Practice Questions Practice Questions
51
2/26/18 1 Practice Questions Practice Questions A G B C D E F • How many steps are there in this reaction?
-
Upload
khangminh22 -
Category
Documents
-
view
4 -
download
0
Transcript of Practice Questions Practice Questions
2/26/18
1
Practice Questions
Practice Questions
A
G
B
C
D
E
F
• How many steps are there in this reaction?
2/26/18
2
Practice Questions
A
G
B
C
D
E
F
• What is the highest-energy transitions state?
Practice Questions
A
G
B
C
D
E
F
• What is the highest-energy intermediate?
2/26/18
3
Practice Questions
A
G
B
C
D
E
F
• Is this reaction exergonic or endergonic?
Practice Questions
A
G
B
C
D
E
F
• Which steps determines the reaction’s rate?
2/26/18
4
https://labs.chem.ucsb.edu/zakarian/armen/courses.html
CHEM 109A Organic Chemistry
Chapter 6
1. The Reactions of Alkenes 2. The Stereochemistry of Addition Reactions
summary of reactions
HBr Br
H2O, H2SO4 OH
CH3OH, H2SO4 OCH3
addition of H ⎯ X
addition of H2O
addition of ROH
1. R2B-H 2. H2O2, HO⎺BR2 OH
hydroboration/oxidation
electrophilic addition; H element
2/26/18
5
summary of reactions
electrophilic addition; element element
Br2Br
Br
Br2, H2O OHBr
peroxyacid
RO
O OHO
O3; CH3SCH3
O O
H
H+
halogen addition
bromohydrin formation
epoxidation
ozonolysis
summary of mechanisms
E+ or E δ+
E = electrophiles
many form reactive intermediates in the first step: carbocations now, are these nucleophiles or electrophiles?
E+
E
new bond
products
2/26/18
6
summary of reaction selectivity
1. regioselectivity
HBr BrBr+
which is the major product?
Br2Br
Br+
which is the major product?
BrBr
2. stereoselectivity
summary of reaction selectivity
3. stereospecificity
Br2Br
Br+
racemic mixture of enantiomers
BrBr
Br2
Z
E
Br
Br
meso productstereoisomers
2/26/18
7
The Mechanism of an Electrophilic Addition Reaction
Addition of Hydrogen Halides
2/26/18
8
Which sp2 Carbon Gets the H+?
The Mechanism
Formation of the carbocation is the rate-limiting step.
2/26/18
9
Relative Stabilities of Carbocations
Alkyl groups decrease the concentration of positive charge on the carbon.
Hyperconjugation
2/26/18
10
Why the Difference in Rate?
The more stable carbocation is formed more rapidly.
The Hammond Postulate
The transition state resembles what it is closer to in energy.
exergonic reaction: The transition state resembles the reactants.
endergonic reaction: The transition state resembles the products.
2/26/18
11
Similar Energies = Similar Structures
TheRelativeRatesofCarbocationFormationDeterminetheRelativeAmountsofProducts
2/26/18
12
What Product Is Formed?
TheElectrophileAddstothesp2CarbonBondedtotheMostHydrogens
2/26/18
13
Regioselective Reaction
A regioselective reaction is a reaction that forms more of one constitutional isomer than of another.
The reaction can be: moderately regioselective highly regioselective completely regioselective
Not Regioselective
2/26/18
14
Acid-Catalyzed Addition of Water
A reaction takes place if an acid is added.
Mechanism for the Acid-Catalyzed Addition of Water
2/26/18
15
Acid-Catalyzed Addition of an Alcohol
Mechanism for the Acid-Catalyzed Addition of an Alcohol
2/26/18
16
TheMajorProductisaSurprise
TheMajorProductisaSurprise
2/26/18
17
A Carbocation Rearrangement (a 1,2-hydride shift)
A Carbocation Rearrangement (a 1,2-methyl shift)
2/26/18
18
TheCarbocationDoesNotRearrange(NoImprovementinCarbocationStability)
BH3isanElectrophile
2/26/18
19
Hydroboration–Oxidation
TheElectrophileAddstothesp2CarbonBondedtotheMostHydrogens
2/26/18
20
Mechanism for Hydroboration
AdditionofBH3andAdditionofHBrFollowtheSameRule
The electrophile adds to the sp2 carbon bonded to the most hydrogens.
2/26/18
21
BH3HasThreePotentialHydrideIons,soaDialkylboraneandaTrialkyleboraneCanBeFormed
OnlyOneHydrideIonisNeededforHydroboration
Because of its bulky R groups, R2BH has a stronger preference for the less substituted sp2 carbon.
2/26/18
22
TheMechanismistheSame
HydroborationisFollowedbyOxidation
In the oxidation reaction, OH replaces BR2
It is an oxidation reaction because the number of C-O bonds increases.
