Organic chemistry reactions

1. Organic Reactions
Kinds of Reactions
Mechanisms (polar, non-polar)
Bond Dissociation Energy
Reaction Profiles

2. Types of Reactions
• Addition Reactions
• Elimination Reactions
C=C
H
H
H
H
+ HBr C C
H
H
Br
H
H
H
C=C
H
H
H
H
+ NaOH
C C
H
H
Br
H
H
H + H2O
+ NaBr

3. Types of Reactions
• Substitution:
– Polar
– Non-polar
H
Br
H
H + KCN
H
NC
H
H + KBr
CH4 + Cl2 CH3Cl + HCl
light (h )

4. Rearrangement

6. 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.

7. Reaction Profile (Exothermic)
rate = kr[A] [B]
a b

8. 2nd Order Reaction
CH3Br + OH CH3OH + Br
Rate = k[CH3Br][OH ]
second order rate kinetics

9. 1st Order Reaction
(CH3)3CBr + H2O (CH3)3OH + HBr
Rate = k[(CH3)3CBr]
First order rate kinetics

10. Bond Breaking:
Non-polar and Polar

11. Bond Forming:
Non-polar and Polar

12. Non-polar Reaction Involves
Free Radicals

13. Free Radicals are Neutral, but
Electron-Deficient

14. Free Radical Chlorination

15. 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).

16. Free Radical Species are Constantly
Generated Throughout the Reaction
Propagation

17. Termination: Reaction of any 2 Radicals

19. Enthalpy of Reaction (DHo)
Measures Difference in Strength of
Bonds Broken and Bonds Formed
Bond Dissociation Energy

21. DHo = Sbonds broken-Sbonds formed

22. DHrxn = -105 kJ/mol
- 351
+ 435 - 431
+ 242
= - 109 KJ/mol
= + 4 KJ/mol

24. Why Not This Mechanism?

26. Chlorination of Propane
60%
40%

27. H’s are not abstracted at the same rate.
Reactivity of Primary (1 o) H abstraction
40%
6H
= 6.7
Reactivity of Secondary (2 o) H abstraction
60%
2H
= 30
Rate of 2 o H abstraction : 1 o H abstraction
= 4.5:1

30. Chlorination of Methylpropane
C
CH3
CH3
CH3 H H
CH3
CH3
CH2
C CH3
CH3
CH3
C.
+
+ Cl
.
.
.
C
CH2
CH3
CH3 H
.
C
CH3
CH3
CH3
+ Cl2
+ Cl2 C
CH3
CH3
CH3 Cl
C
CH2Cl
CH3
CH3 H 65%
35%
+ Cl
+ Cl
.
.

31. Tertiary H’s removed 5.5 times more
readily than primary H’s in
chlorination reactions

32. 3o Radicals are Easiest to Form

33. Stability of Free Radicals

35. Bromination is Very Selective

37. RDS in Bromination is highly
endothermic

38. 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

39. Chlorofluorocarbons and the
Depletion of Ozone
.
O3 O2 + O
h
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)
h
ultraviolet
a CFC

40. Polar Reactions:
Nucleophiles & Electrophiles

42. Nucleophiles are Bases
Electrophiles are Acids

43. Addition of HBr to Ethylene

44. Reactions Often Go Through
Intermediates

45. Transition State

47. Addition Reaction is a Two-
Step Mechanism

49. How Many Mechanistic Steps?
How Many Intermediates?
How Many Transition States?
Which Step is Rate-Determining?