Unit 10 - NCERT

1 The replacement of hydrogen atom(s) in an aliphaticor aromatic hydrocarbon by halogen atom(s) resultsin the formation of alkyl halide (haloalkane) and arylhalide (haloarene), respectively. Haloalkanes containhalogen atom(s) attached to the sp3 hybridised carbonatom of an alkyl group whereas haloarenes containhalogen atom(s) attached to sp2 hybridised carbonatom(s) of an aryl group. Many halogen containingorganic compounds occur in nature and some ofthese are clinically useful. These classes of compoundsfind wide applications in industry as well as in day-to-day life. They are used as solvents for relativelynon-polar compounds and as starting materials forthe synthesis of wide range of organic containing antibiotic, chloramphenicol,produced by microorganisms is very effective for thetreatment of typhoid fever.

2 Our body produces iodinecontaining hormone, thyroxine, the deficiency of whichcauses a disease called goiter. Synthetic halogencompounds, viz. chloroquine is used for the treatmentof malaria; halothane is used as an anaestheticduring surgery. Certain fully fluorinated compoundsare being considered as potential blood substitutesin this Unit, you will study the important methodsof preparation, physical and chemical properties anduses of organohalogen studying this Unit, you will beable to name haloalkanes and haloarenesaccording to the IUPAC system ofnomenclature from their givenstructures; describe the reactions involved inthe preparation of haloalkanes andhaloarenes and understandvarious reactions that theyundergo; correlate the structures ofhaloalkanes and haloarenes withvarious types of reactions.

3 Use stereochemistry as a tool forunderstanding the reactionmechanism; appreciate the applications oforgano-metallic compounds; highlight the environmental effectsof polyhalogen andHaloalkanes andHaloalkanes andHaloalkanes andHaloalkanes andHaloarHaloarHaloarHaloarHaloarenesene senesenesenesHaloalkanes andHaloalkanes andHaloalkanes andHaloalkanes andHaloalkanes andHaloarHaloarHaloarHaloarHaloarenesene senesenesenesHalogenated compounds persist in the environment due to theirresistance to breakdown by soil and haloarenes may be classified as follows:These may be classified as mono, di, or polyhalogen (tri-,tetra-, etc.)compounds depending on whether they contain one, two or more halogenatoms in their structures.

4 For example,Monohalocompounds may further be classified according to thehybridisation of the carbon atom to which the halogen is bonded, asdiscussed class includes(a)Alkyl halides or haloalkanes (R X)In alkyl halides, the halogen atom is bonded to an alkyl group (R).They form a homologous series represented by CnH2n+1X. They arefurther classified as primary, secondary or tertiary according to thenature of carbon to which halogen is attached. If halogen is attached toa primary carbon atom in an alkyl halide, the alkyl halide is calledprimary alkyl halide or 1 alkyl halide. Similarly, if halogen is attachedto secondary or tertiary carbon atom, the alkyl halide is calledsecondary alkyl halide (2 ) and tertiary (3 ) alkyl halide, respectively.

5 (b)Allylic halidesThese are the compounds in which the halogen atom is bonded to ansp3-hybridised carbon atom adjacent to carbon-carbon double bond(C=C) to an allylic carbon.(c)Benzylic halidesThese are the compounds in which the halogen atom is bonded to ansp3-hybridised carbon atom attached to an aromatic theBasis ofNumber CompoundsContainingsp3 C XBond (X= F,Cl, Br, I)Allylic carbonAllylic carbon2022-23291 Haloalkanes and HaloarenesThis class includes:(a)Vinylic halidesThese are the compounds in which the halogen atom is bonded toa sp2-hybridised carbon atom of a carbon-carbon double bond(C = C).(b)Aryl halidesThese are the compounds in which the halogen atom is directlybonded to the sp2-hybridised carbon atom of an aromatic learnt the classification of halogenated compounds, let us now learnhow these are named.

6 The common names of alkyl halides are derived bynaming the alkyl group followed by the name of halide. In the IUPAC systemof nomenclature, alkyl halides are named as halosubstituted mono halogen substituted derivatives of benzene, common and IUPAC names are the same. For dihalogen derivatives, the prefixes o-, m-, p- areused in common system but in IUPAC system, as you have learnt in ClassXI, Unit 12, the numerals 1,2; 1,3 and 1,4 are C XBondThe dihaloalkanes having the same type of halogen atoms are namedas alkylidene or alkylene dihalides. The dihalo-compounds having boththe halogen atoms are further classified as geminal halides or gem-dihalideswhen both the halogen atoms are present on the same carbon atom of Nomenclature2022-23292 Chemistrychain and vicinal halides or vic-dihalides when halogen atoms are presenton adjacent carbon atoms.

