Haloalkanes And Haloarenes (Physical And Chemical Properties)

Physical Properties

The physical properties of haloalkanes and haloarenes are influenced by the nature of the carbon-halogen bond and intermolecular forces.

Melting And Boiling Points

Trend in Haloalkanes:

• Increase with Molecular Weight: Melting and boiling points generally increase with increasing molecular weight (i.e., as the size of the alkyl group and the halogen atom increase). This is due to stronger van der Waals forces between larger molecules.
• Branching: Increasing the branching of the alkyl chain tends to lower the boiling point. Branched molecules are more spherical and have a smaller surface area for intermolecular contact, leading to weaker van der Waals forces.
• Halogen Effect: Boiling points increase in the order: $R-F < R-Cl < R-Br < R-I$. This is due to the increasing molecular weight and van der Waals forces.
• Order of Boiling Points: For isomers, boiling points decrease with branching. Example: $n$-pentane ($69^\circ C$) > Isopentane ($36^\circ C$) > Neopentane ($9.5^\circ C$).

Trend in Haloarenes:

• Boiling points of aryl halides are generally higher than those of corresponding haloalkanes due to stronger intermolecular forces (dipole-dipole and van der Waals).
• For di- and polyhalogenated aromatic compounds, the boiling points depend on the position of the halogens. Generally, para isomers have higher melting points than ortho and meta isomers due to their symmetrical structure allowing better crystal packing.

Solubility

In Water:

• Haloalkanes have low solubility in water. Although the $C-X$ bond is polar, the larger nonpolar alkyl chain dominates the molecule's character, making it hydrophobic. Also, the energy required to break the strong hydrogen bonds between water molecules is not sufficiently compensated by the energy released from forming weak dipole-dipole interactions between haloalkanes and water.
• Haloarenes are practically insoluble in water for similar reasons.

In Organic Solvents: Haloalkanes and haloarenes are generally soluble in organic solvents (like ether, benzene, alcohol, chloroform) due to similar intermolecular forces.

Density

Trend: The density of haloalkanes generally increases with:

• Increasing molecular weight (increasing size of the alkyl group).
• Increasing atomic number of the halogen atom ($density \ increases \ in \ the \ order \ R-F < R-Cl < R-Br < R-I$).

Comparison: Many haloalkanes, especially polyhalogenated ones (like $CCl_4$, $CH_2Cl_2$), are denser than water.

Haloarenes: Aryl halides are generally denser than the parent hydrocarbon (benzene) and many haloalkanes.

Chemical Reactions

The chemical reactivity of haloalkanes and haloarenes is primarily governed by the carbon-halogen bond ($C-X$).

Reactions Of Haloalkanes

Haloalkanes undergo primarily three types of reactions:

1. Nucleophilic Substitution Reactions ($S_N$):

• Description: A nucleophile ($Nu^-$) replaces the halogen atom (leaving group, $X^-$) attached to the $sp^3$ hybridized carbon atom.
• Mechanism: Occurs via two main pathways: $S_N1$ and $S_N2$.
• $S_N2$ Mechanism: Bimolecular nucleophilic substitution. Favored by primary alkyl halides, strong nucleophiles, polar aprotic solvents, and results in inversion of configuration at the chiral center.
• $S_N1$ Mechanism: Unimolecular nucleophilic substitution. Favored by tertiary alkyl halides, weak nucleophiles, polar protic solvents, and involves carbocation intermediates, leading to racemization if the carbon is chiral.
• Reactivity Order of Alkyl Halides:
• Towards $S_N1$: Tertiary > Secondary > Primary
• Towards $S_N2$: Primary > Secondary > Tertiary
• Reactivity Order of Halides: Generally, $R-I > R-Br > R-Cl >> R-F$ (due to decreasing bond strength and better leaving group ability of larger halogens).

2. Elimination Reactions (E):

• Description: Involves the removal of two atoms or groups from adjacent carbon atoms, leading to the formation of a double bond (alkene). These reactions often compete with nucleophilic substitution.
• Mechanism: Occurs via two main pathways: $E1$ and $E2$.
• $E2$ Mechanism: Bimolecular elimination. Favored by strong bases and higher temperatures. Requires anti-periplanar geometry.
• $E1$ Mechanism: Unimolecular elimination. Occurs via carbocation intermediates, often favored by tertiary substrates and higher temperatures, similar conditions to $S_N1$.
• Saytzeff's Rule: Predicts the major product in elimination reactions involving unsymmetrical alkyl halides; the alkene with the greater number of alkyl substituents on the double bond is the major product.

3. Reaction with Metals:

• Formation of Grignard Reagents: Reaction with magnesium metal in dry ether forms Grignard reagents ($R-MgX$). These are extremely useful in organic synthesis for forming new carbon-carbon bonds.

$R-X + Mg \xrightarrow{dry \ ether} R-MgX$

• Formation of Organolithium Compounds: Reaction with lithium metal in dry ether.

$R-X + 2Li \xrightarrow{dry \ ether} R-Li + LiX$

• Wurtz Reaction: Reaction with sodium metal to form alkanes.

$R-X + 2Na \xrightarrow{dry \ ether} R-Na + NaX$

Reactions Of Haloarenes

Haloarenes are generally much less reactive than haloalkanes, particularly towards nucleophilic substitution, due to factors related to the aromatic system.

1. Nucleophilic Substitution Reactions:

• Reactivity: Generally do not undergo nucleophilic substitution reactions easily under normal conditions. The $C_{sp^2}-X$ bond is stronger than $C_{sp^3}-X$, and the negative charge on the halide ion is delocalized into the aromatic ring through resonance, making it a poorer leaving group.
• Conditions for Reaction: Nucleophilic substitution requires harsh conditions like very high temperatures and pressures, often with strong nucleophiles and catalysts.

Example: Formation of phenol from chlorobenzene by heating with $NaOH$ at 623 K and 300 atm pressure.

$C_6H_5Cl + 2NaOH \xrightarrow{high \ T, P} C_6H_5ONa + NaCl + H_2O$

$C_6H_5ONa + H_2O \rightarrow C_6H_5OH + NaOH$

2. Electrophilic Substitution Reactions:

• Reactivity: Haloarenes undergo electrophilic aromatic substitution reactions, but are less reactive than benzene itself because halogens are deactivating groups (due to their -I effect, although they are ortho, para directors due to resonance).
• Directive Influence: Halogens are ortho, para directors.

Example: Nitration of chlorobenzene.

$C_6H_5Cl + HNO_3 \xrightarrow{conc. H_2SO_4} o-ClC_6H_4NO_2 + p-ClC_6H_4NO_2 + H_2O$

3. Reaction with Metals:

• Formation of Grignard Reagents: Aryl halides react with magnesium in dry ether to form aryl Grignard reagents.

$C_6H_5Cl + Mg \xrightarrow{dry \ ether} C_6H_5MgCl$

• Wurtz-Fittig Reaction: Reaction of an aryl halide with an alkyl halide in the presence of sodium metal to form alkylbenzenes.

$C_6H_5Cl + CH_3Cl + 2Na \xrightarrow{dry \ ether} C_6H_5CH_3 + 2NaCl$

4. Reduction: Can be reduced to hydrocarbons using strong reducing agents like lithium in liquid ammonia or $H_2$ with catalysts under forcing conditions.