Halogen Derivatives - Comprehensive Notes

Halogen Derivatives

  • Halogen derivatives are formed by replacing hydrogen atoms in hydrocarbons with halogen atoms.
  • The parent family of organic compounds is hydrocarbon. Replacement of hydrogen atom/s in aliphatic or aromatic hydrocarbons by halogen atom/s results in the formation of halogen derivatives of hydrocarbons.
  • General formulas:
    • Haloalkane: CH3 - CH2 - X
    • Haloalkene: CH2 = CH - X
    • Haloalkyne: HC ≡ C - X
    • Haloarene: X

10.1 Classification of Halogen Derivatives

  • Halogen derivatives are classified in two main ways:
    • Based on the hydrocarbon skeleton:
      • Haloalkanes
      • Haloalkenes
      • Haloalkynes
      • Haloarenes
    • Based on the number of halogen atoms:
      • Mono
      • Di
      • Tri
      • Poly halogen compounds

10.1.1 Classification of Monohalogen Compounds

  • Monohalogen compounds are further classified based on:
    • Position of the halogen atom
    • Type of hybridization of the carbon to which the halogen is attached
a. Alkyl Halides or Haloalkanes

  • Halogen atom is bonded to an sp^3 hybridized carbon that is part of a saturated carbon skeleton.

  • Alkyl halides can be primary (1^0), secondary (2^0), or tertiary (3^0) depending on the substitution state of the carbon to which the halogen is attached.

    • Primary Halide (1^0): R - CH2 - X
    • Secondary Halide (2^0): R - CH - X
      R
    • Tertiary Halide (3^0): R - C - X
      R
      R
b. Allylic Halides
  • Halogen atom is bonded to an sp^3 hybridized carbon atom next to a carbon-carbon double bond.
  • General formula: CH2 = CH - CH2 - X
c. Benzylic Halides
  • Halogen atom is bonded to an sp^3 hybridized carbon atom which is further bonded to an aromatic ring.
d. Vinylic Halides
  • Halogen atom is bonded to an sp^2 hybridized carbon atom of an aliphatic chain.
  • Vinylic halides are haloalkenes.
  • General formula: CH2 = CH - X
e. Haloalkynes
  • Halogen atom is bonded to an sp hybridized carbon atom.
  • General formula: CH ≡ C - X
f. Aryl Halides or Haloarenes
  • Halogen atom is directly bonded to the sp^2 hybridized carbon atom of an aromatic ring.

10.2 Nomenclature of Halogen Derivatives

  • Common names of alkyl halides are derived by naming the alkyl group followed by the name of the halogen as a halide (e.g., methyl iodide, tert-butyl chloride).
  • IUPAC system names alkyl halides as haloalkanes.
  • Aryl halides are named as haloarenes in both common and IUPAC systems.
  • For dihalogen derivatives of arenes, prefixes o- (ortho-), m- (meta-), and p- (para-) are used in common names; in IUPAC, numerals 1,2; 1,3; and 1,4 are used, respectively.

10.3 Methods of Preparation of Alkyl Halides

10.3.1 From Alcohols

  • The hydroxyl group of an alcohol is replaced by a halogen atom using:
    • Halogen acid (HX)
    • Phosphorous halide
    • Thionyl chloride
a. Using Halogen Acid (HX)
  • The reactivity of alcohols with a given haloacid is 3^0 > 2^0 > 1^0.
  • Reaction: R - OH + HX \xrightarrow{\text{suitable condition}} R - X + H2O
  • Hydrogen chloride with zinc chloride (Groves' process) is used for primary and secondary alcohols.
  • Tertiary alcohols readily react with concentrated hydrochloric acid without zinc chloride.
  • Constant boiling hydrobromic acid (48%) is used for preparing alkyl bromides.
  • Primary alkyl bromides can also be prepared by reaction with NaBr and H2SO4, where HBr is generated in situ.
  • Reaction: R-CH2-OH + HBr \xrightarrow{\text{NaBr, H2SO4, heat}} R-CH2-Br + H_2O
  • Alkyl iodides are obtained by heating alcohols with sodium or potassium iodide in 95% phosphoric acid, where HI is generated in situ.
  • Reaction: R - OH + HI \xrightarrow{\text{NaI/H3PO4}} R - I + H_2O
b. Using Phosphorous Halide
  • Phosphorous tribromide (PBr3) and triiodide (PI3) are generated in situ by the action of red phosphorous on bromine and iodine, respectively.
  • Phosphorous pentachloride (PCl_5) reacts with alcohol to give alkyl chloride.
  • Reactions:
    • 3R - OH + PX3 \rightarrow 3R - X + H3PO_3
    • R - OH + PCl5 \rightarrow R - Cl + HCl + POCl3
c. Using Thionyl Chloride
  • Thionyl chloride (SOCl_2) reacts with straight chain primary alcohols to give unrearranged alkyl chlorides; the byproducts are gases.
  • Reaction: R - OH + SOCl2 \xrightarrow{\Delta} R - Cl + SO2 \uparrow + HCl \uparrow

