CHE 331-800 10/31/2014

CHE 331-800 10/31/2014 - lowing nucleophilic substitution...

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Unformatted text preview: lowing nucleophilic substitution reactions. or they wou occur by SNI or 5N2. Explain. \ wm‘ v“ N ‘ «u A ‘ A 8 “X18: + (CH3)2CHO UL M f EX. er‘kfiflimxg am B, 7&9; I /l l + CH3OH _,> ‘ Wiwa‘mI/LYLQJ Nn‘ vLfiC «\ M mu, C. La 55 q) KwCLN Elimination Reactions of Alkyl Halides o Dehydrohalogenation I} Used for the synthesis of alkenes. Strong bases such as alkoxides favor elimination. H | l \ / —c”—CL+ :B— we c=c +H:B+:X" l i / \ A base CH—C—Br C I (91%) i sown-11241 Chapter 6 + NaBr + C711 OH \CH, “ 5 10/16/13 , (madf Cmcm 17 —_fi 10/16/13 IDThe alkoxide bases are made from the corresponding alcohols. 2 CHgCHZQH + 2 Na ——> 2 CHJCHQ: Na+ + H2 Ethanol Sodium ethoxide (excess) dissolved in excess ethanol R—é—ll + Na+=H-—> R—oz-Nm + ll—H Max:723 KJ om = '55 Chapter 6 35 \3 } m cm (A; bar O,i’\Q 10 QSDDD SNQ Reaction IbBoth alkyl halide and base concentrations affect rate; therefore the reaction is 2nd order. Rate ac [CH3CHBrCH3HC2H50’] Rate = k[CH3CHBICH3J[C2H50“ J Overall reaction: Br I CH3CH20Na + CH3CHCH3 —> CszCHCH3 + CH3CH20H + NaBr Mechanism: 3? E; t .y ~ ca; 6 flag)? Ebb—“ix A 5104+ Br GD H __ \ . NCL D \ / 1 3 , , m r Chapter 6 H 36 »& fill l'mmm om- i‘w Oi ma in M. 18 10/16/13 O The E1 Reaction I} The E1 reaction competes with the 5:1 reaction and likewise goes through a carbocation intermediate. CH3 CH3 ~ I I 5”] CH3CI—OI'I + CHfi—OCHZCH3 CH1 CH3 CH I 3 tert-Butyl alcohol ‘ tert-Buiyl ethyl ether ‘ 80% (lel‘OH \ I. In" I" I'm-ml_ I I. _-_m__.__l__l (“Hi‘cficl 20% H:() | ,SOC- (83%) CH, -- terr-Butyl chloride /CH3 CH1=C ' \ CH, 2-Methylpropcnc (17%) Chapter 6 37 Substitution versus Elimination o 8N2 versus E2 (-Ilminaliun l.) + l , :, substitution V __C|~_ \ 5‘2 x u l I} Primary substrate 1 7+ ~ t It the base Is unhindered, reaction occurs mainly by 8N2. n"? 96/ {i I S Lu'i ha in argon ‘ t . Md CllJClIIONa‘ + CHKCHflir Cll..(,‘li,()(‘.llZ(.H, + (,HV,=(.H3 ( am 5‘2 r22 (90%) (10%) It Secondary substrate * Approach to carbon Is sterlcally hindered and E2 elimination is favored. t , C.lLOH CHJLHZO‘Nu‘ + CH3C‘HCH3~—z;i-C——> CHJCI‘VHCH‘ + CH3=CHCHl i L ' . ' Br ‘ “‘3” ocnzcu, 8N2 E2 (21%) (79%) Chapter 6 38 . m; swim Jan‘qu mm W (9 C(afis‘c m‘lfldei/Z’C‘ i; m a C —§> in H *1 W‘UZ \ 'C’8 MiMWa13H’ .I’ H H 19 IDTertiary substrate * Approach to carbon is extremely hindered and elimination predominates especially at high temperatures. ‘1 CH3 CH3 CC . I CH3CH20H 1 I l H3C—C— Bl‘ + CH3CH20N3 -————> CH2=CCH3 | 55°C 9 CH3 . (-NaBr) mainly E2 #Temperature ~k —k 'k lncreaslng temperature favors ellmlnatlon over substitution. ~§< IDSize of the Base/Nucleophile * Potasslum ten-butoxlde Is an extremely bulky base and Is routinely used to favor E2 reaction. CHj I tcn‘tcon Br ———-—+ CM. (1.. u ~ 0 + (. H 1w; intuit H: m (:u‘ (1:11, C‘H,(CHQ,.CH=£CH, + CH.(C‘H,)HC‘H.C‘H,hU-—éf—(jllj (Ill) m (85%) 5‘2 awn 39 10/16/13 Basicity and Polarizability of the N ucleophile E2 reaction is enhanced by the use of a strong, slightly polarizable nucleophile such as RNH‘ or R0. $113 CH3CHZO‘ CH3 CH3— CH- Br —> CH3CH20H CH2 H 5N2 reaction is enhanced by the use of a weakly basic ion such as CH3COO' or a weakly basic, highly polarizable ion such as Br, I', or RS“. 0 II 913 CH3CO' $113 CH3—CH-Br —-—> CH3-CH—OIC|:CH3 0 Chapter 6 4O 20 ...
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