chapter 51 reactions of alkenes and alkynes. chapter 5
TRANSCRIPT
Chapter 5 1
Reactions of Alkenes and Alkynes. Reactions of Alkenes and Alkynes.
Chapter 5Chapter 5
Chapter 5 2
Contents of Chapter 4
Electrophilic Addition ReactionsElectrophilic Addition Reactions CarbocationsCarbocations Various Addition ReactionsVarious Addition Reactions
Chapter 5 3
Mechanism for Electrophilic Addition to Alkenes
Reaction of 2-butene with hydrogen bromide is typical of electrophilic addition to alkenes
The reaction starts with the slow addition of an electrophile to an sp2 carbon, resulting in formation of a carbocation
The next step is the rapid addition of a nucleophile to the other sp2 carbon
Chapter 5 4
Addition of Hydrogen Halides to Alkenes
The more substituted carbocation is preferred
Chapter 5 5
Stability of Carbocations
Alkyl groups (“R”s) tend to stabilize the positive charge on the sp2 carbon of a carbocation
Chapter 5 6
Transition State StabilityFormation of a tertiary carbocation should be faster than formation of a primary carbocation
Chapter 5 7
Markovnikov’s Rule Modern equivalent statement: The electrophile adds to the sp2 carbon
that forms the least stable carbocation The other carbon forms the carbocation
intermediate. Predicting product: put + and – signs
under carbons and electrophile and nucleophile and swap partners.
Chapter 5 8
Addition of Water
Chapter 5 9
Addition of Alcohol
C CH2
H3C
H3C+ CH3CH2OH
H+
slow
H3C
CH3
CH2
Helectrophilic attack of H+
Initiating the reaction: generating a carbocation
carbocation2-methylpropene
Chapter 5 10
Addition of Alcohol
+ CH3CH2OH
H3CCH3
CH2
O
H2C CH3
H
H
fast
H3C
CH3
CH2
H
nucleophilic attack of ethanol
The resulting carbocation is attacked by the nucleophilic alcohol
ethanol
Chapter 5 11
Product Analysis
Chapter 5 12
Alkynes
Common names of alkynes are based on substitution of the simplest alkyne, acetylene
Chapter 5 13
IUPAC Nomenclature of Alkynes
• Find the longest chain containing the triple bond and change the corresponding “-ane” ending to “-yne”
• The chain is numbered in direction that gives the triple bond, the lower number
• If the same number for the triple bond is obtained in numbering from both directions, the number for the substituent nearest the chain end is minimized
Chapter 5 14
IUPAC Multifunctional Compound Nomenclature
Chapter 5 15
IUPAC Multifunctional Compound Nomenclature
• The longest chain has to go past the highest-priority functional group
• High prio group has lowest possible number• If not highest priority NH2 is amino and OH is hydroxy
substituent• General form is n-substit-n-alken-n-yn-n-groupsuffix
Chapter 5 16
Reactivity Considerations
The hydrohalogenation product is an alkene which can undergo a second electrophilic addition reaction
First halogen follows alkene hydrohalogenation regioselectivity rules
Second halogen goes on same carbon as first halogen
Chapter 5 17
Relative Stabilities of Carbocations
Vinyl cations are one level less stable than alkyl cations
Chapter 5 18
Halogen Addition to Alkynes Halogens add to alkynes as well as to
alkenes Excess halogen leads to the addition of a
second equivalent
CH3CH2C CCH3
Cl2CH3CH2C CCH3
Cl
Cl
Cl2
CH2Cl2 CH2Cl2CH3CH2C CCH3
Cl Cl
Cl Cl
CH3C CH
Br2
CH3C CH
Br
Br
Br2
CH2Cl2 CH2Cl2
CH3C CH
Br
Br
Br
Br
Chapter 5 19
Addition of Water to Alkynes
Water adds to alkynes in the presence of acid to yield an enol
Chapter 5 20
Addition of Water to Alkynes
However the initially formed enol reacts further to produce a ketone
Such isomers, differing only in the placement of a hydrogen atom, are called tautomers
Chapter 5 21
Addition of Hydrogen to an Alkyne
Chapter 5 22
Acidity of a Hydrogen Bonded to an sp Carbon
The conjugate bases have the following relative base strength because the stronger the acid, the weaker the conjugate base
Chapter 5 23
Syntheses Using Acetylide Ions
Alkylation reactions work best with primary alkyl halides and methyl halides
Chapter 5 24
The thought process is known as retrosynthetic analysis and is indicated by using open arrows
Introduction to Multistep Introduction to Multistep SynthesisSynthesis
Chapter 5 25
Alkene products made from alkynes Carbonyl products made from alkynes Alkane products made from alkenes Understand issues involved in proper
choice of reagents thoroughly
Introduction to Multistep Introduction to Multistep Synthesis - ConsiderationsSynthesis - Considerations
Chapter 5 26
Regioselectivity of HX + alkene/alkyne rxn Regioselectivity of 2 HX + alkyne rxn Stereospecificity of H2/Lindlar reduction H2 with Pd/C, Pt/C, or Ni does complete reduction RX + NaNH2 + HC=CR rxn works best with primary
halide
Introduction to Multistep Introduction to Multistep Synthesis – Reagent IssuesSynthesis – Reagent Issues
Chapter 5 27
Introduction to Multistep Introduction to Multistep Synthesis – PracticeSynthesis – PracticeUse retrosynthetic analysis to make these: