Unit and 10 ·Haloarene.s

HALOALKANES (ALKYL HALIDES) Preparation PCI • Compounds in which one or more hydrogen of alkane is -�---+RC!5 + POCl + HC! (are) replaced by are known as haloalkanes. HCl,ZnC12 3 RC!+ H 0 • Halogen derivatives can be categorized into two types • R-OH - SOC12 2 RC!+ S0 + HCl depending upon the nature of hydrocarbons as Pt+ I 2 c_-.,c_-,..R!2 + H P0 > Halogen derivatives of saturated hydrocarbon 3 3 • H + 2 e.g. CnH2n+l X R - X __lru_,..RX+ HX Acetone RI+ NaCl >- Halogen derivatives of unsaturated hydrocarbons • R- Cl+ Na! • Hunsdiecker reaction : C11H 11_1X and C11H 11_3 X 2 2 CCI X RCOOAg +X +X / +X + Br + CO + AgBr CH4e.g. � CH3-X CT-12 � 2 � \ 4 X Nature of C - X2 Bond RBr methane methane • In haloalkanes, the halogen atom is bonded to an alkyl Methane Monobalo Dihalo /X /X group. The carbon atom is sp3 hybridised. H-C-X �X-C-X ;, They are further classified as primary (1 °), secondary (2°) and tertiary (3°) halides depending upon whether X X Trihalo -H the halogen atom is attached to a primary, secondary \ Tetrahalo methane methane\ or tertiary carbon atom. >- The C -Xbond is highly polarised covalent bond due Nomenclature to large difference in the electronegativities of carbon • According to common or trivialsystem, monohalodeiivatives and halogen atoms. Halogen tends to pull the electrons are named as alkyl halides. away from carbon due to high electronegativity. &+ For example; -C • • • ,y'- CH -Cl (methyl chloride) This positive charge on carbon.makesit susceptible for 3 nucleophilic attack. CH - CH - I (ethyl iodide) ;..- In haloalkanes bond strength of C -Xbond decreases 3 2 • According to IUPAC system, the monohaloderivatives of with an increase in bond length as one moves from alkanes are named as haloalkanes. fluorine to . CH - CH -CH (2-bromopropane) Increasing size of p-orbital 3 3 '­ '­ '­ Br -C-F -C-Cl 'C-Br -C-I / / / / CH - CH -CH -Br ( 1- bromo-2-methylpropane) 3 2 Increasing bond length > CH 3 • 1f different halogen atoms are present in a chain then we Increasing bond strength write them alphabetically and also counting will be done from that side by which halogen () gets the least Physical Properties • Alkyl halides being polar in nature are water insoluble as possible position followedby chlorine, :fluorineand iodine they cannot break H-bonding already existing in water. alphabetically. • Greater the molecular mass, stronger the van der Waals1 1 2 3 4 forces of attraction and hence higher are the melting and CH CH -CH, - CH (2-bromo-4-chlorobutane) boiling point. For the same alkyl group boiling point I - I 3 - Br Cl 2 followsthe order as R-1 > R--Br> R-CI> R-F Haloa/kanes and Ha/oarenes 251

• As on increasing branching, surface area is decreased Path of SN2 reaction : hence van der Waals' forcesof attractionis decreased and H hence boiling point is decreased. H-C-X 1 • Decreasing order of densityamong thealkyl halides will be G � Nu )�, J Nu � � H Transition state • Stability order of -Xis as follows : Methyl halide RI> RBr> RC)> RF. > R-Br> R H • It is alkyl iodide wWch decomposes in the presence of I R-F R-Cl> R-1. ---+Nu-C-He sunlight. Thus alkyl iodide on standing becomes violet -X or brown. HI 2RI Sunlight - + I Chemical reactions : Because of the polar nature of z e The ½ thus liberated dissolves in alkyl iodide to impart it -Xbond, alkyl halides are highly reactive compounds. a dark colour. R R They undergo nucleophilic substitution reactions and Relimination reactions. Chemical Properties ;.