CSUMB
ESSP 311 Organic Chemistry I
Ronald W. Rinehart, Ph.D.

Chapter 6 Reactions of Alkenes: Addition

Chapter 6:  Reactions of Alkenes.  Addition Reactions.

 I.  Reactions of Alkenes. 
            A.  Addition Reactions. 
                        1. In addition reactions the alkene gains two  new groups:

                         2. A variety of reagents can add to the alkene. 
            B.  Reactions of Alkenes. 
                        1.  Hydrogenation of Alkenes. 

                                    a)   Overall reaction   

                                    b)  Catalysts:  Pt, Pd, Ni, Ru, Rh 
                                    c)   These are exothermic reactions, but react slowly without the above catalysts. 
                                    d)   Examples:                                              

                                     e)  Used in the production of margarines. 
                                    f)   Stereochemistry of hydrogenation.

                                        i)  Both H's add to the same face of C=C 
                                        ii)   This is referred to as syn addition. 
                                                 (From opposite sides would be called anti addition.)
                                        iii)   One cannot distinguish syn and anti addition with a reaction such as: 

                                                CH₃CH=CHCH₃  +  H₂ / Pd   à   CH₃CH₂CH₂CH₃ 

                                        iv)  Another stereochemical aspect of hydrogenation is stereoselectivity:
                                                       A reaction can theoretically yield two or more stereoisomers,
                                                       but one is more abundant than the other (or formed exclusively.)
 

                                        iv)  The bridge inhibits the approach  of  a-pinene to the catalyst surface
                                                by that side of the molecule. 
                                                 Only the 'underside' can approach the catalyst and be hydrogenated.
                                                 We say the reaction occurs on the "less hindered" side. 

                                    g)  Heats of hydrogenation can be used to  compare the stability of alkenes in a similar manner
                                        as with heats of  combustion data.                                              

                                      i)  The stability of alkenes with  different number of C's [but same # of C=C] can be compared to
                                                   each other using heats of hydrogenation but not with heats of combustion.  Why?

                         2. Addition of Hydrogen Halides 

                                    a)   Overall reaction    

                                     b) Order of reactivity of HX:      HI  >  HBr  >  HCl  >  HF
                                                i)  HI is the strongest acid of the series; HF is weakest.
                                                ii)  Electrophilic Addition, since H⁺ (an electrophile) first adds to the alkene.    
                                    c)  Solvents:  pentane, benzene, CH₂Cl₂, glacial acetic acid, CHCl₃
                                    d)   Examples.

                                     e)  The reaction is regioselective:  In unsymmetrical alkenes the hydrogen goes to the carbon
                                                already bearing the greater number of hydrogens (of the two pi bonded  carbons): 
                                                            This is Markovnikov's Rule    THE RICH GET RICHER!!!
                                              In texts using the older German transliteration, you may see the spelling Markownikoff

                                    f)  Complete these reactions, showing both major and minor products. Label the major product. 

                                                i)   CH₃CH=CH₂  +  HCl  à   

                                                ii)               +  HBr  à

                                    g)  Mechanism:     

                                    h) Reason for Markovnikov's Rule:  The “rich getting richer” results in the formation of the more
                                                             stable (higher substituted and easier to form) carbocation. 
                                    i)  Exercise:  Complete the following reaction and then show the mechanism for the reaction,
                                             showing the carbocations leading to both major and minor products.

                                                            CH₃CH₂CH=CH₃  + HCl  à 

                        3.  Addition of concentrated H₂SO₄

                                    a)  This reaction works for ethene, monosubstituted alkenes, and vic disubstituted alkenes
                                             (RCH=CHR').
                                    b)    This reaction follows Markovnikov's Rule (complete and show the mechanism):

                         4.   Hydration:  Addition of Water (dilute H₂SO₄)    

                                    a)  Overall reaction.

                                    b)  The reaction follows Markovnikov's Rule.
                                    c)  Complete the following reactions:

                                    d)   Show the mechanism:

                        5.   Addition of Halogens

                                    a)  Overall reaction.

                                     b)   X₂ = Cl₂, Br₂   solvents used include CCl₄, CH₂Cl₂, CH₃CO₂H
                                    c)   F₂ is explosive
                                    d)  Vicinal diiodides lose I₂ and revert to the alkene.     
                                    e)  Anti addition:  The halonium ion.

                                     f)  Mechanism:

                         6.   Addition of Hypohalous Acids:  Formation of Halohydrins 

                                    a)   Overall reaction and mechanism.

                                     b)   X₂ = Cl₂, Br₂   in water   
                                    c)   Important Facts Concerning Addition of "HOX".
                                                i)  Follows Markovnikov's Rule.  (X goes to the carbon richer in H's; OH to the other.)
                                                the results appear as if:  1)  X₂ + H₂O à HOX + X⁻  2) HOX  ⇌   HO⁻ + X⁺

                                                ii)  Anti addition is observed.  
                                                iii) Reaction mechanism involves the halonium ion.
                                    d)   Mechanism:                                                        

                                    e)  Examples:

                                                ii)  Consider the mechanism for this reaction in order to understand which product is preferred:

                                                iii) Notice that the water molecule attacks the more substituted carbon of the halonium ion
                                                        since it carries greater positive character than the less substituted carbon.