10.3.2 From Hydrocarbons

  • Alkyl halides are formed from saturated and unsaturated hydrocarbons.
Addition of Hydrogen Halide to Alkenes
  • Alkyl halides are formed by adding hydrogen halides to alkenes based on Markovnikov's rule and the peroxide effect.
Halogenation of Alkanes
  • Not suitable for preparation of alkyl halides as it forms a mixture of mono and poly halogen compounds.

10.3.3 Halogen Exchange

Finkelstein Reaction
  • Alkyl iodides are prepared by treating alkyl chlorides or bromides with sodium iodide in methanol or acetone.
  • Reaction: R - Cl + NaI \xrightarrow{\text{acetone}} R - I + NaCl\downarrow
Swartz Reaction
  • Alkyl fluorides are prepared by heating alkyl chlorides or bromides with metal fluorides such as AgF, Hg2F2, AsF3, SbF3, etc.
  • Reaction: R - Cl + AgF \rightarrow R - F + AgCl\downarrow

10.3.4 Electrophilic Substitution

  • Aryl chlorides and bromides can be prepared by direct halogenation of benzene and its derivatives through electrophilic substitution.
  • Reaction is carried out in the dark at ordinary temperature in the presence of a Lewis acid catalyst like Fe, FeCl3, or anhydrous AlCl3.
  • When toluene is brominated in the presence of iron, a mixture of ortho- and para-bromotoluene is obtained.

10.3.5 Sandmeyer's Reaction

  • Aryl halides are prepared by replacement of nitrogen of diazonium salt.

10.4 Physical Properties

  • Physical properties of alkyl halides differ considerably from those of corresponding alkanes.
  • Boiling points are determined by polarity of the C-X bond and the size of halogen atoms.

10.4.1 Nature of Intermolecular Forces

  • Halogens are more electronegative than carbon.
  • The carbon-halogen bond in alkyl halide is a polar covalent bond.
  • Alkyl halides are moderately polar compounds.
  • The size of the halogen atom increases from fluorine to iodine, thus the C-X bond length increases.
  • The C-X bond strength decreases with an increase in the size of the halogen.

10.4.2 Boiling Point

  • Boiling points of alkyl halides are higher than those of corresponding alkanes due to higher polarity and higher molecular mass.
  • For a given alkyl group, the boiling point increases with increasing atomic mass of the halogen.
  • Order: RI > RBr > RCl > RF
  • For a given halogen, the boiling point rises with increasing carbon number.
  • For isomeric alkyl halides, the boiling point decreases with increased branching.

10.4.3 Solubility

  • Alkyl halides are moderately polar but insoluble in water due to their inability to form hydrogen bonds with water.
  • Alkyl halides are soluble in non-polar organic solvents.
  • Aryl halides are also insoluble in water but soluble in organic solvents.
  • The melting point of the para isomer is higher than that of ortho or meta isomers due to its symmetrical structure, which allows for closer packing in the crystal lattice.

10.5 Optical Isomerism in Halogen Derivatives

10.5.1 Chiral Atom and Molecular Chirality

  • A carbon atom in a molecule that carries four different groups/atoms is called a chiral carbon atom.
  • A chiral molecule cannot be superimposed perfectly on its mirror image.
  • Isomers having the same bond connectivities, that is, structural formula are called stereoisomers.
  • A chiral molecule and its mirror image both have the same structural formula and, of course, the same molecular formula.
  • The spatial arrangement of the four different groups around the chiral atom, however, is different.
  • The relationship between a chiral molecule and its mirror image is similar to the relationship between left and right hands; this is called handedness or chirality.
  • The stereoisomerism in which the isomers have different spatial arrangements of groups/ atoms around a chiral atom is called optical isomerism.
  • The optical isomers differ from each other in terms of a measurable property called optical activity.

10.5.2 Plane Polarized Light

  • Ordinary light consists of electromagnetic waves having oscillations of electric and magnetic fields in all possible planes perpendicular to the direction of propagation.
  • When ordinary light is passed through Nicol's prism, oscillations only in one plane emerge out. Such a light having oscillations only in one plane perpendicular to direction of propagation of light is known as plane polarized light.