- Nucleophllic substitution reactions : Alkyl halides • Mechanism of substitution reactions : Due to the easily undergo nucleophilic substitution reactions . presence of C -Xbond, alkyl halides are highly reactive Weakly basic halide ion is a good leaving group and compounds. They are known to undergo nucleophilic gets replaced by other nucleophile easily. substitution reactions. Nucleophilic substitutionreactions R-X +Nu----,,.R- Nu+x- are of t\vo types KOH (aq) R-OH+KX SN ! (Nucleophilic substitutionunimolecular) Moist Ag,O or AgOH SN2 (Nucleophilic substitution bimolecular) R-OH+AgX > SNt reaction : As name indicates, it is a first dry Ag,O order reaction rate of reaction depends upon R-0-R+ 2AgX KSH the concentration of substrate only, not upon the RSH+KX i.e. T hio alcohol concentration of nucleophile. This reaction occurs in Na,S two steps. In the firststep there is heterolytic cleavage R-S-R +NaX Thio ether of C -Xbond to generate a carbonium ion which is R'ONa 2 ( alkoxide) R-0-R'+NaX a planar species carbonium ion carbon in (Williamson's Hynthesis) hybridized state. Since it is a slow step, hence it is rate R'C:::c:C-Na+ i.e. is sp R-C=C-R'+NaX determining step too. (Sodium alkynidc) CH NH, (ale.) RX RNH _!!4R NH Ionization step lH, 2 -HX 2 CH -C-CI3 CH -C8 " l O amine 2° amine ::,ix\ (Slow step) 3 t 3 I \CH3 RX CH R4N°X- +--- R3N Carbonium ion Q Tertiarybutyl chloride Quaternary 3 amine 3 ammonium salt

R'COOA• RCOOR'+AgX R-XC.-0-- KN02 CH3 R-0-N=O+KX In second step nucleophile attacks on carbocation to give Alkyl nitrite the substituted product immediately as carbocation is 0 AgN02 ,:, highly reactive species. This step is fast step hence it does R-N +AgX \.0 not affectthe rate of reaction. Nitro a\kane

During SNl reaction optically active reactant will give Dil. acid or alkali optically inactiveproduct racernicmixture is obtained. (Complete RC�OH hydrolysis) Carboxyhc acid All the tertiary alkyl halides undergo S ! reaction. i.e., N Cone. acid or > S 2 reaction : As name indicates, it is a second N KCN (ale.) alkali+ Hz07_ R-CN . RC order reaction. rate of reaction depends upon the KX Tu=-rn�,�1"-'--> ONR:, - A11..7!cyan.id --+-=1 Acid amide concentration of alkyl halide as well as nucleophile. hydrolysis) i.e., During the �2 reaction no carbocation is formed HiPd or LiA1H4 but transition state is achieved as it is a simultaneous RCH2� or NalC2 Hs OH 1° allllne process. Since the nucleophile attacks from the back (Mend.insreduction) side, hence we get product having configuration opposite to the reactant inversion of configuration AgCN � Na/C,H50H RNHCH R-N- C . 2 takes place in S 2 reaction which is also said to be 7 Reductwn N -AgX Alkyl isocyanide 2G amine Walden inversion. i.e., 252

� Elimination reactions-Deh ydroh alogenation : Alkyl - Correy-House reaction : Alkyl halides on halides undergoj3-elimination reaction in the presence reaction with copper dialkyl lithium (R2 CuLi) of potassium hydroxide in ethanol to yield alkene, by form alkanes. E1 or E2 mechanism according to the structure of

