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Stereoselective and stereospecific reactions.
* *CH3CH=CHCH3 + Br 2 CH3CHCHCH3
Br Br 2-butene 2,3-dibromobutane
2 geometric isomers 3 stereoisomers cis- and trans- (S,S )-, ( R,R)-, and ( R,S )-
meso-
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H CH3 CH3 CH3 \ / \ /
C = C C = C
/ \ / \
CH3 H H H
trans-2-butene cis-2-butene
CH3 CH3 CH3
H Br Br H H Br
Br H H Br H Br
CH3 CH3 CH3
(S,S) (R,R) meso
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CH3
H CH3 \ / H Br
C = C + Br 2
/ \ H Br
CH3 H CH3trans-2-butene meso-2,3-dibromobutane
only product
A reaction tat !ie"ds #redominate"! one stereoisomer (or
one #air o$ enantiomers) o$ se%era" diastereomers is ca""ed a
stereose"ecti%e reaction. &n tis case te meso- #roduct is
#roduced and not te oter t'o diastereomers.
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CH3 CH3
H H \ / H Br Br H C = C + Br 2 +
/ \ Br H H Br CH
3
CH3
CH3 CH3cis-2-butene (S,S )- ( R,R)-2,3-dibromobutane
racemic modification only products
A reaction in 'ic stereocemica""! di$$erent mo"ecu"es
react di$$erent"! is ca""ed a stereos#eci$ic reaction. &n tis
case te cis- and trans- stereoisomers gi%e di$$erent
#roducts.
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e $act tat te addition o$ a"ogens to a"enes is bot
stereose"ecti%e and stereos#eci$ic gi%es us additiona"
in$ormation about te stereocemistr! o$ te addition and te
mecanism $or te reaction.
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C C CC anti-addition
C C C C syn-addition
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C C
H
CH3
H
H3C
C C
CH3
H
H
H3C
CH3
H Br
CH3
Br H
CH3
Br H
CH3
H Br
CH3
H Br
CH3
H Br
+
Br 2
Br 2
&s te addition o$ Br 2 syn or anti
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H H
\ / H
C = C H C C CH3 anti-addition o$ 2 / \ CH3 to te cis-isomer
CH3 CH3
ote0 must rotate about C-C to get to te 1iscer #roection
CH3 H H
H C C CH3 C C H H CH3
CH3 CH3 H
CH3
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H CH3
\ / CH3
C = C H C C H anti-addition o$ 2 / \ CH3 to te trans-isomer
CH3 H
ote0 must rotate about C-C to get to te 1iscer #roection
CH3
CH3 H H H C C H C C H
CH3
CH3 CH3 H
CH
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CH3
H Br
CH3
Br H
H
H
H3C
H3C
anti-
Br
H3C H
Br
HH3CBr
H3C H
H
H3C Br
Br 2
H
CH3H
H3CBr 2
anti-
Br
H CH3
Br
HH3C
H
H3C Br
H
H3C Br
CH3
H Br
CH3
H Br
rotate about C2-C3 to get toFischer projection!
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&n determining 'eter a stereose"ecti%e addition is syn- or
anti- !ou cannot sim#"! "oo at te 1iscer #roection.
4emember it is o$ten necessar! to rotate about a carbon-
carbon bond to get a mo"ecu"e into te con$ormation tatcorres#onds to te 1iscer #roection 5se !our mode" it to
%eri$!
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What does the stereochemistry tell us about the mechanismof addition of halogens to alenes
"# C C + $ $
%&'
C C
$
+ $
(halonium ion(
C C
$
2# $ + C C
$
$
anti-addition
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H!dro6!"ation o$ a"enes0
* *CH3CH=CHCH3 + 78n9: CH3CH-CHCH3
9H 9H 2-butene 2,3-butanedio"
2 geometric isomers 3 stereoisomers
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cis-2-butene + 78n9: 2,3-butanedio" m# 3:oC
trans-2-butene + 78n9: 2,3-butanedio" m# ;
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cis-2-butene + 78n9: meso-2,3-di!dro6!butane m# 3:o
CH3
H 9H
H 9H
CH3
trans-2-butene + 78n9: (S,S ) ( R,R)-2,3-di!dro6!butane m# ;<o
CH3 CH3
H 9H + H9 H H9 H H 9H
CH3 CH3
stereose"ecti%e and stereos#eci$ic
C C
H
H3
C CH3
H
C C
H
H3C H
CH3
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&s !dro6!"ation 'it 78n9: syn- or anti-
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H 9 9 CH3 9H 9H
\ / C = C H C C CH3 syn-o6idation o$
/ \ CH3 H te trans-isomer
CH3 H
ote0 must rotate about C-C to get to te 1iscer #roection
9H 9H CH3
H 9H H C C CH3 H9 C C H H9 H
CH3 H
CH3 CH3 H 9H
CH3
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H 9 9 H 9H 9H
\ /
C = C H C C H syn-o6idation o$
/ \ CH3 CH3 te cis-isomer
CH3 CH3
ote0 no rotation necessar! to get to 1iscer #roection
9H 9H CH3 H H
H C C H H9 C C 9H H 9H CH3 CH3
CH3 CH3 H 9H
CH3
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cis-2-butene + HC93H 2,3-butanedio" m# ;<oC
trans-2-butene + HC93H 2,3-butanedio" m# 3:oC
2,3-butanedio" m# ;
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96idation 'it 78n9: syn-o6idation
cis-2-butene meso-2,3-di!dro6!butane
trans-butene (S,S )- ( R,R)-2,3-di!dro6!butane
96idation 'it HC929H gi%es te o##osite
cis-2-butene (S,S )- ( R,R)-2,3-di!dro6!butane
trans-2-butene meso-2,3-di!dro6!butane
96idation 'it HC929H is anti-o6idation.
