Haloalkanes and Haloarenes – NCERT Notes

Introduction-

Haloalkanes (also known as halogenalkanes) are the chemical compounds in which the hydrogen atom is replaced by halogen atom that is attached to the sp3 hybridized carbon atom.  They have the general formula RX, where R is alkyl halide and X is any halogen (F, Cl, Br, I).

Halo – arenes are the aromatic chemical compounds in which the hydrogen atom is replaced by halogen atom that is attached to the sp2 hybridized carbon atom. They have general formula ArX where Ar is phenyl or any substituted phenyl. They are less reactive than alkyl halides.

Classification –

The number of halogen present in aliphatic or aromatic hydrocarbon classified as mono, di, tri, tetra etc.

Methods of preparation of Haloalkanes –

  • From Alcohols –

This method involves the Nucleophilic substitution reaction in which the –OH group of the alcohol is replaced by any halogen atom.

               R — OH + HX   —>   RX + H2O

Reactivity of halogen acid – HI > HBr > HCl

Reactivity of alcohol – 3° > 2° > 1°

Grove’s process – The preparation of chloro -alkanes or alkyl chloride by the action of lucas reagent (anhydrous ZnCl2 + Conc. HCl) on alcohol.

CH3CH2 – OH + HCl (g) +ZnCl2 (anhy)        —>    CH3CH2Cl + H2O

Purpose of addition of ZnCl2

This is a Nucleophilic substitution type reaction which OH has to be replaced with Cl. OH is a poor leaving group due to the high electro-negativity of the oxygen and Cl is also not a good nucleophile. Hence, the Lucas reagent is used in the reaction. ZnCl2 is Lewis base, so it has tendency to accept the electron pair due to empty 4d orbital. The oxygen of the OH group of the alcohol makes the coordinate bond with the zinc in which oxygen gets the positive charge and zinc gets a negative charge. Through this, the poor leaving group is converted into good leaving group.

Note – Primary and secondary alcohols require anhy. ZnCl2 whereas tertiary alcohols readily react with conc. HCl , they do not require anhy. ZnCl2 .

By the action of phosphorus halides –

By the action of thionyl chlorides –

  • This method is called Darzen method. This is the best method to prepare chloroalkanes because in this reaction, both the side- products (SO2 and HCl) are gases which are escape out from the reaction.
  • Alkyl bromides or alkyl iodides cannot be prepared by this method. (Thionyl bromide is unstable and thionyl iodide does not exist)
  • Pyridine is basic in nature, so it absorbs the HCl and SO2. Hence, reaction   is prevented to go in backward direction.

2.From alkenes –

Br2/CCl4  is used to detect the unsaturation (double or triple bond) in an organic molecule.

Allylic halogenation –

By the addition of halogen acids – Markovnikov’s rule

According to this rule, when a unsymmetrical alkene is react with halogen acid gives an alkyl halide, then halogen adds to the carbon of alkene that has least  number of hydrogen substituents.

When the unsymmetrical alkene is treated with hydrogen bromide in the presence of peroxide , then the addition will take place against the Markovnikov’s rule means the negative part of halogen acid (bromide ion) adds to the carbon of the alkene that has greater number of hydrogen substituents.This rule is known as peroxide effect or Kharasch effect. Peroxide effect follows free radical mechanism.

Note –  

  • HF, HCl, and HI do not give the Anti – Mark. addition even when peroxides are present.
  • Peroxide effect is observed only with HBr. It is explained on the basis of given data –

Both steps are exothermic in case of HBr. The more negative enthalpy change of the reaction, more it will be thermodynamically favorable. Hence, the peroxide effect is observed. With HCl or HBr, second step is endothermic and in case of HF, both steps are endothermic. Hence, the peroxide effect is not observed.

3. Halogen Exchange reaction –

Finkelstein Reaction –

Alkyl bromides/chlorides are treated with sodium iodide in dry acetone. This halogen exchange type reaction is known as Finkelstein reaction. This reaction is useful to prepare the iodoalkanes and gives best result for primary halides.

Reason to use acetone –

Acetone is an organic solvent which is widely used in organic reaction. It is used in this reaction to facilitate the reaction in forward direction. According to the Le-chatlier principal, the reaction will move in that direction where the one of the reagents goes outside the solution.

Since, the precipitation of NaCl will drive the reaction in forward direction. For this, it is important to find such a solvent that would dissolve NaI better than NaCl. Then the concentration of iodide ions in the solution increases that will move the equilibrium in forward direction.

