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Haloalkanes & Haloarenes Formulas

SN1 and SN2 nucleophilic substitutions, E1 and E2 eliminations, Saytzeff rule, and Grignard reagent synthesis.

8 Formulas•527+ Questions in Bank

Core Formulas & Mathematical Identities

8
01

SN1 vs SN2 Nucleophilic Substitution

SN1:  Rate=k[R-X]  (3∘>2∘>1∘),SN2:  Rate=k[R-X][Nu−]  (CH3X>1∘>2∘>3∘)\text{S}_\text{N}1: \; \text{Rate} = k[\text{R-X}] \; (3^\circ > 2^\circ > 1^\circ), \quad \text{S}_\text{N}2: \; \text{Rate} = k[\text{R-X}][\text{Nu}^-] \; (\text{CH}_3\text{X} > 1^\circ > 2^\circ > 3^\circ)

Two-step unimolecular vs one-step concerted bimolecular substitution.

Examiner Tip: Polar protic solvents favor SN1; polar aprotic solvents (DMSO, DMF, acetone) favor SN2.
02

Saytzeff vs Hofmann Elimination Rule

Saytzeff: More substituted alkene (more α-H),Hofmann: Less substituted alkene (bulky base like t-BuO−)\text{Saytzeff: More substituted alkene (more } \alpha\text{-H)}, \quad \text{Hofmann: Less substituted alkene (bulky base like } t\text{-BuO}^-)

Regioselectivity in dehydrohalogenation reactions.

Examiner Tip: Alcoholic KOH promotes E2 elimination.
03

Grignard Reagent Preparation & Reactions

R-X+Mg→dry etherR-MgX→H2OR-H+Mg(OH)X\text{R-X} + \text{Mg} \xrightarrow{\text{dry ether}} \text{R-MgX} \xrightarrow{\text{H}_2\text{O}} \text{R-H} + \text{Mg(OH)X}

Organomagnesium halide organometallic reagent acts as powerful carbanion nucleophile and strong base.

Examiner Tip: Reacts with active hydrogen compounds (water, alcohol, amine) instantly to give alkane.
04

Halogen Exchange: Finkelstein & Swarts Reactions

Finkelstein: R-Cl/Br+NaI→acetoneR-I+NaCl↓,Swarts: R-Cl/Br+AgF/SbF3→R-F+AgX↓\text{Finkelstein: } \text{R-Cl}/\text{Br} + \text{NaI} \xrightarrow{\text{acetone}} \text{R-I} + \text{NaCl}\downarrow, \quad \text{Swarts: } \text{R-Cl}/\text{Br} + \text{AgF}/\text{SbF}_3 \to \text{R-F} + \text{AgX}\downarrow

Specific pathways for synthesizing alkyl iodides (precipitation drives equilibrium) and alkyl fluorides (using heavy metal fluorides).

Examiner Tip: Swarts fluorinating agents: AgF, Hg₂F₂, CoF₃, SbF₃.
05

Nucleophilic Aromatic Substitution (S_NAr & Benzyne)

SNAr Rate: Ar-F≫Ar-Cl>Ar-Br>Ar-I  (with −NO2 at o/p),Benzyne: Ph-Cl+NaNH2→liq NH3[C6H4]→Ph-NH2\text{S}_\text{N}\text{Ar Rate: } \text{Ar-F} \gg \text{Ar-Cl} > \text{Ar-Br} > \text{Ar-I} \; (\text{with } -\text{NO}_2 \text{ at } o/p), \quad \text{Benzyne: } \text{Ph-Cl} + \text{NaNH}_2 \xrightarrow{\text{liq } \text{NH}_3} [\text{C}_6\text{H}_4] \to \text{Ph-NH}_2

S_NAr proceeds via Meisenheimer complex (rate-determining addition). Strong base on unactivated haloarene proceeds via benzyne intermediate.

Examiner Tip: Benzyne mechanism leads to cine substitution (50:50 mixture of substituted products).
06

Haloform Reaction Mechanism & Specificity

R-CO-CH3+3X2+4OH−→R-COO−+CHX3↓+3X−+3H2O(CHI3 Yellow ppt)\text{R-CO-CH}_3 + 3\text{X}_2 + 4\text{OH}^- \to \text{R-COO}^- + \text{CHX}_3\downarrow + 3\text{X}^- + 3\text{H}_2\text{O} \quad (\text{CHI}_3 \text{ Yellow ppt})

Characteristic test for methyl ketones (CH₃-C=O) and secondary alcohols oxidizable to them (CH₃-CH(OH)-R), plus ethanol and acetaldehyde.

Examiner Tip: Tertiary alcohols and methanol DO NOT give the haloform test.
07

Elimination Reaction Spectrum (E1, E2, E1cB)

E1: Carbocation intermediate,E2: Anti-periplanar concerted TS,E1cB: Carbanion intermediate (poor leaving group like F)\text{E1: Carbocation intermediate}, \quad \text{E2: Anti-periplanar concerted TS}, \quad \text{E1cB: Carbanion intermediate (poor leaving group like F)}

Mechanistic comparison of β-eliminations. E1cB occurs when strong electron-withdrawing groups stabilize conjugate carbanion base with poor leaving group.

Examiner Tip: E2 dehydrohalogenation requires strictly anti-coplanar alignment of β-hydrogen and α-halogen.
08

Coupling Reactions: Wurtz, Fittig & Ullmann Synthesis

Wurtz: 2R-X+2Na→R-R,Fittig: 2Ar-X+2Na→Ar-Ar,Ullmann: 2Ar-I+Cu→ΔAr-Ar\text{Wurtz: } 2\text{R-X} + 2\text{Na} \to \text{R-R}, \quad \text{Fittig: } 2\text{Ar-X} + 2\text{Na} \to \text{Ar-Ar}, \quad \text{Ullmann: } 2\text{Ar-I} + \text{Cu} \xrightarrow{\Delta} \text{Ar-Ar}

Reductive coupling reactions forming symmetric carbon-carbon bonds. Ullmann reaction specifically synthesizes biaryls using copper powder.

Examiner Tip: Ullmann reaction works best with ortho-halonitrobenzenes.

Top Examiner Traps & Pitfalls

Frequently tested misconceptions in Haloalkanes & Haloarenes

  • •SN1 involves carbocation rearrangement and racemization; SN2 involves Walden inversion with NO carbocation intermediate.
  • •Saytzeff elimination gives more substituted stable alkene; Hofmann elimination gives less substituted alkene (favored by bulky bases).
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