10.5.3 Optical Activity

  • The property of a substance by which it rotates the plane of polarization of incident plane polarized light is known as optical activity.
  • Compounds that rotate the plane of plane polarized light are called optically active compounds.
  • Optical activity is expressed numerically in terms of optical rotation.
  • The angle through which a substance rotates the plane of plane polarized light is called optical rotation.
  • A compound which rotates the plane of plane polarized light towards right is called dextrorotatory and designated by symbol d- or by (+) sign.
  • A compound which rotates plane of plane polarized light towards left is called laevorotatory and designated by symbol l- or by (-) sign.
  • Isomerism in which isomeric compounds have different optical activity is known as optical isomerism.
  • French scientist Louis Pasteur first recognized that optical activity is associated with certain type of 3-dimensional structure of molecules.
  • Pasteur introduced the term enantiomers for the optical isomers having equal and opposite optical rotation.

10.5.4 Enantiomers

  • The optical isomers which are non-superimposable mirror images of each other are called enantiomers, enantiomorphs, or optical antipodes.
  • Enantiomers have identical physical properties (Such as melting point, boiling points, densities, refractive index) except the sign of optical rotation.
  • The magnitude of their optical rotation is equal but the sign of optical rotation is opposite. They have identical chemical properties except towards optically active reagent.
  • An equimolar mixture of enantiomers (dextrorotatory and laevorotatory) is called racemic modification or racemic mixture. A racemic modification is optically inactive because optical rotation due to molecules of one enatiomer is cancelled by equal and opposite optical rotation due to molecules of the other enantiomer.
  • A racemic modification is designated as (dl) or by (\pm) sign.

10.5.5 Representation of Configuration of Molecules

Fischer Projection Formula (Cross Formula)
  • Two representations are used to represent configuration of chiral carbon and the 3-dimensional structure of optical isomers on plane paper.
  • These are (a) wedge formula and (b) Fischer projection formula (also called cross formula).
Wedge Formula
  • When a tetrahedral carbon is imagined to be present in the plane of paper all the four bonds at this carbon cannot lie in the same plane.
  • The bonds in the plane of paper are represented by normal lines, the bonds projecting above the plane of paper are represented by solid wedges (or simply by bold lines) while bonds going below the plane of paper are represented by broken wedges (or simply by broken lines).

10.6 Chemical Properties

10.6.1 Laboratory Test of Haloalkanes

  • Haloalkanes are of neutral type in aqueous medium. On warming with aqueous sodium or potassium hydroxide the covalently bonded halogen in haloalkane is converted to halide ion.
  • Reaction: R - X + OH \xrightarrow{\Delta} R - OH + X
  • When this reaction mixture is acidified by adding dilute nitric acid and silver nitrate solution is added a precipitate of silver halide is formed which confirms presence of halogen in the original organic compound.
  • Reaction: Ag^{\oplus} \text{ (aq)} + X^{\text{-}} \text{ (aq)} \rightarrow AgX \downarrow \text{ (s)}

10.6.2 Nucleophilic Substitution Reactions of Haloalkanes

  • When a group bonded to a carbon in a substrate is replaced by another group to get a product with no change in state of hybridization of that carbon the reaction is called substitution reaction.
  • The C-X bond in alkyl halides is a polar covalent bond and the carbon in C-X bond is positively polarized. In other words, the C-X carbon is an electrophilic centre. It has, therefore, a tendency to react with a nucleophile.
  • Alkyl halides react with a variety of nucleophiles to give nucleophilic substitution reactions (SN).
  • Reactivity order: tertiary alkyl halide (3^0) > secondary alkyl halide (2^0) >primary alkyl halide (1^0) and R - I > R - Br > R - Cl

10.6.3 Mechanism of SN Reaction

  • The halogen atom of alkyl halide is called ‘leaving group’ in the context of this reaction. Leaving group is the group which leaves the carbon by taking away the bond pair of electrons.
  • Two mechanisms are observed in various SN reactions: SN1 and SN2 mechanisms.
a. SN2 Mechanism

  • SN_2: Substitution Nucleophilic Bimolecular

  • The reaction between methyl bromide and hydroxide ion is second order kinetics.

