Mechanism Oxidation Alkene
-
Upload
jackie-peccadillo-lim -
Category
Documents
-
view
167 -
download
1
Transcript of Mechanism Oxidation Alkene
Sec. 10 - alkenes/alkynes 1
Addition Reactions to AlkenesAddition of Hydrogen Halides to Alkenes.
C CH
H
H
H
R2C=CR2 + HX R2HC-CR2X
Nucleophile(weak base)
+ H +
Electrophile(acid)
C CH
H
H
H
H+
General Reaction
X
Nucleophile(base)
Electrophile(acid)
C C
H
H
H
H
HX
Order of reactivity = HI > HBr > HCl > HF Why?
Slow
Fast
Reactivity parallels acid strength. The alkene is a very weak base so you need
a very strong acid to get a reaction.
Why?
Unsymmetrical alkene
Sec. 10 - alkenes/alkynes 2
Markovnikov’s Rule
In the ionic addition of an unsymmetrical reagent to a double bond, the positive portion of the adding reagent attaches itself to a carbon atom to the double bond as to yield the more stable carbocation as an intermediate (You already knew this from SN1 reactions. The most stable carbocation intermediate forms)
orHydrogen becomes
attached to the carbon atom of the double
bond with the greater number of hydrogens
Sec. 10 - alkenes/alkynes 3
Examples
C C
H
CH3H3C
H3CH Br+
nucleophile(weak base)
electrophile(acid)
C CH
CH3H3C
H3C H+
C CH2CH3
H3C
H3C
Br
sp2
sp2
sp3
sp3
major product
CH3
CH3
H Br+ CH3
CH3
H H
+
CH3
CH3
Br
Br
CH3
CH3
(R)
(R)
(R)
(S)
diastereomers
bottom
top
top
bottom
+
Sec. 10 - alkenes/alkynes 4
C
H3C
H3C
CH2
Examples
I – Cl
C
H3C
H3C
CH2
H2SO4
cold concentrated
S
O
O
O
H
OH
nucleophile(weak base)
electrophile(acid)
very weak basepoor nucleophile
no water
weak bondHSO4¯ great leaving group
Sec. 10 - alkenes/alkynes 5
C
H3C
H3C
CH2
dilute H2SO4
hydrationwater present
S
O
O
O
H
OH
O
H
H
H
or+
S
O
O
O OH
H2O - weak basepoor nucleophile but better
than HSO4¯ also in greater concentration
Examples
H2O
Sec. 10 - alkenes/alkynes 6
C CH2
H3C
H3C
C CH3
H3C
H3C+
coldconcentrated
H2SO4HSO4¯ C CH3
CH3
H3C
OSO3H
C CH3
CH3
H3C
OH concentratedH2SO4
C CH3
CH3
H3C
OH2
C CH3
H3C
H3C+ E1
diluteH2SO4
heat
NaBr
C CH3
CH3
H3C
Br
addH2Oheat
C CH3
H3C
H3C+
C CH3
CH3
H3C
OH+ HSO4¯
C CH3
H3C
H3C+
H2O-H+
SN1
SN1
+
-H2O
H2O-H+
dehydration
C CH2
H3C
H3C
hydration
hydration
What is the difference between hydration, elimination of alcohols and substitution reactions with alcohols, they all seem the same?
C CH3
H3C
H3C+
C CH2
H3C
H3C
C CH2
H3C
H3Cconcentrated
H2SO4
Don’t forget anytime a carbocation is formedrearrangement of the carbonskeleton can occur
cold
-H+
NaBr
Sec. 10 - alkenes/alkynes 7
Anti-Markovnikov addition
The hydrogen atom becomes attached to the carbon atom with the fewer hydrogen atomsThis is a free radical reaction not an ionic reaction. You do not have to memorize the
statement above if you understand the mechanism.
CH3CH=CH2
HBrROOR
a peroxide
CH3CH2CH2BrMechanism
RO OR 2 RO RO H Br BrROH +
C CH
H
CH3
H
CHH3C CH2
Br
CHH3C CH2 Br2° most stable than 1°same as carbocations
H Br CH2H3C CH2 Br Br+propagation
Br
C
H
C
HH C
H
HH
2°
Sec. 10 - alkenes/alkynes 8
Addition of Bromine and Chlorine to Alkenes
Addition produces vicinal dihalides
Sec. 10 - alkenes/alkynes 9
Mechanism
C CR
R
R
R
C CR
R
R
R
nucleophile(base)
Br
Br
δ+
δ−
C C
R
R
R
R
Br
C CR
R
R
R
Br +
electrophile(acid)
δ+
δ−Br
Brpolarized
not true diagrams of orbitalsbut okay for our needs
+ Br
Br
C C
R
R
RRBr
Br
electrophile(acid)
nucleophile(base)
+
bromonium ion intermediate anti addition
forms trans products
Sec. 10 - alkenes/alkynes 10
Example
Br2
CCl4
(solvent)
trans-1,2-dibromocyclohexane
+ enantiomer
Unsymmetrical Alkene
C CH2
H3C
H3C
+ Br – Br C CH3C
H3CBr
H
H
δ+
δ+
stronger bond
weaker bond
3° carbon can bear more of the positive charge.
3°
1°+ Br C C
H3CH3C
H
HBr
Br
Sec. 10 - alkenes/alkynes 11
Bromonium Ion
Sec. 10 - alkenes/alkynes 12
Halohydrin Formation (halogen + alcohol)
H2O(solvent)
Br – Br
Major product
Br2
H2O
OH
Br
Na OH
H2O
+
Sec. 10 - alkenes/alkynes 13
Sec. 10 - alkenes/alkynes 14
Oxidation States of Carbon
A reaction of an organic molecule usually corresponds to increasing its hydrogen content or decreasing its oxygen content.
