Transcript الشريحة 1
Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza Alkenes and Alkynes Hydrocarbons (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 H o H 180 o bond angle 109.5 120 bond length 154 pm 134 pm 121 pm rotation possible restricted restricted geometry tetrahedral triagonal planer linear Hypridization sp3 Bond Length in Benzene sp2 sp 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 • water insoluble ~ alkanes • 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 + Br sec-butyl bromide KOH(alc) CH3CH2CH=CH2 1-butene 19% + CH3CH=CHCH3 2-butene 81% RCH=CH2 RCH=CHR Mechanisms of Elimination E2 with concentrated base 3>2>1 second order rate = K[RX][B] Mechanisms of Elimination E1 with dilute or weak base 3>2 first order rate = K[RX] • Order of reactivity in E2: 3o > 2o > 1o • CH3CH2-X • CH3CHCH3 • 3 adj. H’s CH3CH=CH2 6 adj. H’s & more stable alkene X • CH3 • CH3CCH3 • CH2=CH2 X CH3 CH=CCH3 9 adj. H’s & most stable alkene Evidence for the E2 mechanism 1) second order 2) No Rearrangement 3) 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 mechanism The 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 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 E1 Mechanism Elimination, unimolecular a) b) c) d) e) f) g) E1 • RX: 3o > 2o > 1o • rearragement possible • may yield mixtures • Saytzeff orientation • element effect • no isotope effect • 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 CH3 CH3 H3C EtOH / H2O oC CH3 CH3 H3C 80 Br CH3 + OH CH2 H3C 19% H CH3 H3C Br 80 oC H H EtOH / H2O CH3 H3C OH + H3C CH2 5% 2. dehydration of alcohols: a) b) c) d) e) f) ROH: 3o > 2o > 1o acid is a catalyst rearrangements are possible mixtures are possible Saytzeff mechanism is E1 80 oC Mechanism of Dehydration (E1) Dehydration (Rearrangement) E1 Mechanism, Rearrangement Synthesis of 1-butene from 1-butanol: • CH3CH2CH2CH2-OH + HBr • SN2 • • • • CH3CH2CH2CH2-Br 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.