Solution - Career Point

More documents

Recommendations

Info

KEY CONCEPTOrganicChemistryFundamentalsAROMATICHYDROCARBONHalogenation of Benzene :The function of the Lewis acid can be seen in step 1.–H:Br – FeBr 3Br: Br:+ H – Br: + FeBr 3Benzene does not react with bromine or chlorineunless a Lewis acid is present in the mixture. (As aThe ferric bromide reacts with bromine to produce apositive bromine ion, Br + (and FeBr – 4 ). In step 2 thisconsequence, benzene does not decolorize a solution Br + ion attacks the benzene ring to produce anof bromine in carbon tetrachloride.) When Lewisacids are present, however, benzene reacts readilyarenium ion. Then, finally in step 3 a proton isremoved from the arenium ion by FeBr – 4 . This resultswith bromine or chlorine, and the reactions give in the formation of bromobenzene and hydrogenbromobenzene and chlorobenzene, respectively, in bromide the products of the reaction. At the samegood yields :time this step regenerates the catalyst, FeBr 3 .ClFeCl 3+ Cl 2+ HClNucleophilic Aromatic Substitution through an25ºCElimination – Addition Mechanism : BenzyneChlorobenzene (90%)Although aryl halides such as chlorobenzene andBrbromobenzene do not react with most nucleophilesFeBr 3+ Br 2 + HBrunder ordinary circumstances, they do react underheathighly forcing conditions. Chlorobenzene can beBromobenzene (75%)converted to phenol by heating it with aqueousThe Lewis acids most commonly used to effectsodium hydroxide in a pressurized reactor at 350ºC .chlorination and bromination reactions are FeCl 3 ,FeBr 3 , and AlCl 3 , all in the anhydrous form.ClONa OHA mechanism for the reaction :350ºCElectrophillic Aromatic Bromination :+ NaOHH 2OH 3O +Step 1Phenol+–Br – Br : + FeBr 3 ⎯→ :Br – Br – FeBr 3Bromobenzene reacts with the very powerful base,⎯→ :Br + –+ :Br – FeBr 3NH – 2 , in liquid ammonia :BrNH 2Bromine combines with FeBr 3 to form a complex that dissociates–to form a positive bromine ion and FeBr 4+ – -33ºC+ K :NHStep 22NH 3+ KBr+ slowAniline+ Br:These reactions take place through an elimination –HBr:HBr:+ H addition mechanism that involves the formation of anBr:interesting intermediate called benzyne (or+ +dehydrobenzene). We can illustrate this mechanismwith the reaction of bromobenzene and amide ion.Arenium ionThe positive bromine ion attacks benzene toform an arenium ionStep 3+A proton is removed from the arenium ionto become bromobenzeneIn the first step, the amide ion initiates an eliminationby abstracting one of the ortho protons because theyare the most acidic. The negative charge thatdevelops on the ortho carbon is stabilized by theinductive effect of the bromine. The anion then losesa bromide ion. This elimination produces the highlyunstable, and thus highly reactive, benzyne. Benzynethen reacts with any available nucleophile (in thiscase, an amide ion) by a two-step addition reaction toproduce aniline.XtraEdge for IIT-JEE 30 APRIL 2010
The Benzyne Elimination – Addition Mechanism :BrH–:NH 2(–NH 3)EliminationAdditionBr(–Br – )–Benzyne(or dehydrobenzene)–:NH 2NH 2–:NH 3NH 2 –+ :NH 2HEvidence for an elimination-addition mechanism :When 14 C-labeled (C*) chlorobenzene is treated withamide ion in liquid ammonia, the aniline that isproduced has the label equally divided between the 1and 2 positions.* NH 2* Cl K + –NH 2*– NH2(50%)NH 3*NH 2Elimination Addition (50%)When the ortho derivative 1 is treated with sodiumamide, the only organic product obtained ism-(trifluoromethyl) aniline :CF 31ClNaNH 2NH 3CF 3NH 2m-(Trifluoromethy)anilineThis result can also be explained by an elimination –addition mechanism. The first step produces thebenzyne 2 :CF 3CF 3ClNaNH 2+ Cl –NH 31 2This benzyne then adds an amide ion in the way thatproduces the more stable carbanion 3 rather than theless stable carbanion 4 :CF 3CF 3:NH 2NH 2–4Less stable carbanionCF 3–CF 32– :NH 3–NH 2NH 2 +:NH32More stable carbanion(The negative charge is closer to theelectronegative trifluoromethyl group)Carbanion 3 then accepts a proton from ammonia toform m-(trifluoromethyl) aniline.Carbanion 3 is more stable than 4 because the carbonatom bearing the negative charge is closer to thehighly electronegative trifluoromethyl group. Thetrifluoromethyl group stabilizes the negative chargethrough its inductive effect. (Resonance effects arenot important here because the sp 2 orbital thatcontains the electron pair does not overlap with the πorbitals of the aromatic system.)The Birch Reduction :Benzene can be reduced to 1, 4-cyclohexadiene bytreating it with an alkali metal (sodium, Lithium, orpotassium) in a mixture of liquid ammonia and analcohol.NaNH 3 , EtOHBenzene1, 4-cyclohexadieneA Mechanism for the Reaction :Brich Reduction :Naetc.