Haloalkanes and Haloarenes

This chapter introduces haloalkanes and haloarenes, which are organic compounds formed when a halogen atom replaces a hydrogen atom in an aliphatic or aromatic hydrocarbon, respectively. You will learn to classify these compounds based on the halogen's position, name them using IUPAC rules, and understand the polar nature of the carbon-halogen bond that governs their reactivity. The chapter covers various methods of preparation, such as from alcohols and hydrocarbons, and through specific named reactions like Finkelstein and Swarts reactions.

A significant portion of the chapter is dedicated to chemical reactions. For haloalkanes, the focus is on nucleophilic substitution (SN1 and SN2 mechanisms) and elimination reactions. Understanding the stereochemical aspects of these reactions, including concepts like chirality, inversion, and racemisation, is essential. For haloarenes, you will study their lower reactivity towards nucleophilic substitution and their characteristic electrophilic substitution reactions like halogenation and nitration. The chapter also covers reactions with metals, leading to important reagents like the Grignard reagent.

The most challenging part for many students is mastering the conditions that favour SN1 versus SN2 mechanisms and predicting the products of competing substitution and elimination reactions. A strong foundation in General Organic Chemistry and the Hydrocarbons chapter from Class 11, particularly concepts like isomerism, electronic effects, and basic reaction intermediates, is necessary to do well in this chapter.

Formulas for Haloalkanes and Haloarenes

Preparation from alcohol (Darzen's process): $R-OH + SOCl_2 \xrightarrow{\text{Pyridine}} R-Cl + SO_2 \uparrow + HCl \uparrow$ A preferred method for preparing alkyl chlorides as the byproducts are gases.
Finkelstein reaction: $R-X + NaI \xrightarrow{\text{Dry Acetone}} R-I + NaX$ A halogen exchange method to prepare alkyl iodides. X is typically Cl or Br.
Swarts reaction: $R-X + AgF \rightarrow R-F + AgX$ A halogen exchange method to prepare alkyl fluorides using metallic fluorides.
Saytzeff's rule for elimination: $CH_3CH_2CH(Br)CH_3 \xrightarrow{\text{alc. KOH}, \Delta} CH_3CH=CHCH_3 \text{ (Major Product)}$ In β-elimination, the more substituted alkene is the major product.
Grignard reagent formation: $R-X + Mg \xrightarrow{\text{Dry Ether}} R-Mg-X$ Forms a highly reactive organometallic compound. R can be alkyl or aryl.
Wurtz reaction: $2R-X + 2Na \xrightarrow{\text{Dry Ether}} R-R + 2NaX$ Couples two alkyl halides to form a symmetrical alkane.
Wurtz-Fittig reaction: $C_6H_5X + R-X + 2Na \xrightarrow{\text{Dry Ether}} C_6H_5-R + 2NaX$ Couples an aryl halide with an alkyl halide to form an alkylarene.
Fittig reaction: $2C_6H_5X + 2Na \xrightarrow{\text{Dry Ether}} C_6H_5-C_6H_5 + 2NaX$ Couples two aryl halides to form a biphenyl compound.
Sandmeyer reaction: $ArN_2^+X^- \xrightarrow{Cu_2X_2/HX} Ar-X + N_2$ Prepares aryl halides from diazonium salts. X is Cl or Br.
Friedel-Crafts alkylation of haloarene: $C_6H_5Cl + CH_3Cl \xrightarrow{\text{Anhydrous } AlCl_3} C_6H_4(Cl)(CH_3) + HCl$ Gives a mixture of ortho and para isomers, with the para isomer being major.