123.202 lecture 11 - alkynes
DESCRIPTION
123.202 lecture 11 alkynes and dienesTRANSCRIPT
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dienes & alkynes...LECTURE ELEVEN
PREVIOUSLY...
radicals...Br
➓dienes12
CH
H HH
dienediene
isolated conjugateddiene
cumulated
allene
dienescumulatedallenes
CH
Ph HPh
CH
PhHPh
mirror
CH
H
Ph
PhC C
CH
PhHPh
π(2py+2py)
orbitals
CH
H
Ph
PhC C
CH
PhHPh
π(2py+2py)
π(2pz+2pz)
sp2
sp
90°shape
dienesconjugated
stabilisation
stabilisationresonance
ObservedkJmol–1
ExpectedkJmol–1
127
253 254
254 254
254239
stabilisationresonance
to conjugated dienes➓electrophilic addition13
HX
X
X
H
H
H
HX
X
regioisomers
HX
XH
XH
HX
HH
XX
H
X?regioisomershow many
?
XH
XH
HX
HH
XX
H
X
regioisomershow many
?
XH
XH
HX
HH
XX
H
X
regioisomershow many
?
favouredleast
to conjugated dienes➓electrophilic addition13
Br2
2 x Br2 BrBr
BrBr
to conjugated dienes➓electrophilic addition13
regioisomers
Br2
Br2
BrBr
BrBr
mechanism
BrBr
Br
Br
Br
BrBr
BrBr
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alkynes
➓acidity of alkynes14
R H Na NH2
R Na NHH2
base
very strong
XR
H
H R
H
HH
notacidic
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sp3
(25% s)sp2
(33% s)sp
(50% s)
molecularorbitals
HCl >R OH
O>
OH
>HO
H>
HO
R>R
H
> >HN
HH
H
R
Cl <R O
O<
O
< OH
< OR
<R
< <HNH R
decreasing acid strength
increasing base strength
strong acid strong
weaker
base
weaker
+
acid + base
strong acid
strong
weak
base
weakacid
base
R
OH
R
HO
strong strong
weakbaseweak
acidbase
R
H O
R
OH
Xacid
http://www2.lsdiv.harvard.edu/labs/evans/
best pKa table I know
nucleophile
R H Na NH2 RBr
R
good
SN2
DANGER
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R
strong base
RBr
R
SN2 E2X
side reaction
H3O...Hg(OAc)2
HgOAc
HgOAc
OHH2O
➓hydration of alkynes15
additionMarkvonikov
tautomerisation
OH O
ketoneenol
hydroboration/oxidation
BH3H
BH2
H2O2NaOH
HOH
O
additionMarkvonikovanti
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goodluck