alkyl halides. KOH (sic.) 2 CHI - CH -CH CH,-CH=CH + HBr + R CuLi---> - + RCu + LiX 2 . , , � Optical Rotation R'X R' R Br • Substances which rotate the path of plane polarised light In case of unsymmetrical alkyl halides, thecourse of to either left or right are known as optically active and elimination is determined by Saytzeff's rule according the rotationis knownas optical rotation. to which hydrogen is eliminated preferentially from • If the substance rotates the plane polarised light in the carbon atom which has less number of hydrogen (i.e., atoms and thus highly substituted alkene is the major clockwise direction to the right) it is called product. For example, dextrorotatory or d form and if the substance rotate the plane polarized light in anticlockwise direction (i.e., to Br l � I � KOH(alc.) the left), it is called laevorotatory or form. CH -CH -CH-CH • The optical rotation of an optically active compound is a 3 2 3 expressed in terms of specificrotation. CH -CH -CH=CH + CH -CH=CH-CH 1-Butene (20%) 2-Butene (80%) . . Observed rotation in degrees 3 2 2 3 3 specific rotat10n � 1 ength (d m )x cone. ( g/ mL ) Order of reactivity of alkyl halides towards ° ° 0 • The compounds which differ only in the behaviour dehydrohalogenation is(both El and E2) 3 > 2 > I • towards polarised light are called optical isomers and the � Reduction : Alkyl halides can be reduced byZn-Cu couple and alcohol or zinc and hydrochloric acid or phenomenon is called optical isomerism. lithium aluminium hydride to give alkanes. • Alkyl halides show optical isomerism. Basic requirement for the presence of optical isomerism is :i-- Compounds must have a chiral or asymmetric carbon atom i.e., a carbon atom should be linked to four Alkyl iodides can be reduced with HI and red different groups. > Mirror images must be non-superimposable (non phosphorus. RedP HI superirnposable mirror images are said to be CH,CH, - I+ lSO"C CH,- CH, + I, enantiomers). >- Reaction with metals : • Lowest molecular mass, optically active must With Magnesium : Alkyl halides on reaction with contain four carbon atom. magnesium in ether solution form alkyl magnesium CH3 halides commonly known as Grignard I Reagent (Named after Victor Grignard). .,.*C (RMgX) ff' 1'x C2Hs CH3 I + Mg ------J> CH3MgI Ether In the given structurethere is presence of one chiral carbon Reaction ,,,th soMethydiuml magnesium (Wurtzioilide R ea ction) : Alkyl atom which is linked to four different groups. These are halides on reaction with sodium in presence of and dry ether form alkanes. The reaction is commonly knownas . + 2Na + Drye

- (enantiomers have same physical and chemical properties). CH3 H - CH3 + 2Li--+ CH3 - H- CH3 + LiBr ? 7 Also they rotate the plane of polarized light upto the same Br Li extent, but in opposite directions. - Wurtz-Fittigreaction : Alkyl halides on reaction HALOARENES-H, -:_r�;l (ARYL HALIDES)-r�� with aryl halides and sodium in presence of dry ether give substituted benzene. Nature of C - X Bond �1:'. : <_:(-@ CH3 -\

bond length Presenc e of stron g electron withdr awing gro up at ortho (177 pm) or para -positio n enhanc es the reac tiv it y of arylhalides Cl towards nucleo philic substit ut ion . Few strong electron /� r! ) , , 3 , 2 CH -CH 2 -Cl wit hdr awing gro ups ar e-N (CH3 -NO2 -CN, -SO3 H, bo nd len gt h l)sp hybrid __} -COO H , -CHO, etc. (169 pm) carbon 3 sp hybrid·carbon ° The C - X bond in halo ar en es is less po lar than in NaOH, 300 C halo alk an es due to higher electron egativity of sp 2 High pressure hybridized C-ato m. @ Resonance effe ct : In halo ar en es, the electron pair on NaOH, !60°C the halo gen atom