Chemistry 241
Chapter 4: The Reactions of Alkenes
Homework: 37, 38a,c, 40a, 42, 44, 45, 46, 47, 48, 50
I.
Addition reaction revisited
A. Why it happens
1. electrophile (H δ+) and nucleophile (π bond) attraction
B. Mechanism
1. 2 steps
H Cl
a.
Cl+
H
b.
2. reaction diagram
C. Non-symmetrical alkene
1.
+
H Cl
Cl
+
?
Cl
2. Why? Back to mechanism
a. cation formation is slowest step (rds)
+
more stable/
forms faster than
+
b.
D. Carbocation stability
1. What is 1º, 2º, 3º cation?
a. much like amines → how many alkyl substituents
2. relative stability
a. methyl < 1º < 2º < 3º
3. why? (hyperconjugation)
a. overlap of empty p orbital on cation w/ neighbor sp3 orbital
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E. Transition state (ts)
1. ts is in between reactant and product (or intermediate)
a. top of curve in reaction diagram
b. is ts closer to reactant or product
2. Hammond postulate
a. ts state is closer in structure to whichever is closer in energy
b. since ts highest in energy, closer to reactant or product, which
ever of two is higher in energy
i. closer to reactant in one step exergonic
ii. closer to product in one step endergonic
iii. since carbocation intermediate is high in energy, ts
closer to carbocation
c. therefore, more stable carbocation determines more stable ts
d. if both possible carbocations equally stable (both 2º, e.g.), mix
of products possible
F. Markovnikov’s rule
Cl
HCl
1.
2.
3.
4.
5.
6.
not
Cl
one constitutional isomer favored over another = regioselective
selectivity could be 100% (1º vs 3º)
or just major product and minor product (2º vs 3º)
or no preference at all (2º vs 2º)
Markovnikov’s rule
a. the more stable cation determines product
b. restated: the side that has more H’s gets the H
examples:
HBr
HBr
HBr
HBr
II.
Synthesis strategy
A. How to plan for a product
2
HCl
?
Cl
1.
Cl
HCl
?
2.
3. use regioselectivity to find best reactant for your product
examples:
HCl
HCl
III.
Cl
Cl
Other acid catalyzed addition reactions
A. alkene + HOH → no rxn
1. but add acid catalyst and create much better electrophile
H+ (H2SO4)
OH
+ HOH
2.
B. Mechanism (3 steps)
1. CANNOT use OH- as base in acidic solution
C. Alcohol addition and mechanism
3
OH
1.
H2SO4
Example:
Draw mechanism for rxn of 1-butene with methanol in sulfuric acid
IV.
Carbocation rearrangement
A. Weird rxns
HBr
HBr
1.
2.
3.
4.
5.
1,2 Hydride shift
1,2 Methyl shift
shift makes carbocation intermediate more stable (and ts)
no 1,3 shift ever observed
shift only makes carbocation more stable
a. 1º → 3º
b. 1º → 2º
c. 2º → 3º
d. not 2º → 2º
6. ring expansion
4
HBr
a.
b. 3º better than 2º cation AND 5 member ring better than 4
example:
V.
Addition of halogen to alkene
Br2
A.
1. vicinal dibromide or dichloride
B. Mechanism
1. Bridged bromonium ion
2. Iodine and fluorine not used
C. Format for rxns in organic chemistry
other reactants
catalyst
organic reactant
organic product
catalyst
solvent
rxn conditions (heat, etc)
D. If water is solvent instead of methylene chloride (CH2Cl2)
1. methylene chloride inert
2. if water used, water is also nucleophile
a. not as good as Br-, but a lot greater concentration
3. Halohydrin is result
4. Mechanism
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5. Other nucleophiles
VI.
Oxy-mercuration reduction
A. Reaction
1. Hg(OAc)2, H2O, THF
2. NaBH4
1. Downside: mercury not very green
2. Upside: no carbocation rearrangement, no acid catalyst
3. mechanism p180 (not required)
B. Can also be done in alcohol
1. Hg(O2CCF3)2, methanol
2. NaBH4
VII.
Peroxy acid and alkene
O
R
A.
O
O
H
+
peroxy acid
epoxide
1. mechanism p182 (not required)
a. concerted (several things happen in one step)
2. Nomenclature of epoxides
a. common
b. IUPAC
3. Purpose: great oxidizers! More in later chapters
VIII. Anti-Markovnikov addition
A. Hydroboration Oxidation
1. BH3/THF
2. OH-. H2O2, HOH
1. mechanism p187 (not required)
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2. great way for anti-Markovnikov addition of alcohol
IX.
Hydrogenation of alkenes
A. Add H to both sides of alkene with metal catalyst
H2
Pt/C or Pd/C or Ni
B. Can be used to rank stability of alkenes
>
>
>
=
>
>
Objectives
Knowledge
Define new terms: regioselectivity, Hammond postulate, electrophile and nucleophile,
electrophilic addition reaction
Remember stability order for cations and alkenes (more substituted = more stable)
Remember Markovnikov’s rule for addition reactions
Comprehension
Understand why Markovnikov’s rule is true for those rxns where you have to do the
mechanism
Use Hammond’s postulate and carbocation stability to explain Markovnikov’s rule
Application
Given any two of reactant, reaction conditions, and product predict the third for all
reactions on p194 and 195
Name simple epoxides W2008
Analysis
Draw mechanisms for addition of hydrogen halides, water, and alcohols, vicinal
halogenation, and hydrohalogenation reactions
Be able to predict hydride and methyl shifts in mechanisms where products can only be
explained by these shifts
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