RCH3
[O]
[H]RCH2OH
[O]
[H]R C
O
H[O]
[H]R C
O
OH
LowestOxidation State
HighestOxidationState
R CH ROH
R C R
O[O]
[H]
Oxidation → broad definition → a reaction that increases its content of any element more electronegative than carbon
CH3[O]
[H]CH2Cl
How to figure out the oxidation state of a carbon atom
a bond to hydrogen or anything less electronegative than carbon is electron donating → -1a bond to nitrogen, oxygen or anything more electronegative than carbon is electron withdrawing → +1a bond to a carbon → 0
C
H
H
H
H
C OOC
H
H
H
C
H
OH
H
OS = oxidation state
Sec. 10 - alkenes/alkynes 15
Oxidation States of Carbon
C C
H
HH
HH2/Pd
C
H
H
H
C
H
H
H
LEO goes GER – Loss of Electrons Oxidation, Gain of Electrons Reduction
C
H
H
H
C
H
OH
H
KMnO4C
H
H
H
C
O
OH
Reduction Oxidation
Oxidation Alkenes: Syn 1,2-dihydroxylation
Sec. 10 - alkenes/alkynes 16
Mechanism
one oxygenacts like anelectrophile
(acid) the other oxygenacts like annucleophile
(base)
Syn addition(same side)
causes cleavage without altering stereochemistry
COLD
Sec. 10 - alkenes/alkynes 17
Example
• OsO4, ether
2) Na2SO3
OOs
O
O
O
H
H
intermediate(if you remember this you know the answer)
Oxidative Cleavage of Alkenes (KMnO4 or K2Cr2O7)
BondCleavage
Sec. 10 - alkenes/alkynes 18
Mn
O O
O O
C(CH3)2H3CH2C
Mn
O O
O O
C C
CH3
CH3H3C
H
OS =-1 OS = 0
OS = +7 OS = +5
OS = 0 OS = +1
O
C
CH3H
O
CH3C CH3
cleavage
OS = +1 OS = +2
MnO O
OS = +3
Mechanism
further oxidation of carbon while manganese is
reduced
+
can’t break C-C bondsno further oxidation
O
C
CH3HO
OS = +3
+
In an acidic solutionMn2+ will be the finalmanganese product
In a basic solution Mn(IV)O2
is formed not MnO3¯ and is the final manganese
product
Do not need to know just for interest
Ox
carbonic acid
Ox Ox
decomposition
formaldehydeformic acidno further
oxidation
4 electron transfer
Sec. 10 - alkenes/alkynes 19
Example
1) KMnO4/NaOH heat
2) H3O+
Ox
intermediate
Ozonolysis of AlkenesOzone is in a class of compounds called 1,3-dipole reagents. They are so called
because good neutral resonance forms cannot be written for them.
Diazo Nitrile oxide Azides Nitrones
R2C N N+ +
RC N O+
R N N N+
R2C NO
R
+
Sec. 10 - alkenes/alkynes 20
Ozonolysis of Alkenes
O O
O +
O O
O
+
O O
O
+ 1,3-dipolar addition reaction
O
O O
+
( C H 3 ) C C C
C H 3
H C ( C H 3 ) 3
( C H 3 ) C C C
C H 3
H C ( C H 3 ) 3
O O
O
ozonide not stable
C ( C H 3 ) 3 C
H O
O O C
C H 3 C ( C H 3 ) 3 +
reverse 1,3-dipolar
addition
1,3-dipolar addition C
C H 3
C ( C H 3 ) 3
O
H
C ( C H 3 ) C
O O
ozinide more stable
This form is favoured because of the formation of stronger C-O bonds as opposed to weak O-O bonds
oxidative work up
H 2 O 2
( C H 3 ) 3 C
O
O H C H 3 C
O
C ( C H 3 ) +
a carboxylic acid a ketone
reductive work up
Z n , H 2 O o r Z n , C H 3 C O O H
( C H 3 ) 3 C
O
H
a ketone
C H 3 C
O
C ( C H 3 ) +
an aldehyde
1,3-dipolar addition
same products as KMnO4 or K2Cr2O7
Sec. 10 - alkenes/alkynes 21
Examples
H
H
O OO O
H+
O OO O
OH+
1) O3, CH2Cl2, -78°C
2) Zn/HOAc
1) O3, CH2Cl2, -78°C
2) H2O2
Could also use1) KMnO4/NaOH heat2) H3O+
What is A?
1) O3, CH2Cl2, -78°C
2) Zn/HOAc
1) O3, CH2Cl2, -78°C
2) H2O2
A
A
O
O O
O
H
O
OO
O
OH
H
OH
Sec. 10 - alkenes/alkynes 22
Addition of Bromine and Chlorine to Alkynes
Addition of halogen to alkynes can occur once or twice depending on how many equivalents of the halogen are added
Addition of one equivalent usually proceeds to give the trans dihalide
Sec. 10 - alkenes/alkynes 23
Addition of Hydrogen Halides to Alkynes
Addition of hydrogen halides occurs once or twice depending on how many molar equivalent of hydrogen halide are addedBoth additions are Markovnikov and give gem-halides
HBr can be generated by reaction of acetyl bromide and alumina
Anti-Markovnikov addition of HBr occurs in the presence of peroxide
Sec. 10 - alkenes/alkynes 24
Oxidative Cleavage of Alkynes
Reaction of alkynes with ozone or basic potassium permanganate both lead to formation of carboxylic acids
Sec. 10 - alkenes/alkynes 25
Not covered
Not covered
Sec. 10 - alkenes/alkynes 26
Not covered Not covered
H2O2