ar e in conj ugation with n-electrons of but theri ng. $NO, • Nudeo philic aro matic substitution-Ben zyne �- me chanismor Elimin ation- addi tion re action th rough benzyne me chanism : Ar yl halides can un dergo Due to resonanc e the C - X bo nd ac quires partial nucleo philic ev en in absenc e of do uble bon d character, therefor e cleavage of C - X electron wit hdr awing gr oup bytr eatment with ve1y strong bond becomes diffic ult an d halo aren es sho w lesser reac tiv it y to wards substitution reaction s as co mpared base suc h as amide ion °NH in liquid ammon ia. to halo alk anes. 2 @ Directive infl uence of halo gen : The halo gen at om N,NH,, liq. NH, viz. 6 att ac hed to the carbon at om can sho w t\vo effect s. -I effect ( due to higher clectron egativ it y) Aniline + M eff ect (dueto pr esenc e oflone pairs). > Me chanism: The abov e reaction co mpletes by two But +M effect is do minating over -I eff ect. And due to st ep mec hanism an d addition invo lv in g for mation of +M effect the electron den sity increases at a, p- position highly un st able in termediat e, benzyn e, (-vecha rge on a, p- position in resonating str uc ture) thus - electro philic attack will be mor e pr ior to a, p-po si tion. El imination : In this step, stron g base amide Thus, the halo gen s ar e kno wn as o, p-dire cting. ion -NH:z abstr acts a pr oton fro m ortho-posi ti on . Cl Preparation Cl/¢, � a Clg + NH3 j + NH1 -----+ Cl b h II Anion • @ Cl/FeC1 + 3 @ HCl An ion thus for med eliminates the halide ion resultin g in for mation of ben zyne, N = NCI Cl 1"1 +Cl ) + N " b , b 0 (Benzyne) Substituti,:m Reactions of Haloarenes - • Nucle o ph ilk subs titution re actions : Aryl halides Addition : The in co min g nucleo phile can at tack un dergo nucleo philic substit ut ion react ion at hi gh an y of the triplybon ded carbon ato m, temperature an d pr essureon ly. (})' ONa O 1 +"NH, H b NaOH, 00°C 3 " b Pressure @�@ Direct substitution Cine substitution Cl CN CuCN, 200°C @ Pressure @ NH2 NH , CI¼O 3 @ 200°C, Pressure + 2CuCl + H,O 254 • El ectrophilic sub stitution reactions : Aryl halides )- Reduction : Aryl halides can be reduced to benzene . undergo usual electrophilic substitution reactions, but these Ni- :N,OH are less reactive than benzene because of deactivating effect @- cl +2(H] N @+HC] of . Halogens are a, p-directing, thus a mixture of and p-products are obtained, in which p-isomer Reaction wi th chl oral )- : ChlorobenzeneH SO on reactionDDT. predominateso due to steric effect. 4 Some of common examples of electrophilic substitution with chloral in presence of cone. 2 yield reactions are as follows C! Cl-@-H C Cl 3 J +O=CH- CCl - , orFeCl3A' --1 I, Cl rc_i _+ _Anh_ y'-· Conc H,S0J Halogenation ]-@- ---> @ + $ C H 1 Cl Cl C + Cl 3 1 CHy l �CH- CCl '--- CH + \--c�FriedelA�oo� �d -Crafts�.NrC�½ � ' alkylarion @J c1-@V (4-chlorophenyl)-I .l.l-trichloroethane Cf!, (D.D.T.) Cl 2,2-Bis COC!o Cl � CH, C O, POLYHALOGEN COMPOUNDS C'-- '-- ) COC!f, f-'( H� ' O-'"-, --+ L8J� + Anhyd. A!Cl3 @J + • Dihalogens - obtained by replacing two hydrogen atoms Friedel-Crafts acylation by halogens. 