Haloalkanes and Haloarenes

Haloalkanes (Alkyl halides): Hydrocarbons in which one or more H atoms replaced by halogens (X = F, Cl, Br, I).

Classification:

Nomenclature (IUPAC): halo-X-...alkane.

• $CH_3CH_2Br$: bromoethane
• $(CH_3)_2CHCl$: 2-chloropropane
• $CH_3CHBrCH_2CH_3$: 2-bromobutane

Preparation:

1. From Alcohols:

• $R-OH + HX \to RX + H_2O$ (HX = HCl with $ZnCl_2$ as catalyst; HBr; HI)
• $3R-OH + PX_3 \to 3RX + H_3PO_3$
• $R-OH + PCl_5 \to RCl + POCl_3 + HCl$
• $R-OH + SOCl_2 \to RCl + SO_2 + HCl$ (Darzen's method — clean reaction, gases released easily)

Reactivity of HX: $HI > HBr > HCl$. Reactivity of ROH: $3° > 2° > 1°$ (carbocation stability).

Lucas Test: $ZnCl_2$ + conc. HCl distinguishes alcohols:

• $3°$ alcohol: cloudy immediately
• $2°$: cloudy in $\sim 5$ min
• $1°$: no cloudiness at room T (needs heating)

2. From Alkanes (Free Radical Halogenation): $CH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + HCl$ (and further products). Mix of products; not selective in industry.

3. From Alkenes:

• HX addition (Markovnikov): $CH_2=CH_2 + HBr \to CH_3CH_2Br$
• Halogenation: $CH_2=CH_2 + Br_2 \to BrCH_2CH_2Br$ (1,2-dibromoethane)

4. Halogen Exchange:

• Finkelstein Reaction: $RCl + NaI \xrightarrow{\text{acetone}} RI + NaCl$ (NaCl insoluble in acetone, drives equilibrium)
• Swarts Reaction: $RCl + AgF \to RF + AgCl$ (or $Hg_2F_2, CoF_2$)

Physical Properties:

• C-X bond polar (X is more EN); molecules polar; have higher BP than corresponding alkanes
• BP order: $RF < RCl < RBr < RI$ (larger size → more London forces; polarizability ↑)
• BP among same X: $primary > secondary > tertiary$ (more compact = less surface area)
• Insoluble in water; soluble in organic solvents
• Density: $RF, RCl < $ water; $RBr, RI > $ water (denser due to heavy halogen)

Nucleophilic Substitution Reactions ($S_N$):

Nucleophile (Nu⁻) replaces leaving group (X⁻) at $sp^3$ C: $R-X + Nu^- \to R-Nu + X^-$

Two main mechanisms: $S_N1$ and $S_N2$.

SN2 Mechanism (Bimolecular):

• Concerted single-step: Nu attacks while X leaves
• Transition state: Nu and X partially bonded to same C
• Inversion of configuration (Walden inversion) — like umbrella turning inside out

Rate Law: Rate = $k[R-X][Nu^-]$ — second order; depends on both.

Reactivity: $1° > 2° > 3°$ (steric hindrance increases with branching → 3° hardest).

• Methyl > primary > secondary >> tertiary

Factors Favoring SN2:

• Strong nucleophile
• Polar aprotic solvent (acetone, DMSO, DMF)
• Less steric hindrance (1° preferred)
• Good leaving group ($I^- > Br^- > Cl^- > F^-$)

SN1 Mechanism (Unimolecular):

• Two-step:

1. Slow: $R-X \to R^+ + X^-$ (ionization; rate-determining)
2. Fast: $R^+ + Nu^- \to R-Nu$

• Rate Law: Rate = $k[R-X]$ — first order; independent of Nu concentration.
• Stereochemistry: Racemization (planar $R^+$ → attack from both faces equally).

Reactivity: $3° > 2° > 1°$ (more stable carbocation; opposite of $S_N2$).

• Allyl, benzyl > tertiary > secondary > primary > methyl (carbocation stability)

Factors Favoring SN1:

• Stable carbocation (3° alkyl)
• Polar protic solvent (water, alcohol — stabilizes ionic intermediates by H-bonding)
• Weak/moderate nucleophile
• Good leaving group

Summary Comparison:

Elimination Reactions:

E1 Mechanism: Two-step. Carbocation intermediate, then loss of $\beta$-H by base.

• Reactivity: $3° > 2° > 1°$ (same as $S_N1$)
• Often competes with $S_N1$ in 3° alkyl halides

E2 Mechanism: Concerted. Base removes $\beta$-H as X⁻ leaves simultaneously.

• Reactivity: $3° > 2° > 1°$ (more substituted alkene more stable — Saytzeff)
• Requires anti-periplanar arrangement of H and X
• Rate = $k[R-X][B^-]$

SN vs E Competition:

Hofmann's Rule: Less substituted alkene (more H-rich double bond) preferred when:

• Bulky base used (e.g., $t$-butoxide)
• Quaternary ammonium hydroxide (Hofmann degradation)

Haloarenes (Aryl Halides): Halogen on aromatic ring carbon, e.g., chlorobenzene ($C_6H_5Cl$), bromobenzene.

Structure: $sp^2$ C-X bond. The lone pair on X overlaps with $\pi$ system of ring.

