Download - 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College
![Page 1: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/1.jpg)
1
CH 7: Alkenes, Reactions and Synthesis
Renee Y. BeckerCHM 2210
Valencia Community College
![Page 2: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/2.jpg)
2
Diverse Reactions of Alkenes
• Alkenes react with many electrophiles to give useful products by addition (often through special reagents)
![Page 3: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/3.jpg)
3
Why this chapter?
• To begin a systematic description of major functional groups
• Begin to focus on general principles and patterns of reactivity that tie organic chemistry
![Page 4: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/4.jpg)
4
Preparation of Alkenes: A Preview of Elimination Reactions
• Alkenes are commonly made by– elimination of HX from alkyl halide
(dehydrohalogenation)• Uses heat and KOH
– elimination of H-OH from an alcohol (dehydration) • require strong acids (sulfuric acid, 50 ºC)
![Page 5: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/5.jpg)
5
Addition of Halogens to Alkenes
• Bromine and chlorine add to alkenes to give 1,2-dihaldes, an industrially important process– F2 is too reactive and I2 does not add
• Cl2 reacts as Cl+ Cl-
– Br2 is similar
![Page 6: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/6.jpg)
6
Addition of Br2 to Cyclopentene
• Addition is exclusively trans
![Page 7: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/7.jpg)
7
Mechanism of Bromine Addition
• Br+ adds to an alkene producing a cyclic ion• Bromonium ion, bromine shares charge with carbon
– Gives trans addition
![Page 8: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/8.jpg)
8
Bromonium Ion Mechanism
• Electrophilic addition of bromine to give a cation is followed by cyclization to give a bromonium ion
• This bromoniun ion is a reactive electrophile and bromide ion is a good nucleophile
![Page 9: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/9.jpg)
9
![Page 10: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/10.jpg)
10
Addition of Hypohalous Acids to Alkenes: Halohydrin Formation
• This is formally the addition of HO-X to an alkene to give a 1,2-halo alcohol, called a halohydrin
• The actual reagent is the dihalogen (Br2 or Cl2 in water in an organic solvent)
![Page 11: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/11.jpg)
11
Mechanism of Formation of a Bromohydrin
• Br2 forms bromonium ion, then water adds
– Orientation toward stable C+ species
– Aromatic rings do not react
– Show correct mech!
![Page 12: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/12.jpg)
12
An Alternative to Bromine
• Bromine is a difficult reagent to use for this reaction• N-Bromosuccinimide (NBS) produces bromine in
organic solvents and is a safer source• Markov., Br goes to C with more H’s
![Page 13: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/13.jpg)
13
Addition of Water to Alkenes: Oxymercuration
• Hydration of an alkene is the addition of H-OH to to give an alcohol
• Acid catalysts are used in high temperature industrial processes: ethylene is converted to ethanol
![Page 14: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/14.jpg)
14
Oxymercuration Intermediates
• For laboratory-scale hydration of an alkene
• Use mercuric acetate in THF followed by sodium borohydride
• Markovnikov orientation (H to C with more H)– via mercurinium ion
![Page 15: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/15.jpg)
15
Addition of Water to Alkenes: Hydroboration
• Herbert Brown (HB) invented hydroboration (HB)
• Borane (BH3) is electron deficient is a Lewis acid
• Borane adds to an alkene to give an organoborane
• Non Markov., (H to C with less H)
![Page 16: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/16.jpg)
16
• Addition of H-BH2 (from BH3-THF complex) to three alkenes gives a trialkylborane
• Oxidation with alkaline hydrogen peroxide in water produces the alcohol derived from the alkene
Hydroboration-Oxidation Forms an Alcohol from an Alkene
![Page 17: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/17.jpg)
17
Orientation in Hydration via Hydroboration
• Regiochemistry is opposite to Markovnikov orientation– OH is added to carbon with most H’s
• H and OH add with syn stereochemistry (cis), to the same face of the alkene (opposite of anti addition (trans))
![Page 18: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/18.jpg)
18
Mechanism of Hydroboration
• Borane is a Lewis acid
• Alkene is Lewis base
• Transition state involves anionic development on B
• The components of BH3 are added across C=C
• More stable carbocation is also consistent with steric preferences
![Page 19: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/19.jpg)
19
![Page 20: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/20.jpg)
20
Addition of Carbenes to Alkenes
• The carbene functional group is “half of an alkene”• Carbenes are electrically neutral with six electrons in the
outer shell• They add symmetrically across double bonds to form
cyclopropanes
![Page 21: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/21.jpg)
21
Formation of Dichlorocarbene
• Base removes proton from chloroform
• Stabilized carbanion remains
• Unimolecular elimination of Cl- gives electron deficient species, dichlorocarbene
![Page 22: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/22.jpg)
22
Reaction of Dichlorocarbene
• Addition of dichlorocarbene is stereospecific cis
![Page 23: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/23.jpg)
23
Simmons-Smith Reaction
• Equivalent of addition of CH2: • Reaction of diiodomethane with zinc-copper alloy
produces a carbenoid species• Forms cyclopropanes by cycloaddition
![Page 24: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/24.jpg)
