Chemistry 125: Lecture 54 February 21, 2011 Acetylenes Allylic Intermediates & Dienes Linear and...
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Transcript of Chemistry 125: Lecture 54 February 21, 2011 Acetylenes Allylic Intermediates & Dienes Linear and...
Chemistry 125: Lecture 54February 21, 2011
AcetylenesAllylic Intermediates & DienesLinear and Cyclic Conjugation
(4n+2) Aromaticity This
For copyright notice see final page of this file
Generalization to
Acetylenes
e.g. J&F Sec. 10.6-10.11 pp. 444-455
Stepwise / Markovnikov
“Keto-Enol Tautomerism”Regioselection
Addition of HBr
Addition of H2O
Addition of H2 Stepwise / Stereoselection
Acidity and base-catalyzed isomerization
Stepwise Addition of HBr to Alkyne
1-Hexyne + HBr 2-Bromo-1-hexene
FeBr3
15°C
with “inhibitor”to trap radicals isolated in 40% yield
100 to 1000x slower than comparable ionic addition to alkene, because vinyl cation is not so great.
CH3-CH2-Cl CH3-CH2+ + Cl-
Gas Phase Ionization
193 kcal/mole
CH2=CH-Cl CH2=CH+ + Cl-225 kcal/mole
Stepwise Addition of HBr to Alkyne
1-Hexyne + HBr 2-Bromo-1-hexene
FeBr3
15°C
with “inhibitor”to trap radicals isolated in 40% yield
HBr can add again to the bromoalkene (obviously more slowly) to give a second Markovnikov addition
If the bromo substituent slows addition to an alkene, why is there Markovnikov orientation?
2,2-Dibromohexane
Br is a “schizophrenic” substituent: both electron withdrawing (), and electron-donating ().
+Hg(OAc)2
H+ / H2OHC CR +
HC CR
HgOAc HgOAc
CO
R
CHH2O -H+H+
NaBH4
CO
R
CH H
H
H
Markovnikov EnolH
+
H
Ketone
an easy allylicrearrangement
“Keto-Enol Tautomerism”
+
(favors ketoneCf. Lecture 37)
ve Bond Energies
Can one sum bond energies to getaccurate"Heats of Atomization"?
H C
O
H
CCH H
H
H
H C
O
H
CCH H
H
HKetone "Enol"
C
O
CH
C
O
C
H
C=O 179
C-C 83
C-H 99
sum 361
C-O 86
C=C 146
O-H 111
sum 343
Kcalc = 10-(3/4) 18 = 10-13.5
Kobs = 10-7 = 10-(3/4) 9.3
Bonds that change(the others should cancelin taking the difference)
H C
O
H
CCH H
H
H
H C
O
H
CCH H
H
HKetone "Enol"
H
Why is Enol9 kcal/mole
"Too" Stable?
O
C=O 179
C-C 83
C-H 99
sum 361
C-O 86
C=C 146
O-H 111
sum 343
Kcalc = 10-(3/4) 18 = 10-13.5
Kobs = 10-7 = 10-(3/4) 9.3
••
C(sp2)-Hstronger than
C(sp3)-H
(they shouldn’t actually cancel)
IntramolecularHOMO-LUMO
Mixing
H C
O
H
CCH H
H
H+"ResonanceStabilization”
from
Markovnikov Enol
+Hg(OAc)2
H+ / H2OHC CR +
HC CR
HgOAc
CO
R
CH
H2O
-H+H +
H
Ketone
R’2B-HHC CR C
R
R’2B
CH
H
Anti-Markovnikov Enol
Aldehyde
HOOH
HO-
CR
HO
CH
H
H
vinylborane(hindered R’2BHadds only once)
BH3 + 2
e.g. “disiamylborane”
Hydration with Either Regiospecificity
(what is R’?)
R-C C-R
Hydrogenation with Either Stereospecificity
( Pd / CaCO3 / Pb )
H2
Lindlar CatalystC
R
H
C
H
R
deactivate Pd to stop at alkene
n-Pr-C C-n-PrNa / NH3
C
n-Pr
H
CHn-Pr
“dissolving metal reduction”
syn addition
H H
anti addition
H
H
97% forR = (CH2)3CO2CH3
80-90%
solvated electronNa
NH3
e-(NH3)n+ Na+
R-C C-R
First H+
R-C C-R
R-C C-R e-First e-
C C
R
RH
C C
R R
H
Vinyl radicals are sp2
but they invert easily
H
NH2NH2
Second H+ e-H
NH2
NH2
C C
R
RHVinyl anions are sp2
and invert very slowly(remember XH3)
Second e-
C C
R
RH
C C
R R
H
Vinyl radicals are sp2
but they invert easily
C C
R
RH
H
anti addition(because of radical isomerism)
H
H
Approximate “pKa” Values
CH3-CH2CH=CHH ~ 44
CH3-CH2C CH ~ 25
CH3-CH=C=CHH
CH3-C C-CH2H ~ 38
sp3 C_
sp2 C_ (no overlap)
sp C_ (no overlap)
C_ HOMO - overlap
CH3-CH2CH2CH2H ~ 52
~ 34 H2NH
= 16 HOH
(better E-match N-H)
(bad E-match O-H)
(best E-match C-H)50
40
30
20
10
pKa
*
:
:
(allylic)
(e.g. J&F Acidity of 1-Alkynes Secs. 3.14 p. 129; 12.4 p. 516-518)
H+(aq) +
Equilibrium & Rate
kcal
/mol
40
30
20
10
-10
50
0 CH3-CH=C=CH2
CH3-C C-CH3
CH3-CH2C CH
CH3-CH2C C
CH3-CH=C=CHCH3-C C-CH2
pKa 38
Ka 10-38
G 4/3 38 = 51
pKa 25
Ka 10-25
G 4/3 25 = 33
4.1 4.8
0.1% 0.03%
k 1013 10-38 /sec
t1/2 = 0.69/k 1025 sec = 1017 yrs 104 time since Big Bang
[0]
at equilibrium
H+(aq) +
+ HO-
favors dissn. by 21 kcal
(4/3 16)
Equilibrium & Rate
kcal
/mol
40
30
20
10
-10
50
0 CH3-CH=C=CH2
CH3-C C-CH3
CH3-CH2C CH
CH3-CH2C C
CH3-CH=C=CHCH3-C C-CH2
t1/2 30 yrs @ 300K
-7.20.0001%
2 min @ 150°C + H2N
-
favors dissn. by 45 kcal (4/3 34)
at equilibrium
Trick to obtain terminal acetylene:
Equilibrate with RNH_
base(in RNH2 solvent at room temp)
to form terminal anion.“Quench” by adding water which donates H+ to terminal anion and to RNH_, leaving OH_, which is too weak to allow equilibration.Or add H+, so even [OH
_] is very low.
