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Chapter 6. Alkenes II 1. Types of Rxn.: elimination, addition, substitution 2. Mechanisms: How rxn’s of molecules occur. 3. Electrophilic (Markovnikov) addition A. Addition of HX, regioselective, Inductive effects, most stable R + , R 3 C + > R 2 HC + > RH 2 C + > H 3 C + - PowerPoint PPT Presentation
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6.1-1
Alkenes IIAlkenes II1. Types of Rxn.: elimination, addition, substitution2. Mechanisms: How rxn’s of molecules occur.3. Electrophilic (Markovnikov) addition A. Addition of HX, regioselective, Inductive effects,
most stable R+, R3C+ > R2HC+ > RH2C+ > H3C+
B. Hydration H+/H2O C. Rearrangements D. Bromonium ions, (anti addition) E. Halohydrin formation, (anti) F. H+ - hydration , Hg+2-hydration - (anti)4. Hydroboration syn addition5. Oxidation (syn)6. Reduction (syn), olefin and diene stability 7. Reactions relative to stereocenters
Br
H+
H
H
BrH
HH
HH
H
HH
H
Br
X
Chapter 6
21
6.1-2
Chapter 6
H3C
CH3
H
H OHC
H
H3C FHO
CH
H3CF
O
CO
H3C
HO H
O
CO
H3CO H
H
+
+
XH
H2 / M
Cl
IC
H
H3C D
NCC
HH3CD
(-):CN:
CH2
OH O
HCH3
CH3
H
Br Mg CH3
δ+ δ−
LiCu
2
reaction typesolefin additions
mechanismsenergy
progress of rx
specific additionsand reactions
6.1-3
Chapter 6 reaction types
olefin additions
energyprogress of rx
specific additionsand reactions
6.1-4
Reaction (Rx): A Reaction (Rx): A --->---> BB
Rx mechanism describes:• how rx occurs• which bonds - broken/formed
Mechanism: “how”Mechanism: “how”AA
XX YYZZ
BB
6.1-5
What is a mechanism?
C CH
H
H
H
Br H +conditions
starting material(SM)
C CH
H
H
H
HBr
products
Addition Reaction:Addition Reaction:
C CH
H
H
H
H
Br
What is a rx?
HOW
6.1-6
primary 1o secondary
2o tertiary 3o
“R” = any alkyl group; i.e. methyl, ethyl, etc.
Electrophilic AdditionsElectrophilic Additions
R CH
HCl
R CR'
HCl
R CR'
R"Cl
6.1-7
Rxn’s of Alkenesadditions to the C=C
break 1-π, form 2- bonds (Table 6.1)
Three Classes of Reaction:
1. Addition - increase in ‘atoms bonded’ at 2 (adjacent) carbons.
2. Elimination - decrease in ‘atoms bonded’ at 2 (adjacent) atoms.
3 . Substitution - replace 1 group/atom by another.
6.1-8
Hydrohalogenation
Electrophilic AdditionsElectrophilic Additions
HydrationHalogenationHalohydrinationOxymercurationHydroborationDiol formation (oxidation)Hydrogenationetc.
6.1-9
E DiagamsE Diagams graph showing the changes in energy during a reaction
starting materials products
potential energy
ene r
gy
reaction coordinate (progress of reaction)
BBAA
6.1-11
reaction coordinate
heat of reaction (energy given off)
ener
gy
A + X-Ystarting materials
A-X + Yproducts
activation energy Ea
6.1-12
ene r
gy
SM C + Z-X
products C-Z + X
reaction coordinate
C Z Xtransition state
activation energy
heat of reaction energy absorbed
transition state (t) = an energy maximum “believed” to be formed - unstable species of maximum energy
6.1-13
SM products
reaction coordinate
Hpote
ntia
l en
ergy
t1 t2 2nd transition state
PE diagrams
intermediate1st transition state
intermediates - energy minima between t#s.
6.1-15
SM products
reaction coordinate
heat of reactionpote
ntia
l en
ergy
large Ea
PE diagramslarge Ea, few collisions, slow reactions
t1 t2
small Ea
small Ea, fast rx
6.1-16A. Addition of HCl, HBr, HI
MAJOR
Markovnikov’s rule - H+ (electrophile) to carbon with most H’s- less substituted carbon
“regioselective” (explain next).
