1. The Reactions of Alkenes and Alkynes University of California,
Chapter 6
The Reactions of Alkenes and Alkynes
The Stereochemistry of Addition Reactions
Paula Yurkanis Bruice
University of California,
Santa Barbara

2. Contents of Chapter 6 Electrophilic Addition Reactions Carbocations
Skip Carbocation Rearrangements
Various 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 Stability
Formation 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 Initiating the reaction: generating a carbocationp h i l i c a t t a c k o f H + H C H + H C 3 C H 3 2 C C H + C H C H O H 2 3 2 H C s l o w C H 3 3
2-methylpropene
carbocation
Chapter 5

10. Addition of Alcohol The resulting carbocation is attacked by
the nucleophilic alcohol H n u c l e o p h i l i c a t t a c k o f e t h a n o l C H H C 2 H 3 H C C H 3 C H 3 2 H O + C H C H O H 3 2 f a s t H C C H C H
ethanol 2 3 3
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
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. Introduction to Multistep Synthesis
The thought process is known as retrosynthetic analysis and is indicated by using open arrows
Chapter 5

25. Introduction to Multistep Synthesis - Considerations
Alkene products made from alkynes
Carbonyl products made from alkynes
Alkane products made from alkenes
Understand issues involved in proper choice of reagents thoroughly
Chapter 5

26. Introduction to Multistep Synthesis – Reagent Issues
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 + H-C=C-R rxn works best with primary halide
Chapter 5

27. Introduction to Multistep Synthesis – Practice
Use retrosynthetic analysis to make these:
Chapter 5