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C C !dro6!"ation 'it 78n9: is syn- because o$ an intermediate
9 9 #ermanganate addition #roduct.
8n
9 9
C C !dro6!"ation 'it HC929H 9 is anti- because o$ an intermediate
e#o6ide.
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* *CH2-CH-CH-CH=9
9H 9H 9H
1our carbon sugar, an a"dotetrose.
'o cira" centers, $our stereoisomers
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CH9 CH9
H 9H H9 H
H 9H H9 H
CH29H CH29H
D-erythrose L-erythrose
CH9 CH9
H9 H H 9H
H 9H H9 H
CH29H CH29H
D-threose L-threose
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erythro-”
threo-”
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* *C>H?CHCHC>H? + 79H(a"c) C>H?CH=CC>H? Br CH3 CH3
;-bromo-;,2-di#en!"#ro#ane ;,2-di#en!"#ro#ene
: stereoisomers 2 stereoisomers
( E )- ( Z )-
dehydrohalogenation of an alkyl halide ia @2 mechanism
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C>H? C>H? C>H? C>H?
CH3
H H CH3
CH3
H H CH3
Br H H Br H Br Br H
C>H? C>H? C>H? C>H?
erythro- threo-
C>H? CH3 C>H? C>H? \ / \ /
C = C C = C
/ \ / \ H C>H? H CH3
( E )- ( Z )-
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C>H? C>H?
CH3 H H CH3 79H(a"c)
Br H H Br
C>H? C>H?
erythro- C>H? C>H? \ / C = C / \ H CH3
( Z )-
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C>H? C>H?
CH3 H H CH3 79H(a"c)
H Br Br H
C>H? C>H?
threo- C>H? CH3 \ / C = C / \ H C>H?
( E )-
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@2 is bot stereose"ecti%e and stereos#eci$ic.
; anti-e"imination o$ te H Br 0
C>H?
Br
CH3 H CH3 CH3 H C>H? C C H \ /
Br H C>H? C = C
H / \
C>H? C>H? C>H? H9-
erythro- ( Z )-
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C>H? Br
CH3 H CH3 CH3 C>H?
C>H? C C C>H? \ /
H Br H C = C H / \
C>H? C>H? CH3 H9-
threo- ( E )-
9nce again, !ou must rotate about te CC bond in te
1iscer #roection to get te H Br anti to one anoter.
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@2 is an anti-e"imination. e !drogen and te a"ogen must
be on o##osite sides o$ te mo"ecu"e be$ore te @2 e"imination
can tae #"ace. is maes sense as bot te base and te
"ea%ing grou# are negati%e"! carged. ere$ore te! 'ou"dtr! to be as $ar a#art as #ossib"e. &n addition, te "ea%ing
grou# is "arge and tere is more room $or te remo%a" o$ te
adacent #roton i$ it is on te o##osite side $rom te "ea%ing
grou#.
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8ecanism = e"imination, bimo"ecu"ar E2
; anti-e"imination
base)
C
$
C
H
C C + H)base + )$
%&'
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CH3
H &
CH3
H Br
CH3
& H
CH3
Br H
base C
C
C
CCH3H
H3
C H & CH3
H CH3
CH3
& H
CH3
H Br
CH3
H &
CH3
Br H
baseC
C
C
CCH
3H
H3C & H CH3
H CH3
+
+
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Addition o$ a"ogensto a"enes
anti-addition
H!do6!"ation 'it78n9
:
syn-o6idation
H!dro6!"ation 'itHC929H
anti-o6idation
e!droa"ogenationo$ a"!" a"ides @2 anti-e"imination
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stereos#eci$ic and stereose"ecti%e #rob"ems
tt#0//cemistr!2.csud.edu/organic/s!nanti/starts!nanti.tm"
tt#0//cemistr!2.csud.edu/organic/s!nanti/starts!nanti.tm"
http://chemistry2.csudh.edu/organic/synanti/startsynanti.htmlhttp://chemistry2.csudh.edu/organic/synanti/startsynanti.htmlTop Related