Evidence for the E2 mechanism The Absence of Hydrogen Exchange

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Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza

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Alkenes and Alkynes Hydrocarbons (contain only carbon and hydrogen) 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

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Facts about double and triple bonds

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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.

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Physical Properties

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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

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The Chemistry of Vision

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The more substituted alkene will form

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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%

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Mechanisms of Elimination E2 with concentrated base 3>2>1 second order rate = K[RX][B]

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Mechanisms of Elimination E1 with dilute or weak base 3>2 first order rate = K[RX]

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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

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Evidence for the E2 mechanism 1) second order 2) No Rearrangement 3) Show a large hydrogen isotope effectPrimary 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

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Evidence for the E2 mechanism The Absence of Hydrogen ExchangeThe carbanion mechanism (E1cB elimination unimolecular of the conjugate base)

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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

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Evidence for the E2 mechanism The 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

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E1 MechanismElimination, 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]

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The E1 reaction: Orientation

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Elimination vs. substitution

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Last Updated: 8th March 2018

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