1	Alkenes: Structure and Reactivity David Oliver
2	Goals After this chapter you should be able to: Describe bonding in alkenes Describe and name isomers using cis-trans and Z,E systems Calculate stability of alkenes Describe electrophilic addition and predict products using Markovnikov’s rule Know relative stability of carbocation and use stability to predict products
3	Introduction Alkenes = olefin = unsaturated sp² hybridization = trigonal planar carbons Overlapping p-orbitals form p bonds p bonds can NOT rotate without breaking Cis-trans isomerism possible Interactions occur across multiple alternating double bonds systems through resonance
4	Degree of Unsaturation C_nH_2n+2 for alkane How many fewer pairs of hydrogens Other considerations Add halogens to hydrogens Ignore the Oxygens Subtract Nitrogens from hydrogens Number of Double Bonds + Rings
5	Degree of Unsaturation
6	Degree of Unsaturation
7	Nomenclature Number longest continuous carbon chain that contains the double bond beginning with end closest to double bond. Indicate the position of the double bond with a number- Drop ane ending and use –ene suffix As a substituent group the double bond takes precedence over substituent groups in numbering.
8	Nomenclature
9	Nomenclature
10
11
12	Nomenclature
13	Nomenclature For poly-enes use position numbers for each double bond and use suffix… -diene -triene -tetraene Start numbering from end with lowest numbered double bond.
14	Nomenclature
15	Nomenclature For cycloalkenes Numbering begins with double bond Lowest number to substituent groups
16	Nomenclature
17	Nomenclature: Cis –Trans No free rotation about double bond can occur without breaking p bond
18	Nomenclature E,Z system More general than cis-trans
19	Nomenclature E,Z system Consider carbons of double bond separately Rank atoms attached to each carbon by atomic weight with larger having higher priority For chains move further down attached chains until atomic weight difference can distinguish priority Multiple bonds count as if two atoms are attached.
20	Nomenclature E,Z system
21	Nomenclature E,Z system
22	Alkene Stability Cis-trans isomers can interconvert upon treatment with strong acid Given that the equilibrium mixture is 76% trans calculate the energy difference between the two states. DG= -RT lnK DG= -8.314J/Kmol x 298K x ln(24/76) DG= -2.86kJ/mol
23	Alkene Stability Relative stability of cis-trans isomers can also be found by the difference in heats of combustion of the two isomers. -2682.2 -(- 2685.5) = 3.3kJ/mol
24
25	Alkene Stability The stability of intermediates of addition reactions are stabilized by hyperconjugation
26	Electrophilic Addition: C=C + HX Markovnikov’s Rule: “Them as has, gets.” (From Huck Finn) The carbon with the most hydrogens gets the hydrogen.
27	Stability of the Carbocation (C⁺) Intermediate. The most highly substituted carbon can stabilize the carbocation to the largest degree. Partly because of inductive effect Direct attraction of electrons from adjacent carbons Partly because of hyperconjugation. The more hydrogens attached to adjacent carbons, the greater number of hyperconjugation possibilities with the vacant p orbital
28	Stability of the Carbocation (C⁺) Intermediate.
29	The Hammond Postulate The structure of a transition state (activated complex) resembles the structure of the nearest stable species. Transition state for endergonic steps resemble products Transition state for exergonic step resembles reactant
30	Energy Profile for Protonation in Addition Reaction Mechanism
31	Carbocation Rearrangement
32	Carbocation Rearrangement
33
34	Carbocation Rearrangement: Alkyl Shift for More Stable Carbocation
35	Carbocation Rearrangement Driven by more stable carbocation. Carbocation Rearrangement
36	Goals After this chapter you should be able to: Describe bonding in alkenes Describe and name isomers using cis-trans and Z,E systems Calculate stability of alkenes Describe electrophilic addition and predict products using Markovnikov’s rule Know relative stability of carbocation and use stability to predict products