WELCOME TO MODERN ORGANIC CHEMISTRY. Organic Chemistry , 5 th Edition L. G. Wade, Jr. Chapter 4 The Study of Chemical Reactions. WHAT IS A REACTION MECHANISM . A DESCRIPTION OF STRUCTURES AN ENERGIES OF STARTING MATERIALS AND PRODUCTS OF A REACTION AS WELL AS ANY REACTION INTERMEDIATES.
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Organic Chemistry, 5th EditionL. G. Wade, Jr.
A DESCRIPTION OF STRUCTURES AN
ENERGIES OF STARTING MATERIALS AND
PRODUCTS OF A REACTION AS WELL AS
ANY REACTION INTERMEDIATES
IN ADDITION, ALL OF THE TRANSITION
STATES (ENERGY MAXIMA) SEPARATING
THE REACTANTS FROM THE PRODUCTS
(ENERGY MINIMA) MUST BE DETERMINED.
1. ISOLATE THEM
2. TAKE THEIR SPECTRA
a. IR - FUNCTIONAL GROUPS
b. NMR - environment of hydrogen and carbon atoms
c. MS - actual MW
CAN'T MEASUREMENT DIRECTLY
MEASURE THE RATE LAW TELLS NUMBER AND KIND OF MOLECULES INVOLVED IN TS
CERTAIN ENZYMES RECOGNIZE TS -
THAT IS THEY BOND WITH IT!!
DGo = DHo-TDSo
HOW FAST DOES A REACTION GO? KINETICS
RATE IS INVERSELY PROPORTIONAL TO ACTIVATION ENERGY (EA)
Requires heat or light for initiation.
Some ethane is formed
THESE FACTORS ARE CHARACHERISTICS OF FREE-
A chlorine molecule splits homolytically into chlorine atoms (free radicals)
THE CHLORINE ATOM IS ELECTROPHILIC - LEWIS ACID
SEEKS AN ELECTRON TO REGAIN OCTET OF ELECTRONS
USUALLY ABSTRACTS A HYDROGEN ATOM
The chlorine atom collides with a methane molecule and abstracts (removes) a H, forming another free radical and one of the products (HCl).
Methyl radicals is also electrophilic -seeks e-
Methyl radical is a member of the highly reactive
The methyl free radical collides with another chlorine molecule, producing the other product (methyl chloride) and regenerating the chlorine radical.
chlorine atom then attacks another methyl step 1 and 2 repeat over and over again
Can you suggest others? =>
DGo = -RT(lnKeq) where R = 1.987 cal/K-mol and T = temperature in kelvins
Since chlorination has a large Keq, the free energy change is large and negative.
G = H - TS
Which entropy value shown below is correct?
Table 4.2 gives BDE for homolytic cleavage of
bonds in a gaseous molecule.
We can use BDE to estimate H for a reaction.=>
THIS IS THE OFFICIAL MECHANISM
58Which is more likely?
Estimate DH for each step using BDE.
Trapping agents are used to determine intermediate
Transition states are at energy maxima.
The reaction step with highest Ea will be the slowest,
therefore rate-determining for the entire reaction.
reactivity of 3o = 36/1 = 36% per 1 H
Rel reactive of 3o/ 1o = 36/7.1 =
reactivity of 1o = 64/9 =
RelativeReactivity = 3o > 2o > 1o
Lower Ea, faster rate, so more stable intermediate is formed faster.