Organic Chemistry II Week 2 Notes
Organic Chemistry II Week 2 Notes CHE 256
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This 10 page Class Notes was uploaded by Bethany Shay Edgeworth on Thursday August 27, 2015. The Class Notes belongs to CHE 256 at University of Southern Mississippi taught by Janice Buchanan in Summer 2015. Since its upload, it has received 50 views. For similar materials see Organic Chemistry II in Chemistry at University of Southern Mississippi.
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Date Created: 08/27/15
Organic Chemistry II Dr Buchanan Week 2 August 24 2015August 28 2015 o Radical halogenation substitution reaction in which alkanes react with molecular halogens to produce alkyl halides haloalkane Example R H X2 9 R X H X heat or light 3A MULTIPLE HALOGEN SUBSTITUTION 0 An alkane is combined with excess Cl by heat or light to produce different mixtures byproducts 0 Cl amp Br are the only 2 effective halogens when dealing with free radical halogenation Example I H H H 3B LACK OF CHLORINE SELECTIVITY Clz 9 CH2C2 o Chlorination gives a mixture of isomeric monochlorinated products as well as more highly halogenated compounds Chlorine relatively unselective does not discriminate effectively among the different types of H atoms primary secondary amp tertiary in an alkane use if all atoms are the same amp there 1 possible product Because alkane chlorinations usually yield a complex mixture of products they are typically not useful synthetically when the goal is the preparation of a specific alkyl halide Ch Cl 31 Polychlorinated prods HCl Isobutane isobutyl CI 48 tertButyl CI 29 o Bromine more thermodynamically stable heavier use if there are 2 possible products less reactive toward alkanes than chlorine more selective in the site of attack when it does attack El h HFCI Neopentane excess Neopentyl chloride ATOM STABILITY EACTIVITY SELECTIVITY Bromine High Low High Chlorine Low High Low 4 CHLORINATION OF METHANE MECHANISM OF REACTION 0 Most radical reactions include 3 steps 1 Chain initiation initiate radicals I ll CI hv gt 2 CI homolytic cleavage 2 Chain propagation radicals make radicals other than CH3CI other chlorination products can be formed in this step H N H mmn ii CH3 CI CI gt CH3C 039 1 2 3 3 Chain termination radicals are destroyed ClnmCH3 gt Cl CH3 ocm gt H3C CH3 ClnmCH2CI gt Cl CHZCI CHCI2 CCI3 gt CIZHC CCI3 5 HALOGENATION OF HIGHER ALKANES Mechanism for radical halogenation of ethane Chain initiation Step 1 light Clz I 2 CI Chain propagation Step2 W CH3ch Cl n CH3CIH2 HC Step 3 h CH3CH239 CHE quot CH3CH2CI 39Cl Chain termination V39N CH3CH239 39Cl Igt CH3CH2 CI V h v l l a l v l CH3CH2 39CH3CH2 gt CH3CH2CH2CH3 Ah CI 39Cl gt CICI 5A SELECTIVITY OF BROMINE Bromination slower than chlorination because the 1st propagation step is more endothermic overall reaction is exothermic As a result bromination is more selective than chlorination h H Br 9900 lt 100 A hv Br Br gt 2 Br gt20 gt gt39 HBr major 3 radical more stable gt H Br gt H Br minor 1 radical less stable 6 THE GEOMETRY OF ALKYL RADICALS R CR R o Sp2 hybridized planar similar to carbocation 7 REACTIONS THAT GENERATE TETRAHEDRAL CHIRALITY CENTERS CI CIZ Cl M M gt achkal Pentane 1Chloropentane i2Choropentane achiral achiral a racemic form 4 CI 3Chloropentane achiral o Stereochemistry of Chlorination at C2 of pentane 41 41 CH3 H3C CI IIII H H CI CH2CH2CH3 H3CH2CH2C S2Choropentane R2Choropentane 5000 5000 ENANTOMERS 7A GENERATION OF A SECOND CHIRALITY CENTER IN A RADICAL HALOGENATION g El M 2535 C39 Dichloropentane 253R ChiraD Dichloropentane chiral from bottom Clz fromtop CIZ face face DIASTEREOMERS 8 ALLYLIC SUBSTITUTION amp ALLYLIC RADICALS o Allylic group atom or group that is bonded to an sp3 hybridized carbon adjacent to an alkene double bond group said to be bonded at the allylic position 0 Examples Br 0 As you add alkyl groups electron donating becomes more stable inductive more likely for hyperconjugation to occur 0 Hyperconiugation stealing electron density from bond next to it o Stereoelectronic effect orientation requirement because orbitals have to line up for stealing to occur 0 Tertiary radicals more stable than primary radicals o If carbocation is more stable radical more stable 0 Eastituiion reaction high temperature amp low concentration of X2 hihtem XZ g pgtXHX low conc Propene of X2 0 Addition reaction low temperature A X2 Iowtemp gt CCI4 Propene X 8A ALLYLIC CHLORINATION HIGH TEMPERATURE O A Clz 400C gt W0 HCI gas phase Propene 3Chloropropene allyl chloride SB ALLYLIC BROMINATION WITH NBROMOSUCCINIMIDE LOW CONCENTRATION OF Br2 light or l39 N ROOR A OW Ww NBromosuccinimide 3Bromopropene Succinimide NBS allyl bromide o romosgccinimid solid that provides a constant but very low concentration of bromine in the reaction mixture Br I HBr gt O NBS ROOR A O Brz gt 8287 8C ALLYLIC RADICALS ARE STABILIZED BY ELECTRON DELOCALIZATION gt HX Eact is low allyl radical gt HX Eact is high vinylic radical 0 Relative Stability Allylic or allyl gt tertiary gt secondary gt primary gt vinyl or vinylic Vinyl radical 1 Radical 2 Radical 30 Radical A Allyl radical DHO 465 A DHO 423 kJ mol 1 DH 413 kJ mol 1 DHO 400 kJ mol 1 DH 369 kJ mol 1 kJ mol1 H Y H H H H 2014 John Wiley 8 Sons Inc All rights reserved 9 BENZYLIC SUBSTITUTION amp BENZYLIC RADICALS o Benzylic group atom or group bonded to an sp3 hybridized carbon adjacent to a benzene ring groups said to be bonded at the benzylic position 0 Examples CF Cc o v 10 RADICAL ADDITION TO ALKENES THE ANTlMARKOVNIKOV ADDITION OF HYDROGEN BROMIDE o AntiMarkovnikov addition of HBr to alkenes peroxide effect addition of HBr to alkenes usually follows Markovnikov s rule HBr Br H HBr not o In the presence of peroxides ROOR antiMarkovnikov addition is observed HBr H Br not Br H RO OR heat 0 Radical Mechanism heat ROOR gt 2 RO39 homolytlc cleavage chain initiation F V Wrx RO H Br gt ROH Br39 Br lt gt 39lt Br tertiary radical more stable nm v Br not Br gt primary radical least stable WNWV H 39 H Br gt K Br Br Br 0 Hydrogen bromide is the only hydrogen halide that gives antiMarkovnikov addition when peroxides are present 0 HF HCl amp H DO NOT give antiMarkovnikov addition even when peroxides are present 11 RADICAL POLYMERIZATION OF ALKENES CHAINGROWTH POLYMERS peroxide n CH2 CH2 gt CHZCHZ heat monomer polymer
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