Haloalkanes And Haloarenes (Introduction)

Classification

Haloalkanes and haloarenes are organic compounds where one or more hydrogen atoms in a hydrocarbon are replaced by halogen atoms (F, Cl, Br, I).

On The Basis Of Number Of Halogen Atoms

Organic compounds containing halogens can be classified based on the number of halogen atoms present in the molecule:

• Monohaloalkanes/Monohaloarenes: Contain one halogen atom (e.g., $CH_3Cl$, $C_6H_5Cl$).
• Dihaloalkanes/Dihaloarenes: Contain two halogen atoms (e.g., $CH_2Cl_2$, $C_6H_4Cl_2$).
• Trihaloalkanes/Trihaloarenes: Contain three halogen atoms (e.g., $CHCl_3$, $C_6H_3Cl_3$).
• Polyhaloalkanes/Polyhaloarenes: Contain several halogen atoms.

Compounds Containing $sp^3$ C—X Bond (X = F, Cl, Br, I)

Description: In these compounds, the halogen atom is attached to a carbon atom that is $sp^3$ hybridized. This carbon atom is typically part of an alkyl group or an alkyl side chain of an aromatic ring.

Classification based on the carbon attached to halogen:

1. Primary Alkyl Halides (1°): The halogen atom is attached to a primary carbon atom (a carbon atom attached to only one other carbon atom).

Example: $CH_3CH_2Cl$ (Chloroethane)

2. Secondary Alkyl Halides (2°): The halogen atom is attached to a secondary carbon atom (a carbon atom attached to two other carbon atoms).

Example: $CH_3CH(Cl)CH_3$ (2-Chloropropane)

3. Tertiary Alkyl Halides (3°): The halogen atom is attached to a tertiary carbon atom (a carbon atom attached to three other carbon atoms).

Example: $(CH_3)_3CCl$ (2-Chloro-2-methylpropane)

Allylic and Benzylic Halides: A special category where the halogen is attached to a carbon atom adjacent to a double bond (allylic) or an aromatic ring (benzylic). These are particularly reactive due to resonance stabilization of the resulting radical or carbocation.

• Allylic Halide: $CH_2=CHCH_2Cl$ (3-Chloroprop-1-ene)
• Benzylic Halide: $C_6H_5CH_2Cl$ (Benzyl chloride)

Compounds Containing $sp^2$ C—X Bond

Description: In these compounds, the halogen atom is attached to a carbon atom that is $sp^2$ hybridized. This occurs in two main types:

1. Vinylic Halides: Halogen atom is attached directly to a doubly bonded carbon atom.

Example: $CH_2=CHCl$ (Chloroethene)

2. Aryl Halides: Halogen atom is directly attached to a carbon atom of an aromatic ring.

Example: $C_6H_5Cl$ (Chlorobenzene)

Reactivity: Vinylic and aryl halides are generally much less reactive towards nucleophilic substitution reactions compared to alkyl halides due to the $sp^2$ hybridization of the carbon atom and the electron-withdrawing nature of the $\pi$ system, which makes the $C-X$ bond stronger and harder to break.

Nomenclature

The nomenclature of haloalkanes and haloarenes follows the general IUPAC rules, with the halogen atom treated as a substituent.

Haloalkanes:

• Parent Chain: The longest continuous carbon chain containing the halogen atom.
• Numbering: Number the chain from the end that gives the halogen substituent the lowest possible number.
• Name: Prefix the name of the halogen (fluoro-, chloro-, bromo-, iodo-) to the parent alkane name.
• Multiple Halogens: Use prefixes di-, tri-, tetra- and indicate positions.

Examples:

• $CH_3Br$: Bromomethane
• $CH_3CH_2Cl$: Chloroethane
• $CH_3CH(I)CH_3$: 2-Iodopropane
• $CH_3CH_2CH(Br)CH_3$: 2-Bromobutane
• $CH_2Cl_2$: Dichloromethane
• $CHCl_3$: Trichloromethane (Chloroform)
• $CCl_4$: Tetrachloromethane (Carbon tetrachloride)

Haloarenes:

• The halogen atom is considered a substituent on the benzene ring.
• Common names like Chlorobenzene, Bromobenzene, Iodobenzene are accepted.
• For disubstituted benzenes, ortho (o-), meta (m-), and para (p-) prefixes are used, or numbering is employed.

Examples:

• $C_6H_5Cl$: Chlorobenzene
• $C_6H_4Br_2$ (ortho/meta/para): o-Dibromobenzene, m-Dibromobenzene, p-Dibromobenzene

Nature Of C—X Bond

The carbon-halogen bond ($C-X$) is a polar covalent bond due to the significant difference in electronegativity between carbon and the halogen atom.

Polarity:

• Halogens are more electronegative than carbon.
• This results in a partial positive charge on the carbon atom ($\delta^+$) and a partial negative charge on the halogen atom ($\delta^-$).

$C^{\delta+} - X^{\delta-}$

Effect of Electronegativity:

• Electronegativity of halogens: $F > Cl > Br > I$.
• Therefore, the polarity of the $C-X$ bond decreases in the order: $C-F > C-Cl > C-Br > C-I$.

Bond Strength:

• The strength of the $C-X$ bond generally decreases down the halogen group ($C-F > C-Cl > C-Br > C-I$).
• This is because the size of the halogen atom increases, leading to poorer overlap between the carbon orbital and the halogen orbital.

Reactivity:

• Polarity: The polarity of the $C-X$ bond makes the carbon atom susceptible to nucleophilic attack.
• Bond Strength: The decreasing bond strength down the group influences the ease of bond cleavage in reactions like nucleophilic substitution and elimination. Generally, compounds with weaker $C-X$ bonds are more reactive in these processes.
• Vinylic/Aryl Halides: The $C-X$ bond in vinylic and aryl halides is stronger than in alkyl halides due to $sp^2$ hybridization of carbon and some double bond character, making them less reactive towards nucleophilic substitution.

Dipole Moment: Haloalkanes are polar molecules and exhibit dipole moments, which influence their physical properties like boiling points and solubility.