Haloalkanes and Haloarenes — Revision Notes

⚡ 30-Second Revision

Haloalkanes: — R-X (alkyl halides)
Haloarenes: — Ar-X (aryl halides)
C-X bond: — Polar, C is electrophilic.
S\_N1: — 2 steps, carbocation, racemization, 3° > 2° > 1°, polar protic solvents.
S\_N2: — 1 step, transition state, inversion, 1° > 2° > 3°, polar aprotic solvents.
E1: — 2 steps, carbocation, Saytzeff's rule, high temp.
E2: — 1 step, concerted, Saytzeff's rule, strong base, high temp.
Markovnikov's Rule: — H to C with more H, X to C with fewer H (addition to alkenes).
Peroxide Effect: — Anti-Markovnikov for HBr only.
Finkelstein: — R-Cl/Br + NaI $\xrightarrow{\text{acetone}}$ R-I.
Swarts: — R-Cl/Br + AgF $\rightarrow$ R-F.
Sandmeyer: — Ar-N\_2^+Cl^- + CuX/HX $\rightarrow$ Ar-X.
Wurtz: — 2R-X + 2Na $\rightarrow$ R-R.
Fittig: — 2Ar-X + 2Na $\rightarrow$ Ar-Ar.
Wurtz-Fittig: — Ar-X + R-X + 2Na $\rightarrow$ Ar-R.
Grignard: — R-X + Mg $\xrightarrow{\text{dry ether}}$ R-MgX.
Haloarene reactivity: — Less reactive to S\_N, more reactive to E\_S (deactivating, o,p-directing).

2-Minute Revision

Haloalkanes (R-X) and haloarenes (Ar-X) are fundamental organic compounds. The C-X bond is polar, making the carbon susceptible to nucleophilic attack. Haloalkanes primarily undergo nucleophilic substitution (S\_N1 and S\_N2) and elimination (E1 and E2) reactions.

S\_N1 is a two-step process involving a carbocation, leading to racemization, and is favored by tertiary halides and protic solvents. S\_N2 is a one-step, concerted reaction with backside attack, resulting in inversion of configuration, favored by primary halides and aprotic solvents.

Elimination reactions (E1, E2) remove HX to form alkenes, often following Saytzeff's rule for the major product. Haloarenes are significantly less reactive towards nucleophilic substitution due to resonance and sp\_2 hybridization, but undergo electrophilic substitution on the ring (deactivating but ortho/para directing).

Key preparation methods include reactions of alcohols, free radical halogenation, addition to alkenes (Markovnikov's, anti-Markovnikov for HBr/peroxide), and halogen exchange (Finkelstein, Swarts). Important name reactions like Sandmeyer, Wurtz, Fittig, and Wurtz-Fittig are crucial.

Stereochemistry, including chirality and optical activity, is also a vital aspect, particularly concerning the outcomes of S\_N1 and S\_N2 reactions.

5-Minute Revision

Haloalkanes and haloarenes are organic compounds where hydrogen atoms are replaced by halogens. The C-X bond is polar, with carbon being partially positive, making it an electrophilic center. This polarity drives their characteristic reactions.

Haloalkanes primarily undergo:

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Nucleophilic Substitution (S\_N1 and S\_N2):

* S\_N1: Two steps, carbocation intermediate. Reactivity: 3° > 2° > 1°. Leads to racemization if chiral. Favored by polar protic solvents (e.g., H\_2O, alcohols) and weak nucleophiles. * S\_N2: One step, concerted backside attack. Reactivity: 1° > 2° > 3°. Leads to inversion of configuration (Walden inversion) if chiral. Favored by polar aprotic solvents (e.g., acetone, DMSO) and strong nucleophiles.

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Elimination (E1 and E2): — Dehydrohalogenation to form alkenes.

* E1: Two steps, carbocation intermediate. Favored by 3° halides, weak bases, high temperature. Follows Saytzeff's rule. * E2: One step, concerted. Favored by 1°/2° halides, strong bases (e.g., alcoholic KOH), high temperature. Follows Saytzeff's rule.

Preparation of Haloalkanes:

From alcohols: R-OH + HX, PCl\_3/PCl\_5, SOCl\_2 (Darzen's method).
From alkenes: H-X (Markovnikov's rule), HBr/peroxide (anti-Markovnikov's rule), X\_2.
Halogen exchange: Finkelstein (R-I from R-Cl/Br + NaI/acetone), Swarts (R-F from R-Cl/Br + AgF).

Haloarenes are less reactive towards nucleophilic substitution due to resonance (partial double bond character of C-X) and sp\_2 hybridization. They undergo:

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Electrophilic Substitution: — Halogens are deactivating but ortho-para directing. Examples: halogenation (with Lewis acid), nitration, sulfonation, Friedel-Crafts.

