br)$$
• Swarts Reaction: Alkyl fluorides are synthesized by heating alkyl chlorides/bromides in the presence of metallic fluorides such as $AgF$, $Hg_2F_2$, $CoF_2$, or $SbF_3$. $$H_3C-Br + AgF \rightarrow H_3C-F + AgBr$$

Aryl chlorides and bromides are prepared by direct halogenation of arenes with chlorine or bromine in the presence of a Lewis acid catalyst like iron or iron(III) chloride ($FeCl_3$) in the dark.

• The ortho and para isomers can be separated due to a large difference in their melting points.
• Reactions with iodine are reversible and require the presence of an oxidizing agent ($HNO_3$, $HIO_4$) to oxidize the $HI$ formed.

When a primary aromatic amine dissolved in cold aqueous mineral acid is treated with sodium nitrite ($NaNO_2$), a diazonium salt is formed. Shaking the freshly prepared diazonium salt with cuprous chloride or cuprous bromide results in the replacement of the diazonium group by $-Cl$ or $-Br$.

1. Diazotisation: $$Ar-NH_2 + NaNO_2 + 2HX \xrightarrow{273-278 K} Ar-N_2^+X^- + NaX + 2H_2O$$
2. Sandmeyer Reaction: $$Ar-N_2^+X^- \xrightarrow{Cu_2X_2} Ar-X + N_2 \quad (X = Cl, Br)$$
3. Iodide Replacement: Shaking the diazonium salt with potassium iodide ($KI$) yields aryl iodides without requiring cuprous halide. $$Ar-N_2^+X^- + KI \rightarrow Ar-I + K+ + N_2$$

A nucleophile attacks the electron-deficient carbon bonded to the halogen, displacing the halide ion (leaving group):

Mechanisms of Substitution:

• $S_N2$ (Substitution Nucleophilic Bimolecular):
  • Occurs in a single step via a transition state with no intermediate.
  • The incoming nucleophile attacks from the side opposite to the leaving group, resulting in complete inversion of configuration (Walden inversion).
  • Order of reactivity: $\text{Primary } (1^\circ) > \text{Secondary } (2^\circ) > \text{Tertiary } (3^\circ)$ (governed by steric hindrance).
• $S_N1$ (Substitution Nucleophilic Unimolecular):
  • Occurs in two steps. Step I (slow, rate-determining) involves the cleavage of the C—X bond to form a stable carbocation. Step II (fast) involves the attack of the nucleophile.
  • Accompanied by racemisation due to the planar nature of the carbocation intermediate.
  • Order of reactivity: $\text{Tertiary } (3^\circ) > \text{Secondary } (2^\circ) > \text{Primary } (1^\circ)$ (governed by carbocation stability).

When a haloalkane with a $\beta$-hydrogen atom is heated with an alcoholic solution of potassium hydroxide ($KOH$), elimination of a hydrogen atom from the $\beta$-carbon and a halogen atom from the $\alpha$-carbon occurs, forming an alkene.

• Zaitsev (Saytzeff) Rule: In dehydrohalogenation reactions, the preferred product is the highly substituted alkene (the one with the greater number of alkyl groups attached to the doubly bonded carbon atoms).

• Formation of Grignard Reagents: Alkyl halides react with magnesium metal in dry ether to form alkyl magnesium halides ($RMgX$), known as Grignard Reagents. $$R-X + Mg \xrightarrow{\text{dry ether}} RMgX$$ Grignard reagents are highly reactive and decompose in the presence of any proton source (like water, alcohols) to form hydrocarbons: $$RMgX + H_2O \rightarrow R-H + Mg(OH)X$$
• Wurtz Reaction: Alkyl halides react with sodium in dry ether to give hydrocarbons containing double the number of carbon atoms. $$2R-X + 2Na \xrightarrow{\text{dry ether}} R-R + 2NaX$$

Aryl halides are extremely unreactive towards nucleophilic substitution due to:

1. Resonance Effect: The electron pairs on the halogen atom are conjugated with the $\pi$-electrons of the benzene ring, giving the C—X bond a partial double bond character, making it difficult to break.
2. Difference in Hybridisation: The carbon attached to the halogen is $sp^2$-hybridised in haloarenes (shorter, stronger bond) compared to $sp^3$-hybridised in haloalkanes.
3. Instability of Phenyl Cation: Self-ionization to form a phenyl cation is highly unstable.

Dow's Process (Drastic Conditions): Chlorobenzene can be converted to phenol under high temperature and pressure: $$\text{Chlorobenzene} \xrightarrow{\text{(i) NaOH, 623K, 300 atm, (ii) } H^+} \text{Phenol}$$ The presence of electron-withdrawing groups (like $-NO_2$) at ortho- and para-positions significantly increases the reactivity of haloarenes.

The halogen atom on the benzene ring is deactivating but $o, p$-directing due to resonance.

• Halogenation: Reaction with $Cl_2$ in anhydrous $FeCl_3$ yields 1,4-dichlorobenzene (major) and 1,2-dichlorobenzene (minor).
• Nitration: Reaction with concentrated $HNO_3$ and $H_2SO_4$ yields 1-chloro-4-nitrobenzene (major).
• Sulphonation: Heating with concentrated $H_2SO_4$ yields 4-chlorobenzenesulfonic acid (major).
• Friedel-Crafts Alkylation/Acylation: Reaction with alkyl/acyl halides in the presence of anhydrous $AlCl_3$.

• Wurtz-Fittig Reaction: A mixture of an alkyl halide and aryl halide treated with sodium in dry ether gives an alkylarene. $$Ar-X + Na + R-X \xrightarrow{\text{dry ether}} Ar-R + 2NaX$$
• Fittig Reaction: Aryl halides treated with sodium in dry ether yield diaryl compounds. $$2Ar-X + 2Na \xrightarrow{\text{dry ether}} Ar-Ar + 2NaX$$