Haloalkanes and Haloarenes

They are used as solvents for relatively non-polar compounds and as starting materials for the synthesis of wide range of organic compounds. (i) Chlorine containing compound produced by soil microorganisms which is effective in treating typhoid fever (ii) Iodine containing hormone produced in our body deficiency of which causes goitre. ( abnormal swelling of neck). (iii) Synthetic halogen compound which is used in the treatment of malaria (iv) It is used as a dental anaesthetic in surgery( CF3-CHClBr) (v) Fully fluorinated compounds that are potential blood substitutes during long surgeries

These may be classified as mono, di, or polyhalogen (tri-,tetra-, etc.) compounds depending on whether they contain one, two or more halogen atoms in their structures.

I) sp3 hybridised i) Alkyl halides In alkyl halides, the halogen atom is bonded to an alkyl group (R). They form a homologous series represented by CnH2n+1X. They are further classified as primary, secondary or tertiary according to the nature of carbon to which halogen is attached. If halogen is attached to a primary carbon atom in an alkyl halide, the alkyl halide is called primary alkyl halide or 1° alkyl halide. Similarly, if halogen is attached to secondary or tertiary carbon atom, the alkyl halide is called secondary alkyl halide (2°) and tertiary (3°) alkyl halide, respectively. ii)Allylic halides These are the compounds in which the halogen atom is bonded to an sp3 -hybridised carbon atom adjacent to carbon-carbon double bond (C=C) i.e. to an allylic carbon. iii) Benzylic halides These are the compounds in which the halogen atom is bonded to an sp3 -hybridised carbon atom attached to an aromatic ring.

(a) Vinylic halides These are the compounds in which the halogen atom is bonded to a sp2 -hybridised carbon atom of a carbon-carbon double bond (C = C). (b) Aryl halides These are the compounds in which the halogen atom is directly bonded to the sp2 -hybridised carbon atom of an aromatic ring.

Halogen atoms are more electronegative than carbon, therefore, 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.

As we go down the group in the periodic table, the size of halogen atom increases. Fluorine atom is the smallest and iodine atom is the largest. Consequently the carbon-halogen bond length also increases from C—F to C—I. Bond length is inversely proportional to bond enthalpy. Longer the bond, lesser is the energy required to cleave it. So bond enthalpy decreases down the group from C—F to C—I

The dipole moment of the CH3-F is lesser than CH3-Cl. The magnitude of -ve charge on the F atom is greater, however the C-F bond has a smaller bond legnth due to the smaller size of fluorine atom. Dipole moment = charge x distance of seperation since as bond legnth decreases, distance of seperation of charges decreases, dipole moment of CH3F decreases.

Thionyl chloride is preferred because in this reaction alkyl halide is formed along with gases SO2 and HCl. The two gaseous products are escapable, hence, the reaction gives pure alkyl halides. R-OH + SOCl2 ——–> R-Cl + SO2↑ + HCl↑

The reactions of primary and secondary alcohols with HCl require the presence of a catalyst, ZnCl2 . With tertiary alcohols, the reaction is conducted by simply shaking the alcohol with concentrated HCl at room temperature. R-OH + HCl ——> R-Cl + H2O Bromoalkanes can be prepared by refluxing a suitable alcohol with constant boiling hbr(48%) R-OH + NaBr + H2SO4 ——> R-Br + NaHSO4 + H2O Iodoalkanes can be prepare dby refluxing alcohol with constant boiling hi. ROH + HI —–> RI + H2O

ZnCl2 helps in breaking of the carbnon-oxygen bonds. It is a lewis acid and coordinates with the oxygen atom of the alcohols, hence the carbon-oxygen bond weakens and breaks to form the carbocation.

Good yields of R—I may be obtained by heating alcohols with sodium or potassium iodide in 95% orthophosphoric acid. The order of reactivity of alcohols with a given haloacid is 3°>2°>1°. R-OH + NaI/KI + H3PO4 ——> R-I + NaH2PO4 + H2O

Sulphuric acid is a strong oxidising agent. There h2so4 will oxidise the produced HI to I2 and hence the reaction will not proceed.

. Phosphorus tribromide and triiodide are usually generated in situ (produced in the reaction mixture) by the reaction of red phosphorus with bromine and iodine respectively. R-OH —red P/ Br2/I2——> R-Br/ R-I

3R-OH + PX3 ——> 3R-X + H3PO3 ( X= cl/br) R-OH + PCl5 ——-> R- Cl + POCl3 + HCl

The preparation of alkyl chloride is carried out either by passing dry hydrogen chloride gas through a solution of alcohol or by heating a mixture of alcohol and concentrated aqueous halogen acid. The above methods are not applicable for the preparation of aryl halides because the carbon-oxygen bond in phenols has a partial double bond character and is difficult to break being stronger than a single bond.

Free radical chlorination or bromination of alkanes gives a complex mixture of isomeric mono- and polyhaloalkanes, which is difficult to separate as pure compounds, due to their similar boiling points. Consequently, the yield of any single compound is low. CH3CH2CH2CH3 + Cl2 —–UV light or heat —-> CH3CH2CH2CH2Cl + CH3CH2CHClCH3

(i) Addition of hydrogen halides: An alkene is converted to corresponding alkyl halide by reaction with hydrogen chloride, hydrogen bromide or hydrogen iodide. Propene yields two products, however only one predominates as per Markovnikov’s rule. (ii) Addition of halogens: In the laboratory, addition of bromine in CCl4 to an alkene resulting in discharge of reddish brown colour of bromine constitutes an important method for the detection of double bond in a molecule. The addition results in the synthesis of vic-dibromides, which are colourless

Alkyl iodides are often prepared by the reaction of alkyl chlorides/ bromides with NaI in dry acetone. This reaction is known as Finkelstein reaction. R-Cl/Br + NaI —- dry acetone—> R-I + NaCL/Br NaCl/ Br being ionic gets precipitated out of organic solvent dry acetone.It facilitates the forward reaction according to Le Chatelier’s Principle.

The synthesis of alkyl fluorides is best accomplished by heating an alkyl chloride/bromide in the presence of a metallic fluoride such asAgF, Hg2F2, CoF2 or SbF3 . The reaction is termed as Swarts reaction. H3C-Cl/Br + AgF —-> H3C-F + Agcl/br

When alkenes are heated at 773K with cl2/br2, the h atom of the allylic carbon ( ie carbon atom next to the double bond), is substituted by a halogen atom to form allylic halides. CH3- CH=CH2 + Cl2 —–773K—->CH2Cl- CH=CH2

At high temperature, addition reactions does not take place. Substitution takes place. Addition reactions are irreversible.