Organic Reactions Kinds of Reactions Mechanisms (polar, non-polar) Bond Dissociation Energy Reaction Profiles
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Organic Reactions
Kinds of Reactions
Mechanisms (polar, non-polar)
Bond Dissociation Energy
Reaction Profiles
Types of Reactions
• Addition Reactions
• Elimination Reactions
C=CH
H
H
H+ HBr C C
H
H
Br
H
H
H
C=CH
H
H
H
+ NaOHC C
H
H
Br
H
H
H + H2O
+ NaBr
Types of Reactions
• Substitution:
– Polar
– Non-polar
H
BrH
H + KCN
H
NCH
H + KBr
CH4 + Cl2 CH3Cl + HCllight (hν)
Rearrangement
Definitions
• Mechanism: Complete step-by-step of exactly which bonds break and which bonds form and in what order.
• Thermodynamics: The study of the energy changes that occur in chemical transformations. This allows for comparison of stability of reactants and products.
• Kinetics: The study of reaction rates, determining which products are formed most rapidly. One can predict how the rate will change with changing conditions.
Reaction Profile (Exothermic)
rate = kr[A] [B]a b
2nd Order Reaction
CH3Br + OH CH3OH + Br
Rate = k[CH3Br][OH ]
second order rate kinetics
1st Order Reaction
(CH3)3CBr + H2O (CH3)3OH + HBr
Rate = k[(CH 3)3CBr]
First order rate kinetics
Bond Breaking:Non-polar and Polar
Bond Forming:Non-polar and Polar
Non-polar Reaction Involves
Free Radicals
Free Radicals are Neutral, but
Electron-Deficient
Free Radical Chlorination
Experimental Evidence Helps
to Determine Mechanism• Chlorination does not occur at room
temperature in the dark.
• The most effective wavelength of light is
blue that is strongly absorbed by Cl2 gas.
• The light-initiated reaction has a high
quantum yield (many molecules of product
are formed from each photon of light).
Free Radical Species are Constantly
Generated Throughout the Reaction
Propagation
Termination: Reaction of any 2 Radicals
Enthalpy of Reaction (∆∆∆∆Ho)
Measures Difference in Strength of
Bonds Broken and Bonds Formed
Bond Dissociation Energy
∆∆∆∆Ho = ΣΣΣΣbonds broken-ΣΣΣΣbonds formed
∆∆∆∆Hrxn = -105 kJ/mol
- 351
+ 435 - 431
+ 242
= - 109 KJ/mol
= + 4 KJ/mol
Why Not This Mechanism?
Chlorination of Propane
60%
40%
H’s are not abstracted at the same rate.
Reactivity of Primary (1o) H abstraction
40%
6H= 6.7
Reactivity of Secondary (2o) H abstraction
60%
2H= 30
Rate of 2o H abstraction : 1o H abstraction
= 4.5:1
Chlorination of Methylpropane
C
CH3
CH3
CH3 H HCH3
CH3
CH2
C CH3
CH3
CH3
C.++ Cl
.
.
.
Tertiary H’s removed 5.5 times more
readily than primary H’s in
chlorination reactions
3o Radicals are Easiest to Form
Stability of Free Radicals
Bromination is Very Selective
RDS in Bromination is highly
endothermic
Consider the free radical monochlorination of
2,2,5-trimethylhexane. Draw all of the
unique products (ignore stereoisomers; use
zig-zag structures please) and predict the ratio
or percent composition of the products.
The relative reactivity of H abstraction in a
chlorination reaction: 1o: 2o: 3o = 1: 4.5: 5.5
Chlorofluorocarbons and the
Depletion of Ozone.O3 O2 + O
hν
C Cl
F
F
Cl Cl
F
F
C . .
. .
. .
Cl+
Cl + O3 ClO + O2
ClO + O Cl . + O2
O3 + O 2 O2.net reaction
i)
ii)
hν
ultraviolet
a CFC
Polar Reactions:
Nucleophiles & Electrophiles
Nucleophiles are Bases
Electrophiles are Acids
Addition of HBr to Ethylene
Reactions Often Go Through
Intermediates
Transition State
Addition Reaction is a Two-
Step Mechanism
How Many Mechanistic Steps?
How Many Intermediates?
How Many Transition States?
Which Step is Rate-Determining?
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