ORGANIC REACTIONS OVERVIEWDr. ClowerCHEM 2411Spring 2014

McMurry (8th ed.) sections 6.1, 6.2, 6.4-6, 6.8-10, 7.10, 10.8

Organic Reactions• Types of Reactions:

• Addition• Elimination• Substitution• Rearrangement• Oxidation• Reduction

• See handout

Reaction Mechanisms• The details of how reactions occur

• Bonds broken• Bonds formed• Electron rearrangement• Order of steps• Kinetics (rate)• Thermodynamics (energy)• Role of solvent, catalysts, etc.

Bond Breaking• Symmetrical/radical/homolytic

• One electron to each atom• Fishhook arrow• Result in formation of free radicals

• Unsymmetrical/polar/heterolytic• Both electrons to one atom• Regular curved arrow• Electrons move to more electronegative atom

Bond Formation• Symmetrical/radical/homogenic

• One electron from each atom

• Unsymmetrical/polar/heterogenic• Both electrons from one atom

• What is the nucleophile? What is the electrophile?

Nucleophiles and Electrophiles• Nucleophile

• Electron pair donor• Contain lone pair or p e-

• Electrophile• Electron pair acceptor• Positive or partial positive charge

• Remember electrons always move from nucleophile to electrophile

Nucleophile or Electrophile?

Br CH3 C CH3

CH3

C

O

CH3 CH3

Drawing Mechanisms• Curved arrows• Some guidelines:1. Electrons move from nucleophile to electrophile2. Nucleophile is negative or neutral (Nu: or Nu:-)3. Electrophile is positive or neutral (E or E+)4. Obey the octet rule

• See Mechanisms worksheet

Energy Diagrams• Change in energy as reaction proceeds• A one-step reaction: Label:

• Axes• Starting material• Product• Transition state• DG/DH• DGǂ/Ea

• Where does bond breaking occur?

• Where does bond making occur?

• How do you know if the reaction is endothermic or exothermic?

Transition State• One transition state per step• Highest energy species in the step• Unstable; cannot be isolated• Resembles species (starting material or product) that is

closest in energy• Hammond’s postulate• In an endothermic step the TS resembles the product• In an exothermic step the TS resembles the reactant/starting material

Activation Energy• DGǂ or Ea

• Energy difference between starting material and transition state

• Minimum energy needed for reation to occur• High activation energy = slow reaction

• Rate-determining step (RDS)• The slowest step• The step with the largest activation energy

Energy Diagrams• A two-step reaction:

Label:• Axes• Starting material• Product• Transition states• DG/DH• DGǂ/Ea for each

step• Intermediate

Intermediate• Energy minimum between two transition states• Higher energy than starting material or product• Usually cannot isolate (unstable)• Types of intermediates:

1. Free radicals

2. Carbocations

H C H

H

R C H

H

R C H

R

R C R

R

methyl primary secondary tertiary

H C H

H

R C H

H

R C H

R

R C R

R

methyl primary secondary tertiary

Intermediates• Which carbocation is most stable? Least stable?

• Why?1. Inductive effect

• Donation of electrons through bonds (R groups)2. Hyperconjugation

• Donation of electrons through orbitals

• Other stable carbocations are resonance-stabilized

H C H

H

R C H

H

R C H

R

R C R

R

methyl primary secondary tertiary

C C

CC

allylic benzylic

An Example Reaction• HBr + ethylene → bromoethane

• What type of reaction is this?• What is the nucleophile? Electrophile? Look at structure:

• Ethylene C=C has high electron density (4 e-); relatively easy to break p bond (weaker than s bond)

• HBr is a strong acid (H+ donor) with partial positive charge on H

• Electrons are donated from p bond of ethylene to H of HBr• Sigma bond of ethylene is not broken

Mechanism• Two steps• Step 1:

• Step 2:

Mechanism • The mechanism can be written as one scheme:

Energy Diagram

Label:• Axes• Starting material• Intermediate• Product• DG/DH• DGǂ/Ea for each

step

Radical Reactions• Homolytic reactions• Not as common as polar reactions (heterolytic)• Mechanisms involve three steps

1. Initiation: start of the reaction; usually catalyzed by something2. Propagation: continuation of the reaction; there can be many of

these steps3. Termination: end of the reaction

• An example reaction: chlorination of methane

• What type of reaction is this?

Chlorination of Methane• Initiation

• Caused by irradiation with UV light• Break s bond to create reactive radicals

Chlorination of Methane• Propagation

• Chlorine radical reacts with methane to create methyl radical• Methyl radical reacts with Cl2 to give product and more Cl radical• New Cl radical repeats this propagation process (a chain reaction)

Chlorination of Methane• Termination

• Two radicals collide to form stable product• Break the reaction cycle

Radical Halogenation• Used to synthesize alkyl halides from alkanes• One of only two alkane/cycloalkane reactions

1. Radical halogenation2. Combustion (alkanes as fuel)

• Requires heat (Δ) or light (hn) to initiate radical formation• Chlorination (Cl2) or bromination (Br2)

• Iodine is too endothermic; fluorine is too reactive• Typically results in mixtures of products

Halogenation of Alkanes• Ex: ethane

• Ex: butane

• Why is this? Consider the intermediate structure…

CH3 CH3 + Br2hn

CH3 CH2 + H-Br

Br

Halogenation of Alkanes• Substitution is favored at more substituted carbons

• Tertiary > secondary > primary • The tertiary radical is more stable than the secondary radical• Regiochemistry

Stereochemistry of Halogenation• If the product contains a stereocenter, what is the

stereochemistry?

• This reaction will produce a racemic mixture. Why?• Look at radical intermediate: CH3─CH─CH2─CH3

CH3 CH2 + Br2hn

CH3 CH + H-Br

Br

CH2 CH3 CH2 CH3*

C CH2CH3CH3

Hplanar; reaction can occur on either face

Br

CCH2CH3CH3

H

Br

CCH2CH3

HCH3

R enantiomer

S enantiomer

Draw products for the following reactions:

a)

b)

c)

Br2

hnBr2

hnBr2