What are Haloalkanes and Haloarenes ?

  • In aliphatic or aromatic compounds, the replacement of hydrogen atom(s) by halogen atoms(s) results in the formation of alkyl halide (haloalkane) and aryl halide (haloarene), respectively.
  • In case of haloalkanes, halogen atom is attached to the sp3 hybridised carbon atom of an alkyl group whereas in haloarenes, halogen atom is attached to sp2 hybridised carbon atom of an aryl group.
  • Haloalkanes or haloarenes are classified as mono, di- tri-, tetra- haloalkanes or haloarenes, etc., according to the one, two, three, four, etc., halogen atoms respectively present in their molecule.
Haloalkanes and Haloarenes
  • On the basis of the halogen atom attached to primary, secondary and tertiary carbon atoms alkyl halides are further classified as classified as 1°, 2°, 3°, allylic, benzylic, vinylic and aryl derivatives.
Haloalkanes and Haloarenes

Classification of Haloalkane s and Haloar enes

Compounds containing sp3 C–X Bond
  • Alkyl halides or haloalkanes (R–X)
  • Allylic halides This have X bonded to the carbon atom adjacent to C–C double bond.
  • Benezylic halidesThis have X bonded to the carbon atom adjacent to a benzene ring.
Classification of Haloalkane s and Haloar enes

Compounds containing sp2 C–X Bond

  • Vinylic halides This have a halogen atom (X) bonded to a C–C double bond.
  • Aryl halides This have a halogen atom bonded to a benzene ring.
Classification of Haloalkane s and Haloar enes

Nature Of C-x Bond

Since halogen atoms are more electronegative than carbon, the carbon-halogen
bond of alkyl halide is polarised; the carbon atom bears a partial positive
charge whereas the halogen atom bears a partial negative charge.


Methods Of Preparation Of Haloalkanes

Haloalkanes can be prepared from displacement of alcoholic group in alkyl alcohol by halogen acid, PCl5 and PCl3. Haloalkanes can also be prepared by addition of halogen acids or halogens on alkene and alkyne. Alkyl halides can aslo be prepared by free radical halogenation of alkene.

Finkelstein Reaction

From Alcohols

Finkelstein Reaction

$$R-X+NaI\xrightarrow[X = Cl, Br]{acetone} R—I + NaX$$

Swarts Reaction

$$\\ H_3C- Br + AgF\longrightarrow H_3C -F + AgBr$$
$$\\ Hg_2F_2,\space COF_2 \space and \space SbF_3 \space \text{can also be used as a reagent for Swarts reaction.}$$

Hunsdiecker Reaction

$$\\CH_3COOAg + Br_2\xrightarrow[CCl_4]{}CH_3Br + AgBr + CO_2$$

Free Radical halogenat ion of alkane s

Free Radical halogenat ion of alkane s

Addition of hydrogen halides on alkene s

Addition of hydr ogen halides on alkene s

Physical Properties Of Haloalkanes

  • Boiling point orders 1. R-I > R - Br > R-Cl > R -F
  • CH3–(CH2)2–CH2Br > (CH3)2CHCH2Br > (CH3)3CBr
  • CH3CH2CH2 > CH3CH2X > CH3X
  • Bond strength of haloalkanes decreases as the size of the halogen atom increases. Thus, the order of bond strength is CH3F > CH3Cl > CH3Br > CH3I.
  • Dipole moment decreases as the electronegativity of the halogen decreases.
  • Haloalkanes though polar but are insoluble in water as they do not form hydrogen bonding with water.
  • Density order is RI> RBr > RCl > RF (for the same alkyl group) CH3I > C2H5I > C3H7I

Chemical Properties Of Haloalkanes

Relative reactivity of alkyl halides for same alkyl group is RI > RBr > RCI> RF

(I) Nucleophilic Substitution R eactions (S N reactions)

(I) N ucleophilic Substitution R eactions (S N reactions)

Nucleophilic Substitution R eactions

Characteristics of SN1 Mechanism Characteristics of SN2 Mechanism
Kinetics: First-order kinetics; rate = k[RX]  Kinetics: Second-order kinetics; rate = k[RX][:Nu]
Mechanism: Two steps Mechanism: One step
Stereochemistry: Trigonal planar carbocation intermediate. Racemization at a single stereogenic center Stereochemistry: Backside attack of the nucleophile. Inversion of configuration at a stereogenic center
Identity of R: More substituted halides react fastest. Identity of R: Unhindered halides react fastest.
Rate : R3CX > R2CHX > RCH2X > CH3X Rate : CH3X > RCH2X > R2CHX > R3CX

