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Unformatted text preview: l. Substitution and Elimination are competing pathways iii 3‘ . '7' B1 R3 R1
Rs-s-Q-X + N” Brass-Nu + >=<
R4 R2 R4 R2 R4 R2
Substitution product Elimination product '
(SN-1 or SN-2) (E1 or E-2) Product distribution is determined by nature of nucleophile and nature of R-X ll. General Rules (and exceptions) 1. If the nucleophile is a strong base, E-2 elimination is favored
Exceptions: If the R-X is primary, SN-2 is favored with strong’base.
Note-1: if the nucleophile is hindered, E2 is still favored for primary R-X
Note-2: if the primary R-X has a branch at the B—carbon, E-2 is still favored 2. If the nucleophile is a weak base, substitution (SN-1 or SN-2) is favored Exceptions: If the R-X is secondary and a di or trisubstittued alkene can be formed, thenE-1 is favored
if the R-X is tertiary and a tri- or tetrasubstituted alkene can be formed, then E1 is favored lll. Definitions: Strong base: Conjugate base of a “weak acid" with pKa 2 15
Examples: CHSO‘; CH30H20'; tBuO‘; HO‘ Weak base: Conjugate base of a “strong acid“ with pKa < 15
Examples: CH3002‘; NC“; N3"; HS‘; l‘; Br‘; Cl"; F" Leaving groups: E? e S? e i? G) e e e e e
CPS—s—o >CH3—s—o >‘CH3 s—o >r > Br > Cl > F > R002 0 O ('5 Polar Protic Solvents CH30H, H20, CH30H20H, CHacoza, H002H Polar Aprotic Solvents o
_H- — O O O 9 I
H C EHCHS CH3-i'5—CH3 CHs—s—CH3 CHa-CN g 7 (CH3)2N-lf'~N(CH3)2 3 N(CH3)2 DMF Acetone I _ DMSO Acetonitrile THF HMPA DMF Dimethylformamide DMSO Dimethylsulfoxide THF ' Tetrahydrofuran HMPA Hexamethylphosphorous triamide E'Z 3 [7: IA" 5% [UM/£67 gel—PF 2’ — 541—2: PA é—Z‘ : [9M
Sit/“(3 FF. Weak (04w EP‘ I (of, W¢ﬁj¢ LW 0 3 ’- VP2AMQW
9‘28“ FHA} sméw D _
0430(cher Br + Med . Mp alga; 0Q cdl— ox! SN»? \\ L\ A— NQOOL‘g SN- '2
n n ,r Ma 075% haw—i? CUEaLchcéll (2—; >—">r + K: 2L? >—I sA/«z f0” + K: ffiL; f: + > SN-l + C”!
Maw mil/1W >/P7v + C3043 Eel-014 ;>_0d§ + > 5M4 1L 2-2.
w»in Maw ...
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This note was uploaded on 03/08/2012 for the course CHEM 352 taught by Professor Staff during the Winter '08 term at BYU.
- Winter '08