Mechanisms
One of the most practical aspects of organic chemistry is the study and application of chemical reactions.
Due to the large number of reactants that can be used, it is virtually impossible to memorize all possible reactions.
Organic reactions are often organized into groups based on their mechanisms and the intermediates that are involved.
Mechanisms
Mechanism: a step-by-step pathway from reactants
to products that shows which bonds break, which bonds form and the order in which they happen
includes structures of all reactants, intermediates and products and curved arrows showing the movement of electrons
Mechanisms
Your success in this class depends in large part on learning the mechanisms of key reactions and applying these mechanisms to predict the products formed from starting materials you have not used before.
Mechanisms
Halogenation of alkanes:
alkane + halogen alkyl halide(s) + HX
CH4 (g) + Cl2 (g) CH3Cl + CH2Cl2 + CHCl3 + CCl4 +
HCl
or h
or h
Mechanisms
The halogenation of alkanes is a substitution reaction that occurs via a chain reaction mechanism.
Substitution reaction: a reaction in which one atom
substitutes for or replaces another atom
In the chlorination of methane, a chlorine atom replaces a hydrogen atom.
Mechanisms
Three types of steps occur in all chain reactions: Initiation:
generates a reactive intermediate
Reactive intermediate: a short-lived species that reacts as quickly as it is formed
never present in high concentration
Mechanisms
Three types of steps occur in all chain reactions: propagation
reactive intermediate reacts with a stable molecule to form a new reactive intermediate and a new stable molecule
continues until reactants are exhausted or reactive intermediate is destroyed
termination side reaction that destroy the reactive intermediate
slows or stops reaction
Mechanisms
Chlorination of Methane (Mechanism) Initiation:
Chlorine absorbs h generating two free radicals.
Use half arrows to show movement of one electron.
Cl + Clh
ClCl
Mechanisms
Free radical a reactive intermediate with one or
more unpaired electrons also called a radical electron deficient (doesn’t have octet)
Mechanisms
Chlorination of Methane (Mechanism) cont. Propagation:
First propagation step: Chlorine radical collides with a methane molecule and abstracts a hydrogen atom
H C
H
H
H + Cl H C
H
H
+ H Cl
Mechanisms
Chlorination of Methane (Mechanism) cont. Second Propagation step:
methyl radical reacts with chlorine molecule, generating product and another reactive species
The new chlorine radical continues the chain by abstracting another hydrogen atom from methane, etc.
H C
H
H
ClH C
H
H
+ Cl Cl + Cl
Mechanisms Chlorination of Methane (Mechanism)
con’t. Termination:
Any reaction that produces fewer reactive intermediates than it uses will slow or stop the reaction:
Cl ClCl+Cl
CH3Cl+ ClH C
H
H
Mechanisms
More possible termination reactions:
H C
H
HH C
H
H
+ CH3CH3
H C
H
H
collides with wall H3C wall
Cl collides with wall Cl wall
Thermodynamics
Information about chemical reactions is obtained using thermodynamics and kinetics.
Thermodynamics: used to study the stability of reactants and products predicts which compounds are favored
by the equilibrium
Thermodynamics
For an equilibrium reaction:
a A + b B c C + d D
Go = -RTlnKc
Spontaneous reaction (favors products): Kc > 1 G = neg
Nonspontaneous reaction (favors reactants): Kc < 1 G = pos
Thermodynamics
Two thermodynamic quantities contribute to G:
G = H - TS
H = enthalpy change (amount of heat gained or lost) H = positive (endothermic: heat
gained) H = negative (exothermic: heat lost)
S = entropy change change in the randomness or disorder
Thermodynamics
For many organic reactions, TS is small relative to H G ~ H
Therefore, most exothermic organic reactions tend to favor the formation of products.
The Hrxn can be estimated using the bond dissociation energies of the bonds broken and formed during the reaction.
Thermodynamics
Bond dissociation energy: the amount of energy required to break
a bond homolytically equally each atom in the bond being broken gets one electron
forms free radicals
As BDE increases, more energy is needed to break the bond: stronger bond
Thermodynamics
Example: Which of the following bonds is the strongest? The weakest?
F - F, Cl - Cl, CH3 - F, CH3 - Cl, H - F, or H - Cl
F - FCl - ClCH3 - FCH3 - ClH - FH - Cl
38 kcal/mol5810984136103
Thermodynamics
Example: Rank the following C-H bonds in order from the easiest to the hardest to break homolytically.
Methyl H104 kcal/mol
1o H98 kcal/mol
2o H95 kcal/mol
3o H91 kcal/mol
H3C H CH3CH2C
H
H
H
CH3CHCH3
H CH3CCH3
CH3
H
Thermodynamics
As BDE increases, it is harder to break the bond: Ease of homolytic cleavage:
3o > 2o > 1o > methyl
The stability of methyl, 1o, 2o, and 3o free radicals follows the same trend:
3o > 2o > 1o > methyl
i.e. 3o free radicals are the most stable and methyl radicals are the least stable
(easiest) (hardest)
Kinetics Many reactions that have favorable energy
changes (G = neg or H = neg) occur so slowly that the reaction is imperceptible. Very slow reaction rate
For the general reaction:a A + b B c C + d D
Rate = k [A]m[B]n
where k = rate constantm = reaction order with respect to An = reaction order with respect to B
Kinetics
The reaction rate depends on: collision frequency a probability or orientation factor activation energy (Ea)
The reaction rate increases as the number of collisions between reacting species increase. Concentration temperature
Cl .
Kinetics
Collisions must occur in a particular orientation for reactions to occur.
For the reaction: Cl. + H - Br H - Cl + Br.
Br H No HCl formed
Cl . BrH HCl can form
Cl . Br
H
No HCl formed
Kinetics
Collisions must occur with a specific minimum amount of energy in order for a reaction to take place.
Activation energy (Ea) the minimum energy the reactants must have for a reaction to occur
the energy difference between the reactants and the transition state
Kinetics
Transition state: a particular arrangement of atoms of
the reacting species in which bonds are partially broken and partially formed
the state of highest energy between reactants and products
a relative maximum on the reaction-energy diagram
Reaction Energy Diagram
Reaction energy diagram: a plot of potential energy changes that
occur as reactants are converted to products
Hammond Postulate
What does the transition state look like? The appearance of the transition state
depends on whether the reaction is endothermic or exothermic.
governed by the Hammond Postulate
Hammond Postulate: Related species that are closer in energy
are also closer in structure. The structure of the transition state resembles the structure of the closest stable species.
Hammond Postulate
For an endothermic reaction, the transition state more closely resembles the products.
For an exothermic reaction, the transition state more closely resembles the reactants.
endothermicexothermic
Rate Determining Step
Chlorination of methane has two propagation steps. The first propagation step controls the
rate of the overall reaction and is called the rate-determining step.
Rate-determining step (rate-limiting step): the slowest step in a multi-step process the step with the highest energy
transition state
Rate Determining Step
intermediate
ClH C
H
H
HClH C
H
H
Cl