CCH3
CH
HH CCH3
CHHH
ClH
“trace”
+ CCH3
CHHH
HCl
+ HCl
6.1-17
base -system
Mechanism of Electrophilic AdditionMechanism of Electrophilic AdditionBase is C=C functional groupH+ (E+, electrophile) is acids
+ H(+)H
CH
CH3C H
E+ (others later)
H CH
CH3C H
Hcarbocation
R+
6.1-18
HC
H
H3CH
H
C
Carbocation
Lewis base + Lewis acid
trigonal/120o/sp2
(+)H
CH
CH3C H
+ HH C
H
CH3C H
H
[ E(+) ]
6.1-19Carbocation
Lewis base + Lewis acid
(+)H
CH
CH3C H
+ HH C
H
CH3C H
H H CH
CH3C H
H
X
X
Nu
Br
6.1-20How RegioselectiveWhich intermediate carbocation is easier to form?
CC
C
H
HH
H HH
CCC
H
HH
H HH
HH+
CCC
H
H H
H HH
HC
CC
H
H H
H HH
H
6.1-21Why RegioselectiveWhich intermediate carbocation is easier to form?
PE
SM Product(s)
ts1
ts2
CCC
H
H H
H HH
H
CCC
H
HH
H HH
H
6.1-22
HC
H
Hno alkyl gps H
CC
H
HH
H
1 alkyl gp
<
1o
Alkyl - groups slightly electron donating - stabilize + charge.
<H-C
2o
CH3
CH3 < H3CC
CH3
CH3
3o
6.1-23B. Addition of H2O: Acid-Catalyzed Hydration of Alkenes
1. Catalyst H2SO4 (or H3PO4)2. H+ adds to least substituted end of alkene 3. “ -OH” adds to more substituted end
regioselective
CC
CH3
H
H
H+ H2SO4H2O C
C
CH3
H
H
H
HHO
6.1-24Hydration - Addition of H2O (~HX addition)
Rx:
Mechanism:H
H + OH
H
HO
H acid-base
CH3
C C HH
HH
+O
H
H CH3
C C HH
HHO
HH
2.
OH
H
H
CH3
C C HH
H+
CH3
C C HH
HH
+1. H+ transferOH
H
-H+
product
CC
CH3
H
H
H+ H2SO4H2O C
C
CH3
H
H
H
HHO
6.1-25
Selective for 2o R+ over 1o R+
Why regioselective?
CC
CH
H H
HH
H
H
OHH
CC
C
H
HH
H HH
CCC
H
HH
H HH
H
H+
CCC
H
H H
H HH
H
HOH
Slow or “rate determining,” highest Ea’s
R3C > R2CH > RCH2 > CH3
+ + + +
6.1-26
Chapter 6
H3C
CH3
H
H OHC
H
H3C FHO
CH
H3CF
O
CO
H3C
HO H
O
CO
H3CO H
H
+
+
XH
H2 / M
Cl
IC
H
H3C D
NCC
HH3CD
(-):CN:
CH2
OH O
HCH3
CH3
H
Br Mg CH3
δ+ δ−
LiCu
2
hydrationrearrangements
halogenationhalohydration
mercury hydrationhydroborationother additions
6.1-27C. carbocation characteristic - rearrangements
majornormal and rearranged product
mechanism?
H CH3C
CH3
CC
H
HH+ H-Cl H C
H3C
CH3
CC
H
H H
HCl Cl C
H3C
CH3
CC
H
H H
+H
H
H3C CH3C
CH3
CC
H
HH+ O
H3C CH3C
CH3
CC
H
H HH
OH
H
HH
O CH3C
CH3
CC
H
H H
+H
CH3
H
major2o R+
3o R+E
6.1-29rx
H3C C
H3C
HC
C
H
H
HH Cl H3C C
H3C
HC
C
HH
HH3C C
H3C
ClC
C
HH
HHH
Cl H
+
H3C C
H3C
HC
C
HH
HH
mech.Cl
H3C C
H3C
CC
HH
HH
H1,2- H:migration
Cl
2o R+
3o R+E
normal rearranged
6.1-30Hydration, Rearrangements can occur - R+
H3CCH3C
H3CC
C
H
HH H2SO4
HOH
H3CCH3C
H3CCC
OH
HH
H
H
+HO
CH3C
H3CC
C
H
HH
HH3C
major
Rearrangements - problems in the book and a web worksheet
(mechanism)?
