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Alkenes

Alkenes. C n H 2n. Alkenes. contain carbon - carbon double bonds. called unsaturated hydrocarbons also known as olefins ( oleum , latin, oil; facere , latin, make) C n H 2n C n H 2n + H 2  C n H 2n+2 - one degree of unsaturation. Degree of unsaturation.

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Alkenes

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  1. Alkenes CnH2n

  2. Alkenes • contain carbon - carbon double bonds • called unsaturated hydrocarbons • also known as olefins (oleum, latin, oil; facere, latin, make) • CnH2n • CnH2n + H2 CnH2n+2 - one degree of unsaturation

  3. Degree of unsaturation Degree of unsaturation = (2NC - NX + NN – NH + 2)/2 NC = number of carbons NX = number of halogens NN = number of nitrogens NH = number of hydrogens

  4. Nomenclature – the E/Z system 1. To name alkenes, select the longest carbon chain which includes the carbons of the double bond. Remove the -ane suffix from the name of the alkane which corresponds to this chain. Add the suffix -ene. a derivative of octene not nonane

  5. Nomenclature – the E/Z system 2. Number this chain so that the first carbon of the double bond has the lowest number possible.

  6. Nomenclature – the E/Z system trans cis

  7. cis/trans problems This molecule is a 1-bromo-1-chloropropene but is it cis or trans!

  8. Nomenclature – the E/Z system (Z)-1-bromo-1-chloropropene • use the Cahn-Ingold-Prelog system to assign priorities to the two groups on each carbon of the double bond. • then compare the relative positions of the groups of higher priority on these two carbons. • if the two groups are on the same side, the compound has the Z configuration (zusammen, German, together). • if the two groups are on opposite sides, the compound has the E configuration (entgegen, German, across).

  9. E-Z designations

  10. Relative stabilities of alkenes • Cis isomers are generally less stable than trans isomers due to strain caused by crowding of the two alkyl groups on the same side of the double bond • Stabilities can be compared by measuring heats of hydrogenation of alkenes.

  11. Overall relative stabilities of alkenes

  12. Synthesis of alkenes by elimination reactions dehydrohalogenation: dehydration:

  13. Dehydrohalogenation of alkyl halides Reactivity: RX 3o > 2o > 1o a 1,2 elimination reaction

  14. Dehydrohalogenation

  15. Alkoxide ions – bases used in dehydrohalogenation

  16. Dehydrohalogenation of alkyl halides - no rearrangement

  17. The mechanism In the presence of a strong base, the reaction follows second order kinetics: rate = k[RX][B-] However, with weak bases at low concentrations and as we move from a primary halide to a secondary and a tertiary, the reaction becomes first order. There are two mechanisms for this elimination: E1 and E2.

  18. E2 mechanism

  19. E1 mechanism slow fast

  20. Evidence for the E1 mechanism • Follows first order kinetics • Same structural effects on reactivity as for SN1 reactions - 3 > 2 > 1 • Rearrangements can occur indicative of the formation of carbocations

  21. Evidence for the E2 mechanism • The reaction follows second order kinetics • There are no rearrangements • There is a large deuterium isotope effect • There is an anti periplanar geometry requirement

  22. Isotope effects A difference in rate due to a difference in the isotope present in the reaction system is called an isotope effect.

  23. Isotope effects If an atom is less strongly bonded in the transition state than in the starting material, the reaction involving the heavier isotope will proceed more slowly. The isotopes of hydrogen have the greatest mass differences. Deuterium has twice and tritium three times the mass of protium. Therefore deuterium and tritium isotope effects are the largest and easiest to determine.

  24. Primary isotope effects These effects are due to breaking the bond to the isotope. Thus the reaction with protium is 5 to 8 times faster than the reaction with deuterium.

  25. Evidence for the E2 mechanism - a large isotope effect

  26. Further evidence for the E2 mechanism RI > RBr > RCl > RF

  27. Orientation and reactivity KOH CH CH CHCH CH CH=CHCH 3 2 3 3 3 C H OH Cl 2 5 80% The ease of alkene formation follows the sequence:- R2C=CR2 > R2C=CHR > R2C=CH2, RHC=CHR > RHC=CH2 This is also the order of alkene stability. Therefore the more stable the alkene formed, the faster it is formed. Why?

  28. Orientation and reactivity Let’s look at the transition state for the reaction: The double bond is partially formed in the transition state and therefore the transition state resembles an alkene. Thus the factors which stabilize alkenes will stabilize this nascent alkene. A Zaitsev elimination.

  29. anti elimination

  30. anti elimination KOH ?

  31. anti elimination

  32. Formation of the less substituted alkene Dehydrohalogenation using a bulky base favours the formation of the less substituted alkene:

  33. Substitution vs elimination SN2 v E2 substitution elimination

  34. Substitution vs elimination SN1 v E1

  35. Substitution vs elimination

  36. Dehydration of alcohols

  37. Dehydration of alcohols - the mechanism

  38. Dehydration of alcohols - orientation

  39. The Zaitsev product predominates

  40. The Zaitsev product predominates The transition state explains the orientation:

  41. Dehalogenation of vicinal dihalides

  42. Hydrogenation of alkynes

  43. Synthesis of alkynes by elimination reactions

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