(II) Eliminat ion Reactions

Dehydrohalogenation is a -elimination reaction in which halogen is from α carbon atom and the hydrogen from the  β carbon according to Saytzeff rule, e.g.,

(II) Eliminat ion R eactions

(III) Reduction

$$\text{C}_2\text{H}_5 — \text{Br} + \text{H}_2\xrightarrow[]{\text{Ni, 575 K}}\text{C}_2\text{H}_6 + \text{HBr}\\\text{C}_2\text{H}_5\text{I} + \text{HI}\xrightarrow[]{\text{Red P, 420 K}}\text{C}_2\text{H}_6 + \text{I}_2$$

(IV) R eaction with Metals

(i) Wurtz reaction: $$\\ RX + 2Na + XR \xrightarrow[]{Dry ether}R—R (alkane) + 2NaX$$
(ii) Wurtz-Fitting reaction: $$\\C_6H_3—Cl + 2Na + Cl CH_3\xrightarrow[]{Dry ether}C_6H_5—CH_3+ 2NaCl$$
(iii) Reaction with Mg: $$\\C_2H_5Br + Mg\xrightarrow[]{Dry ether}\underset{\mathrm{(Grignard’s reagent)}}{C_2H_5—Mg—Br}$$

Methods Of Preparation Of Haloarenes

(a) By Electrophilic Substitution Reaction:

By Electrophilic Substitution Reaction

(b) Sandmeyer’s Reaction

Sandmeyer’s Reaction

(c) Gattermann Reaction:

Gattermann Reaction:

(d) In the presence of KI

Gattermann Reaction:

(e) Balz-Schiemann reaction:

Balz-Schiemann reaction

Chemical Properties Of Haloarenes

Aromatic nucleophilic s ubstitution reaction Haloarenes do not undergo nucleophilic substitution reaction readily. This is due to C–X bond in aryl halide is short and strong and also the aromatic ring is a centre of high electron density. The halogen of haloarenes can be substituted by OH-, NH2 -, or CN- with appropriate nucleophilic reagents at high temperature and pressure. For Example :

$$\\\underset{\mathrm{Chlorobenzene}}{C_6H_5Cl + NaOH}\xrightarrow[300 atm]{350°C}\underset{\mathrm{Chlorobenzene}}{C_6H_5OH + NaCl}$$

$$\\\underset{\mathrm{Chlorobenzene}}{C_6H_5Cl + 2NH_3}\xrightarrow[50 atm]{250°C}\underset{\mathrm{Aniline}}{C_6H_5NH_2 + NH_4Cl}$$

$$\\\underset{\mathrm{Chlorobenzene}}{C_6H_5Cl + CuCN}\xrightarrow[Pyridine]{250°C}\underset{\mathrm{Phenyl cyanide}}{C_6H_5CN + CuCl}$$

Polyhalogen Compounds: Uses And Environmental Effects

(i) Dichloromethane (Methylene chloride)

  • It is used as solvent, paint remover, propellant in aerosols, process solvent in
    the manufacture of drugs and in the metal cleaning and finishing solvent.
Methylene chloride

(ii) Triiodomethane (Iodoform)

  • They are used as an antiseptic due to the liberation of free iodine. It is not because of Iodoform itself.

(iii) Trichloromethane (Chloroform)

  • It is used as anesthetic because when pure chloroform is inhaled it affects
    the heart due to which after mixing with ether and other suitable anesthetics
    chloroform can be used as anesthetic.

(iv) Tetra Chloromethane (Carbon Tetrachloride)

  • They are used in manufacturing refrigerants, propellants for aerosol cans and for the synthesis of chlorofluorocarbons, pharmaceutical etc.
Carbon te-trachloride

(v) Freons

  • The Chlorofluorocarbon compounds of methane and ethane are jointly called freons.
  • They are very stable, non-corrosive, non-toxic, and unreactive liquefiable gases.

(vi) P,P'-Dichlorodiphenyltrichloroethane (DDT)

  • DDT stands to be the first chlorinated organic insecticides. It is highly
    poisonous to all living organisms as it does not get metabolized
    rapidly by animals and gets deposited and stored in the fatty tissues.