2o R+
3o R+E
6.1-31examples:
CH3
O-H
C + H2OH2SO4(cat.)
H
H
CH3
O-H
number ofregioisomers/stereoisomers
1 / 0
1 / 0
1 / 4
2 / 8 (4 each)
H+ = proton from H2SO4, H3PO4, HClO4, etc. (or H3O+)
CH3 + H2OH+
(cat.)
H
CH3 + H2OH+
(cat.)
H
H
CH3
H-O
HH
CH3+ H2O
H+
(cat.)
HH
H
CH3
H-O
CH3
O-H
+
H
H
HH
plus RR
6.1-32
H3CCH3C
H3CC
C
H
HH
H
Hydration, Rearrangements can occur - R+
H3CCH3C
H3CC
C
H
HH H2SO4
HOH
H3CCH3C
H3CCC
OH
HH
H
H
+HO
CH3C
H3CC
C
H
HH
HH3C
majorMech.:
H3CCH3C
H3CC
C
O
HH
H
H H
HHO
H
-H+
CH3CH3C
CC
H
HH
HH3CO
CH3C
H3CC
C
H
HH
HH3CHH
-H+
6.1-33D. Bromination & Chlorination
(F2 too reactive, I2 poor to OK)
+ Br Br
Br
Br
anti addition
6.1-34
H
C
H3C
H3C C
C
H
H
H
H
C
H3C
H3C C
C
H
HH
Br
Br
Br
Br+
Antiaddition
Br-Br has very weak bond
mechanism, then stereochemistry
Nu:- + Br Br: Nu Br: + :Br:-
6.1-35Antiaddition
H
C
H3C
H3C C
C
H
H
H
Br
Br Br(2) No rearrangement
Notice: (1) No stereochemistry
H
C
H3C
H3C C
C
H
H
H
H
C
H3C
H3C C
C
H
HH
Br
Br
Br
Br+
H
C
H3C
H3C C
C
H
H
H
Br
6.1-36Anti Addition: Stereochemistry
trans-diaxial addition - ‘stereospecific’,
HH
Br Br+
HH
Br
Br
Br HH
Br
bromonium ion controls anti addition
6.1-37
Chapter 6
H3C
CH3
H
H OHC
H
H3C FHO
CH
H3CF
O
CO
H3C
HO H
O
CO
H3CO H
H
+
+
XH
H2 / M
Cl
IC
H
H3C D
NCC
HH3CD
(-):CN:
CH2
OH O
HCH3
CH3
H
Br Mg CH3
δ+ δ−
LiCu
2
halohydration
mercury hydrationhydroboration other additions
6.1-38HOBr
Issues:Why selective for 2o alcohol (vs 1o)?Why no RR (note -H)?
H CH3C
CH3
CC
H
HH
+ OH C
H3C
CH3
CC
H
H HBr
O
H
HH
xs
Br Br + + H Br
6.1-39
Why 2o - not 1o?
HOBr
H CH3C
CH3
CC
H
HH
+ OH C
H3C
CH3
CC
H
H HBr
O
H
HH
xs
Br Br + + H Br
1 or 2 steps
H CH3C
CH3
CC
H
H H
Br
OH
H
H CH3C
CH3
CC
H
H H
BrO
HH
-H
6.1-40HOBr
H CH3C
CH3
CC
H
HH
+ OH C
H3C
CH3
CC
H
H HBr
O
H
HH
xs
Br Br + + H Br
1 or 2 steps
H CH3C
CH3
CC
H
H H
Br
OH
HH C
H3C
CH3
CC
Br
HH
O
HH
H
-H 1o OHH C
H3C
CH3
CC
Br
HH
O
HH
H
δ+
δ+
6.1-41
H CH3C
CH3
CC
Br
HH
H
δ+
δ+
HOBr
H CH3C
CH3
CC
H
HH
+ OH C
H3C
CH3
CC
H
H HBr
O
H
HH
xs
Br Br + + H Br
1 or 2 steps
H CH3C
CH3
CC
H
H H
Br
OH
H
H CH3C
H3C
CC
H
H H
BrO
HH
H CH3C
H3C
CC
Br
HH
Hδ+ δ+
OH
H
-H
2oR+ like > 1oR+-like end
