10.1 Classification
- Haloalkanes and haloarenes are classified based on their structure.
- Haloalkanes are alkanes where one or more hydrogen atoms are replaced by halogen atoms.
- Haloarenes are aromatic compounds where one or more hydrogen atoms are replaced by halogen atoms.
- They can be further classified into primary, secondary, and tertiary haloalkanes based on the carbon atom to which the halogen is attached.
- Haloarenes can be classified based on the position of the halogen on the aromatic ring, such as ortho, meta, and para positions.
10.1.1 On the Basis of Number of Halogen Atoms
- Halogen compounds are classified based on the number of halogen atoms: mono, di, or polyhalogen (tri-, tetra-, etc.).
- Monohalocompounds contain one halogen atom.
- Dihalocompounds contain two halogen atoms.
- Polyhalocompounds contain three or more halogen atoms.
- Monohalocompounds can be further classified based on the hybridization of the carbon atom bonded to the halogen.
10.1.2 Compounds Containing sp3 C—X Bond (X= F, Cl, Br, I)
- Compounds containing sp3 C—X bonds (X = F, Cl, Br, I) include alkyl halides or haloalkanes (R—X).
- Alkyl halides form a homologous series represented by CnH2n+1X and are classified as primary (1°), secondary (2°), or tertiary (3°) based on the carbon to which the halogen is attached.
- Allylic halides have the halogen atom bonded to an sp3-hybridised carbon adjacent to a carbon-carbon double bond (C=C).
- Benzylic halides have the halogen atom bonded to an sp3-hybridised carbon attached to an aromatic ring.
10.1.3 Compounds Containing sp2C−X
- Compounds containing sp2 C-X bonds include vinylic halides and aryl halides.
- Vinylic halides have a halogen atom bonded to an sp2-hybridised carbon atom of a carbon-carbon double bond (C=C).
- Aryl halides have a halogen atom directly bonded to an sp2-hybridised carbon atom of an aromatic ring.
10.2 Nomenclature
- (CH3)2CBrCH2CH3 is 2-Bromo-2-methylbutane.
- CH3CH2CH(CH3)CH2Br is 1-Bromo-2-methylbutane.
- (CH3)3CCH2Br is 1-Bromo-2,2-dimethylpropane.
- Example 10.2 involves writing IUPAC names for given compounds.
- Solutions include names like 4-Bromopent-2-ene, 3-Bromo-2-methylbut-1-ene, and others.
10.3 Nature of C-X Bond
- Halogen atoms are more electronegative than carbon, making the carbon-halogen bond in alkyl halides polarized.
- The carbon atom bears a partial positive charge, while the halogen atom bears a partial negative charge.
- As you move down the group in the periodic table, the size of the halogen atom increases, leading to an increase in carbon-halogen bond length from C-F to C-I.
- Typical bond lengths, bond enthalpies, and dipole moments are provided in Table 10.2.
- Alkyl halides are best prepared from alcohols, which are easily accessible.
10.4 Methods of Preparation of Haloalkanes
- Haloalkanes can be prepared by the free radical halogenation of alkanes.
- They can also be synthesized through the addition of hydrogen halides to alkenes.
- Another method involves the halogen exchange reaction, such as the Finkelstein reaction.
- Alcohols can be converted to haloalkanes using reagents like PCl5, PCl3, or SOCl2.
- The Sandmeyer reaction is used to prepare haloarenes from diazonium salts.
10.4.1 From Alcohols
- The hydroxyl group of an alcohol can be replaced by a halogen using concentrated halogen acids, phosphorus halides, or thionyl chloride.
- Thionyl chloride is preferred as it produces pure alkyl halides along with escapable gases SO2 and HCl.
- Primary and secondary alcohols react with HCl in the presence of a catalyst, ZnCl2, while tertiary alcohols react with concentrated HCl at room temperature.
- Alkyl bromides are prepared using constant boiling with HBr(48, and alkyl iodides by heating alcohols with sodium or potassium iodide in 95% orthophosphoric acid.
- The reactivity order of alcohols with haloacids is 3∘>2∘>1∘.
- Key reactions include: R−OH+HClZnCl2R−Cl+H2O R−OH+NaBr+H2SO4⟶R−Br+NaHSO4+H2O 3R−OH+PX3⟶3R−X+H3PO3(X=Cl,Br) R−OH+PCl5⟶R−Cl+POCl3+HCl R−OH+X2=Br2,I2redP/X2R−X R−OH+SOCl2⟶R−Cl+SO2+HCl
- These methods are not suitable for preparing aryl halides due to the partial double bond character of the carbon-oxygen bond in phenols.
10.4.2 From Hydrocarbons
- The given text contains chemical formulas.
- The first formula is (CH3)2C(Cl)CH2CH3.
- The second formula is CH3CH(CH2Cl2CH2CH3).
10.4.3 Halogen Exchange
- Alkyl iodides are prepared by reacting alkyl chlorides/bromides with NaI in dry acetone, known as the Finkelstein reaction.
- Reaction: R−X+NaI⟶R−I+NaX where X=Cl,Br
- NaCl or NaBr formed is precipitated in dry acetone, facilitating the forward reaction according to Le Chatelier’s Principle.
- Alkyl fluorides are synthesized by heating an alkyl chloride/bromide with a metallic fluoride (e.g., AgF, Hg2F2, CoF2, SbF3).
- This reaction is known as the Swarts reaction.
10.5 Preparation of Haloarenes
- Haloarenes can be prepared from hydrocarbons by electrophilic substitution using chlorine or bromine in the presence of Lewis acid catalysts like iron or iron(III) chloride.
- The ortho and para isomers of haloarenes can be separated due to their different melting points.