C C
Conjugation & Aromaticity
Conjugated Pi Systems
OC
Yoke
Jungere
Jugóm
(to Join)
e.g. J&F Ch. 12-13
The Localized Orbital Picture(Pairwise MOs and Isolated AOs)
Is Our Intermediate betweenH-like AOs and Computer MOs
When must we think more deeply?
Allylic Stabilization:Cation
R-Cl R+ + Cl-(gas phase kcal/mol)
Cl
Cl
Cl
193
172
171
Anion
pKa
OH
OH
16
10
5OHO
Radical
Bond Dissociation
Energy (kcal/mol)
H
H
101
89
Conjugation worth ~ 13 kcal !
as good as secondary
4/3 6 = 8 kcal
Br-
-78°C
20%
80%
85%
15%
Allylic Cation Intermediates:Addition of HX to Butadiene
HBr-78°C
e.g. J&F Sec. 12.9-12.10 pp. 534-541
H
+
+
H
+
H
H
Br
H
BrKineticvs.
Thermodynamic Control
Reason for Kinetic
Distribution?
FeBr3
+17.6
-21.4
HOMO-4HOMOLUMO+1LUMOHOMO-1LUMO+1LUMOHOMO
Butadiene Propenyl CationH+
hyperconjugated C-H
bestoverlap
bestpotential
best productbest overlap
symmetrical(but for D)
p. 1288
3.1 : 1-78°
1.6 : 125°
Cl-
DCl
rapid ion-pair collapse competes with motion
Allylic Transition States:SN1
e.g. J&F Sec. 12.11a,b pp. 541-543
krel for solvolysis in 1:1 EtOH/H2O at 45°C
Cl<< 0.01
Cl0.01
Cl6300
Cl43
Cl0.05
Cl
0.07
Cl[100]
Cl39
++
methylation is effective where charge is (C1,C3)
Allylic Transition States:SN2
e.g. J&F Sec. 12.11a,b pp. 541-543
krel for Displacement by EtO- in EtOH at 45°C
Cl[1]
Cl560
Cl97
Cl37
Cl
33
Cl1.9
Allylic Anion Intermediates:RH Acidity
allylic
benzylic
e.g. J&F Sec. 12.11d pp. 543-544 and Sec. 13.12
pKa ~52
HCH2
pKa 43
HCH2
pKa 41
HCH2
4/3 x 9 ≈ 11 kcal/mole
4/3 x 12 ≈ 16 kcal/mole
Allylic Free-Radical Intermediates:Allylic Bromination
Cf. J&F Sec. 11.8 pp. 497-500, Sec. 12.11c p. 543
N-Bromosuccinimide(NBS)
N
O
O
Br
58% yieldK. Ziegler (1942)
Et2O30 min.
h
Ionic Preparation and Destruction of NBS
pKa 9.5
N
O
O
HNaOH
0°C N-
O
O
N
O
O
BrBr2
+ NaBr
N
OH
O
Br+
N
OH
O
undo with HBr
N
OH
O
Br
+
Br-
“enol” to “ketone”
Br-
+ Br2
BrBr
How to controlAddn. vs. Subst.?
Rate [Br2]2
Br2 helps Br- leave from “Br+” in nonpolar solvent
(like protonation of OH)
Whenever a Br2 molecule is consumed, one new Br2 molecule is created.
Allylic Reactivity - Radical
Automatically maintains minimal [Br2].
+ Br2 + Br-Br-Br-
Br+
CH2Cl2
25°C Dark
H
H
or H
Br
Br
H
Br2 + Br
N
O
O
Br N
O
O
H
Addition
Substitution
2
HBr•
initiator(h, peroxide, etc.)
Keep dark
+ HBrslow
(selective)
or minimize [Br2] (tedious to impossible?)
Cl• also attacks this CH2 group
Conjugation worth
~7 kcal
Conjugation worth
~8 kcal
Hcombustion
768.9 ±0.3
761.6 ±0.2
17.7
25.4
Hformation
Diene Stabilization:
End of Lecture 54February 21, 2011
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