reactions and reaction mechanisms
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Reactions and Reaction Mechanisms
A reaction mechanism shows the actualflow of electrons and movement of the atoms during the reaction.
If one can understand a reaction mechanismone can predict the course of other reactions.
B
F
FFH N
H
H
Attack of a Lewis Base on a Lewis Acid
N B
H
FHH
FF
It is also useful to correlate changesin energy with the movement of atoms.
To do this one creates an energy profileof the reaction showing how the energychanges as the atoms move.
+
H3O+ Cl-+
H2O HClH
Energy Profile of a Exothermic Reaction
Potential Energy
H2O + HCl H3O+ + Cl-
+
H3O+ Cl-+
H2O HClH
Ea
-+H
O
H
H ClTransitionState
H = D (bonds broken) - D (bonds formed)
H = DH-H + DC=C - 2 DC-H - DC-C
C CH
CH3
H
H
H
HH2+
HC
CH3
CH
H
H = 432 + 614 - 2 x 413 - 347 = -127 kJ / mol
C CH
CH3
H
H
H
HH2+
HC
CH3
CH
H
But if you mix propene and H2 nothing happens!
C CH H
HCH3
H HMolecules repeleach other.
No good mechanismfor the reaction.
H = -127kJ/mol
CH3CH=CH2 H2+
CH3CH2CH3
? Ea too high thusno reaction
Try to find a new mechanismwith a lower Ea
How do you dothat? Use acatalyst.
Palladium metal can be used as a catalystfor the hydrogenation of alkenes.
H2 will actually dissolvein Pd metal. The H atomsdissociate and go intothe octahedral holes.
The bond of an alkene will bond to thesurface atom of the Pdmetal.
This is all we need!
C CH H
HCH3
Pd Pd PdPd Pd
PdPd
H H
C C
PdH
CH3
H HH
H
C
Pd
HH
H
H
CH3
H
C HH
H
H
H3C
H
Pd
C C
PdH
CH3
H HH
H
C
Pd
HH
H
H
CH3
H
C CCH3
H HH
+
H2Pd
C HH
H
H
H3C
HPd
C CCH3
H HH
+
H2
Pd
C HH
H
H
H3C
H
Palladium metalis a catalyst forthe hydrogenationof alkenes.
H2
Pd
H2
Pd
H2
Pd
BrBr
or
C C
H
H H
HC C
Br
H
HH
HH
Addition of HBr to ethylene gives bromoethane
But what does HBr give when it reacts with2-methylpropene?
BrBr
or
But what does HBr give when it reacts with2-methylpropene?
For this we need to know the mechansism.
The reaction involves a carbocationintermediate.
CH3
CCH3C
H
H
CH3
CCH3C
H
HH
CH3
CCH3C
H
HH
H Bror
Br
Br
Which carbocationis formed?
C
C
HH
H
HH
C
C
HH
HH
H
Alkyl groups stabilizecarbocations viaHyperconjugation.
It involves “no bond”resonance structures.
C
C
HH
H
H
H
C
C
HH
H
HH
C
HHC
H
HH
Empty p orbitalFilled sp3
hybrid orbital
Electrons from the C-H bond helpstabilize the empty p orbital
Order of Carbocation Stability
C
H
HHC
CH3
HH
C
CH3
CH3HC
CH3
CH3CH3
LeastStable
MostStable
<<<
primary
secondarytertiary
CH3
CCH3C
H
H
CH3
CCH3C
H
HH
CH3
CCH3C
H
HH
H Bror
Br
Br
Which carbocationis formed?
Tertiary most stable
C
CH3
CH3CH3
How does acarbocation react?
Br
C
CH3
BrCH3
CH3
Since the carbocationis a Lewis acid It can react with aLewis base.
The electron pair onthe base attacks theelectron deficient center
General Reaction:
The addition of a hydrogen halide, HCl, HBr orHI to an alkene gives an alkyl halide. Theregiochemistry is determined by which carbocation is the most stable.
HBr
Br
H
HCH3
H
more stable
H I
I CH3
H HH
H HH
more stable
HCl
Cl
Cl
and
Both carboncation intermediateswould be secondary, little selectivity
C C
H
H H
H H2SO4
H2OC C
H
HO
H
HH
H
What happens if there is no halide?
Hydration of alkenes to give alcohols.
H2SO4 is a catalyst
C C
H
H CH3
H H2SO4
H2OC C
H
HO
H
HH
CH3
or C C
H
H
H
HOH
CH3
What is the regiochemistry of the reaction?
What is the mechanism of the reaction?
CH3
CCH3C
H
H
CH3
CCH3C
H
HH
CH3
CCH3C
H
HH
or
The H2SO4 dissociatesto give hydronium ion.
