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 (C n H 2N + 2 ) Cycloalkanes (C n H 2N ) b) Unsaturated: contain Alkenes: double bonds (,,,C n H 2N ) Alkynes: triple bonds ((C n H 2N - 2 ) Aromatic: benzene like compounds

Facts about double and triple bonds

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: R 2 C=CR 2 > R 2 C=CHR > R 2 C=CH 2, RCH=CHR > RCH=CH 2 > CH 2 =CH 2 Stability of alkenes: R 2 C=CR 2 > R 2 C=CHR > R 2 C=CH 2, RCH=CHR > RCH=CH 2 > CH 2 =CH 2 CH 3 CH 2 CHCH 3 + KOH(alc)  CH 3 CH 2 CH=CH 2 RCH=CH 2 Br 1-butene 19% sec-butyl bromide+ CH 3 CH=CHCH 3 RCH=CHR 2-butene 81%

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: 3 o > 2 o > 1 o CH 3 CH 2 -X  CH 2 =CH 2 3 adj. H’s CH 3 CHCH 3  CH 3 CH=CH 2 6 adj. H’s & more stable X alkene CH 3 CH 3 CH 3 CCH 3  CH=CCH 3 9 adj. H’s & most stable X 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) K H / K D = 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 E1 a) RX: 3 o > 2 o > 1 o 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

2.dehydration of alcohols: a)ROH: 3 o > 2 o > 1 o b)acid is a catalyst c)rearrangements are possible  d)mixtures are possible  e)Saytzeff f)mechanism is E1

Mechanism of Dehydration (E1)

Dehydration (Rearrangement)

E1 Mechanism, Rearrangement

Synthesis of 1-butene from 1-butanol: CH 3 CH 2 CH 2 CH 2 -OH + HBr  CH 3 CH 2 CH 2 CH 2 -Br S N 2 E2  KOH(alc) CH 3 CH 2 CH=CH 2 only! To avoid the rearrangement in the dehydration of the alcohol the alcohol is first converted into an alkyl halide.