Preparation of Haloarenes:

From benzene: Electrophilic substitution (e.g., C\_6H\_6 + Cl\_2 $\xrightarrow{\text{FeCl\_3}}$ C\_6H\_5Cl).
From diazonium salts: Sandmeyer (Ar-X from Ar-N\_2^+Cl^- + CuX/HX), Gattermann (similar, uses Cu powder), Balz-Schiemann (Ar-F).

Name Reactions involving Metals:

Wurtz: — 2R-X + 2Na $\rightarrow$ R-R (alkanes).
Fittig: — 2Ar-X + 2Na $\rightarrow$ Ar-Ar (diaryls).
Wurtz-Fittig: — Ar-X + R-X + 2Na $\rightarrow$ Ar-R (alkylarenes).
Grignard Reagent: — R-X + Mg $\xrightarrow{\text{dry ether}}$ R-MgX (highly reactive nucleophile).

Stereochemistry: Understand chiral centers, enantiomers, diastereomers, and the concepts of inversion (S\_N2) and racemization (S\_N1). Polyhalogen compounds like chloroform, DDT, and Freons have specific uses and environmental impacts.

Prelims Revision Notes

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Classification: — Haloalkanes (R-X) - 1°, 2°, 3°, allylic, benzylic. Haloarenes (Ar-X) - halogen directly on aromatic ring, vinylic.
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C-X Bond: — Polar, C is electrophilic ($C^{\delta+}-X^{\delta-}$). Bond length: C-I > C-Br > C-Cl > C-F. Bond strength: C-F > C-Cl > C-Br > C-I.
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Preparation of Haloalkanes:

* From Alcohols: R-OH + HX (reactivity: HI > HBr > HCl; 3° > 2° > 1° alcohols), PCl\_3, PCl\_5, SOCl\_2 (Darzen's, best for R-Cl). * From Alkenes: H-X (Markovnikov's rule). HBr + peroxide (anti-Markovnikov's rule, free radical mechanism). X\_2 (vicinal dihalides). * Halogen Exchange: Finkelstein (R-I from R-Cl/Br + NaI/acetone). Swarts (R-F from R-Cl/Br + AgF).

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Preparation of Haloarenes:

* Electrophilic Substitution: Ar-H + X\_2 $\xrightarrow{\text{Lewis Acid}}$ Ar-X (e.g., Cl\_2/FeCl\_3). * From Diazonium Salts: Sandmeyer (Ar-X from Ar-N\_2^+Cl^- + CuX/HX). Gattermann (similar, uses Cu powder). Balz-Schiemann (Ar-F).

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Physical Properties: — Higher boiling points than hydrocarbons (dipole-dipole). Density > water. Insoluble in water, soluble in organic solvents.
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Chemical Reactions of Haloalkanes:

* Nucleophilic Substitution: * S\_N1: 2 steps, carbocation intermediate. Rate = k[R-X]. Reactivity: 3° > 2° > 1°. Racemization. Favored by polar protic solvents, weak nucleophiles. * S\_N2: 1 step, concerted.

Rate = k[R-X][Nu^-]. Reactivity: CH\_3X > 1° > 2° > 3°. Inversion (Walden inversion). Favored by polar aprotic solvents, strong nucleophiles. * Elimination (Dehydrohalogenation): Alcoholic KOH. Saytzeff's rule (more substituted alkene is major product).

* Reaction with Metals: Wurtz (R-R), Grignard (R-MgX).

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Chemical Reactions of Haloarenes:

* Nucleophilic Substitution: Very difficult (resonance, sp\_2 C-X bond). Activated by electron-withdrawing groups at o/p positions. * Electrophilic Substitution: Halogens are deactivating but ortho-para directing. * Reaction with Metals: Wurtz-Fittig (Ar-R), Fittig (Ar-Ar), Ullmann (Ar-Ar from Ar-I + Cu).

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Stereochemistry: — Chiral center (4 different groups). Enantiomers (non-superimposable mirror images). Racemic mixture (equimolar enantiomers, optically inactive). Inversion (S\_N2). Racemization (S\_N1).
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Polyhalogen Compounds: — CH\_2Cl\_2 (solvent), CHCl\_3 (solvent, anesthetic), CHI\_3 (antiseptic), CCl\_4 (solvent, fire extinguisher), DDT (insecticide), Freons (refrigerants, ozone depleting).

Vyyuha Quick Recall

S.N.A.P.E.S.: S\_N1: No Attack, Planar Enemy (carbocation), Solvent (protic). S\_N2: Nucleophile Attacks Primary Enemy (backside), Solvent (aprotic). This helps remember the key features of S\_N1 (carbocation, protic solvent) and S\_N2 (backside attack, primary substrate, aprotic solvent).