Swarts reaction –

                          R — Br + Hg2F2  ——–> R — F  +   Hg2Cl2

AgF, CoF2, SbF3 can also be used for the Swarts reaction

4. Hunsdiecker Reaction –

This reaction is also known as Borodine Hundsdiecker reaction. In this reaction, we will get the one carbon less.

5.From alkynes –

Methods of preparation of Haloarenes –

  1. Nuclear halogenation –

This is electrophilic substitution type reaction which is used to prepare alkyl chlorides or alkyl bromides by direct chlorination or bromination.

  • From diazonium salt –

Diazonium salt is very reactive compound. Alkyl chlorides or alkyl bromides are easily prepared by this method.

  • By side chain halogenation –

If Cl2 is passed for the longer time –

  • From Phenol –
  • Hundsdiecker reaction –
  • Physical Properties of Haloalkanes –

    • Boiling Point –
      Boiling point of halogen – R — I  >  R — Br >  R — Cl > R —F 

    (R = alkyl group)

    As we move from iodine to fluorine –

    • Size increases
    • Molecular weight and mass increases
    • Magnitude of vanderwaal forces increases
    • As the size of alkyl group increases, boiling point increases in case of same halogen atom. For example –

          CH3Br > CH3CH2Br > CH3(CH2)2Br > CH3(CH2)3Br

    The boiling point of alkyl chlorides, alkyl bromides and alkyl iodides generally increases with increase in the number of halogen atom.

    • As the branching increases, boiling points decreases because branching decrease the surface area and make the molecule more compact. This is because the magnitude of vanderwaal force decreases.
    • Solubility –

    They are slightly soluble in water but readily soluble in organic solvents. This is because due to the release of less energy when new attractions are setup between halo alkane and water molecules. The hydrogen bonding in water is more strong than haloalkane.

    • Color –

    In pure state, alkyl halides are colorless but bromides and iodides develop color when they expose to the light.

    • Density –
    • For the same alkyl group, density increases as the atomic mass of the halogen atom increases for ex. – R —I  > R — Br > R — Cl > R — F
    • Density increases with increases in the number of halogen atom.

    CCl4 > CHCl3 > CH2Cl2 > CH3Cl

    • As the number of carbon atom increases, the density increases.

    Physical Properties of Haloarenes –

    • Boiling Point and melting point –
      The boiling point of monohalogen derivative of benzene –

    Iodo > bromo > chloro > fluoro

    • As the size of aryl group increases, the melting and boiling point increases in case of same halogen atom.
    • The para – isomer has more melting point than ortho and meta – isomer due to the symmetrical nature of para – isomer, it can easily fit into crystal lattice. Therefore, the intermolecular force of attraction is getting strong. So, the more energy is break to break that lattice.
    • Color –

    These are generally colorless liquids or crystalline solids.

    Chemical Properties of Halo – alkanes –

    1. Nucleophilic Substitution reaction –

    Those reactions in which a stronger nucleophile displaces a weaker nucleophile, called Nucleophilic substitution reaction.

                Rδ+ — Xδ-   +   Nu        —–>               R — Nu + X

    Nucleophilic substitution reaction mechanism may take place in two ways –

    • SN1 mechanism
    • SN2 mechanism

    SN1 type reaction ( Nucleophilic substitution Unimolecular reactions) –

    Mechanism –

    • The SN1 reactions occur in two steps – first step involves the formation of carbocation and a second step involves the attack of nucleophile on carbocation.
    • The first step is the slow step. So, it is rate determining step.
    • A SN1 reaction follows first order kinetics.
    • Rate = K[ (CH3)3CX]
    • Generally favored by polar protic solvent such as water, alcohol, acetic acid etc.
    • Rearrangement is commonly observed in this reaction.
    • The order of reactivity of alkyl halides –

    3° > 2° > 1° > CH3X

    More stable will be the carbocation intermediate; faster will be the SN1 reaction.

    • Racemic mixture is obtained.