24
Reduction of Alkenes: Hydrogenation
• Addition of H-H across C=C• Reduction in general is addition of H2 or its equivalent • Requires Pt or Pd as powders on carbon and H2
• Hydrogen is first adsorbed on catalyst• Reaction is heterogeneous (process is not in solution)
![Page 25: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/25.jpg)
25
Hydrogen Addition- Selectivity
• Selective for C=C. No reaction with C=O, C=N
• Polyunsaturated liquid oils become solids
• If one side is blocked, hydrogen adds to other
![Page 26: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/26.jpg)
26
Mechanism of Catalytic Hydrogenation
• Heterogeneous – reaction between phases• Addition of H-H is syn
![Page 27: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/27.jpg)
27
Oxidation of Alkenes: Epoxidation and Hydroxylation
• Epoxidation results in a cyclic ether with an oxygen atom
• Stereochemistry of addition is syn
![Page 28: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/28.jpg)
28
Osmium Tetroxide Catalyzed Formation of Diols
• Hydroxylation - converts to syn-diol
• Osmium tetroxide, then sodium bisulfate
• Via cyclic osmate di-ester
![Page 29: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/29.jpg)
29
Oxidaton of Alkenes:Cleavage to Carbonyl Compounds
• Ozone, O3, adds to alkenes to form molozonide
• Reduce molozonide to obtain ketones and/or aldehydes
![Page 30: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/30.jpg)
30
Examples of Ozonolysis of Alkenes
• Used in determination of structure of an unknown alkene
![Page 31: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/31.jpg)
31
Permangante Oxidation of Alkenes
• Oxidizing reagents other than ozone also cleave alkenes
• Potassium permanganate (KMnO4) can produce carboxylic acids and carbon dioxide if H’s are present on C=C
![Page 32: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/32.jpg)
32
Cleavage of 1,2-diols
• Reaction of a 1,2-diol with periodic (per-iodic) acid, HIO4 , cleaves the diol into two carbonyl compounds
• Sequence of diol formation with OsO4 followed by diol cleavage is a good alternative to ozonolysis
![Page 33: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/33.jpg)
33
Addition of Radicals to Alkenes: Polymers
• A polymer is a very large molecule consisting of repeating units of simpler molecules, formed by polymerization
• Alkenes react with radical catalysts to undergo radical polymerization
• Ethylene is polymerized to poyethylene, for example
![Page 34: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/34.jpg)
34
Free Radical Polymerization: Initiation
• Initiation - a few radicals are generated by the reaction of a molecule that readily forms radicals from a nonradical molecule
• A bond is broken homolytically
![Page 35: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/35.jpg)
35
Polymerization: Propagation
• Radical from initiation adds to alkene to generate alkene derived radical
• This radical adds to another alkene, and so on many times
![Page 36: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/36.jpg)
36
Polymerization: Termination
• Chain propagation ends when two radical chains combine
• Not controlled specifically but affected by reactivity and concentration
![Page 37: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/37.jpg)
37
Other Polymers
• Other alkenes give other common polymers
![Page 38: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/38.jpg)
38
Biological Additions of Radicals to Alkenes
• Severe limitations to the usefulness of radical addition reactions in the lab
• In contrast to electrophilic additions, reactive intermediate is not quenched so it reacts again and again uncontrollably
![Page 39: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/39.jpg)
39
Biological Reactions
• Biological reactions different than in the laboratory
• One substrate molecule at a time is present in the active site of an enzyme
• • Biological reactions are more controlled, more specific
than other reactions
![Page 40: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/40.jpg)
Mechanism Examples
40
Group 1: Halogenation
Cl2
CH2Cl2
Regioselective? Why, why not?Cyclic, trans or cis? Why?
Br2
CH2Cl2+
+
Group 1: Halogenation
Cl2
CH2Cl2
Regioselective? Why, why not?Cyclic, trans or cis? Why?
Br2
CH2Cl2+
+
![Page 41: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/41.jpg)
Mechanism Examples
41
Group 2: dehydrohalogenation
Regioselective? Why, why not?
KOH
alcohol
major Minor KBr H2O
How many prod? Major and minor? KBr H2OKOH
alcohol
Br
Br
++
G r o u p 2 : d e h y d r o h a l o g e n a t i o n
R e g i o s e l e c t i v e ? W h y , w h y n o t ?
K O H
a l c o h o l
m a j o r M i n o r K B r H
2O
How many prod? Major and minor? KBr H2OKOH
alcohol
Br
Br
++
![Page 42: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/42.jpg)
Mechanism Examples
42
Group 3: dehydration
Regioselective? Why, why not?H2O
OHH3O+
THF 50C
Major Minor
OH
Regioselective? Why, why not?
H2OH3O+
THF 50C
Group 3: dehydration
Regioselective? Why, why not?H2O
OHH3O+
THF 50C
Major Minor
OH
Regioselective? Why, why not?
H2OH3O+
THF 50C
![Page 43: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/43.jpg)
Mechanism Examples
43
H2O
Group 4: Halohydrin formation
Br2
H2O
Regioselective? Why, why not?Cyclic, trans or cis? Why?
Br2
+
+
H2O
Group 4: Halohydrin formation
Br2
H2O
Regioselective? Why, why not?Cyclic, trans or cis? Why?
Br2
+
+
![Page 44: 1 CH 7: Alkenes, Reactions and Synthesis Renee Y. Becker CHM 2210 Valencia Community College](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfc31a28abf838ca554e/html5/thumbnails/44.jpg)
Mechanism Examples
44
H2O
H2SO4
250 CH2O
Group 5: Hydration
250 C
H2SO4
Regioselective? Why, why not?
+
+
H
2O
H2SO4
250 CH2O
Group 5: Hydration
250 C
H2SO4
Regioselective? Why, why not?
+
+