Haloalkanes — Revision Notes

⚡ 30-Second Revision

General Formula — R-X (R=alkyl, X=F, Cl, Br, I)
C-X Bond — Polar, C is electrophilic, X is leaving group.
Classification — 1\textdegree, 2\textdegree, 3\textdegree (based on \(\alpha\)-carbon substitution).
Preparation

- Alcohols + HX/SOCl\_2/PCl\_3. - Alkenes + HX (Markovnikov's; HBr/peroxide for anti-Markovnikov's). - Finkelstein: R-Cl/Br + NaI \(\rightarrow\) R-I. - Swarts: R-Cl/Br + AgF \(\rightarrow\) R-F.

S\textsubscript{N}1 — 2 steps, carbocation, racemization, 3\textdegree > 2\textdegree > 1\textdegree, polar protic solvent.
S\textsubscript{N}2 — 1 step, concerted, inversion, 1\textdegree > 2\textdegree > 3\textdegree, polar aprotic solvent.
E1 — 2 steps, carbocation, Saytzeff, 3\textdegree > 2\textdegree > 1\textdegree.
E2 — 1 step, concerted, anti-periplanar, Saytzeff, 3\textdegree > 2\textdegree > 1\textdegree.
Competition — Strong bulky base/high temp favors E. Strong small nucleophile favors S.
Wurtz Reaction — 2R-X + 2Na \(\rightarrow\) R-R (symmetrical alkanes).
Grignard Reagent — R-X + Mg \(\rightarrow\) R-Mg-X.

2-Minute Revision

Haloalkanes (R-X) are crucial organic compounds with a polar C-X bond, making the carbon susceptible to nucleophilic attack. They are classified as primary, secondary, or tertiary, which dictates their reactivity.

Key preparation methods include converting alcohols with HX or SOCl\_2, adding hydrogen halides to alkenes (following Markovnikov's rule, or anti-Markovnikov with HBr/peroxides), and halogen exchange reactions like Finkelstein (for iodides) and Swarts (for fluorides).

The most important reactions are nucleophilic substitution (S\textsubscript{N}1 and S\textsubscript{N}2) and elimination (E1 and E2). S\textsubscript{N}1 is a two-step process via a carbocation, leading to racemization and favoring 3\textdegree haloalkanes in polar protic solvents.

S\textsubscript{N}2 is a one-step concerted reaction, causing inversion of configuration and favoring 1\textdegree haloalkanes in polar aprotic solvents. Elimination reactions (E1/E2) form alkenes, generally following Saytzeff's rule.

E2 is concerted and bimolecular, while E1 is two-step and unimolecular. Understanding the competition between S\textsubscript{N} and E reactions, influenced by substrate, reagent, and solvent, is vital for predicting major products.

Named reactions like Wurtz (for alkane synthesis) and the formation of Grignard reagents are also significant.

5-Minute Revision

Haloalkanes, or alkyl halides, are compounds where a halogen (F, Cl, Br, I) is bonded to an sp\textsuperscript{3} hybridized carbon. The C-X bond is polar, with carbon being partially positive, making it an electrophilic center. Their classification (1\textdegree, 2\textdegree, 3\textdegree) is based on the substitution of the carbon bearing the halogen, which profoundly influences their reactivity.

Preparation Methods:

1
From Alcohols — R-OH + HX \(\rightarrow\) R-X (reactivity 3\textdegree > 2\textdegree > 1\textdegree for alcohols, HI > HBr > HCl for HX). R-OH + SOCl\_2 \(\rightarrow\) R-Cl (Darzen's process, high yield). R-OH + PCl\_5 \(\rightarrow\) R-Cl.
2
From Alkenes — Addition of HX. For unsymmetrical alkenes, Markovnikov's rule applies (H to more H-rich carbon, X to less H-rich). Exception: HBr in presence of peroxides follows anti-Markovnikov's rule.

* *Example*: Propene + HCl \(\rightarrow\) 2-chloropropane.

1
Halogen Exchange

* Finkelstein Reaction: R-Cl/Br + NaI \(\xrightarrow{\text{Acetone}}\) R-I + NaCl/NaBr. (For iodoalkanes). * Swarts Reaction: R-Cl/Br + AgF/Hg\_2F\_2 \(\rightarrow\) R-F + AgCl/Hg\_2Cl\_2. (For fluoroalkanes).

Chemical Reactions: The two main types are nucleophilic substitution (S\textsubscript{N}) and elimination (E).

Nucleophilic Substitution (S\textsubscript{N}1 vs S\textsubscript{N}2)

* S\textsubscript{N}1: Two steps, carbocation intermediate, rate = k[R-X], reactivity 3\textdegree > 2\textdegree > 1\textdegree, racemization if chiral, favored by polar protic solvents (e.g., H\_2O, alcohols). * S\textsubscript{N}2: One step (concerted), transition state, rate = k[R-X][Nu\textsuperscript{-}], reactivity 1\textdegree > 2\textdegree > 3\textdegree, inversion of configuration (Walden inversion) if chiral, favored by polar aprotic solvents (e.g., DMSO, acetone).

