Organic chemistry B Chapter 12 Alkynes By Prof. Dr. Adel M. Awadallah Islamic University of Gaza.
Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza
description
Transcript of Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza
![Page 1: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/1.jpg)
Organic A
Chapter 8Alkenes (I)
By Prof. Dr.Adel M. Awadallah
Islamic University of Gaza
![Page 2: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/2.jpg)
Alkenes and AlkynesHydrocarbons (contain only carbon and hydrogen)
a) Saturated: (Contain only single bonds)
Alkanes (CnH2N + 2 )
Cycloalkanes (CnH2N )
b) Unsaturated: contain
Alkenes: double bonds (,,,CnH2N)
Alkynes: triple bonds ((CnH2N - 2)
Aromatic: benzene like compounds
![Page 3: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/3.jpg)
Facts about double and triple bonds
HH
bond angle 109.5 120 o 180 o
bond length 154 pm 134 pm 121 pm
rotation possible restricted restricted
geometry tetrahedral triagonal planer linear
Hypridization sp3 sp2 sp
Bond Length in Benzene 139 pm (plannar, sp2 hypridized)
![Page 4: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/4.jpg)
A pi bond is one in which the electrons in the p orbitals are held above and below the plane of the molecule.The sigma bond is stronger than the pi bond.A double bond is formed from a sigma bond and a pi bond, and so it is stronger than a single bond.
![Page 5: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/5.jpg)
Physical Properties
![Page 6: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/6.jpg)
• Physical properties:
• non-polar or weakly polar
• no hydrogen bonding
• relatively low mp/bp ~ alkanes
• water insoluble
• Importance:
• common group in biological molecules
• starting material for synthesis of many plastics
![Page 7: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/7.jpg)
The Chemistry of Vision
![Page 8: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/8.jpg)
![Page 9: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/9.jpg)
![Page 10: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/10.jpg)
The more substituted alkene will form
![Page 11: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/11.jpg)
• Saytzeff orientation:• In dehydrohalogenation the preferred product is the alkene that has
the greater number of alkyl groups attached to the doubly bonded carbon atoms
• (the more substituted alkene will form)
• Ease of formation of alkenes:• R2C=CR2 > R2C=CHR > R2C=CH2, RCH=CHR > RCH=CH2 >
CH2=CH2
• Stability of alkenes:
• R2C=CR2 > R2C=CHR > R2C=CH2, RCH=CHR > RCH=CH2 > CH2=CH2
• CH3CH2CHCH3 + KOH(alc) CH3CH2CH=CH2 RCH=CH2
• Br 1-butene 19%• sec-butyl bromide +• CH3CH=CHCH3 RCH=CHR• 2-butene 81%
![Page 12: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/12.jpg)
Mechanisms of EliminationE2 with concentrated base 3>2>1second order rate = K[RX][B]
![Page 13: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/13.jpg)
Mechanisms of EliminationE1 with dilute or weak base 3>2
first order rate = K[RX]
![Page 14: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/14.jpg)
![Page 15: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/15.jpg)
• Order of reactivity in E2: 3o > 2o > 1o
• CH3CH2-X CH2=CH2 3 adj. H’s
• CH3CHCH3 CH3CH=CH2 6 adj. H’s & more stable
• X alkene
• CH3 CH3• CH3CCH3 CH=CCH3 9 adj. H’s & most stable• X alkene
![Page 16: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/16.jpg)
Evidence for the E2 mechanism1) second order2) No Rearrangement3) Show a large hydrogen isotope effect
Primary hydrogen isotope effect:
A bond to hydrogen (protium) is broken faster than a bond to deuterium (D) KH / KD = 5 - 8
This means that the breaking of hydrogen is in the rate determining step
![Page 17: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/17.jpg)
Evidence for the E2 mechanismThe Absence of Hydrogen Exchange
The carbanion mechanism (E1cB elimination unimolecular of the conjugate base)
![Page 18: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/18.jpg)
Run the reaction until about half the substrate had been converted into alkene. Unconsumed 2-phenylethyl bromide was recovered. It contained no deuterium. So, the reaction was not acompanied by hydrogen exchange. This rules out the carbanion mechanism
![Page 19: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/19.jpg)
Evidence for the E2 mechanismThe Element Effect (is the breaking of the C-X bond in the rate determining step????)
Strength of the bond
R-F > R-Cl > R-Br > RI
Reactivity toward SN2, SN1, E2 and E1
R-I > R-Br > R-Cl > R-F
So, R-X bond breaking is in the rate determining step
![Page 20: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/20.jpg)
E1 Mechanism
•Elimination, unimolecular E1
•a) RX: 3o > 2o > 1o •b) rearragement possible
•c) may yield mixtures •d) Saytzeff orientation
•e) element effect•f) no isotope effect
•g) rate = k [RW]
![Page 21: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/21.jpg)
![Page 22: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/22.jpg)
![Page 23: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/23.jpg)
![Page 24: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/24.jpg)
The E1 reaction: Orientation
![Page 25: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/25.jpg)
Elimination vs. substitution
![Page 26: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/26.jpg)
![Page 27: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/27.jpg)
Substitution is generally the main reaction, but, E1 Elimination occurs more with 3 > 2 >1
CH3CH3
Br
CH3
CH3CH3
Br
H
CH3CH3
OH
CH3
CH3CH3
OH
H
CH3 CH2
CH3
CH3 CH2
H
+
+
EtOH / H2O
EtOH / H2O
80 oC
80 oC
19%
5%
![Page 28: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/28.jpg)
2. dehydration of alcohols:a) ROH: 3o > 2o > 1o
b) acid is a catalystc) rearrangements are possible d) mixtures are possible e) Saytzefff) mechanism is E1
![Page 29: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/29.jpg)
80 oC80 oC
![Page 30: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/30.jpg)
Mechanism of Dehydration (E1)
![Page 31: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/31.jpg)
Dehydration (Rearrangement)
![Page 32: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/32.jpg)
E1 Mechanism, Rearrangement
![Page 33: Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza](https://reader035.fdocuments.in/reader035/viewer/2022062315/56815854550346895dc5b06c/html5/thumbnails/33.jpg)
Synthesis of 1-butene from 1-butanol:
• CH3CH2CH2CH2-OH + HBr CH3CH2CH2CH2-Br• SN2• E2 KOH(alc)•• CH3CH2CH=CH2
• only!
• To avoid the rearrangement in the dehydration of the alcohol the alcohol is first converted into an alkyl halide.