                                    f)   Halohydrins are converted by base to epoxides [see # 10]

                        7.  Oxymercuration-Demercuration. Synthesis of Alcohols. 
                                                [not in the text, but you should know it anyway!]
                                    a)   Overall reaction.
 

                                     b)   Important Facts Concerning Oxymercuration/Demercuration:
                                                i)   Follows Markovnikov's Rule
                                                ii)  No rearrangements occur even though a carbocation intermediate is involved in
                                                         the mechanism (the carbocation is stabilized by Hg).
                                                iii)  A water/tetrahydrofuran (THF) mixture is used as solvent: 
                                                        good solvent for organics and water.     
                                                iv)   Mechanism for the 2nd step of the reaction (demercuration) is uncertain: 
                                                        free radicals have been suggested.

                                    c)  Example:

                                     d)  Mechanism:

                         8.  Free Radical Addition of HBr.  Anti-Markovnikov Addition 

                                    a)  Recall the normal addition of HBr to an alkene:

                                                            H₂C=CHCH₃ + HBr à CH₃-CHBrCH₃        2-bromopropane           

                                                 i)  Markovnikov Addition is observed.                            

                                    b)  BUT in the presence of peroxides [in nonpolar solvents]: 

                                                H₂C=CHCH₃  +  HBr  +RO-OR  à   BrCH₂CH₂CH₃    1-bromopropane

                                                 i)  Anti-Markovnikov addition  is observed.
                                                            [also called counter-Markovnikov or Kharasch addition]  

                                    c)   Mechanism:

                                     d)  Complete the following reactions:

                                    e)  Synthesize the following compounds, using methods which will give maximum yields of product:                                                                         

                                    f)  Solutions: 

                                                i)  CH₃CH₂CH₂CH=CH₂  +  HBr à 

                                                ii)  CH₃CH₂CH₂CH=CH₂  +  HBr  + peroxides  à

                        9.   Hydroboration-Oxidation:  Synthesis of Alcohols from Alkenes. 

                                    a)   Some important observations concerning hydroboration:

                                                i)  Anti-Markovnikov addition.    [hydration]
                                                ii)  Syn addition.
                                                iii) No rearrangements.
                                                iv)  Solvents: 

                                                 v)  Retention of configuration is observed in the oxidation step.

                                     b)  Example:  

                                                CH₃CH=CH₂   à { 1. BH₃, diglyme   2. H₂O₂, OH^-   } à          CH₃CH₂CH₂OH

                                     c)  Mechanism:                                                                             

                                     d)  Reason for regioselectivity: 

                                                i)   The C-B bond that forms is the one favoring the more stable transition state
                                                                        (so that the positive character is on the more substituted C).  

                                                ii)  Steric Factors:  The bulky BH₃  group is attracted to the less hindered (less substituted) carbon.

                        10.  Epoxidation 

                                    a)  Epoxidation is achieved directly by the reaction between an alkene and a peroxy acid
                                                            (and indirectly by forming a halohydrin, then treating with base  [6f].)

                                       b)  Nomenclature:
                                                i)   Epoxides can be named as epoxy derivatives of alkanes: 

                                                 ii)  Simple epoxides can also be named as oxiranes, where oxirane is the parent name of the simplest
                                                             epoxide.  Number 1 = O on the ring.  See bottom row in table just above.  

                                    b)   Complete the following reactions and name the product: 

                                      5,5-dimethyl-3,4-epoxyheptane                                      2-methyl-2,3-epoxyhexane

                                     c)  Mechanism: 

                        11.  Ozonolysis:  (1) Ozonation followed by (2) hydrolysis. 
                                    [not in the text, but you should know it anyway!]

                                    a)  Overall reaction. “Snip & cap”

                                    b)  Zn is a reducing agent, added to prevent the oxidation of aldehydes to carboxylic   acids.  

                                    c) Ozonolysis is used to deduce the structure of alkenes by analyzing the structure of the resulting
                                                 carbonyl compounds. 

                                    d)  Examples for two isomers of C₅H₁₀:

                             12.   Permanganate Cleavage of Alkenes. [not in the text, but you should know it anyway!]

                                     a)  This reaction is an alternative to ozonolysis.        

                                    b)  Carboxylic acids are formed in place of  aldehydes.

                                    c)   Examples.  (Complete the reactions):

                         13.  Acid-catalyzed Polymerization:  Alkenes + Alkenes. 

                                    a)  Overall reaction.

                                    b)  Occurs when 3^o carbocations can form [e.g., from R₂C=CH₂, R₂C=CHR, R₂C=CR₂ with H⁺]. 
                                    c)  Mechanism: (Cationic Polymerization).

                         14.  Free-Radical Polymerization. 

                                    a)  Overall reaction [for ethylene].                                                                            

                                    b)  n can equal thousands!  
                                    c)   Mechanism:

                                      d)   Some common monomers/polymers:  [See my 30B polymers page]