REACTION MECHANISMS

Reaction Equation

describes what happens in a chemical reaction

Reaction Mechanism

a detailed step-by-step description of how a chemical reaction occurs

Reaction Mechanism

sequence of bond-making and bond-breaking steps

Reaction Mechanism

involve the movement of electrons

Reaction Mechanism

provides a rationalization for chemical reactions

Reaction Mechanism

allows one to predict the likely outcome of a reaction

Heterolytic cleavage

asymmetrical cleavage of a bond (Ionic reactions)

Ionic reactions

Heterolytic cleavage – asymmetrical cleavage of a bond

Radical reactions

Homolytic cleavage – symmetrical cleavage of a bond

Radical reactions

Radical – high-energy species carrying an unpaired electron

Radical

– high-energy species carrying an unpaired electron (Radical reactions)

Homolytic cleavage

- symmetrical cleavage of a bond (Radical reactions)

heterolytic cleavage

In _, one atom (B in this case) takes both electrons from the broken bond. This creates a positively charged cation (A+) and a negatively charged anion (B-).

homolytic cleavage

In _, each atom takes one electron from the broken bond. This creates two radicals, which are species with unpaired electrons.

Nucleophiles

(Nu− or Nu:)

Electrophiles

(E+

Nucleophiles

electron-rich, nucleus-seeking reagents

Nucleophiles

typically have a negative charge (anions), lone pair, or multiple bonds

Electrophiles

electron-deficient, electron-seeking reagents

Electrophiles

typically have a positive charge (cations), or are polarizable molecules that

can develop an electron-deficient center

LEAVING GROUP (L−, L:)

ions or neutral molecules that are displaced from a reactant as part of

a mechanistic sequence

LEAVING GROUP (L−, L:)

displacement when a nucleophile attacks an electrophile that carries

a suitable leaving group (C-L)

rate-determining step

the slowest transformation in the sequence

rate of reaction

Rate = k [A] [B]

Rate

_ = k [A] [B]

rate of reaction:

dependence of the reaction rate on the concentration of reagents and

other variables indicates the number and nature of the molecules

involved in the rate-determining step

molecularity

number of reactant molecules involved in the rate-

determining step; usually equivalent to the kinetic reaction order

first

k[A] Reaction order:

A →

k[A] Probable reaction:

unimolecular

k[A] Molecularity:

second

k[A][B] Reaction order:

A + B →

k[A][B] Probable reaction:

bimolecular

k[A][B] Molecularity:

second

k[A]² Reaction order:

A + A →

k[A]² Probable reaction:

bimolecular

k[A]² Molecularity:

Transition State

cannot be isolated, or even

detected; energy maximum

Intermediate

stable and can be isolated;

energy minimum

• Substitution

• Elimination

• Addition

• Rearrangement

• Radical Reactions

TYPES OF ORGANIC

REACTIONS 5

reaction (one step)

reaction (several steps)

equilibrium

equilibrium (right-hand product favoured)

transformation in either direction (but not equilibrium)

resonance

curly arrow - movement of two electrons

curly arrow - movement of one electron

reaction with a converts A into B

reaction with a in the presence of b converts A into B

reaction with a in suitable solvent converts A into B

reaction with a at t °C, for h hours converts A into B

reaction with a first, then with b converts A into B

reagent a achieves conversion A → B, reagent b achieves conversion B → A