Key Points

Haloalkanes contain a halogen atom bonded to an $sp^3$ hybridized carbon atom of an alkyl group. Haloarenes have a halogen atom bonded to an $sp^2$ hybridized carbon atom of an aryl group. They are classified as primary ($1^{\circ}$), secondary ($2^{\circ}$), or tertiary ($3^{\circ}$) based on the carbon to which the halogen is attached.

The C-X bond is polar because halogens are more electronegative than carbon, creating a partial positive charge ($\delta^+$) on carbon and a partial negative charge ($\delta^-$) on the halogen. Bond length increases and bond enthalpy decreases down the group from F to I.

Alcohols can be converted to haloalkanes using concentrated halogen acids ($HX$ with $ZnCl_2$), phosphorus halides ($PX_3, PCl_5$), or thionyl chloride ($SOCl_2$). The reaction with thionyl chloride is preferred as the byproducts ($SO_2$ and $HCl$) are gases, yielding a pure product.

When adding a hydrogen halide ($HX$) to an unsymmetrical alkene, the negative part (halogen) attaches to the carbon atom with fewer hydrogen atoms. For example, propene reacts with HBr to form 2-bromopropane: $CH_3-CH=CH_2 + HBr \rightarrow CH_3-CH(Br)-CH_3$.

This reaction prepares alkyl iodides from alkyl chlorides or bromides by reacting them with sodium iodide (NaI) in dry acetone. The reaction is $R-X + NaI \rightarrow R-I + NaX$, where $X = Cl, Br$.

Alkyl fluorides are synthesized by heating an alkyl chloride or bromide with a metallic fluoride like $AgF$, $Hg_2F_2$, or $SbF_3$. The reaction is $CH_3-Br + AgF \rightarrow CH_3-F + AgBr$.

This method converts a primary aromatic amine into a haloarene by first forming a diazonium salt ($ArN_2^+X^-$) and then treating it with a cuprous halide ($CuX$). The reaction is $Ar-N_2^+Cl^- \xrightarrow{CuCl/HCl} Ar-Cl + N_2$.

Substitution Nucleophilic Bimolecular ($S_N2$) is a single-step reaction where the rate depends on both the haloalkane and the nucleophile. It results in inversion of configuration. The reactivity order is primary > secondary > tertiary ($1^{\circ} > 2^{\circ} > 3^{\circ}$) due to steric hindrance.

Substitution Nucleophilic Unimolecular ($S_N1$) is a two-step reaction that proceeds through a planar carbocation intermediate. The rate depends only on the haloalkane concentration and results in racemisation. The reactivity order is tertiary > secondary > primary ($3^{\circ} > 2^{\circ} > 1^{\circ}$) due to carbocation stability.

A chiral carbon is bonded to four different groups, leading to non-superimposable mirror images called enantiomers. Enantiomers are optically active, rotating plane-polarized light in opposite directions.

When a haloalkane with a $\beta$-hydrogen is heated with an alcoholic solution of potassium hydroxide (KOH), it undergoes dehydrohalogenation to form an alkene. This is known as a $\beta$-elimination reaction.

In dehydrohalogenation reactions, the major product is the more substituted alkene, meaning the alkene with the greater number of alkyl groups attached to the double-bonded carbon atoms.

Alkyl magnesium halides ($R-Mg-X$), known as Grignard reagents, are formed by reacting haloalkanes with magnesium in dry ether. They are highly reactive and act as strong nucleophiles or bases, reacting with any source of protons to form hydrocarbons.

The Wurtz reaction couples two alkyl halides using sodium in dry ether to form a higher alkane ($2RX + 2Na \rightarrow R-R$). The Wurtz-Fittig reaction couples an alkyl halide and an aryl halide to form an alkylarene ($ArX + RX + 2Na \rightarrow Ar-R$).

Haloarenes are extremely less reactive towards nucleophilic substitution than haloalkanes. This is due to resonance, which gives the C-X bond a partial double-bond character, making it stronger and harder to break.

Halogen atoms are deactivating but ortho, para-directing for electrophilic aromatic substitution. The deactivation is due to the electron-withdrawing inductive effect (-I), while the o,p-direction is due to the electron-donating resonance effect (+R).

Compounds like chloroform ($CHCl_3$), carbon tetrachloride ($CCl_4$), and DDT have been widely used as solvents, refrigerants, and insecticides. However, many are toxic or cause environmental problems like ozone layer depletion.