Chapter 51 Reactions of Alkenes and Alkynes. Chapter 5.

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: