Haloalkanes And Haloarenes (Preparation Methods)

Methods Of Preparation Of Haloalkanes

Haloalkanes (or alkyl halides) are organic compounds where a halogen atom is bonded to an $sp^3$ hybridized carbon atom.

From Alcohols

Alcohols are versatile starting materials for the synthesis of alkyl halides. The hydroxyl group (-OH) acts as a leaving group, usually after being converted into a better leaving group.

1. Using Hydrogen Halides (HX):

• Alcohols react with hydrogen halides ($HCl$, $HBr$, $HI$) to form alkyl halides and water. The reaction is typically catalyzed by Lewis acids like $ZnCl_2$ (Lucas reagent) for $HCl$.

$RXH + HX \rightarrow RX + H_2O$

• Reactivity Order of Alcohols: Tertiary > Secondary > Primary
• Reactivity Order of HX: $HI > HBr > HCl$
• Lucas Test: The Lucas reagent ($ZnCl_2$ in conc. $HCl$) is used to distinguish between primary, secondary, and tertiary alcohols.
• Tertiary alcohols react immediately to form alkyl halide (turbidity).
• Secondary alcohols react in 5-10 minutes.
• Primary alcohols react slowly and require heating to form alkyl halide.

2. Using Phosphorus Halides:

• Phosphorus Trichloride ($PCl_3$): Reacts with primary and secondary alcohols to form alkyl chlorides. Three moles of alcohol react with one mole of $PCl_3$.

$3R-OH + PCl_3 \rightarrow 3R-Cl + H_3PO_3$

• Phosphorus Pentachloride ($PCl_5$): Reacts with alcohols to form alkyl chlorides, phosphoryl chloride ($POCl_3$), and $HCl$.

$R-OH + PCl_5 \rightarrow R-Cl + POCl_3 + HCl$

3. Using Thionyl Chloride ($SOCl_2$):

• Reacts with alcohols to form alkyl chlorides, sulfur dioxide ($SO_2$), and hydrogen chloride ($HCl$). This is a convenient method because the by-products are gases and escape easily, leaving pure alkyl halide.

$R-OH + SOCl_2 \rightarrow R-Cl + SO_2(g) + HCl(g)$

From Hydrocarbons

1. Free Radical Halogenation of Alkanes:

• Alkanes react with halogens (especially $Cl_2$ and $Br_2$) in the presence of UV light or heat via a free radical mechanism.
• This reaction is generally not selective and can produce a mixture of mono-, di-, and polyhalogenated products.
• Reactivity order of halogens: $F_2 > Cl_2 > Br_2 > I_2$.
• Reactivity order of hydrogens: Tertiary > Secondary > Primary.

$CH_4 + Cl_2 \xrightarrow{UV \ light} CH_3Cl + HCl$

$CH_3Cl + Cl_2 \xrightarrow{UV \ light} CH_2Cl_2 + HCl$

2. Addition of Hydrogen Halides to Alkenes and Alkynes:

• Alkenes and alkynes react with $HX$ via electrophilic addition, following Markovnikov's rule.

$CH_3CH=CH_2 + HBr \rightarrow CH_3CHBrCH_3$

$CH \equiv CH + HCl \rightarrow CH_2=CHCl$ (first addition) $\rightarrow CH_3CHCl_2$ (second addition)

3. Addition of Halogens to Alkenes and Alkynes:

• Alkenes react with halogens ($Cl_2$, $Br_2$) via addition across the double bond.

$CH_2=CH_2 + Br_2 \rightarrow CH_2BrCH_2Br$

• Alkynes react similarly, adding two molecules of halogen.

$CH \equiv CH + Br_2 \rightarrow CHBr=CHBr \xrightarrow{Br_2} CHBr_2CHBr_2$

Halogen Exchange

Finkelstein Reaction: This reaction involves the conversion of one alkyl halide into another by reacting it with an alkali metal halide in a suitable solvent. It is particularly useful for preparing alkyl iodides and fluorides.

• Preparation of Alkyl Iodides: By treating an alkyl chloride or bromide with sodium iodide ($NaI$) in dry acetone. Since $NaI$ is soluble in acetone, but $NaCl$ and $NaBr$ are insoluble, the equilibrium is driven towards the formation of alkyl iodide (Le Chatelier's principle).

$R-Cl + NaI \xrightarrow{Acetone} R-I + NaCl(s)$

• Preparation of Alkyl Fluorides: By treating alkyl chlorides or bromides with silver fluoride ($AgF$) or mercury(II) fluoride ($Hg_2F_2$) or antimony trifluoride ($SbF_3$) (Swarts reaction).

$2R-Cl + Hg_2F_2 \rightarrow 2R-F + Hg_2Cl_2$

Preparation Of Haloarenes

Haloarenes are compounds where a halogen atom is directly attached to an aromatic ring.

From Hydrocarbons By Electrophilic Substitution

Description: This is the primary method for preparing aryl halides. A halogen molecule ($X_2$) acts as an electrophile (or generates an electrophile) which attacks the electron-rich aromatic ring, substituting a hydrogen atom.