Elimination Reactions (E1 vs E2)

* E1: Two steps, carbocation intermediate, rate = k[R-X], reactivity 3\textdegree > 2\textdegree > 1\textdegree, forms Saytzeff product, favored by polar protic solvents. * E2: One step (concerted), rate = k[R-X][Base], reactivity 3\textdegree > 2\textdegree > 1\textdegree, requires anti-periplanar H and X, forms Saytzeff product (unless bulky base), favored by strong bases and high temperatures.

Competition — Strong, bulky bases favor E2. Strong, small nucleophiles favor S\textsubscript{N}2. Tertiary haloalkanes favor S\textsubscript{N}1/E1. High temperatures favor elimination.

Reactions with Metals

* Wurtz Reaction: 2R-X + 2Na \(\xrightarrow{\text{Dry Ether}}\) R-R + 2NaX (for symmetrical alkanes). * Grignard Reagents: R-X + Mg \(\xrightarrow{\text{Dry Ether}}\) R-Mg-X (highly useful synthetic reagents).

Stereochemistry: Chiral haloalkanes exhibit optical activity. S\textsubscript{N}2 leads to inversion, while S\textsubscript{N}1 leads to racemization. Identifying chiral centers is key.

Prelims Revision Notes

Haloalkanes (R-X) are alkyl halides. The C-X bond is polar, making the carbon electrophilic.

Nomenclature & Classification:

IUPAC: Haloalkanes (e.g., chloromethane). Common: Alkyl halides (e.g., methyl chloride).
1\textdegree, 2\textdegree, 3\textdegree: Based on the number of carbons attached to the \(\alpha\)-carbon (the one bearing the halogen).

Preparation:

From Alcohols — R-OH + HX (Lucas test for 1\textdegree, 2\textdegree, 3\textdegree alcohols). R-OH + SOCl\_2 (Darzen's, best for R-Cl). R-OH + PCl\_3/PCl\_5/PBr\_3.
From Alkenes — Alkene + HX (Markovnikov's rule: H to more H-rich carbon, X to less H-rich). Exception: HBr in presence of peroxides (anti-Markovnikov's).
Halogen Exchange

* Finkelstein Reaction: R-Cl/Br + NaI \(\xrightarrow{\text{Acetone}}\) R-I. (NaI is soluble in acetone, NaCl/NaBr precipitates). * Swarts Reaction: R-Cl/Br + metallic fluorides (AgF, Hg\_2F\_2, CoF\_2, SbF\_3) \(\rightarrow\) R-F.

Physical Properties:

Boiling Point: R-I > R-Br > R-Cl > R-F (for same R). Increases with R size. Decreases with branching.
Density: R-I > R-Br > R-Cl (denser than water). R-F generally less dense than water.
Solubility: Sparingly soluble in water, soluble in organic solvents.

Chemical Reactions:

Nucleophilic Substitution (S\textsubscript{N}1 & S\textsubscript{N}2)

* S\textsubscript{N}1: 2 steps, carbocation intermediate, rate = k[R-X]. Reactivity: 3\textdegree > 2\textdegree > 1\textdegree. Stereochemistry: Racemization. Solvent: Polar protic (H\_2O, alcohols).

Weak nucleophiles. * S\textsubscript{N}2: 1 step (concerted), transition state, rate = k[R-X][Nu\textsuperscript{-}]. Reactivity: 1\textdegree > 2\textdegree > 3\textdegree (steric hindrance). Stereochemistry: Inversion (Walden inversion).

Solvent: Polar aprotic (DMSO, acetone). Strong nucleophiles.

Elimination Reactions (E1 & E2) — (Dehydrohalogenation):

* E1: 2 steps, carbocation intermediate, rate = k[R-X]. Reactivity: 3\textdegree > 2\textdegree > 1\textdegree. Product: Saytzeff's rule (more substituted alkene). Favored by polar protic solvents, weak bases. * E2: 1 step (concerted), rate = k[R-X][Base]. Reactivity: 3\textdegree > 2\textdegree > 1\textdegree. Requires anti-periplanar H and X. Product: Saytzeff's rule (unless bulky base). Favored by strong bases, high temperature.

Competition — High temperature, strong bulky base favors E. Strong small nucleophile favors S. Tertiary halides favor S\textsubscript{N}1/E1.
Reaction with Metals

* Wurtz Reaction: 2R-X + 2Na \(\xrightarrow{\text{Dry Ether}}\) R-R + 2NaX. (For symmetrical alkanes). * Grignard Reagents: R-X + Mg \(\xrightarrow{\text{Dry Ether}}\) R-Mg-X. (Highly reactive, used in synthesis).

Reduction — R-X \(\rightarrow\) R-H (e.g., with LiAlH\_4, NaBH\_4, or H\_2/Pd).

Vyyuha Quick Recall

Steric Nuisance 2 (SN2) means 1st degree is best, Inversion is the outcome. Stable Nice 1 (SN1) means 3rd degree is best, Racemization is the outcome.