© 2014 by John Wiley & Sons, Inc. All rights reserved. Chapter 8 Alkenes and Alkynes II: Addition Reactions

1.Addition Reactions of Alkenes © 2014 by John Wiley & Sons, Inc. All rights reserved.

Classification of reagents and alkenes by symmetry with regard to addition reaction

Carbocation

Hydrogen halides, for example, react with alkenes by donating a proton to the  bond. The proton uses the two electrons of the  bond to form a  bond to one of the carbon atoms. This leaves a vacant p orbital and a +ve charge on the other carbon. The overall result is the formation of a carbocation and a halide ion from the alkene and HX:

Being highly reactive, the carbocation then combines with the halide ion by accepting one of its electron pairs:

Electrophiles Are Lewis Acids Electrophiles are molecules or ions that can accept an electron pair. Nucleophiles are molecules or ions that can furnish an electron pair (i.e. Lewis Bases).

ADDITION OF HYDROGEN HALIDES TO ALKENES: MARKOVNIKOV ’ S RULE The addition of HX to an unsymmetrical alkene could conceivably occur in two ways. The addition of HBr to propene could lead to either 1-bromopropane or 2-bromopropane. The main product, however, is 2-bromopropane. 

Alkyl groups stabilize carbocations by donating electron density from C--H and C--C sigma bonds that can line up with the empty p orbital on the positively charged carbon atom

BASE GRAPH: Reaction energy diagram for the addition of HBr to ethene

BALL-AND-STICK MODEL OF TRANSITION STATE 1: Reaction energy diagram for the addition of HBr to ethene

BALL-AND-STICK MODEL OF TRANSITION STATE 2: Reaction energy diagram for the addition of HBr to ethene

BALL-AND-STICK MODEL OF CARBOCATION INTERMEDIATE: Reaction energy diagram for the addition of HBr to ethene

BALL-AND-STICK MODEL OF REACTANTS: Reaction energy diagram for the addition of HBr to ethene

BALL-AND-STICK MODEL OF PRODUCT: Reaction energy diagram for the addition of HBr to ethene

Reaction energy diagram for formation of the isopropyl and propyl cations from propene

1A.How To Understand Additions to Alkenes  This is an addition reaction: E – Nu added across the double bond Bonds brokenBonds formed  -bond  -bond 2  -bonds © 2014 by John Wiley & Sons, Inc. All rights reserved.

1.Addition Reactions of Alkenes © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Since  bonds are formed from the overlapping of p orbitals,  electron clouds are above and below the plane of the double bond  electron clouds © 2014 by John Wiley & Sons, Inc. All rights reserved.

 In an electrophilic addition, the  electrons seek an electrophile, breaking the  bond, forming a  bond and leaving a positive charge on the vacant p orbital on the adjacent carbon. Addition of a nucleophile to form a  bond provides an addition product. © 2014 by John Wiley & Sons, Inc. All rights reserved.

2.Electrophilic Addition of Hydrogen Halides to Alkenes: Mechanism & Markovnikov’s Rule  Mechanism © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Mechanism ●Sometimes do not go through a “free carbocation”, may go via © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Markovnikov ’ s Rule ●For symmetrical substrates, no problem for regiochemistry © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Markovnikov ’ s Rule ●But for unsymmetrical substrates, two regioisomers are possible © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Markovnikov ’ s Rule ●In the electrophilic addition of an electrophile across an unsymmetrical alkene, the more highly substituted and more stabilized carbocation is formed as the intermediate in preference to the less highly substituted and less stable one © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Markovnikov ’ s Rule ●Thus Note: carbocation stability  3 o > 2 o > 1 o © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Addition of Hydrogen Halides ●Addition of HCl, HBr and HI across a C=C bond ●H + is the electrophile © 2014 by John Wiley & Sons, Inc. All rights reserved.  x

2A.Theoretical Explanation of Markovnikov ’ s Rule  One way to state Markovnikov ’ s rule is to say that in the addition of HX to an alkene, the hydrogen atom adds to the carbon atom of the double bond that already has the greater number of hydrogen atoms © 2014 by John Wiley & Sons, Inc. All rights reserved. 

☓ Step 1Step 2 © 2014 by John Wiley & Sons, Inc. All rights reserved. 

 x

 Examples © 2014 by John Wiley & Sons, Inc. All rights reserved.  

2B.General Statement of Markovnikov ’ s Rule  In the ionic addition of an unsymmetrical reagent to a double bond, the positive portion of the added reagent attaches itself to a carbon atom of the double bond so as to yield the more stable carbocation as an intermediate © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Examples © 2014 by John Wiley & Sons, Inc. All rights reserved. 

 Examples © 2014 by John Wiley & Sons, Inc. All rights reserved. 

ADDITION OF SULFURIC ACID TO ALKENES The addition of sulfuric acid is regioselective, and it follows Markovnikov’s rule

Alcohols from Alkyl Hydrogen Sulfates Alkyl hydrogen sulfates can be easily hydrolyzed to alcohols by heating with water.

2C.Regioselective Reactions  When a reaction that can potentially yield two or more constitutional isomers actually produces only one (or a predominance of one), the reaction is said to be regioselective © 2014 by John Wiley & Sons, Inc. All rights reserved. 

2D.An Exception to Markovnikov ’ s Rule  Via a radical mechanism (see Chapter 10)  This anti-Markovnikov addition does not take place with HI, HCl, and HF, even when peroxides are present © 2014 by John Wiley & Sons, Inc. All rights reserved. With peroxides

4.Addition of Water to Alkenes: Acid-Catalyzed Hydration  Overall process ●Addition of H–OH across a C=C bond ●H + is the electrophile ●Follows Markovnikov’s rule © 2014 by John Wiley & Sons, Inc. All rights reserved.