Reaction Conditions:

• Halogen: $Cl_2$ or $Br_2$.
• Catalyst: A Lewis acid catalyst such as anhydrous $FeCl_3$, $AlCl_3$, $FeBr_3$, or $SbCl_5$. The catalyst polarizes the halogen molecule, making it a stronger electrophile.
• Conditions: Usually carried out at room temperature.

Mechanism: Electrophilic aromatic substitution.

Examples:

• Chlorobenzene formation: Benzene reacts with chlorine in the presence of anhydrous $FeCl_3$.

$C_6H_6 + Cl_2 \xrightarrow{FeCl_3} C_6H_5Cl + HCl$

• Bromobenzene formation: Benzene reacts with bromine in the presence of $FeBr_3$.

$C_6H_6 + Br_2 \xrightarrow{FeBr_3} C_6H_5Br + HBr$

Iodination: Direct iodination is difficult because $HI$ formed is a reducing agent and can reduce $I_2$ back to $I^-$. It requires an oxidizing agent like $HNO_3$ or $HIO_3$ along with $I_2$ to oxidize the $HI$ formed.

$2C_6H_6 + I_2 \xrightarrow{HNO_3} 2C_6H_5I + 2H_2O$

Fluorination: Direct fluorination is too vigorous and explosive. Methods involving specific fluorinating agents are used.

From Amines By Sandmeyer’s Reaction

Description: This reaction is used to replace the amino group ($-NH_2$) of an aromatic amine with a halogen atom (Cl, Br, or CN) via a diazonium salt intermediate.

Steps:

1. Diazotization: Primary aromatic amines react with nitrous acid ($HNO_2$, generated in situ from $NaNO_2$ and $HCl$) at low temperatures (0-5°C) to form diazonium salts.

$Ar-NH_2 + NaNO_2 + 2HCl \xrightarrow{0-5^\circ C} [Ar-N_2]^+Cl^- + NaCl + 2H_2O$

2. Sandmeyer Reaction: The diazonium salt solution is then treated with cuprous chloride ($CuCl$) for chlorobenzene, cuprous bromide ($CuBr$) for bromobenzene, or potassium iodide ($KI$) for iodobenzene.

For Chlorobenzene: $[Ar-N_2]^+Cl^- \xrightarrow{CuCl} Ar-Cl + N_2(g)$

For Bromobenzene: $[Ar-N_2]^+Cl^- \xrightarrow{CuBr} Ar-Br + N_2(g)$

3. For Iodobenzene: Treat the diazonium salt solution with potassium iodide ($KI$). $CuI$ is not as effective.

$[Ar-N_2]^+Cl^- + KI \rightarrow Ar-I + N_2(g) + KCl$

Gattermann Reaction: A similar reaction can be carried out using copper powder instead of cuprous salts, though yields are usually lower.

Significance: This method is very useful for introducing halogens specifically onto an aromatic ring, especially when direct electrophilic substitution is difficult or gives unwanted isomeric mixtures.

Nature Of C—X Bond

The nature of the carbon-halogen bond ($C-X$) in haloalkanes and haloarenes is crucial in determining their reactivity.

Polarity:

• Halogens (F, Cl, Br, I) are more electronegative than carbon.
• This electronegativity difference results in a polar covalent bond, with a partial positive charge on the carbon atom ($\delta^+$) and a partial negative charge on the halogen atom ($\delta^-$).

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

• The polarity of the $C-X$ bond decreases in the order: $C-F > C-Cl > C-Br > C-I$, due to the decreasing electronegativity difference.

Bond Strength:

• The strength of the $C-X$ bond decreases down the halogen group ($C-F > C-Cl > C-Br > C-I$).
• This is primarily due to the increasing size of the halogen atom, which leads to poorer overlap between the carbon orbital and the halogen orbital, resulting in a weaker bond.

Reactivity:

• Nucleophilic Substitution ($S_N1$ and $S_N2$): The polarity of the $C-X$ bond makes the carbon atom electrophilic and susceptible to attack by nucleophiles. The ease of cleavage of the $C-X$ bond influences the reactivity in these reactions. The order of reactivity of alkyl halides in nucleophilic substitution is generally: Tertiary > Secondary > Primary (for $S_N1$) and Primary > Secondary > Tertiary (for $S_N2$). This order is influenced by both bond polarity and bond strength, as well as steric factors.
• Elimination Reactions (E1 and E2): The $C-X$ bond also needs to be broken in elimination reactions, so bond strength is a factor.
• Vinylic and 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 (from resonance in aryl halides). This makes them much less reactive towards nucleophilic substitution compared to alkyl halides.

Dipole Moment: Due to the polar nature of the $C-X$ bond, haloalkanes are polar molecules and possess dipole moments. The dipole moment generally decreases in the order $CH_3F > CH_3Cl > CH_3Br > CH_3I$.