1. Chapter 7
Alkenes and Alkynes I: Properties and Synthesis Elimination Reactions of Alkyl Halides

2. Heat of Hydrogenation
The hydrogenation of alkenes is an exothermic reaction.
The relative stabilities of alkenes can be measured using the heat of hydrogenation reactions.
Heats of hydrogenation of three butene isomers:
∆H°≈ -120 kJ mol-1

3. Overall Relative Stabilities of Alkenes
The greater the number of attached alkyl groups (i.e. the more highly substituted the carbon atoms of the double bond), the greater the alkene’s stability.

4. Consider the two alkenes 2-methyl-1-pentene
and 2-methyl-2-pentene and decide which would be
most stable.
2-methyl-1-pentene
2-methyl-2-pentene
disubstituted
trisubstituted
Less stable
More stable

5. Cycloalkenes
The rings of cycloalkenes containing five carbon atoms or fewer exist only in the cis- form.
The introduction of a trans- form into small rings if it were possible, cause greater strain than the cis- forms.
trans-cyclooctane still be stable at room temperature, so it has been isolated.

6. Synthesis of Alkenes via Elimination Reactions
Alkenes are commonly made by Elimination Reactions
Dehydrohalogenation (- HX)
Dehydration (- H2O)
Dehalogenation (- X2)
Synthesis of Alkenes via Hydrogenation Reactions of Alkynes

7. A. Dehydrohalogenation of Alkyl Halides
Dehydrohalogenation is elimination (loss) of HX from an alkyl halide using strong base (KOH)

8. Zaitsev’s Rule: Formation of the Most Substituted Alkene is Favored with a Small Base
Some hydrogen halides can eliminate to give two different alkene products.
Zaitsev’s Rule: when two different alkene products are possible in an elimination, the most highly substituted (most stable) alkene will be the major product.

9. Using Zaitsev’s Rule, predict which would be the major product of the following reaction:
monosubstituted
trisubstituted
Less stable
More stable
According to the Zaitsev’s Rule
The product B would be the major product

10. Acid Catalyzed Dehydration of Alcohols
Recall that elimination is favored over substitution at higher temperatures.
Typical acids used in dehydration are sulfuric acid and phosphoric acid.
The temperature and concentration of acid required to dehydrate depends on the structure of the alcohol.
Primary alcohols are the most difficult to dehydrate.

11. More easier to dehydrates
Secondary alcohols usually dehydrate under milder conditions.
Tertiary alcohols are usually so easily dehydrated that extremely mild conditions.
The relative ease with which alcohols undergo dehydration is in the following order:
More easier to dehydrates

12. Mechanism for Dehydration of Secondary 2° and Tertiary 3° Alcohols: An E1 Reaction
E1 mechanism is a stepwise need acidic catalytst
nitializing step
formation of
Protonated alcohol
(Oxonium ion)
Step 1
formation of
Carbonium ion C+
(intermediate)
Step 2
formation of
Alkene (product)
and acid

13. Energy diagram of E1 mechanism:
A two-step reaction with one intermediate

14. decreasing of stability
Carbocation Stability and the Transition State
Recall the stability order of carbocations is:
The second step of the E1 mechanism in which the carbocation forms is rate determining.
The transition state for this reaction has carbocation character.
Tertiary alcohols react the fastest because they have the most stable tertiary carbocation-like transition state in the second step.
decreasing of stability

15. In E1 mechanism the intermediate for 2° or 3° carbocation is formed.
The relative heights of DG‡ for the second step of E1 dehydration indicate that primary alcohols have a prohibitively large energy barrier.
In E1 mechanism the intermediate for 2° or 3° carbocation is formed.
The 3° carbocation intermediate has the lowest energy of activation than the 2° one.
1° C+
less stable
3° C+
most stable

16. Transition state which give an Alkene
Mechanism for Dehydration of Primary Alcohols 1° :
An E2 Reaction
Primary alcohols cannot undergo E1 dehydration because of the instability of the carbocation-like transition state in the 2nd step.
In the E2 dehydration the first step is again protonation of the hydroxyl to yield the good leaving group water.
initializing step
formation of
Protonated alcohol
Step 1
formation of
Transition state which give an Alkene

17. Energy diagram of E2 mechanism:
a one-step reaction with no intermediate

18. Dehalogenation of vicinal dihalides
| | | |
— C — C — Zn  — C = C — ZnX2
| |
X X
eg.
CH3CH2CHCH Zn  CH3CH2CH=CH ZnBr2
Br Br

19. Arrange the following alcohols in order of their reactivity toward acid-catalyzed dehydration (start with the most reactive) 3
1-Pentanol
1° alcohol
2-methyl-2-butanol 1
3° alcohol
3-methyl-2-butanol
2° alcohol 2

20. Synthesis of Alkenes via Hydrogenation Reactions of Alkynes

21. Synthesis of alkyne By the reaction of calcium carbide with water
+ 2 H+ OH-

22. Synthesis of Alkynes via Elimination Reactions
Dehalogenation of vicinal dihalides
Alkynes can be prepared by elimination reactions of halogen from a vicinal dihalide in the presence of strong base such as sodium amide (NaNH2).
step1
formation of
Alkene via E2
Step2
formation of
Alkyne via E2

23. Dehydrohalogenation of Vicinal Dihalides
Alkenes can be converted to alkynes by bromination and two consecutive dehydrohalogenation reactions.
- 2HBr

24. Dehydrohalogenation of Geminal Dihalides
Geminal dihalides can also undergo consecutive dehydrohalogenation reactions to yield the alkyne
Show how you might synthesize ethynylbenzene from
methyl phenyl ketone

25. Outline all steps in a synthesis of propyne from each of the following:
a) CH3COCH3
b) CH3CH2CHBr2
c) CH3CHBrCH2Br
d) CH3CH=CH2

26. The Acidity of Terminal Alkynes
Recall that acetylenic hydrogens are much more acidic than most other C-H bonds.
The higher the percent of s-character of the hybrid orbital, the closer the lone pair is held to the nucleus, and the more stable the conjugate base.
The relative acidity of acetylenic hydrogens in solution is:

27. Bond length (pm) 27 27

28. 7.29 p.324 without consulting tables, arrange the following compounds in order of decreasing acidity:
Pentane 4
1-pentene 3
1-pentyne 2
1-pentanol 1

29. The products are called alkynides
Acetylenic hydrogens can be deprotonated with relatively strong bases (sodium amide is typical)
The products are called alkynides 29

30. Replacement of the Acetylenic Hydrogen Atom of Terminal Alkynes
Sodium alkynides can be used as nucleophiles in SN2 reactions
New carbon-carbon bonds are the result
Only primary alkyl halides can be used or else elimination reactions predominate

31. Outline a synthesis of 4-phenyl-2-butyne
from 1-propyne

32. Your goal is to synthesize 4,4-dimethyl-2-pentyne
Your goal is to synthesize 4,4-dimethyl-2-pentyne. You have a choice of beginning with any of the following:

33. 7.31 p.324 Outline the syntheses of Propene from each of the following: