1. Reactions of Alkanes & Alkenes
Unit 8

2. Introduction Overall reaction: reactants  products
Mechanism: Step-by-step pathway.
Learn more about a reaction using:
Thermodynamics
Kinetics

3. Reactions and Their Mechanisms
Almost all organic reactions fall into one of four categories:
Substitutions – characteristic reactions of saturated compounds—one group replaces another
Additions—characteristic of compounds with multiple bonds—all parts of the adding reagent appear in the product; two molecules become one
Eliminations—one molecule loses the elements of another small molecule—provide a method for preparing compounds with double and triple bonds
Rearrangements—a molecule undergoes a reorganization of its constituent parts

4. Substitution Reactions

5. Addition Reactions

6. Elimination Reactions

7. Rearrangement

8. Chlorination of Methane
Requires heat or light for initiation.
The most effective wavelength is blue, which is absorbed by chlorine gas.
Many molecules of product are formed from absorption of only one photon of light (chain reaction).

9. The Free-Radical Chain Reaction
Initiation generates a radical intermediate.
Propagation: The intermediate reacts with a stable molecule to produce another reactive intermediate (and a product molecule).
Termination: Side reactions that destroy the reactive intermediate.

10. Initiation Step: Formation of Chlorine Atom
A chlorine molecule splits homolytically into chlorine atoms (free radicals).

11. Propagation Step: Carbon Radical
The chlorine atom collides with a methane molecule and abstracts (removes) an H, forming another free radical and one of the products (HCl).

12. Propagation Step: Product Formation
The methyl free radical collides with another chlorine molecule, producing the organic product (methyl chloride) and regenerating the chlorine radical.

13. Overall Reaction C

14. Termination Steps
A reaction is classified as a termination step when any two free radicals join together producing a nonradical compound.
Combination of free radical with contaminant or collision with wall are also termination steps.

15. Alkene Characteristic Reactions

16. Alkene Characteristic Reactions

17. Reaction Mechanisms
A reaction mechanism describes how a reaction occurs
which bonds are broken and which new ones are formed
the order and relative rates of the various bond- breaking and bond-forming steps
if in solution, the role of the solvent
if there is a catalyst, the role of a catalyst
the position of all atoms and energy of the entire system during the reaction

18. Developing a Reaction Mechanism
How it is done
design experiments to reveal details of a particular chemical reaction
propose a set or sets of steps that might account for the overall transformation
a mechanism becomes established when it is shown to be consistent with every test that can be devised
this does not mean that the mechanism is correct, only that it is the best explanation we are able to devise

19. Why Mechanisms?
they are the framework within which to organize descriptive chemistry
they provide an intellectual satisfaction derived from constructing models that accurately reflect the behavior of chemical systems
they are tools with which to search for new information and new understanding

20. Electrophilic Additions
hydrohalogenation using HCl, HBr, HI
hydration using H2O in the presence of H2SO4
halogenation using Cl2, Br2
halohydrination using HOCl, HOBr
oxymercuration using Hg(OAc)2, H2O followed by reduction

21. Addition of HX
Carried out with pure reagents or in a polar solvent such as acetic acid
It is a two-step mechanism:
The first step is the slow, rate-determining step.
The second step is fast.

22. Addition of HX Addition is regioselective
regioselective reaction: an addition or substitution reaction in which one of two or more possible products is formed in preference to all others that might be formed
Markovnikov’s rule: in the addition of HX, H2O, or ROH to an alkene, H adds to the carbon of the double bond having the greater number of hydrogens

23. Addition of H2O addition of water is called hydration
acid-catalyzed hydration of an alkene is regioselective; hydrogen adds preferentially to the less substituted carbon of the double bond
HOH adds in accordance with Markovnikov’s rule

24. Addition of Cl2 and Br2
carried out with either the pure reagents or in an inert solvent such as CH2Cl2
addition of bromine or chlorine to a cycloalkene gives a trans-dihalocycloalkane
addition occurs with anti stereoselectivity; halogen atoms add from the opposite face of the double bond

25. Addition of HOCl and HOBr
Treatment of an alkene with Br2 or Cl2 in water forms a halohydrin
Halohydrin: a compound containing -OH and -X on adjacent carbons

26. Oxymercuration/Reduction
Oxymercuration followed by reduction results in hydration of a carbon-carbon double bond
oxymercuration
reduction

27. Hydroboration/Oxidation
Hydroboration: the addition of borane, BH3, to an alkene to form a trialkylborane
Borane dimerizes to diborane, B2H6 H C H 2 3 H B + 3 C H 2 = C H 3 2 B H C H 2 3
Borane
Triethylborane
(a trialkylborane) 2 B H 3 B 2 H 6
Borane D
iborane

28. Oxidation/Reduction Oxidation: the loss of electrons
alternatively, the loss of H, the gain of O, or both
Reduction: the gain of electrons
alternatively, the gain of H, the loss of O, or both
Recognize using a balanced half-reaction
1. write a half-reaction showing one reactant and its product(s)
2. complete a material balance; use H2O and H+ in acid solution, use H2O and OH- in basic solution
3. complete a charge balance using electrons, e-

29. Oxidation/Reduction
Three balanced half-reactions

30. Reduction of Alkenes
Most alkenes react with H2 in the presence of a transition metal catalyst to give alkanes
commonly used catalysts are Pt, Pd, Ru, and Ni
the process is called catalytic reduction or, alternatively, catalytic hydrogenation Pd
+ H 2
25°C, 3 atm
Cyclohexene
Cyclohexane

31. Diels–Alder Reaction
Named after Otto Diels and Kurt Alder. They received the Nobel prize in 1950.
Produces a cyclohexene ring.
The reaction is between a diene with an electron-deficient alkene (dienophile).
The Diels-Alder is also called a [4+2] cycloaddition because a ring is formed by the interaction of four pi electrons of the alkene with two pi electrons of the alkene or alkyne.

32. Mechanism of the Diels–Alder Reaction
One-step, concerted mechanism.
A diene reacts with an electron-poor alkene (dienophile) to give cyclohexene or cyclohexadiene rings.

33. Examples of Diels–Alder Reactions