Physical Properties & Methods of Preparation Haloalkanes & Haloarenes -

Physical Properties of Haloalkanes

Haloalkanes (alkyl halides) are derivatives of alkanes in which one or more hydrogen atoms have been replaced by halogen atoms (group 17 elements). Their overall physical behaviour is that of covalent molecular compounds. Differences from the parent hydrocarbons arise mainly because of the higher molecular mass, greater polarizability of halogen atoms and the polarity of the C-X bond. The following subsections describe important physical properties with reasons and examples useful for Class 12 syllabus and competitive examinations.

Colour

Pure alkyl halides are generally colourless liquids. Bromides and iodides may develop colour on prolonged exposure to light because they can slowly decompose to release elemental bromine or iodine, which are coloured.

Melting and Boiling Points

• The melting and boiling points of haloalkanes are higher than those of the corresponding hydrocarbons because halogen atoms increase molecular mass and polarizability, which strengthen van der Waals (dispersion) forces.
• For a given alkyl group the boiling point order is R-I > R-Br > R-Cl > R-F, because polarizability and molecular mass increase down the halogen group, increasing dispersion forces.
• The boiling points of isomeric haloalkanes decrease with increasing branching. Branching reduces the surface area of molecules, lowers van der Waals attraction and thus decreases boiling point.
• The boiling points of isomeric dihalobenzenes are usually close to one another. The melting point of the para isomer is higher than the ortho and meta isomers because the para isomer is more symmetrical and packs more efficiently in the crystal lattice, giving stronger lattice forces.

Solubility

• Although the C-X bond is polar, haloalkanes and haloarenes are practically immiscible with water. They cannot form hydrogen bonds with water and the favourable interactions with water are insufficient to overcome the strong water-water hydrogen bonding.
• Haloalkanes dissolve well in organic solvents of low polarity such as ether, benzene, carbon tetrachloride, etc., because intermolecular interactions in those solvents are of the same type and strength (dispersion forces and weak dipole-dipole interactions).

Density

• Relative densities increase with heavier halogens. Alkyl fluorides and chlorides are generally lighter than water, whereas many alkyl bromides and iodides are heavier than water. The order of densities is generally RI > RBr > RCl > RF for similar alkyl groups.
• For a homologous series of alkyl halides the density generally decreases as the size of the alkyl group increases because the proportion of the heavy halogen atom to the whole molecule decreases.

Stability (C-X Bond Strength)

The thermal and chemical stability related to the carbon-halogen bond follows the order R-F > R-Cl > R-Br > R-I. This order reflects the bond dissociation energies: the C-F bond is the strongest (short and strong overlap) while C-I is the weakest (long bond and poorer overlap). Therefore, haloalkanes with heavier halogens are more prone to homolysis and nucleophilic substitution through bond cleavage.

Dipole Moment

Dipole moment depends on both bond polarity and bond length (distance between centres of opposite partial charges). For methyl halides the observed order is CH₃Cl > CH₃F > CH₃Br > CH₃I. Although fluorine is most electronegative, the very short C-F bond reduces the charge separation and hence CH₃F has a slightly lower dipole moment than CH₃Cl. As the halogen gets larger the bond polarity decreases and so does the dipole moment.

Methods of Preparation of Haloalkanes & Haloarenes

Haloalkanes and haloarenes are prepared by a number of well-established reactions. Important methods include conversion of alcohols to alkyl halides, addition of halogen or hydrogen halides to alkenes, free radical halogenation of alkanes and halogen exchange (substitution) reactions. The following sections explain these methods, typical reagents, the mechanistic idea and useful remarks for exams.

Common preparative routes:

1. From alcohols (replacement of -OH by halogen)
2. From hydrocarbons (free radical halogenation)
3. From alkenes by addition of hydrogen halides or halogens
4. Halogen exchange reactions (Finkelstein, Swarts)

Preparation of Haloalkanes

1. From Alcohols

Alcohols react with hydrogen halides or with halogenating reagents to give alkyl halides. Mechanistically, primary alcohols usually undergo an SN2 type substitution (concerted nucleophilic attack and loss of leaving group), while tertiary alcohols follow an SN1 pathway via a carbocation intermediate. Secondary alcohols may follow either route depending on conditions.

• Reaction with HCl (Lucas reagent): Treatment of alcohols with conc. HCl in presence of ZnCl₂ (Lucas reagent) converts alcohols to alkyl chlorides. Tertiary alcohols react immediately, secondary alcohols react within minutes to hours and primary alcohols do not react readily. This difference is used to classify alcohols.
• Reaction with HBr: Alcohols are converted to alkyl bromides by HBr or by PBr₃ (which is better for primary and secondary alcohols). Example: CH₃CH₂OH + HBr → CH₃CH₂Br + H₂O.
• Reaction with phosphorus halides: PCl₅, PCl₃ or PBr₃ convert alcohols to corresponding alkyl halides. These reagents are especially useful for primary and secondary alcohols and give good yields.
• Reaction with thionyl chloride (SOCl₂): SOCl₂ converts alcohols to alkyl chlorides with formation of gaseous by-products (SO₂ and HCl) which are easily removed. Often pyridine is used as a base to prevent side reactions and to obtain inversion of configuration in chiral centres (SN2).

