1. Reactions of Alkenes and Alkynes (Part I)
Essential Organic Chemistry
Paula Yurkanis Bruice
Chapter 5
Reactions of Alkenes
and Alkynes (Part I)

2. Electrophilic Addition
Electrophilic addition to the double bond is a
two-step process R + Y Z -
electrophile
nucleophile
First, the  bond reacts with an electrophile.
Second, the resulting carbocation reacts with
a nucleophile

3. Figure: UN
Title:
General Example of Electrophilic Addition to an Alkene
Caption:
This is a general reaction illustrating the electrophilic addition of a YZ compound to an alkene. In this reaction the YZ compound acts like an electrophile and a nucleophile. The electrons of the pi bond are attracted to the electrophile.
Notes:
The pi bond is broken and two new sigma bonds with the electrophile and nucleophile are formed in this reaction.

4. 5.1 Addition of a Hydrogen Halide to an Alkene
Electrophilic reagents we can use are hydrogen halides:
HF, HCl, HBr, HI
They all follow the same reaction scheme.

5. Figure: UN
Title:
Electrophilic Addition to an Unsymmetrical Alkene
Caption:
When the alkene is unsymmetrical, more than one product can possibly be formed. The major product will be the product that produces the more stable intermediate (in this case a carbocation).
Notes:
The 2-chloro-2-methyl propane product is major. It forms a stable tertiary carbocation as an intermediate. This is an example of Markovnikov's rule.

6. Reaction coordinate diagram

7. Electrophilic Addition
Reactions of alkene with identical substituents
on both carbons lead to one product. H H B r B r
cyclopentene
bromo-cyclopentane

8. Electrophilic Addition
In case the alkene has different, or different
number of substituents, two products
are possible. H B r + 1 - b o m e t h y l c p n a A
Is there a preference for one product?
Which is the preferred product?

9. Electrophilic Addition
Experiment shows that (A) is preferred over (B) H B r A B + B r H H B r
The preference for (A) is explained by the mechanism

10. We could obtain two structurally different
Carbocations; however, A is formed much faster. H B r s l o w C 2 + A C H 2 B r - f a s t

11. 5.2 Carbocation Stability
Comparing many reactions with different alkenes leads
to a classification of carbocation intermediates.
The more substituents, the more stable the cation

13. 5.3 Regioselectivity of Electrophilic Addition Reactions

14. Regioselective reactionH I 1 - m e t h y l c o x n i d a 2
Major product
Minor product +
We obtain both products; however, one is the
major product.
We have a regioselective reaction.

15. Constitutional isomers1 - i o d m e t h y l c x a n 2
The molecular formula of both products is identical:
C7H13I
However, the way the atoms are connected is different:
Constitutional isomers

16. Markovnikov’s Rule The regioselectivity of this reaction was first
observed by Markovnikov.
He found that the hydrogen, which is the
electrophile, adds to the carbon that contains
the larger number of hydrogens.

24. 5.4 Addition of Water to an AlkeneC H C H 2 2 H C + H O X C H 3 2 1 - b u t e n e
When adding water to alkenes
NO reaction takes place.

25. Hydration When small amounts of acid are present,2 C H 2 2 2 H C C H + H O H C 3 2 C H 3 1 - b u t e n e 2 - b u t a n o l O H
When small amounts of acid are present,
hydration of the double bond takes place.

26. Hydration- 1st step 1st step is electrophilic attack of H+ H C H C H Cw 2 2 2 2 + H + H C H C C H 3 C H 3
1st step is electrophilic attack of H+

27. Hydration- 2nd step 2nd step is nucleophilic attack of H2O H H C H C H+ C H 2 f a s t 2 H C C H H C C H 3 3 O H H a d d i t i o n o f n u c l e o p h i l e p r o t o n a t e d a l c o h o l
2nd step is nucleophilic attack of H2O

28. Hydration- 3rd step 3rd step is deprotonation to generate the alcohol.+ H + p r o t o n a t e d a l c o h o l
3rd step is deprotonation to generate the alcohol.
H+ is regenerated,
so that only catalytic amounts of acid are necessary.

29. Figure: UN
Title:
Electrophilic Addition of Water to Propene
Caption:
This process is called hydration. It needs an acid catalyst to take place, since water is too weakly acidic to allow the hydrogen to act as an electrophile in the reaction. The acid acts as the electrophile.
Notes:
The water becomes the nucleophile in this reaction. A protonated alcohol is formed, which rapidly loses its proton to form the product.

30. Figure: UN
Title:
Mechanism for the Electrophilic Addition of Water to Propene
Caption:
The acid catalyst acts as the electrophile in this reaction. The water is then the nucleophile. The first step is the rate-determining step of the mechanism.
Notes:
A protonated alcohol is formed, which rapidly loses its proton to form the product.

31. Figure: 05-02
Title:
Reaction Coordinate Diagram
Caption:
A reaction coordinate diagram for the acid-catalyzed addition of water to an alkene.
Notes:
Note the formation of two different intermediates.

34. 5.5 Addition of an Alcohol to an Alkene

35. Addition of an Alcohol to an Alkene
Alcohols react the same way as water C H 2 3 + O - m e t h y l p r o n a
tert b u
Note: the reaction works only in the
presence of acid.

36. Addition of an Alcohol to an Alkene
Initiating the reaction: generating a carbocation H e l e c t r o p h i l i c a t t a c k o f H + + H C H C H 3 C H 3 2 C C H + C H C H O H 2 3 2 H C s l o w C H 3 3
2-methylpropene
carbocation

37. Addition of an Alcohol to an Alkene
The resulting carbocation is attacked by
the nucleophilic alcohol H n u c l e o p h i l i c a t t a c k o f e t h a n o l C H H C 2 H 3 H C C H 3 C H 3 2 H O + C H C H O H 3 2 f a s t H C C H C H
ethanol 2 3 3

38. Addition of an Alcohol to an Alkene
Completion of the reaction:
deprotonation of the “alkoxonium species” C H C H H C 3 H C 3 3 3 C H C H 3 3 H O O + + H H C C H H C C H 2 3 2 3
alkoxonium species
t-butyl-ethyl ether

43. 5.6 Introduction to Alkynes
Alkynes are molecules containing a triple bond: R
The triple bond consists of
one  bond R
sp-hybridized carbon
and two  bonds: py R px

44. Alkynes The sp-hybridization results in a 1800 CCC-bond angle
Consequently, four atoms are arranged linearly. 1 8 H H H C C C C H H H

45. Alkynes Ene-diyne part of Neocarzinostatin isolated fromH 3 C N
Ene-diyne part of
Neocarzinostatin isolated from
Streptomyces carzinostaticus
aponeocarzinostatin

46. Alkynes O C C C C H C Capillin, fungicide isolated from3 c a p i l l i n
Capillin, fungicide isolated from
Artemisia nilagirica Clarke

47. Alkynes Found in Amazon basin frogs and in a number of plants,H O H C H C C C C C C C C 3 O H i c h t h y o t h e r e o l
Found in Amazon basin frogs and in a number of plants,
used by Amazon Indians as arrow poison.

48. CnH2n+2- 4P-2R Molecular Formula Alkyne: P = number of πbonds
R = number of rings.

50. 5.7 Nomenclature of Alkynes
Systematic names of alkynes are derived from alkanes by
replacing the ending –ane with –yne.

51. Alkynes: Nomenclature
What is the name of the following compound ? C H B r 2 C H 3 H C H C C C C H 3 2 2

52. Alkynes: Nomenclature
What is the name of the following compound?
1. Find longest chain: 6 C H B r 1 2 C H 2 3 5 H C H C C C C H 3 2 2 4 3

53. Alkynes: Nomenclature
What is the name of the following compound?
1. Find longest chain.
2. Enumerate such that substituents obtain lowest number. 1 C H B r X 6 6 2 C H 3 2 4 X X 3 5 H C H C C C C H 1 X 3 2 2 3 4 X 5 X 2

54. Alkynes: Nomenclature
What is the name of the following compound?
1. Find longest chain.
2. Enumerate such that substituents obtain lowest number.
3. Determine name of substituents.
Bromo 1 C H B r X 6 6 2 C H 3 2 4 3 5 X X H C H C C C C H 1 X 3 2 2 3 4 X 5 X 2
methyl

55. Alkynes: Nomenclature
What is the name of the following compound?
1. Find longest chain.
2. Enumerate such that substituents obtain lowest number.
3. Determine name of substituents.
4. Name 1 C H B r X 6 6 2 C H 3 2 4 X X 3 5 H C H C C C C H 1 X 3 2 2 3 4 5 X X 2
1-bromo-2-methyl-3-hexyne