Reactions of Alkenes and Alkynes (Part I) Essential Organic Chemistry Paula Yurkanis Bruice Chapter 5 Reactions of Alkenes and Alkynes (Part I)

Electrophilic Addition Electrophilic addition to the double bond is a two-step process R + Y Z - electrophile nucleophile First, the  bond reacts with an electrophile. Second, the resulting carbocation reacts with a nucleophile

Figure: 05-00-03UN Title: General Example of Electrophilic Addition to an Alkene Caption: This is a general reaction illustrating the electrophilic addition of a YZ compound to an alkene. In this reaction the YZ compound acts like an electrophile and a nucleophile. The electrons of the pi bond are attracted to the electrophile. Notes: The pi bond is broken and two new sigma bonds with the electrophile and nucleophile are formed in this reaction.

5.1 Addition of a Hydrogen Halide to an Alkene Electrophilic reagents we can use are hydrogen halides: HF, HCl, HBr, HI They all follow the same reaction scheme.

Figure: 05-00-05UN Title: Electrophilic Addition to an Unsymmetrical Alkene Caption: When the alkene is unsymmetrical, more than one product can possibly be formed. The major product will be the product that produces the more stable intermediate (in this case a carbocation). Notes: The 2-chloro-2-methyl propane product is major. It forms a stable tertiary carbocation as an intermediate. This is an example of Markovnikov's rule.

Reaction coordinate diagram

Electrophilic Addition Reactions of alkene with identical substituents on both carbons lead to one product. H H B r B r cyclopentene bromo-cyclopentane

Electrophilic Addition In case the alkene has different, or different number of substituents, two products are possible. H B r + 1 - b o m e t h y l c p n a A Is there a preference for one product? Which is the preferred product?

Electrophilic Addition Experiment shows that (A) is preferred over (B) H B r A B + B r H H B r The preference for (A) is explained by the mechanism

We could obtain two structurally different Carbocations; however, A is formed much faster. H B r s l o w C 2 + A C H 2 B r - f a s t

5.2 Carbocation Stability Comparing many reactions with different alkenes leads to a classification of carbocation intermediates. The more substituents, the more stable the cation

5.3 Regioselectivity of Electrophilic Addition Reactions

Regioselective reaction H I 1 - m e t h y l c o x n i d a 2 Major product Minor product + We obtain both products; however, one is the major product. We have a regioselective reaction.

Constitutional isomers 1 - i o d m e t h y l c x a n 2 The molecular formula of both products is identical: C7H13I However, the way the atoms are connected is different: Constitutional isomers

Markovnikov’s Rule The regioselectivity of this reaction was first observed by Markovnikov. He found that the hydrogen, which is the electrophile, adds to the carbon that contains the larger number of hydrogens.

5.4 Addition of Water to an Alkene C H C H 2 2 H C + H O X C H 3 2 1 - b u t e n e When adding water to alkenes NO reaction takes place.

Hydration When small amounts of acid are present, 2 C H 2 2 2 H C C H + H O H C 3 2 C H 3 1 - b u t e n e 2 - b u t a n o l O H When small amounts of acid are present, hydration of the double bond takes place.

Hydration- 1st step 1st step is electrophilic attack of H+ H C H C H C w 2 2 2 2 + H + H C H C C H 3 C H 3 1st step is electrophilic attack of H+

Hydration- 2nd step 2nd step is nucleophilic attack of H2O H H C H C H + C H 2 f a s t 2 H C C H H C C H 3 3 O H H a d d i t i o n o f n u c l e o p h i l e p r o t o n a t e d a l c o h o l 2nd step is nucleophilic attack of H2O

Hydration- 3rd step 3rd step is deprotonation to generate the alcohol. + H + p r o t o n a t e d a l c o h o l 3rd step is deprotonation to generate the alcohol. H+ is regenerated, so that only catalytic amounts of acid are necessary.