    SN 2 type reaction ( Nucleophilic Substitution Bimolecular reaction) –

    • In SN2 reactions, the incoming nucleophile (OH-) approaches towards the alkyl halide molecule from the back side and the halide ion starts leaving from the front side. These two steps take place in the single step and no intermediate is formed.
    • Single step reaction.
    • A SN2 reaction follows second order kinetics.
    • Rate = K[ CH3Cl][Nu]
    • Generally favored by aprotic solvent.
    • Rearrangement is not observed in this reaction.
    • The order of reactivity of alkyl halides –

    3° < 2° < 1° < CH3X

    The rate of reaction is depending on the stearic hinderence of the alkyl group. As the length of the alkyl group increases, the rate of reaction decreases.

    • Inversion of configuration is observed.

    2. Elimination reactions

    Dehydrohalogenation –

    It is the β – elimination reaction in which halo – alkane with β – hydrogen atom is heated with alcoholic potassium hydroxide, then the elimination occurs from β – carbon and a halogen atom from the α – carbon, then there is the formation of alkene. The elimination reaction follows the saytzeff rule (which is also known as Zaitsev rule). According to this rule, the preferred alkene is that which has the greater number of alkyl groups attached to the doubly bonded carbon atom.

    The elimination reaction introduces the multiple bonds and it can classify into E1 and E2 reactions.

    E1 Reaction

    It is the Unimolecular type reaction occurs in two steps. The first step involves the formation of carbocation intermediate and the second step involves the loss of proton by base. The first step is the slow step, so it is rate determining step.

                 (CH3)3C — I + H2O         —->             CH2 ═ C(CH3)2 + H3O+  + I-

    More stable is the carbocation, faster will be the reaction. In this reaction, the rate is depending on the concentration of tert. – butyl iodide.

                                  Rate = K [ (CH3)3CI]

    Mechanism –

    Step – 1 –

    Step-2-

    • Order of reactivity of E1 reaction –  3° > 2°  > 1°
    • E1 reaction favored by protic solvents.
    • It is observed in the presence of weak bases.
    • Commonly, rearrangement is observed in this reaction.
    • This E1 reaction leads to the stable stereochemistry.

    E2 Reaction

    It is the bimolecular type reaction occurs in only one step. The rate depends on the concentration of substrate and base.

                                  Rate = K [ RX][Base]

    Example –

    • No intermediate is formed in this reaction. The reaction is goes through the transition state.
    • Single step reaction.
    • E2 reaction favored by aprotic solvents.
    • It is observed in the presence of strong bases.
    • Commonly, rearrangement is not observed in this reaction.
    • This E2 reaction may not lead to the more stable stereochemistry.

    3. Reaction with metals –

    Reaction with sodium – Wurtz Reaction

                 R — X + 2Na + X — R     —->               R — R + 2Na X

    Reaction with Magnesium –

                                  R — X + Mg    ——->            R — MgX ( Grignard Reagent )

       4. Reaction with CN – 

           R — X + KCN                  —–>                 R — CN + KX

           R — X + AgCN                 —->                  R — CN + AgX

    Explanation –

    CN is an ambient nucleophile which means it can form the bond from C and N atoms. KCN is an ionic compound, so C and N both are available for the formation of bond. Since, C –C bond is more stable and stronger than C –N bond due to which cyanide is formed.

    AgCN is covalent compound, so it can not dissociate completely. In this, only N is available for the formation of bond, therefore isocyanide is formed in case of AgCN.

    Chemical properties of Halo – arenes –

    1. Nucleophilic Substitution reaction –

    Aryl halides are less reactive towards the Nucleophilic substitution reaction because –

    • In haloarenes, the halogen atom is attached to sp2 hybridized carbon atom due to which C – X bond get partial double bond character. Therefore, the C – X bond is stable in haloarenes and difficult to break.
    • The lone pair present on the halogen atom are delocalized on the benzene ring due to resonance.
    • Instability of phenyl cation.

    Stable Resonating Structure unstable Resonating Structure

    • The presence of electron withdrawing group like NO2 group increases the reactivity of aryl halide towards Nucleophilic substitution reaction.

    2. Dow‘s Process –

    • The presence of NO2 group at ortho and para position increases the reactivity of aryl halide due to the increase in the delocalization of negative charge.

    The nitro group at ortho position –

    The nitro group at meta position –

    The nitro group at para position –

    3. Electrophilic Substitution reaction –

    Aryl halides are deactivating but ortho and para directing.

    Halogenation

    (Major Product) (Minor Product)

    Fridel craft Reaction –

    (Major product) (Minor Product)

    Sulphonation –

    Nitration –

    4. Reaction with metals

    Wurtz Fittig Reaction –

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