Reactions of Halogenoalkanes

Bond Polarity: Halogens are more electronegative than carbon, creating a polar bond where the carbon atom carries a partial positive charge ($\delta+$) and the halogen carries a partial negative charge ($\delta-$). This electron deficiency makes the carbon atom a target for nucleophiles, which are electron-pair donors.

Bond Enthalpy: While polarity suggests fluoroalkanes should be the most reactive due to the highest $\delta+$ charge, the bond enthalpy (strength) is the actual determining factor for reactivity. The $C-F$ bond is the strongest ($467 \text{ kJ mol}^{-1}$) and hardest to break, while the $C-I$ bond is the weakest ($228 \text{ kJ mol}^{-1}$), making iodoalkanes the most reactive.

Mechanism: In an elimination reaction, the $OH^-$ ion acts as a base rather than a nucleophile. It removes a proton ($H^+$) from a carbon atom adjacent to the carbon bonded to the halogen, leading to the formation of a $C=C$ double bond.

Conditions: Elimination is favored when using ethanolic sodium hydroxide and high temperatures (hot). The small molecule $HX$ (hydrogen halide) is removed, resulting in an alkene, water, and a halide salt.

Regioselectivity: If the halogenoalkane is unsymmetrical, elimination can occur from different adjacent carbons, potentially leading to a mixture of isomeric alkene products.

Hydrolysis Rate: The rate of reaction can be measured by adding aqueous silver nitrate and ethanol to halogenoalkanes. The ethanol acts as a common solvent to allow the reactants to mix.

Precipitate Formation: As the reaction progresses, halide ions ($X^-$) are released and react with $Ag^+$ ions to form colored precipitates: $AgCl$ (white), $AgBr$ (cream), and $AgI$ (yellow).

Observation: The yellow precipitate ($AgI$) forms the fastest, while the white precipitate ($AgCl$) forms the slowest. This confirms that bond enthalpy (weakest $C-I$ bond) is the dominant factor in determining the rate of reaction.

Curly Arrow Precision: Always start curly arrows from a lone pair of electrons or the center of a bond. Ensure the arrow points exactly to the atom receiving the electrons.

Solvent Awareness: Always check if the solvent is aqueous or ethanolic. This is the most common way examiners distinguish between substitution and elimination pathways.

Nitrile Chain Length: When drawing the product of a reaction with $CN^-$, remember that the total number of carbons in the organic product increases by one. A common mistake is to keep the same number of carbons.

Ammonia Conditions: For amine formation, specify 'excess' ammonia and 'sealed tube' or 'under pressure' to ensure full marks for conditions.