- Iodination requires an oxidizing agent (e.g., HNO3, HIO4) to oxidize the HI formed, while fluoro compounds are not prepared this way due to fluorine’s high reactivity.
- Haloarenes can also be prepared from amines by Sandmeyer’s reaction, where a diazonium salt is formed and then replaced by −Cl or −Br using cuprous chloride or cuprous bromide.
- Replacement of the diazonium group by iodine can be done by shaking the diazonium salt with potassium iodide without the need for cuprous halide.
10.6 Physical Properties
- Alkyl halides are colorless when pure, but bromides and iodides develop color upon light exposure.
- Boiling points of alkyl halides increase with the size and mass of the halogen atom: RI>RBr>RCl>RF.
- Boiling points of isomeric haloalkanes decrease with increased branching; para-isomers have higher melting points due to better crystal lattice fitting.
- Density of bromo, iodo, and polychloro derivatives is higher than water and increases with more carbon atoms, halogen atoms, and higher atomic mass of halogens.
- Haloalkanes are slightly soluble in water due to weaker new attractions compared to original hydrogen bonds but dissolve well in organic solvents.
10.7 Chemical Reactions
- Chemical reactions involve the transformation of reactants into products.
- The general form of a chemical equation is: Reactants → Products.
- Balancing chemical equations ensures the conservation of mass.
- Types of chemical reactions include synthesis, decomposition, single replacement, and double replacement.
- Energy changes, such as exothermic and endothermic reactions, are associated with chemical reactions.
10.7.1 Reactions of Haloalkanes
- Haloalkanes undergo three main types of reactions: nucleophilic substitution, elimination, and reactions with metals.
- Nucleophilic substitution reactions: A nucleophile replaces an existing nucleophile in a haloalkane. Two mechanisms are involved:
- SN2 mechanism: Bimolecular, second-order kinetics, involves a single step with inversion of configuration.
- SN1 mechanism: Unimolecular, first-order kinetics, involves two steps with the formation of a carbocation intermediate, often leading to racemisation.
- Elimination reactions: When a haloalkane with a β-hydrogen atom is heated with alcoholic KOH, an alkene is formed through β-elimination. The major product is usually the more substituted alkene (Zaitsev’s rule).
- Reaction with metals: Haloalkanes react with metals to form organo-metallic compounds, such as Grignard reagents (RMgX), which are highly reactive and used to form hydrocarbons.
- Grignard reagents: Formed by reacting haloalkanes with magnesium in dry ether, they are highly polar and react with water to form hydrocarbons.
- Wurtz reaction: Alkyl halides react with sodium in dry ether to form hydrocarbons with double the number of carbon atoms.
10.7.2 Reactions of Haloarenes
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Nucleophilic Substitution: Aryl halides are less reactive towards nucleophilic substitution due to resonance effect, hybridization differences, instability of phenyl cation, and repulsion from electron-rich nucleophiles. The C−Cl bond in haloarenes has partial double bond character and is shorter (169 pm) compared to haloalkanes (177 pm).
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Replacement by Hydroxyl Group: Chlorobenzene can be converted into phenol by heating in aqueous sodium hydroxide at 623K and 300 atmospheres. The presence of electron-withdrawing groups like (−NO2) at ortho- and para-positions increases reactivity.
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Electrophilic Substitution Reactions: Haloarenes undergo halogenation, nitration, sulphonation, and Friedel-Crafts reactions. Halogen atoms are o, p-directing due to resonance, which increases electron density at ortho- and para-positions, despite their deactivating inductive effect.
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Reaction with Metals:
- Wurtz-Fittig Reaction: A mixture of alkyl halide and aryl halide treated with sodium in dry ether forms alkylarene.
- Fittig Reaction: Aryl halides treated with sodium in dry ether form compounds where two aryl groups are joined together.
10.8 Polyhalogen Compounds
- Polyhalogen compounds are carbon compounds with more than one halogen atom.
- These compounds are significant in various industrial and agricultural applications.
- The section provides descriptions of several polyhalogen compounds.
10.8.1 Dichloromethane (Methylene chloride)
- Dichloromethane (Methylene chloride) is widely used as a solvent, paint remover, propellant in aerosols, and in drug manufacturing.
- It is also utilized as a metal cleaning and finishing solvent.
- Methylene chloride harms the human central nervous system.
- Lower levels of exposure can impair hearing and vision slightly.
- Higher levels of exposure can cause dizziness, nausea, tingling, and numbness in fingers and toes.
- Direct skin contact causes intense burning and mild redness, while eye contact can burn the cornea.
10.8.2 Trichloromethane (Chloroform)
- Chloroform (trichloromethane) is used as a solvent for fats, alkaloids, iodine, and other substances.
- Its major use today is in the production of the freon refrigerant R-22.
- Chloroform was previously used as a general anaesthetic but has been replaced by safer alternatives.
- Inhaling chloroform vapors can depress the central nervous system and cause dizziness, fatigue, and headache.
- Chronic exposure to chloroform can damage the liver and kidneys, and it can cause skin sores.
- Chloroform oxidizes to phosgene (carbonyl chloride) in the presence of light and air, so it is stored in dark, airtight bottles.
10.1 Classification Haloalkanes and haloarenes are classified based on their structure. Haloalkanes are alkanes where one or more hydrogen atoms are replaced by halogen atoms. Haloarenes are aromatic compounds where one or more hydrogen atoms are replaced by halogen atoms. They can be further classified into primary, secondary, and tertiary haloalkanes based on the carbon atom to which the halogen is attached. Haloarenes can be classified based on the position of the halogen on the aromatic ring, such as ortho, meta, and para positions.