– –BenzeneBenzene radical anionThe first electron transfer produces a delocalizedbenzene radical anion.HEtOHHetc.HHCyclohexadienyl radicalProtonation produces a cyclohexadienyl radical(also a delocalized species)NaH–Hetc. EtOHHHHHH–HCyclohexadienyl anion1,4-CyclohexadieneTransfer of another electron leads to the formation of a delocalizedcyclohexadienyl anion, and protonation of this produces the1,4-cyclohexadiene.XtraEdge for IIT-JEE 31 APRIL 2010
Page 3 and 4: Volume - 5 Issue - 10April, 2010 (M
Page 5 and 6: Volume-5 Issue-10April, 2010 (Month
Page 7 and 8: Hospital in Parel and ApolloHospita
Page 9: KNOW IIT-JEEBy Previous Exam Questi
Page 12 and 13: CHEMISTRY6. From the following data
Page 14 and 15: (ii)OHH + CH 3OH⊕PorClPClClOCH 3O
Page 16 and 17: Physics Challenging ProblemsSet #12
Page 18 and 19: 8 QuestionsSolutionSet # 11Physics
Page 20 and 21: 8 QuestionsSolutionSet # 12Physics
Page 22 and 23: PHYSICSSStudents'ForumExpert’s So
Page 24 and 25: If the body loses this heat in time
Page 26 and 27: Perfect gas equation :From the kine
Page 28 and 29: The internal energy of n molecules
Page 30 and 31: * The binding energy per nucleon is
Page 34 and 35: KEY CONCEPTPhysicalChemistryFundame
Page 36 and 37: UNDERSTANDINGOrganic Chemistry1. An
Page 38 and 39: The starting compound (A) reacts wi
Page 40 and 41: `tà{xÅtà|vtÄ V{tÄÄxÇzxá12Se
Page 43 and 44: MATHEMATICAL CHALLENGESSOLUTION FOR
Page 45 and 46: A 2 ⎛ 3s − 2s ⎞ s≤ s ⎜
Page 47 and 48: 5. Let z 1 , z 2 and z 3 are unimod
Page 49 and 50: If I n =∫x n . e ax dx, then I n
Page 51 and 52: The common difference of an A.P. is
Page 53 and 54: XtraEdge for IIT-JEE 51APRIL 2010
Page 55 and 56: 4. Consider the following reactions
Page 57 and 58: 15. What would be the structure of
Page 59 and 60: 14. If line lies inside the region
Page 61: 6. A BTwo containers A & B contain
Page 64 and 65: MOCK TEST FOR IIT-JEEPAPER - IITime
Page 66 and 67: SECTION - IVInteger answer typeThis
Page 68 and 69: X Y Z W0123456789012345678912. If z
Page 70 and 71: (C) Pressure at A > pressureat B(D)
Page 72 and 73: 7. Experimental verification of New
Page 74 and 75: 28. Assertion : A rocket moves forw
Page 76 and 77: 60. Point out the incorrect stateme
Page 78 and 79: MOCK TEST - BIT-SATTime : 3 Hours T
Page 80 and 81: 22. The displacement of interfering
Page 82:
40. A body of mass 2 kg is being dr
Page 85 and 86:
10. The foci of a hyperbola coincid
Page 87 and 88:
LOGICAL REASONING1. Fill in the bla
Page 89 and 90:
SOLUTION FOR MOCK TESTIIT-JEE (PAPE
Page 91 and 92:
or 0.1 v α + 1 = 2 + 2 × 10 -2 ×
Page 93 and 94:
12.[A, B, C, D]13.[B]2x t − 5t +
Page 95 and 96:
R l=sin 45º sin 30ºR sin 45º = =
Page 97 and 98:
1.[C]2.[B]3.[B]SOLUTION FOR MOCK TE
Page 99 and 100:
5.[A,B,D] We have adj A = |A| A -1a
Page 101 and 102:
(1, 1)7λ 5.5× 10t min = =P4n 4×1
Page 103 and 104:
K TFor a cylinder = 2 in case ofK R
Page 105 and 106:
∴17.[C]Potential of shell A is,1
Page 107 and 108:
39.[B] Solution is decinormal, that
Page 109 and 110:
71.[B]which is a H.P.n th term of c
Page 111 and 112:
85.[C] Letters of word 'STATISTICS'
Page 113 and 114:
13. [B] Point P lies on the arms CD
Page 115 and 116:
33. [B] For quarter revolution∆ V
Page 117 and 118:
16.[A]NH 2⎯ 0−5°C+ NaNO 2 + 2H
Page 119 and 120:
11.[B]→ →∴ α & β are two mu
Page 121 and 122:
1/ 21 -2∫2 x 2 dx⇒⇒01 2 - 3 2
Page 124 and 125:
8.[A]9.[A]10.[B]To take off :irrele
Page 126:
CAREER POINTIn associationwithLaunc
show all

Solution - Career Point

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?