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?