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Properties of Haloalkanes

Physical trends and Chemical reactions ($S_N1$, $S_N2$, Elimination).

By chemca Team • Updated Jan 2026

Haloalkanes contain a polar $C-X$ bond. This polarity dictates their physical properties (higher boiling points than alkanes) and their high reactivity towards nucleophiles (Substitution) and bases (Elimination).

1. Physical Properties

Trends

• Boiling Point: Increases with molecular mass ($RI > RBr > RCl > RF$).
  For isomers, BP decreases with branching due to reduced surface area ($n\text{-butyl} > \text{iso-butyl} > \text{tert-butyl}$).
• Solubility: Although polar, haloalkanes are insoluble in water because they cannot form Hydrogen bonds with water. They are soluble in organic solvents.
• Density: Increases with atomic mass of halogen. $RI$ and $RBr$ are generally heavier than water.

2. Nucleophilic Substitution Reactions ($S_N$)

The halogen atom is replaced by a nucleophile ($OH^-, CN^-, NH_2^-, etc.$).

$$ R-X + Nu^- \rightarrow R-Nu + X^- $$

A. $S_N2$ Mechanism (Bimolecular)

A single-step reaction where the nucleophile attacks from the backside, leading to a transition state and Inversion of Configuration (Walden Inversion).

$$ Nu^- + R-X \rightarrow [Nu \cdot\cdot\cdot C \cdot\cdot\cdot X]^\ddagger \rightarrow Nu-R + X^- $$

Reactivity Order: $1^\circ > 2^\circ > 3^\circ$ (Steric Hindrance governs rate)

B. $S_N1$ Mechanism (Unimolecular)

A two-step reaction involving the formation of a stable Carbocation intermediate. If the carbon is chiral, the product is a racemic mixture (Racemization).

1. $R-X \rightleftharpoons R^+ + X^-$ (Slow/RDS)
2. $R^+ + Nu^- \rightarrow R-Nu$ (Fast)

Reactivity Order: $3^\circ > 2^\circ > 1^\circ$ (Carbocation stability governs rate)

Feature	$S_N1$ Reaction	$S_N2$ Reaction
Kinetics	1st Order ($Rate \propto [RX]$)	2nd Order ($Rate \propto [RX][Nu]$)
Steps	Two Steps	One Step
Intermediate	Carbocation	Transition State
Stereochemistry	Racemization	Inversion
Solvent	Polar Protic ($H_2O, ROH$)	Polar Aprotic (Acetone, DMSO)

3. Elimination Reactions (Dehydrohalogenation)

$\beta$-Elimination

When heated with alcoholic KOH, haloalkanes lose a hydrogen from the $\beta$-carbon and the halogen to form alkenes.

$$ CH_3-CH_2-CH(Br)-CH_3 \xrightarrow{alc. KOH, \Delta} \text{But-2-ene (Major)} + \text{But-1-ene (Minor)} $$

Saytzeff Rule: "In dehydrohalogenation, the preferred product is the alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms." (More substituted alkene is more stable).

4. Reaction with Metals

Organometallic Compounds

A. Grignard Reagent: Reaction with Magnesium in dry ether forms Alkyl Magnesium Halide.

$$ R-X + Mg \xrightarrow{\text{Dry Ether}} R-MgX $$

B. Wurtz Reaction: Reaction with Sodium in dry ether couples two alkyl groups.

$$ 2R-X + 2Na \xrightarrow{\text{Dry Ether}} R-R + 2NaX $$

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