  • Reaction: CH3Br + OH^{\text{-}} \rightarrow CH3OH + Br^{\text{-}}

  • Rate: rate = k [CH_3Br] [OH^{\text{-}}]

    Salient features of SN2 mechanism:
    • Single step mechanism with simultaneous bond breaking and bond forming.
    • Backside attack of nucleophile.
    • In the transition state (T.S.) the nucleophile and leaving groups are bonded to the carbon with partial bonds and carry partial negative charge.
    • The T.S. contains pentacoordinate carbon having three σ (sigma) bonds in one plane making bond angles of 120^0 with each other and two partial covalent bonds along a line perpendicular to this plane.
    • SN_2 reaction is found to proceed with inversion of configuration. This is like flipping of an umbrella . It is known as Walden inversion. The inversion in configuration is the result of backside attack of the nucleophile.
b. SN1 Mechanism

  • SN_1: Substitution Nucleophilic Unimolecular

  • The reaction between tert-butyl bromide and hydroxide ion follows a first-order kinetics.

  • The rate of this reaction depends on concentration of only one species, which is the substrate molecule, tert-butyl bromide.

  • It is a two step mechanism.

  • Rate: rate = k [(CH3)3CBr]

    ##### Salient features of SN1 mechanism:

    • Two step mechanism.
    • Heterolyis of C-X bond in the slow and reversible first step to form planar carbocation intermediate.
    • Attack of the nucleophile on the carbocation intermediate in the fast second step to form the product.
    • When SN_1 reaction is carried out at chiral carbon in an optically active substrate, the product formed is nearly racemic which is due to that Nucleophile can attack planar carbocation from either side results in formation of both the enantiomers of the product.

10.6.4 Factors Influencing SN 1 and SN 2 Mechanism

a. Nature of Substrate
  • SN_2: favored in primary halides.
  • SN_1: favored in tertiary halides.
b. Nucleophilicity of the Reagent
  • A more powerful nucleophile attacks the substrate faster and favours SN_2 mechanism.
  • The rate of SN_1 mechanism is independent of the nature of nucleophile.
c. Solvent Polarity
  • SN_1 mechanism proceeds via formation of carbocation intermediate. A good ionizing solvent, polar solvent, stabilizes the ions by solvation.
  • SN_1 proceeds more rapidly in polar protic solvents.
  • SN_2 mechanism favors aprotic solvents or solvents of low polarity.

10.6.5 Elimination Reaction: Dehydrohalogenation

  • When alkyl halide having at least one β-hydrogen is boiled with alcoholic solution of potassium hydroxide, it undergoes elimination of hydrogen atom from β-carbon and halogen atom from α - carbon resulting in the formation of an alkene.
  • Saytzeff's rule: In dehydrohalogenation reaction, the preferred product is that alkene which has greater number of alkyl groups attached to doubly bonded carbon atoms.

10.6 Reaction with active metals

a. Reaction with magnesium
  • When alkyl halide is treated with magnesium in dry ether as solvent, it gives alkyl magnesium halide. It is known as Grignard reagent.
  • R-X + Mg \xrightarrow{\text{dry ether}} R - Mg - X
b. Wurtz reaction
  • Alkyl halides react with metallic sodium in dry ether as solvent, and form higher alkanes containing double the number of carbon atoms present in alkyl halide. This reaction is called Wurtz reaction.
  • 2 R-X + 2 Na \xrightarrow{\text{dry ether}} R - R + 2 NaX

10.6.1 Reaction of haloarenes

a. Reactions of haloarene with metals
  • The reaction of aryl halide with alkyl halide and sodium metal in dry ether to give substituted aromatic compounds is known as Wurtz- Fittig reaction.
  • In case only aryl halide takes part in the reaction, the product is biphenyl and the reaction is known as Fittig reaction.
b. Nucleophilic substitution SN of haloarenes
  • Aryl halides show low reactivity towards nucleophilic substitution reactions, due to Resonance effect and sp^2 hybrid state of C.
  • Presence of electron withdrawing group at ortho and/or para postion greatly increases the reactivity of haloarenes towards subsitution of halogen atom. Electron withdrawing group at meta position has practically no effect on reactivity.
c. Electrophilic substitution (SE) in arylhalides
  • Aryl halides undergo electrophilic substitution reaction slowly as compared to benzene. In resonance structures of chlorobenzene elelctron density is relatively more at ortho and para position. Therefore incoming electrophilic group is more likely to attack at these positions. But due to steric hinderance at ortho position, para product usually predominates.

10.7 Uses and Environmental effects of some polyhalogen compounds

  • Dichloromethane / methylene chloride (CH2Cl2).
  • Chloroform / trichloromethane (CHCl_3).
  • Carbon tetrachloride / tetrachloromethane (CCl_4).
  • Idoform or triiodomethane (CHI_3).
  • Freons: CFC's commonly used as refrigerants. They are used as blowing agents in making foams and packing materials. Chloroflurocarbons are responsible for ozone depletion in stratosphere.
  • Dichlorodiphenyltrichloroethane (DDT).