1. Chapter 11 Alkenes and IR Lect. #2 I.Infrared Spectroscopy A.Energy Absorption and Vibration 1)IR electromagnetic radiation is just less energetic than visible light 2)This energy is sufficient to cause excitation of vibrational energy levels 3)Wavelength ( ) = 2.5-16.7 x 10 -6 m 4) = wavenumbers. Larger = higher energy 5)Excitation depends on atomic mass and how tightly they are bound a) Hooke’s Law for 2 masses connected by a spring b)Many possible absorptions per molecule exist: stretching, bending,… k = constant f = force constant = bond strength m-term = reduced mass

2. Vibrational modes leading to IR absorptions:

3. B.Using IR in Organic Chemistry 1.Functional Groups have characteristic IR absorptions 2.Fingerprint Region (600-1500 cm -1 ) is unique for every molecule and lets us match an unknown with a known spectrum

8. 4.IR of Alkenes a.Alkene C—H absorbs at higher energy than alkanes because the force constant is stronger than alkanes (sp 2 hybridization) b.Substitution pattern of alkenes give characteristic absorptions i.Terminal alkenes give 915, 995 cm -1 ii.Geminal disubstituted gives 890 cm -1 iii.trans disubstituted gives 970 cm -1

9. II.Stability of Alkenes A.Heat of Hydrogenation 1)  H for the hydrogenation of an alkene tells us how stable it was 2)All alkene isomers give the same saturated product, so we can compare

10. B.Stability of alkene isomers 1.Hyperconjugation makes substituted alkenes more stable by stabilizing p-orbitals 2. cis alkenes are less stable than trans alkenes because of steric crowding 3. cis cycloalkenes are more stable than trans for the small rings

11. III.Preparation of Alkenes A.E2 Elimination 1)Regioselectivity = selective reaction at one of multiple sites 2)T.S. is lower energy for the major product 3)Saytzev Rule = most highly substituted product will be favored

12. 4)A bulky base will lead to the other product as major (Hoffman Rule) 5)trans product more stable than cis product 6)Some eliminations are stereospecific due to anti elimination

13. B.Dehydration of Alcohols 1.Mineral acid and heat will dehydrate an alcohol to an alkene 2.Reaction is easier for more substituted products 3.Primary alcohols by E2 4.Secondary and Tertiary by E1 (with rearrangement to most substituted product)