H O
H
H
OH
H
OH
H
CH3
CCH3C
H
HH
O
H
H
O
CC
H
HHH3C
H3C
HH
O
CC
H
HHH3C
H3C
H
OH
H
H3O+
+
C C
H
H CH3
H H2SO4
H2OC C
H
HO
H
HH
CH3
or C C
H
H
H
HOH
CH3
What is the regiochemistry of the reaction?
The regiochemistry is determined by therelative stability of the intermediatecarbocation.
Reactions of alkenes so far:
H2
Pd
XHX
OHH2O
H2SO4
One more reaction for the sake of completeness
Br2 Br
Br
Bromine or chlorine easily add to a doublebond to give dibromo or dichloro compounds.
Br2
Br
Br
C
CH H
C
Cl C
Cl
H H
Cl2
Reactions of alkenes so far:
H2
PdX
HX
OHH2O
H2SO4
Br
Br
Br2
OHH2O
H2SO4
Enough of this simple stuff.
The acid catalyzed addition of water toan alkene gives an alcohol.
The regiochemistry is determined by the relative stability of the intermediate carbocation.
But what if you want 1-propyl alcohol?
What could you do to “trick” the regiochemistry?
The Trick
CH3C
C
H
H
H
CH3C
H
H
HH
H+
The observed regiochemistry is determinedby the first step of the reaction.
But we want an OH- group to add to theprimary carbon. How do we do that?
Add some other Lewis Acid instead of H+
then convert it to an OH group.
The Trick
CH3C
C
H
H
H
BH3
CH3C
H
B
HH
HH
H
CH3C B
HH
H
HH H2
B
tripropylborane
B
But what good is tripropylborane? We are tryingto make 1-propanol?
It turns out that oxidation of alkyl boraneswill give boric acid plus alcohols.
B
OH
OO
H
H
OH3
+
H2O2
-OH
Net Reaction
1. BH32. H2O2, -OH
OH
This is called hydroboration
1. BH32. H2O2, -OH
OH
Thus we have a choice in regiochemistry.
OH
H2O
H2SO4
HO2. H2O2, -OH 1. BH3
OHH2O
H2SO4
H2O
H2SO4
Oxidation states in carbon chemistry.
Oxidation and reduction are important chemicalprocesses. In organic chemistry it is not always obvious that you are carrying out an oxidation or a reduction.
Consider average oxidation states of the C atoms in each of the following molecules.Remember that H is counted as +1 and O as –2.
CH3 CH3
-3 C C
H
H H
H-2
C CH H
-1
CH3CH2
OH
-2
CH3C
H
O
-1
CH3C
O
O
H
0
C C
O
O O
O
H
H +3
Adding H2 to a double bond is a reduction.The reverse reaction ( removing H2) would be an oxidation.
-3-2
H2CH3 CH3C C
H
H H
H
-2-1CH3
CH2O
HH2
CH3C
H
O
Adding H2O is not an oxidation. Removing H2O is not a reduction.
-2CH3
CH2O
H
-2H2SO4
H2OC C
H
H H
H
Adding H2O is not an oxidation. Removing H2O is not a reduction.
-1-1
C C
H
H O
H
HH2O
H2SO4
C CH H
Unstable rearranges
-1CH3
CH
O
To go from an alcohol to an aldehyde (or a ketone )is an oxidation. Going from an aldehyde on to a carboxylic acid is a further oxidation.
SomeOxidant
CH3C
H
O
-1
CH3CH2
OH
-2
CH3C
O
O
H
0
SomeOxidant
Primary alcohols can be oxidized to carboxylic acids using CrO3. Chromium (VI) is a strong oxidizing agent. It is used with an acid catalyst.
OH
OH
O
CrO3
H2SO4
OH
O
OH H2SO4
CrO3
Alcohols can be oxidized to aldehydes or ketonesusing a modified form of CrO3 called PCC for pyridinium chlorochromate, (C5H6NCrO3Cl).
It is a milder reagent and if you use it carefully you can stop the reaction at the intermediate aldehyde step.
PCCO
H
OH
PCCO
HO
PCC
OH
O
Going Backwards: Reduction of aldehydes or ketones to an alcohol. To reduce a carbonyl group, C=O, to an alcohol one could use H2 gas, but a simpler way is to use a hydride as a source of H- and a acid as a source of H+.
The reagent of choice is NaBH4, followed by some acid.
1. NaBH4
2. H3O+O
HO
H
O
Na
B
H
H HH
B
H
HH
Na
H
O
H
H
O
H
Na
O
H
HH
H
O
H
H
O1. NaBH4
2. H3O+
OH
OH
1. NaBH4
2. H3O+O
H
Starting with an alkene how would you make?
Br
1. BH32. H2O2, -OH O
H
HBr
OOH
PCCH3O+
H2SO4
HBr
Br
Can you explain an unexpected result?
82%
+
? 18%
HBrBr
Br
and
82%
? 18%
CH3
H
H
H
H
CH3H
H
H
H
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