3 COCH > Vicinal dihal ides or alkylene halides : When two Cone. HN03 + o halogen atoms are attached to different carbon atoms. Cone. H2S04, t. bN , 2 rk e.g. CH, - CH Br. Nitration > Geminal dihalides or alkylid ene halides : When +� two halogen atoms are attached to same carbon atom. LSJ NO, . e.g. CH3-CH2Cl2 s Lso3H Trihalogen Derivatives '-Coro ro H,-'�o' ,_+_o_ , . __ ho Cl Sulphomuon- - Cb lorof orm(CHCl ) + y 3 LSJ • Preparation ,H ----. so > 2CCl3CHO + Ca(OH)2 2CHC13 + (HC00)2Ca Chloral ;;,, Fittig reaction : Aryl halides react with sodilllll metal hv hv -�- 4 Cl2, 3 Cl 2 2 in dry ether to give biphenyls. The reaction is similar > CH CH C! '' CH Cl hv to Wurtz reaction of alkyl halides. C12, 3 • CHCl @-c1 + 2Na + c1--@ �;;'" Proper ties 1 COCl2 - hv Phosgene @ @+2NaCl CJ,, f-�-- CCl Biphenyl .. 4 3 HN0 3 2 )- Wurtz-Fittig reaction : When a mixture of aryl CHC1,--J --"--c.. CC 1 N0 Chloropicrin halide and alkyl halide reacts with sodium metal in 2 5 C H NR 2 5 dry ether, substituted arenes are obtained. KOH - C H NC Dry e!hoc Ag @-ct +2Na +Cl - Cf!, 3 @-cH +2NaCI L-'-"'--c.. CH = CH Iodofor m (CID3 > Ullmann reaction : Ary! halides especially iodides ) on reaction with copper metal yield biphenyls. • Prepar ation ! 3 -- s� ed tub, CH3CH,OH + 12 + NaOH --,. CHI + HCOONa ©-I + 2Cu + I @ • Proper ties: Compounds containing CH3CO - group give yp yellow ppt ofiodoform with sodium h ohalite. @-@ + 2Cul CH3COCH3 + NaOH + 1,--,.CHI 3 + CH3COONa Biphenyl Ha/oalkanes and Haloarenes 255

Uses and Environmental Effects of Some Important l'olyhalogen Compounds

1. Dichloromethane As a solvent in a paint remover. Have harmful effects on the human central 2 2) (CH Cl An effective solvent for extraction rn nervous system. pharmaceutical in the manufacturing of drugs Cause dizziness, nausea, tingling and numbness and food industries. in the fingers and the toes. As a propellant in aerosols. Direct contact with the eyes can even bum the As refrigerant and dewaxing agent. cornea. 2. Trichloromethane or As an important solvent particularly for fats, Breathing of chloroform for a short time causes Chloroform (CHC13) alkaloids, iodine, waxes, rubber, etc. dizziness, fatigueand headache. As an anaesthetic. May damage liver and kidneys because As a laboratory reagent. chloroform is metabolised to poisonous Used in medicines. phosgene. The use of chloroform as an In the production of freon of refrigerant. anaesthetic has been replaced by less toxicand safer anaesthetic such as ether. 3. Tetrachloromethane In large quantities in the manufacture of Exposure to carbon tetrachloride causes liver (CC14) refrigerants and propellants for aerosol cans. cancer in humans. As a feedstock in the sysnthesis of The most common effects are dizziness, light chlorofluorocarbons. headedness, nausea and vomiting, which can As a solvent for the manufacture of cause permanent damage to nerve cells. pharmaceuticals and oils, fats, waxes, etc. When carbon tetrachloride is released into air, it rises to the atmosphere and depletes the ozone layer. Depletion ofthe ozone layer increases the human exposure to ultraviolet radiations which may lead to increase skin cancer, eye diseases and disorders and possible disruption ?f the immune system. 4. Tri-iodomethane(CHI 3) As an antiseptic and this nature is due to free Causes nausea, vomiting. iodine that it liberates and not due to iodoform Irritating to mucous membranes and respiratory itself. system. In the manufacture of pharmaceuticals. May cause eye irritation. 