2/26/18
23
Mechanism for the Oxidation Reaction
No Carbocation Rearrangement
A carbocation is not formed, so a carbocation rearrangement cannot occur.
2/26/18
24
Addition of Br2 or Cl2
The Mechanism for the Addition of a Halogen
2/26/18
25
A Cyclic Bromonium Ion Intermediate
The carbons atoms connected to the oxygen atom are the most electrophilic atoms.
No Carbocation Rearrangement
A carbocation is not formed, so a carbocation rearrangement cannot occur.
2/26/18
26
Alkenes Do Not Add I2
Formation of Halohydrins
2/26/18
27
Mechanism for Halohydrin Formation
Why Does It Follow the Same Rule?
The electrophile adds to the sp2 carbon bonded to the most hydrogens.
2/26/18
28
More Reactions
Addition of a Peroxyacid
2/26/18
29
AnOxygen–OxygenSingleBondisaWeakBond
MCPBAisaCommonlyUsedPeroxyacid
2/26/18
30
Mechanism for Epoxidation
The mechanism is similar to that for the addition of Br2.
Nomenclature of Epoxides
2/26/18
31
Ozonolysis
Mechanism for Ozonide Formation
2/26/18
32
TheOzonideisConvertedtoKetonesand/orAldehydes
Ketones and Aldehydes
2/26/18
33
What Alkene Formed the Ozonolysis Products?
Ozonolysis
2/26/18
34
A Regioselective Reaction
A Stereoselective Reaction
2/26/18
35
A Stereospecific Reaction
TheProductDoesNotHaveStereoisomers
2/26/18
36
TheProductisaRacemicMixture
Why a Racemic Mixture is Formed
2/26/18
37
Forming a Racemic Mixture
If the reactant does not have an asymmetric center and the reaction forms a product with an asymmetric center,
the product will be a racemic mixture.
Forming a Second Asymmetric Center
If the reactant has an asymmetric center and the reaction forms a product with a new asymmetric center, the product will be a pair of diastereomers.
2/26/18
38
Reactions That Form Two Asymmetric Centers
The stereoisomers obtained as products depend on the mechanism of the reaction.
FourStereoisomersareObtainediftheReactionFormsaCarbocationIntermediate
syn and anti additions
2/26/18
39
The Stereochemistry of Hydrogen Addition
only syn addition
SynAdditiontoaCisIsomerFormsOnlytheErythroStereoisomers
2/26/18
40
IftheSubstituentsaretheSame,theErythroStereoisomerisaMesoCompound
SynAdditiontoaTransIsomerFormsOnlytheThreoStereoisomers
2/26/18
41
SynAdditiontoaCisIsomerFormsOnlytheCisStereoisomers
The products can also be written as mirror images.
IftheSubstituentsaretheSame,TheCisStereoisomerisaMesoCompound
2/26/18
42
CyclicAlkenesareCis,UnlessTheyHaveatLeastEightRingAtoms
The Stereochemistry of Peroxyacid Addition
only syn addition
2/26/18
43
SynAdditiontoaCisIsomerFormsOnlytheCisEnantiomers
The products can also be written as mirror images.
SynAdditiontoaTransIsomerFormsOnlytheTransEnantiomers
The products can also be written as mirror images.
2/26/18
44
IftheSubstituentsaretheSame,theCisStereoisomerisaMesoCompound
The Stereochemistry of Hydroboration–Oxidation
only syn addition
2/26/18
45
The H and OH Add to the Same Side of the Ring
The products can also be written as mirror images.
Stereochemistry of Reactions That Form a Cyclic Bromonium (or Chloronium) Ion Intermediate
only anti addition
2/26/18
46
AntiAdditiontoaCisIsomerFormsOnlytheThreoStereoisomers
AntiAdditiontoaTransIsomerFormsOnlytheErythroStereoisomers
2/26/18
47
IftheSubstituentsaretheSame,theErythroStereoisomerisaMesoCompound
AntiAdditiontoaCisIsomerFormsOnlytheTransStereoisomers
2/26/18
48
CIS-SYN-(ERYTHRO or CIS)
You can change two terms but not one.
TRANS-ANTI-(ERYTHRO or CIS) OK
CIS-ANTI-(THREO or TRANS) OK
CIS-ANTI-(ERYTHRO or CIS) not OK
© 2017 Pearson Education, Inc.
2/26/18
49
AnEnzymeFormsOnlyOneStereoisomer
Only the S stereoisomer is formed.
Enzyme-catalyzed reactions are completely stereoselective.
AnEnzymeCanBlockOneSideoftheReactant
2/26/18
50
AnEnzymeCatalyzestheReactionofOnlyOneStereoisomer
Enzyme-catalyzed reactions are stereospecific.
TheStereospecificityofanEnzyme-CatalyzedReactionAllowsEnantiomerstobeSeparated
2/26/18
51
When A Reacts, B is Synthesized
CompoundsThatCanBeFormedFromanAlkene