2. From Alkenes (Addition Reactions)

Alkenes add hydrogen halides or halogens across the C=C double bond to give haloalkanes or vicinal dihalides respectively.

• Hydrohalogenation (addition of HX): The addition of HX (where X = Cl, Br, I) to alkenes generally follows Markovnikov's rule: the hydrogen adds to the carbon with more hydrogens and the halogen to the more substituted carbon (the site which can better stabilise a positive charge). The reaction proceeds via an electrophilic addition mechanism with formation of a carbocation intermediate.
• Peroxide (Kharasch) effect: In the presence of peroxides the addition of HBr to certain alkenes proceeds by a free radical chain mechanism and gives the anti-Markovnikov product. This peroxide effect is specific to HBr (not generally observed with HCl or HI) and is also called the Kharasch effect.

3. From Free Radical Halogenation (of Alkanes)

Under radical conditions (UV light or heat) halogens add to alkanes to give haloalkanes. The reaction proceeds by a free radical chain mechanism with three stages: initiation, propagation and termination. Because selectivity is limited, a mixture of mono-, di- and poly-substituted products is formed and regioselectivity issues (primary/secondary/tertiary hydrogen abstraction) arise. Therefore this method is not ideal when a single mono-substituted product is desired.

4. Halogen Exchange Reactions

Halogen exchange (nucleophilic substitution at carbon) is used to convert one alkyl halide to another.

• Finkelstein reaction: Exchange of halogens using a soluble salt in an appropriate solvent. For example, R-Cl or R-Br + NaI (acetone) → R-I + NaCl/NaBr (precipitate). The insolubility of NaCl or NaBr in acetone drives the reaction to completion.
• Swarts reaction: Conversion of alkyl chlorides or bromides to alkyl fluorides using metal fluorides such as SbF₃ or AgF under suitable conditions. This method is commonly referred to as the Swarts reaction for preparing alkyl fluorides.

Preparation of Haloarenes

1. From Arenes by Electrophilic Aromatic Substitution

Aryl halides (haloarenes) are commonly prepared by electrophilic aromatic substitution of benzene and its derivatives with halogens in presence of a Lewis acid catalyst (for chlorination and bromination). Typical catalysts are FeCl₃, AlCl₃, FeBr₃, etc.

Mechanistic outline:

• The aromatic π-electrons attack the electrophile (for example Cl⁺) to form a σ-complex (also called an arenium ion or Wheland intermediate) in which aromaticity is temporarily lost.
• Removal of a proton from the σ-complex restores aromaticity and gives the substituted aryl halide.
• This substitution frequently gives a mixture of ortho and para isomers (and minor meta depending on directing effects of substituents). Differences in melting and boiling points among isomers, especially the higher melting point of para isomers due to better symmetry, often allow separation.

2. From Aromatic Amines by Sandmeyer Reaction (via Diazonium Salts)

The Sandmeyer reaction converts aromatic amines to aryl halides through diazonium salts. It is a valuable method for introducing halogen atoms into an aromatic ring.

1. Formation of the diazonium salt: A primary aromatic amine reacts with sodium nitrite in presence of cold mineral acid (HCl or HBr) at 273-278 K to form the corresponding diazonium salt (Ar-N₂⁺ X^-).
2. Replacement of the diazonium group: The diazonium salt reacts with a cuprous halide (CuX, X = Cl, Br, CN for other transformations) to give the corresponding aryl halide (Ar-X) with loss of N₂ gas.

Relevant details and mechanism:

• In the first step NaNO₂ + HCl → HNO₂ + NaCl; HNO₂ in acid forms the nitrosyl cation (NO⁺) which reacts with the amine to eventually form the diazonium salt.
• In the second step the diazonium salt undergoes a single-electron transfer with Cu(I) halide to produce aryl radical or cationic intermediates and finally the aryl halide with evolution of nitrogen gas.

Mechanism summary of Sandmeyer:

• NaNO₂ + HCl → HNO₂ + NaCl
• HNO₂ + H⁺ → NO⁺ (electrophile)
• Aromatic amine uses its lone pair to react with NO⁺ to form N-nitroso intermediate which, on further reaction and loss of water, gives the diazonium salt.
• Diazonium salt + CuX → Ar-X + N₂↑