Revision: Class 12 >> Haloalkanes and Haloarenes NEET (UG) Haloalkanes and Haloarenes

Compounds in which one or more hydrogen atom(s) of an alkane is (are) replaced by halogen.

General formula: R–X.

Compounds in which one or more hydrogen atom(s) directly bonded to an aromatic ring is (are) replaced by halogen.

General formula: Ar–X.

Formation of Alkyl Halide from Alcohols:

From Alkenes

(a) Addition of halogen acids:

(b) Allylic halogenation:

\[\ce{\underset{Propene}{CH3 - CH} = CH2 + Cl2 ->[775K] \underset{\underset{(Allyl chloride)}{3-Chloro-1-propene}}{ClCH2 - CH = CH2 + HCl}}\]

From alkanes (Swarts reaction)

Alkyl chloride or bromide react with AgF, SbF₃, or Hg₂F₂ give alkyl fluoride. This reaction is known Swarts reaction. Antimony trifluoride (SbF₂) is commonly used in this reaction.

2CH₃CH₃CI + Hg₂F₂→ 2CH₃CH₂F+ Hg₂Cl₂

From the halide exchange method (Finkelstein reaction)

Alkyl chloride or bromide react with Nal or KI in presence of acetone give alkyl iodide. It is halide exchange reaction or Finkelstein reaction. It is a S_N2 reaction.

\[\ce{R - Cl + Nal ->[Acetone or Methanol] R - I + NaCl}\]

\[\begin{array}{cc}
\phantom{..........}\ce{Cl}\phantom{................................................}\ce{I}\phantom{..}\\
\phantom{..........}|\phantom{..................................................}|\phantom{.}\\
\phantom{}\ce{CH3 - CH2 - CH - CH3 ->[Nal][Acetone] CH3 - CH2 - CH - CH3}\phantom{}
\end{array}\]

Borodine-Hunsdiecker reaction

Silver salt of carboxylic acid on heating with Br₂ + carbon tetrachloride give alkyl bromide

\[\ce{\underset{Silver acetate}{CH3COOAg} + Br2 ->[CCl4/Reflux] \underset{Methyl bromide}{CH3 - Br} + AgBr + CO2}\]

Note: If silver salt of carboxylic acid is heated with I₂ in presence of CCI₄ ester is obtained. This reaction is known as simonini reaction.

\[\ce{2RCOOAg + I2 ->[CCl4] \underset{Ester}{RCOOR} + 2CO2 + 2AgI}\]

If silver salt of carboxylic acid is heated with I₂ in presence of HgO or lead tetra acetate than alkyl iodide is obtained

\[\ce{2RCOOAg ->[I2/HgO][Δ] 2R - X + 2CO2 + HgI2 + H2O}\]

On the Basis of Number of Halogen Atoms-

Gem dihalide: Both halogens on the same carbon (e.g., 1,1-dichloroethane).
Vic dihalide: Halogens on adjacent carbons (e.g., 1,2-dichloroethane).

On the Basis of Type of Carbon Bearing the Halogen:

Alkyl halide carbon (with X) is sp³ hybridised; aryl halide carbon is sp² hybridised — this is why aryl C–X bond is shorter and stronger.

Common Names:

• Alkyl halide or aryl halide
• e.g., CH₃Cl → Methyl chloride; CH₂=CHCl → Vinyl chloride

IUPAC Names:

• Haloalkane or arylhalide
• Rule 1: Find the longest carbon chain containing the halogen. If a double/triple bond is present, give it priority.
• Rule 2: Number from the end nearer the first substituent. Assign each substituent a position number.
• Multiple same halogens → di-, tri-, tetra- prefix.
• Different halogens → list alphabetically and number to give the alphabetically first halogen the lowest possible number.

Examples:

• CH₃Cl → Chloromethane
• CH₂=CHCl → Chloroethene
• (CH₃)₃CCl → 2-Chloro-2-methylpropane (common: tert-butyl chloride)
• 2-Chloro-1-methylbenzene → o-Chlorotoluene → IUPAC: 1-Chloro-2-methylbenzene