Figure: 05-01-23UN Title: Electrophilic Addition of Water to Propene Caption: This process is called hydration. It needs an acid catalyst to take place, since water is too weakly acidic to allow the hydrogen to act as an electrophile in the reaction. The acid acts as the electrophile. Notes: The water becomes the nucleophile in this reaction. A protonated alcohol is formed, which rapidly loses its proton to form the product.

Figure: 05-01-24UN Title: Mechanism for the Electrophilic Addition of Water to Propene Caption: The acid catalyst acts as the electrophile in this reaction. The water is then the nucleophile. The first step is the rate-determining step of the mechanism. Notes: A protonated alcohol is formed, which rapidly loses its proton to form the product.

Figure: 05-02 Title: Reaction Coordinate Diagram Caption: A reaction coordinate diagram for the acid-catalyzed addition of water to an alkene. Notes: Note the formation of two different intermediates.

5.5 Addition of an Alcohol to an Alkene

Addition of an Alcohol to an Alkene Alcohols react the same way as water C H 2 3 + O - m e t h y l p r o n a tert b u Note: the reaction works only in the presence of acid.

Addition of an Alcohol to an Alkene Initiating the reaction: generating a carbocation H e l e c t r o p h i l i c a t t a c k o f H + + H C H C H 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

Addition of an Alcohol to an Alkene 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

Addition of an Alcohol to an Alkene Completion of the reaction: deprotonation of the “alkoxonium species” C H C H H C 3 H C 3 3 3 C H C H 3 3 H O O + + H H C C H H C C H 2 3 2 3 alkoxonium species t-butyl-ethyl ether

5.6 Introduction to Alkynes Alkynes are molecules containing a triple bond: R The triple bond consists of one  bond R sp-hybridized carbon and two  bonds: py R px

Alkynes The sp-hybridization results in a 1800 CCC-bond angle Consequently, four atoms are arranged linearly. 1 8 H H H C C C C H H H

Alkynes Ene-diyne part of Neocarzinostatin isolated from H 3 C N Ene-diyne part of Neocarzinostatin isolated from Streptomyces carzinostaticus aponeocarzinostatin

Alkynes O C C C C H C Capillin, fungicide isolated from 3 c a p i l l i n Capillin, fungicide isolated from Artemisia nilagirica Clarke

Alkynes Found in Amazon basin frogs and in a number of plants, H O H C H C C C C C C C C 3 O H i c h t h y o t h e r e o l Found in Amazon basin frogs and in a number of plants, used by Amazon Indians as arrow poison.

CnH2n+2- 4P-2R Molecular Formula Alkyne: P = number of πbonds R = number of rings.

5.7 Nomenclature of Alkynes Systematic names of alkynes are derived from alkanes by replacing the ending –ane with –yne.

Alkynes: Nomenclature What is the name of the following compound ? C H B r 2 C H 3 H C H C C C C H 3 2 2

Alkynes: Nomenclature What is the name of the following compound? 1. Find longest chain: 6 C H B r 1 2 C H 2 3 5 H C H C C C C H 3 2 2 4 3

Alkynes: Nomenclature What is the name of the following compound? 1. Find longest chain. 2. Enumerate such that substituents obtain lowest number. 1 C H B r X 6 6 2 C H 3 2 4 X X 3 5 H C H C C C C H 1 X 3 2 2 3 4 X 5 X 2

Alkynes: Nomenclature What is the name of the following compound? 1. Find longest chain. 2. Enumerate such that substituents obtain lowest number. 3. Determine name of substituents. Bromo 1 C H B r X 6 6 2 C H 3 2 4 3 5 X X H C H C C C C H 1 X 3 2 2 3 4 X 5 X 2 methyl

Alkynes: Nomenclature What is the name of the following compound? 1. Find longest chain. 2. Enumerate such that substituents obtain lowest number. 3. Determine name of substituents. 4. Name 1 C H B r X 6 6 2 C H 3 2 4 X X 3 5 H C H C C C C H 1 X 3 2 2 3 4 5 X X 2 1-bromo-2-methyl-3-hexyne