Chapter 6 Reactions of Alkenes: Addition Reactions - PowerPoint PPT Presentation

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Chapter 6 Reactions of Alkenes: Addition Reactions

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Heats of Hydrogenation of Isomers. Ethylene 136. Monosubstituted 125-126 ... most stable isomer. Problem 6.2. 6.3. Stereochemistry of Alkene Hydrogenation ... – PowerPoint PPT presentation

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Title: Chapter 6 Reactions of Alkenes: Addition Reactions


1
Chapter 6Reactions of AlkenesAddition Reactions
2
Reactions of Alkenes
  • The characteristic reaction of alkenes is
    addition to the double bond.

AB
3
6.1Hydrogenation of Alkenes
4
Hydrogenation of Ethylene
s
s
s
p
HH
  • exothermic DH 136 kJ/mol
  • catalyzed by finely divided Pt, Pd, Rh, Ni

5
Example
H2, Pt
(73)
6
Problem 6.1
  • What three alkenes yield 2-methylbutane on
    catalytic hydrogenation?

7
Problem 6.1
  • What three alkenes yield 2-methylbutane on
    catalytic hydrogenation?

H2, Pt
8
Mechanism of Catalytic Hydrogenation Figure 6.1
9
Mechanism of Catalytic Hydrogenation Figure 6.1
B
Y
C
C
A
X
10
Mechanism of Catalytic Hydrogenation Figure 6.1
11
Mechanism of Catalytic Hydrogenation Figure 6.1
B
Y
A
X
C
C
12
Mechanism of Catalytic Hydrogenation Figure 6.1
13
Mechanism of Catalytic Hydrogenation Figure 6.1
14
6.2Heats of Hydrogenation
  • can be used to measure relative stability of
    isomeric alkenes
  • correlation with structure is same as when heats
    of combustion are measured

15
Heats of Hydrogenation of Isomers
126
119
115
CH3CH2CH2CH3
16
Heats of Hydrogenation (kJ/mol)
  • Ethylene 136
  • Monosubstituted 125-126
  • cis-Disubstituted 117-119
  • trans-Disubstituted 114-115
  • Terminally disubstituted 116-117
  • Trisubstituted 112
  • Tetrasubstituted 110

17
Problem 6.2
Match each alkene of Problem 6.1 with its
correctheat of hydrogenation.
18
Problem 6.2
Match each alkene of Problem 6.1 with its
correctheat of hydrogenation.
highest heat ofhydrogenationleast stable isomer
lowest heat ofhydrogenationmost stable isomer
19
6.3Stereochemistry of Alkene Hydrogenation
20
Two spatial (stereochemical) aspects ofalkene
hydrogenation
  • syn addition of both H atoms to double bond
  • hydrogenation is stereoselective, corresponding
    to addition to less crowded face of double bond

21
syn-Additon versus anti-Addition
syn addition
anti addition
22
Example of Syn Addition
CO2CH3
CO2CH3
(100)
23
Stereoselectivity
  • A reaction in which a single starting
    materialcan give two or more stereoisomeric
    productsbut yields one of them in greater
    amounts thanthe other (or even to the exclusion
    of the other)is said to be stereoselective.

24
Example of Stereoselective Reaction
H2, cat
Both productscorrespond tosyn additionof H2.
25
Example of Stereoselective Reaction
H2, cat
But only thisone is formed.
26
Example of Stereoselective Reaction
H2, cat
Top face of doublebond blocked bythis methyl
group
27
Example of Stereoselective Reaction
H2, cat
H2 adds to bottom face of double bond.
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