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
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Transcript of Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza
Organic A
Chapter 8Alkenes (I)
By Prof. Dr.Adel M. Awadallah
Islamic University of Gaza
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
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)
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.
Physical Properties
• 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
The Chemistry of Vision
The more substituted alkene will form
• 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%
Mechanisms of EliminationE2 with concentrated base 3>2>1second order rate = K[RX][B]
Mechanisms of EliminationE1 with dilute or weak base 3>2
first order rate = K[RX]
• 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
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
Evidence for the E2 mechanismThe Absence of Hydrogen Exchange
The carbanion mechanism (E1cB elimination unimolecular of the conjugate base)
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
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
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]
The E1 reaction: Orientation
Elimination vs. substitution
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%
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
80 oC80 oC
Mechanism of Dehydration (E1)
Dehydration (Rearrangement)
E1 Mechanism, Rearrangement
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.