5. Freons (Chloroflouro As refrigerants in refrigerators and air In stratosphere, freons undergo photochemical carbons) conditioners and hence is the name freons. decomposition and initiate radical chain As propellants for aerosols and foams to spray reactions and deplete the protective ozone layer out deodorants, cleansers, shaving creams, hair surrounding our earth. Therefore, the use of sprays and insecticides. freons as propellants and refrigerantshas been drastically discouraged. It has also been banned in many countries. 6. DDT (Dichloro It is widely used as an insecticide for killing D.D.T. was formedto have high toxicity towards diphenyl trichloroethane) mosquitoes and other insects. fish. c1 c-Hc�Cl D.D.T. is not biodegradable. Its residues , v;::;\_&c 1 accumulate in environment and its long term effects could be highly dangerous. D.D.T. is not metabolised very rapidly by animals rather it gets deposited and stored in fatty tissues. N"' CONCEPT MAP "' iif iekiri¥/W•S,m¥i,i18•Mi•*•Si Halogen Derivatives Obtained by replacing one or more H-atoms of hydrocarbons. I ,!, i ,I, · [ Halo.alklinesJ ! !l;doare;n�_s·. I P01YhalOge·n··COmprittlld�;:;1 I ,J, ,I, 4/ Pr operties i I Properties · 1 I I .. .. o·pticai'Iso:ineri.Slli • Dihalogens Trihalogens Substances which can rotate the plane of c11�1nical,, polarizedlight --l-optically active. I Physical I I I Cornbounds with Compounds with Dextrorotatory (+) --l-rotates towards right. • B.P. iodoarenes > bromoarenes > • Low reactivity due to I two halogen atoms three halogen Laevorotatory(-)-----,,rotates towmds left. chloroarenes > fluoroarenes. ksser polar character. atoms e.g. B.P. isomcrio dihalobenzenes --l- • C-X bond acquires some CHC13, CHI3 J. nearly same. double bond character and J. hence stronger. Phy sical I I M.P. ofisomerio dihalobenzencs -t ... I CJ,einlca! p>o>m • Presence ofe-withdrawing .. groups such as -NO,, -CN Vicinal Geminal • Water insoluble e Highly reactive undergoes. I dihalides dihalides • M.P. and B.P. : R-1 > R-Br > etc. at o--· and Jrpositions R-Cl>R-F w.r.t halogen -;, activates Both halogen Both halogen I the halogen towards atoms on atoms on same • B.P. alkyl part size ,!. ex. nucleophilic displacement. different atoms carbon atom • B.P. I/branching Ntrcle opliilkS\Jbsti,. tuti/;n rx: l -�]Irili�·ano1:1: j • Halogen atom is slightly • Bond strength : CH3-Cl > ·. Re action . .· ·. . •·•. I ; Re ac ti on · deactivating and a, p- CH-F> CH-Br> CH -I : directing (p-product + 3 3 1 SNl : I order reaction : EI : In 2 steps. Chlorofluorocarbons • Stability of -Xbond : R-F r+ f-+ usually predominates). I • Reactivity of alkyl f Perfluoro + > R-C l> R-Br> R-I • Rate ex [ substrate J halides 3° >2° >1° Freons BHC • Proceeds via carbonium carbons Polychlorotluoro ) Gammexane ion formation. (PFC , ) c H cl ) alkanes - CCI2F2 c F , , • In 2 steps, I--+formation of E2 : In l steps. C CI F ,, ,,,+2· 4 2 2 4 CF4, C2F6 carbonium ion (slow --+ • via transition rate determining), II --+ I state. . I Attack ofnucleophile. 0 d ether • Favourable --+ I R-X+Mg ry R-Mg-XGrignard's reagent • Racemicmixtureobtained. alkyl halide. • Most favourable substrate --;3 ° alkyl halide. R-X+NaCN 6 R--CN C C,H,OH-H,0 AgCN • R-N ____..,. C 4 S 2 : II order reaction : Alcohol,� N R-I+AgNO, RONO + RNO, -AgI • Rate = [alkylhalide] [nucleophile] Alkyl nitrite Nitro alk.:ine • Converted reaction (in 1 step). • Inversion of configuration occurs (walden inversion), R-X+ R'ONa ROR'+NaX • Most favourable substrate --+1 ° alkyl halide. • Favoured in non-polar solvent.