CHE 331-800 12.1 - M/§z 6/qu 53/122 C I i if «Vi/UL L W...

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Unformatted text preview: M ///§z 6/qu 53/122?) C I, i if «Vi/UL L? W — Q “ “U Kw CNUO PrOoLU/U" 11/24/14 0 Reactions that Generate Tetrahedral Stereogenic Carbons ID A reaction of an achiral starting material which produces a A product with a stereogenlc carbon will produce a racemic mixture. C H C -: I. 1 1 .1 C H 2. 3 W monOC/hl 0% CH3CH1CH2CH3CH.‘ mm“ CH3CH2CH1CH2CH1U + CHJCHZCHECHL ICH, Z LC), Pentane l-Chloropenune (: )-2-Chioropentane p (uchiral) (uchiral) (a racemic form) \ ‘1 + CH,CH;CHCICH1CH, C/ 3-Chloropenuane ‘4 Mechanism: 1 k (achirall 4 , ‘ , ‘ .+ Vomit/a - 3 4 :7 > a , ‘6 ,< \6\ Q96;me m if?‘ 1/: made WO" 9651111715 W or Marge a Generation of a Second Stereogenic Carbon in a Radical Halogenation ID When a molecule with one or more Stereogenic carbons undergoes halogenation to create another stereogenic carbon, the two diastereomeric products are not produced in equal amounts. heat or light CH2 (rxn at C-3) diastereomers formed in unequal amounts _,_——a f ’— ,’— Chapter 10 16 ' OUi’w‘d‘S ‘W 6i SMQO Ceflbr Wm O‘i’iCiC/Lufl 005111011 tux) T100615 'Um n . b _ ’ W (gummy? Tb gu adore; 8 mad? “Tm/’biiim 3+wa no+ 2e 1 10mm , WWU 0U Q-E‘WLS 1% 51d mead/g input/L GWYM C 64/) rdr Mechanism: (‘l l‘] (S)-2-Chlorupemane 1”. (33m gm 0H,, H . Cl H 1 C] H CI \ ‘ / \ / \ - c C c/ . ()12 (:12 ('1‘ + /(t\ <— C —> /C + Cl- “ i H (a) l\ (b) H \(!| cu2 1120': II CH.2 l l (:Ii,I “.10 CH3 (25.39-23-Dichloropentane Trigonal planar (25,3kl-2.3-Dichlornpentane (chiral) radical (chiral) (chiral) [ Diastereomers ; Chapter 10 | 7 Allylic Substitution and Allylic Radicals An atom or group that is bonded to an sp3-hybridized carbon adjacent to an alkene double bond is called an allylic group. The group is said to be bonded at the allylic position. The chlorine and bromine atoms below are bonded at allylic positions. The hydrogen atoms bonded to the highlighted carbons below are allylic hydrogens. 0 /\/Cl /\'/ Br 0 m 4 John win-y 5 Sons, in. All right: «and. /\ 11/24/14 ‘905m0fl “Mt *0 a doubt; 160de is gilijtic, . NYULfl mm 1636a elm/(bu ‘\+/L/\_311( \Oond mrm 0113111» iiticiWCL‘iU/b ciorbr‘g W W5 1" CIOLQQQLQ bond A + X2 Klowctéa'mpnk- 4 Propene X At high temperature (or in the presence of a radical initiator) and low concentration of X2 a substitution reaction occurs. high temp. x /\ + X ————-> /\/ + HX / 2 low conc. / Propene of X2 rem” RM r w (“M (m amen 300 5M0 {rem Allylic Chlorination (High Temperature) 400°C gas phase A + CI2 W0 + HCl Propene 3-Chloropropene (allyl chloride) 11/24/14 10 11/24/14 :* Mechanism H Chain—initiating‘step . )_ -5— u at “Cf VCQCJ C Li W ~-——‘7C”H First chain-propagating step f7 CI. 11% >Q§~H./§ J \S'gfnd chain-propagating step Allylic Bromination with N-Bromosuccinimide (NBS) (low concentration of Br2) 3-Bromopropene Succinimide (allyl bromide) afiwmfl' W 603% (/3613qu 11“ 11 11/24/14 N-Bromosuccinimide is a solid that provides a constant but very low concentration of bromine in the reaction mixture. :, fig. NBS, ROOR '2] 82-87% Allylic Radicals Are Stabilized by Electron Delocalization / + . —> + HX Allyl radical —> + HX Vinylic radical Ea“. : W'qm % acmmm 12 11/24/14 \\ 30531” u Relative stability f ){f\ allylic or allyl > 3° > 2° > 1° > vinyl or vinylic Contributing Resonance Structures be 5+ 001; C A. Vinyl radical CZ”) \/ 1° Radical d - 2° Radical /\, 3° Radical Allyl radical O 2014109!an . Song WK. All light! m. ,M more) PQ‘t 'lfl ‘H/m 1€6§ 13001: if 13‘ v Noféé W 75 V/lflLd/Q raCUCae 13 , (595+ (mtg rcm‘w ,/ 11/24/14 Benzylic Substitution and Benzylic Radicals An atom or group bonded to an sp3-hybridized carbon adjacent to a benzene ring is called a benzylic group. The group is said to be bonded at the benzylic position we” 0‘3 0 CH3 g + N—Br Methylbenzene (Toluene) O NBS ' light ©/\Br + Benzyl bromide (-bromotoluene) 4‘ (64%) (L O éN—H O 14 I ‘Wflqcf’e C\ addafl L,Qlll (Mini Space 15% 99% 0m 0' cow‘ 0 (fl a g;_ heat or light CI i l~ 0 170 (3)61") 21% C» (lidr’l’l Cr") Mew/Mid” fl? Oflé a heat or light Dichloromethyl chloride benzene Benzylic radical (more stable) X. (- HX) 1° Radical (less stable) Trichloromethyl benzene 1-Halo-1-phenylethane (major product) slow x2 ©/\/ —> X 1-Halo-2-phenylethane (minor product) 11/24/14 15 ...
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