7 In common name system, gem-dihalides arenamed as alkylidene halides and vic-dihalides are named as alkylenedihalides. In IUPAC system, they are named as nameIUPAC nameCH3CH2CH(Cl)CH3sec-Butyl chloride2-Chlorobutane(CH3)3 CCH2 Brneo-Pentyl bromide1-Bromo-2,2-dimethylpropane(CH3)3 CBrtert-Butyl bromide2-Bromo-2-methylpropaneCH2 = CHClVinyl chlorideChloroetheneCH2 = CHCH2 BrAllyl bromide3-BromopropeneCH2Cl2 Methylene chlorideDichloromethaneCHCl3 ChloroformTrichloromethaneCHBr3 BromoformTribromomethaneCCl4 Carbon tetrachlorideTetrachloromethaneCH3CH2CH2 Fn-Propyl fluoride1-Fluoropropaneo-Chlorotoluene1- Chloro-2-methylbenzeneor2-ChlorotolueneB enzyl chlorideChlorophenylmethaneTable.

8 Common and IUPAC Names of some HalidesExample the structures of all the eight structural isomers that have themolecular formula C5H11Br. Name each isomer according to IUPAC systemand classify them as primary, secondary or tertiary (1o)CH3CH2CH2CH(Br)CH32-Bromopentane(2o) CH3CH2CH(Br)CH2CH33-Bromopentane (2o)(CH3)2 CHCH2CH2Br1-Bromo-3-methylbutane (1o)Some common examples of halocompounds are mentioned in Table and HaloarenesIntext QuestionIntext QuestionIntext QuestionIntext QuestionIntext Write structures of the following compounds:(i)2-Chloro-3-methylpentane(ii )1-Chloro-4-ethylcyclohexane(iii)4-tert. Butyl-3-iodoheptane(iv)1,4-Dibromobut-2- ene(v)1-Bromo-4-sec. atoms are more electronegative than carbon, therefore,carbon-halogen bond of alkyl halide is polarised; the carbon atom bearsa partial positive charge whereas the halogen atom bears a partialnegative charge.

9 As we go down the group in the periodic table, the size of halogenatom increases. Fluorine atom is the smallest and iodine atom is thelargest. Consequently the carbon-halogen bond length also increasesfrom C F to C I. Some typical bond lengths, bond enthalpies anddipole moments are given in Table halides are best prepared from alcohols, which are easily ofNature ofNature ofNature ofNature ofC-X BondC-X BondC-X BondC-X BondC-X Bond(CH3)2 CHCHBrCH32-Bromo-3-methylbutane(2o)(CH3) 2 CBrCH2CH32-Bromo-2-methylbutane (3o)CH3CH2CH(CH3)CH2Br1-Bromo-2-methylbu tane(1o)(CH3)3 CCH2Br1-Bromo-2,2-dimethylpropane (1o)Write IUPAC names of the following:(i)4-Bromopent-2-ene(ii)3-Brom o-2-methylbut-1-ene(iii)4-Bromo-3-methyl pent-2-ene(iv)1-Bromo-2-methylbut-2-ene( v)1-Bromobut-2-ene(vi)3-Bromo-2-methylpr openeExample FromHydrocarbonsBondBond length/pmC-X Bond enthalpies/ kJmol-1 Dipole moment/DebyeCH3 :Carbon-Halogen (C X) Bond Lengths, BondEnthalpies and Dipole Moments The preparation of alkyl chloride is carried out either by passingdry hydrogen chloride gas through a solution of alcohol or by heatinga mixture of alcohol and concentrated aqueous halogen acid.

10 The above methods are not applicable for the preparation of arylhalides because the carbon-oxygen bond in phenols has a partial doublebond character and is difficult to break being stronger than a singlebond (Unit 11, Class XI).(I)From alkanes by free radical halogenationFree radical chlorination or bromination of alkanes gives a complexmixture of isomeric mono- and polyhaloalkanes, which is difficult toThe hydroxyl group of an alcohol is replaced by halogen on reaction withconcentrated halogen acids, phosphorus halides or thionyl chloride is preferred because in this reaction alkyl halide is formedalong with gases SO2 and HCl. The two gaseous products are escapable,hence, the reaction gives pure alkyl halides.