Resonance Structures: $X$ donates lone pair to ring → 4 resonance structures with double bond character in C-X.

Why aryl halides are unreactive toward $S_N$?

1. C-X bond is shorter and stronger (due to partial double bond from resonance)
2. Resonance stabilization of ground state makes ionization difficult
3. $sp^2$ C is more electronegative than $sp^3$ C — holds halide tightly
4. Direct $S_N1$/$S_N2$ on aryl C is sterically hindered and electronically unfavorable

Preparation of Haloarenes:

1. From Benzene (Electrophilic Aromatic Substitution): $C_6H_6 + Br_2 \xrightarrow{FeBr_3} C_6H_5Br + HBr$

• Lewis acid catalyst polarizes halogen

2. From Diazonium Salt (Sandmeyer Reaction): $C_6H_5N_2^+Cl^- + CuCl \to C_6H_5Cl + N_2 + CuCl$ (or CuBr → ArBr)

• For ArI: just KI; for ArF: Schiemann reaction with $HBF_4$ → $ArF + BF_3 + N_2$

Reactions of Aryl Halides:

1. Electrophilic Aromatic Substitution:

• $-X$ is o/p director, deactivating (due to +R donating but -I dominating)
• $C_6H_5Cl + HNO_3/H_2SO_4 \to o$- and $p$-nitrochlorobenzenes

2. Nucleophilic Substitution (Difficult):

• Requires high temperature and pressure
• $C_6H_5Cl + NaOH \xrightarrow{623K, 300\,atm} C_6H_5ONa \to C_6H_5OH$ (Dow's process for phenol from chlorobenzene)
• Activated by -NO₂, -CN groups at o/p positions (EW groups stabilize Meisenheimer intermediate)

Nucleophilic Aromatic Substitution Mechanism: Addition-elimination via Meisenheimer complex: $ArX + Nu^- \to [Ar(Nu)X]^- \to ArNu + X^-$ (intermediate has sp³ C).

With strong EW groups: $p$-nitrochlorobenzene + NaOH/heat → $p$-nitrophenol; $2,4$-dinitrochlorobenzene reacts at room T.

3. Benzyne Mechanism (in absence of activating groups, with very strong base like $NH_2^-$): $C_6H_5Cl + NaNH_2/NH_3(liq) \to$ benzyne intermediate → addition of Nu. Gives random position addition.

4. Wurtz-Fittig Reaction: $ArX + RX + 2Na \xrightarrow{ether} Ar-R + 2NaX$

• Aryl halide + alkyl halide + Na → alkyl aryl compound.

5. Fittig Reaction: $2ArX + 2Na \xrightarrow{ether} Ar-Ar + 2NaX$

• Two aryl halides + Na → biaryl.

Polyhalogen Compounds: Have more than one halogen atom.

1. Dichloromethane ($CH_2Cl_2$):

• Solvent for paint removal; aerosol propellants (banned now in many countries)
• Slightly soluble in water
• Toxic; carcinogen

2. Chloroform ($CHCl_3$):

• Anesthetic (historical use; now replaced)
• Solvent for fats, alkaloids, etc.
• On exposure to light + air → poisonous phosgene ($COCl_2$): $2CHCl_3 + O_2 \to 2COCl_2 + 2HCl$
• Stored in dark amber bottles with small amount of ethanol (which reacts with phosgene)

3. Iodoform ($CHI_3$):

• Yellow crystalline solid; sweet smell
• Antiseptic (mild; due to release of free iodine)
• Iodoform test: for $CH_3CO-$ or $CH_3CH(OH)-$ groups: react with I₂/NaOH → yellow ppt of $CHI_3$ + carboxylate.
• Positive: acetone, ethanol, acetaldehyde, methyl ketones, secondary alcohols with CH₃-CH(OH)-
• Negative: methanol, formaldehyde

4. Carbon Tetrachloride ($CCl_4$):

• Solvent
• Fire extinguishers (Pyrene; now phased out due to toxicity)
• Depletes ozone layer (limited use now)
• Can cause liver damage; carcinogen

5. Freons (Chlorofluorocarbons, CFCs):

• $CCl_2F_2$ (Freon-12), $CCl_3F$ (Freon-11), etc.
• Refrigerants, propellants
• Banned (Montreal Protocol, 1987) due to ozone layer depletion
• Replaced by HFCs (hydrofluorocarbons): no chlorine, less harmful

Mechanism of Ozone Depletion: $CFCl_3 \xrightarrow{UV} CFCl_2^\bullet + Cl^\bullet$ $Cl^\bullet + O_3 \to ClO^\bullet + O_2$ $ClO^\bullet + O \to Cl^\bullet + O_2$ (Cl regenerated; chain reaction)

6. DDT ($p,p'$-Dichlorodiphenyltrichloroethane):

• $Cl_2C_6H_4-CCl_3-C_6H_4Cl$ structure
• First synthetic insecticide (1939, Müller, Nobel Prize 1948)
• Highly effective against malaria mosquitoes
• Banned in many countries (1970s-2000s) due to:
• Bio-accumulation in food chain
• Toxicity to wildlife (especially birds — eggshell thinning)
• Carcinogenic concerns
• Still used in some places for malaria control

Applications of Haloalkanes: