Physical and Chemical Properties — Revision Notes

⚡ 30-Second Revision

Boiling Point: — RI > RBr > RCl > RF (for same R); decreases with branching. Higher than alkanes due to polarity + mass.
Density: — RI > RBr > RCl (for same R); increases with number of halogens. Often > water.
Solubility: — Sparingly soluble in water (no H-bonds); soluble in organic solvents.
S\(_N\)1 Reactivity: — 3° > 2° > 1° > CH\(_3\)X (carbocation stability). Racemization.
S\(_N\)2 Reactivity: — CH\(_3\)X > 1° > 2° >> 3° (steric hindrance). Inversion.
Leaving Group Ability: — I\(^-^\) > Br\(^-^\) > Cl\(^-^^\) > F\(^-^\).
Elimination (E1/E2): — Forms alkenes. Favored by strong bases, high temp. Saytzeff's rule: more substituted alkene is major.
Grignard Reagent: — R-X + Mg \xrightarrow{\text{dry ether}} R-Mg-X. Powerful nucleophile.
Wurtz Reaction: — 2R-X + 2Na \xrightarrow{\text{dry ether}} R-R + 2NaX.

2-Minute Revision

Haloalkanes exhibit distinct physical and chemical properties due to the polar C-X bond. Physically, their boiling points increase with molecular mass (RI > RBr > RCl > RF) and decrease with branching, generally being higher than alkanes due to dipole-dipole interactions.

They are often denser than water and are sparingly soluble in water but soluble in organic solvents. Chemically, their reactivity is dominated by nucleophilic substitution (S\(_N\)1 and S\(_N\)2) and elimination (E1 and E2) reactions.

S\(_N\)1 favors tertiary halides and proceeds via a carbocation, leading to racemization. S\(_N\)2 favors primary halides, is concerted, and leads to inversion. Elimination, or dehydrohalogenation, forms alkenes, with the major product often predicted by Saytzeff's rule.

Reaction with magnesium in dry ether yields Grignard reagents (R-Mg-X), which are crucial synthetic tools. Understanding the factors influencing these reactions (substrate, reagent, solvent, temperature) is key for NEET.

5-Minute Revision

The physical and chemical properties of haloalkanes are central to their behavior. Physical properties are governed by intermolecular forces. Boiling points increase with the atomic mass of the halogen (RI > RBr > RCl > RF) due to stronger van der Waals forces.

Branching decreases boiling points by reducing surface area. Haloalkanes generally have higher boiling points than alkanes of similar molecular mass due to the additional dipole-dipole interactions from the polar C-X bond.

Densities are typically higher than water (especially for bromides and iodides) and increase with halogen atomic mass and number of halogens. They are sparingly soluble in water because they cannot form strong hydrogen bonds, but readily dissolve in organic solvents.

Chemical properties are dominated by the reactivity of the C-X bond. The two main reaction types are:

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Nucleophilic Substitution (S\(_N\)1 and S\(_N\)2): — The halogen is replaced by a nucleophile.

* S\(_N\)1: Two steps, carbocation intermediate. Reactivity: 3° > 2° > 1°. Favored by weak nucleophiles, protic solvents. Leads to racemization. * S\(_N\)2: One concerted step, backside attack. Reactivity: CH\(_3\)X > 1° > 2° >> 3°. Favored by strong nucleophiles, aprotic solvents. Leads to inversion. * Leaving Group: I\(^-^\) > Br\(^-^\) > Cl\(^-^\) > F\(^-^\).

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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. * E2: One concerted step. Favored by 1°/2° halides, strong bases, high temperature. * Saytzeff's Rule: Major product is the more substituted alkene.

Competition: Strong, bulky bases at high temperatures favor elimination. Strong, unhindered nucleophiles at lower temperatures favor substitution. For example, 2-bromopropane with alcoholic KOH (strong base, high temp) yields propene (elimination), while with aqueous KOH (strong nucleophile, lower temp) yields propan-2-ol (substitution).

Other Reactions:

Grignard Reagents: — R-X + Mg \xrightarrow{\text{dry ether}} R-Mg-X. Highly reactive nucleophiles for C-C bond formation.
Wurtz Reaction: — 2R-X + 2Na \xrightarrow{\text{dry ether}} R-R + 2NaX. Forms higher alkanes.
Reduction: — R-X \(\rightarrow\) R-H (e.g., with LiAlH\(_4\) or Zn/HCl).

Example: Predict the major product when 2-chloro-2-methylpropane reacts with alcoholic KOH. This is a tertiary halide with a strong base (alcoholic KOH) at high temperature, favoring E2 elimination. The only possible alkene product is 2-methylpropene. This demonstrates the application of reaction type identification and product prediction.

Prelims Revision Notes

Physical Properties

Boiling Points:

* Order for same alkyl group: RI > RBr > RCl > RF (due to increasing molecular mass, stronger van der Waals forces). * Order for same halogen: increases with increasing size of alkyl group. * Effect of branching: Boiling point decreases with increasing branching (reduced surface area, weaker van der Waals forces).

* Comparison with alkanes: Haloalkanes have higher boiling points than alkanes of comparable molecular mass due to C-X bond polarity (dipole-dipole interactions) in addition to van der Waals forces.

Density:

* Generally higher than alkanes and water (except some fluoro- and chloroalkanes). * Order for same alkyl group: RI > RBr > RCl (due to increasing atomic mass of halogen). * Increases with the number of halogen atoms (e.g., CH\(_2\)Cl\(_2\) > CH\(_3\)Cl).

Solubility:

* Sparingly soluble in water: Cannot form strong hydrogen bonds with water; energy required to break water's H-bonds is not compensated. * Soluble in organic solvents: 'Like dissolves like' principle.

Chemical Properties

Nucleophilic Substitution (S\(_N\)1 and S\(_N\)2):

* S\(_N\)1: 2 steps, carbocation intermediate. Reactivity: 3° > 2° > 1° > CH\(_3\)X. Favored by weak nucleophiles, protic solvents. Stereochemistry: Racemization. * S\(_N\)2: 1 concerted step, backside attack. Reactivity: CH\(_3\)X > 1° > 2° >> 3°. Favored by strong nucleophiles, aprotic solvents. Stereochemistry: Inversion. * Leaving Group Ability: I\(^-^\) > Br\(^-^\) > Cl\(^-^\) > F\(^-^\) (weak bases are good leaving groups).

Elimination (E1 and E2) / Dehydrohalogenation: — Formation of alkenes.

* E1: 2 steps, carbocation intermediate. Favored by 3° halides, weak bases, high temperature. * E2: 1 concerted step. Favored by 1°/2° halides, strong bases, high temperature. * Saytzeff's Rule: Major product is the more substituted (more stable) alkene.

Competition between S\(_N\) and E:

* Strong, bulky bases + high temperature $\rightarrow$ Elimination. * Strong, unhindered nucleophiles + lower temperature $\rightarrow$ Substitution.

Reaction with Metals:

* Grignard Reagent: R-X + Mg \xrightarrow{\text{dry ether}} R-Mg-X. Highly useful synthetic intermediate (strong nucleophile). * Wurtz Reaction: 2R-X + 2Na \xrightarrow{\text{dry ether}} R-R + 2NaX. Forms higher alkanes.

Reduction: — R-X + [H] \(\rightarrow\) R-H (e.g., LiAlH\(_4\), Zn/HCl).

Vyyuha Quick Recall

HALO-PROPS: Hydrogen-bonding (no in water), Atomic mass (BP increases), Leaving group (I > Br > Cl > F), Organic soluble, Polar (C-X), Reactivity (S\(_N\)1/S\(_N\)2/E), Order (3° S\(_N\)1, 1° S\(_N\)2), Products (Saytzeff's), Synthesis (Grignard).