4A.Mechanism © 2014 by John Wiley & Sons, Inc. All rights reserved.

ADDITION OF WATER TO ALKENES: ACID-CATALYZED HYDRATION Example

The rate-determining step in the hydration mechanism is step 1; the formation of the carbocation.

5.Alcohols from Alkenes through Oxymercuration–Demercuration: Markovnikov Addition  Step 1: Oxymercuration  Step 2: Demercuration © 2014 by John Wiley & Sons, Inc. All rights reserved.

5A.Regioselectivity of Oxymercura- tion – Demercuration  Oxymercuration–demercuration is also highly regioselective and follows Markovnikov’s rule © 2014 by John Wiley & Sons, Inc. All rights reserved.

5C.Mechanism of Oxymercuration  Does not undergo a “free carbocation”  H 2 O attacks the carbon of the bridged Hg ion that is better able to bear the partial +ve charge © 2014 by John Wiley & Sons, Inc. All rights reserved.

 hydroboration-oxidation-hydrolysis ●Overall: anti-Markovnikov addition of H–OH across a C=C bond ●Opposite regioisomers as oxymercuration-demercuration © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Example anti-Markovnikov syn addition This oxidation step occurs with retention of configuration © 2014 by John Wiley & Sons, Inc. All rights reserved.

8A.Regiochemistry and Stereo- chemistry of Alkylborane Oxidation and Hydrolysis  Hydroboration–oxidation reactions are regioselective; the net result of hydro- boration–oxidation is anti-Markovnikov addition of water to an alkene  As a consequence, hydroboration–oxidation gives us a method for the preparation of alcohols that cannot normally be obtained through the acid-catalyzed hydration of alkenes or by oxymercuration–demercuration © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Markovnikov  Anti-Markovnikov © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Examples with rearrangement Markovnikov addition of H 2 O without rearrangement anti-Markovni- kov, syn addition of H 2 O © 2014 by John Wiley & Sons, Inc. All rights reserved.

11.Electrophilic Addition of Bromine & Chlorine to Alkenes  Addition of X–X (X = Cl, Br) across a C=C bond © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Examples © 2014 by John Wiley & Sons, Inc. All rights reserved.

11A. Mechanism of Halogen Addition Br–Br bond becomes polarized when close to alkene (vicinal dibromide) (bromonium) © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Stereochemistry ●Anti addition © 2014 by John Wiley & Sons, Inc. All rights reserved.

ADDITION OF BROMINE AND CHLORINE TO ALKENES

13.Halohydrin Formation  Addition of –OH and –X (X = Cl, Br) across a C=C bond  X + is the electrophile  Follows Markovnikov’s rule © 2014 by John Wiley & Sons, Inc. All rights reserved. 

 Mechanism © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Other variation ●If H 2 O is replaced by ROH, ●ROH will be the nucleophile © 2014 by John Wiley & Sons, Inc. All rights reserved. 

15.Oxidation of Alkenes: Syn 1,2-Dihydroxylation  Overall: addition of 2 OH groups across a C=C bond  Reagents:dilute KMnO 4 / HO ⊖ / H 2 O / cold or OsO 4, pyridine then NaHSO 3, H 2 O © 2014 by John Wiley & Sons, Inc. All rights reserved.

15A. Mechanism for Syn Dihydroxylation of Alkenes © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Both reagents give syn dihydroxylation © 2014 by John Wiley & Sons, Inc. All rights reserved.

OXIDATIONS OF ALKENES: SYN HYDROXYLATION Osmium tetroxide addition to cyclopentene

 Comparison of the two reagents ●KMnO 4 : usually lower yield and possibly side products due to over- oxidation ●OsO 4 : usually much higher yield but OsO 4 is extremely toxic (oxidative cleavage of C=C) © 2014 by John Wiley & Sons, Inc. All rights reserved.

16.Oxidative Cleavage of Alkenes 16A. Cleavage with Hot Basic Potassium Permanganate © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Other examples © 2014 by John Wiley & Sons, Inc. All rights reserved.

16B. Cleavage with Ozone © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Examples © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Mechanism © 2014 by John Wiley & Sons, Inc. All rights reserved.

17.Electrophilic Addition of Bromine & Chlorine to Alkynes © 2014 by John Wiley & Sons, Inc. All rights reserved.

18.Addition of Hydrogen Halides to Alkynes  Regioselectivity ●Follows Markovnikov’s rule © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Mechanism © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Anti-Markovnikov addition of hydrogen bromide to alkynes occurs when peroxides are present in the reaction mixture © 2014 by John Wiley & Sons, Inc. All rights reserved.

19.Oxidative Cleavage of Alkynes  Example OR © 2014 by John Wiley & Sons, Inc. All rights reserved.

 How to synthesize ? ●Retrosynthetic analysis 20A. Retrosynthetic Analysis © 2014 by John Wiley & Sons, Inc. All rights reserved.

●Synthesis Markovnikov addition of H 2 O © 2014 by John Wiley & Sons, Inc. All rights reserved.

 How to synthesize ? ●Retrosynthetic analysis ●Synthesis anti-Markovnikov addition of H 2 O © 2014 by John Wiley & Sons, Inc. All rights reserved.

Summary of Addition Reactions of Alkenes

Summary of Addition Reactions of Alkynes

END OF CHAPTER 8 © 2014 by John Wiley & Sons, Inc. All rights reserved.