Slide Number 1Slide Number 2Review Alkenes and Alkynes: Pi BondsSlide Number 4The (E)-(Z) System for Designating Alkene Diastereomers Slide Number 6ExamplesSlide Number 8Slide Number 9Slide Number 10Slide Number 11Slide Number 12Slide Number 13Slide Number 14Slide Number 15Slide Number 16How to Favor an E2 MechanismZaitsev’s RuleSlide Number 19Slide Number 20Slide Number 21Slide Number 22AnnouncementsSlide Number 24Slide Number 25Slide Number 26Slide Number 28Small vs. Bulky BaseSlide Number 30Slide Number 31Slide Number 32Case Study Experimental ResultsNeomenthyl Chloride Slide Number 36Slide Number 37Slide Number 38Slide Number 39Primary AlcoholsSecondary AlcoholsTertiary AlcoholsSummarySlide Number 44Slide Number 45Slide Number 46Slide Number 47Slide Number 48Slide Number 49RearrangementsSlide Number 51Slide Number 52Slide Number 53Other Commons Carbocations RearrangementsSlide Number 55Slide Number 56Slide Number 57Slide Number 58Slide Number 59Slide Number 60Slide Number 61Slide Number 62Slide Number 63Note: Anti-additionSlide Number 65Slide Number 66Hydrogenation of AlkynesSlide Number 68Slide Number 69Slide Number 70Slide Number 71Slide Number 72Slide Number 73Slide Number 74Slide Number 75How would you prepare the target molecule below?Retrosynthetic AnalysisSynthesisSlide Number 79E2 Elimination from 1,2-dibromo-diphenylethaneChapter 7 Alkenes and Alkynes I: Properties and Synthesis Elimination Reactions of Alkyl Halides Alkenes Alkynes 1In this chapter : 1. The properties of alkenes and alkynes and how they are named 2. How alkenes and alkynes can be transformed into alkanes 3. How to plan the synthesis of any organic molecule 2Review Alkenes and Alkynes: Pi Bonds Two electrons clouds 2 (Two electrons clouds) 3The (E)-(Z) System for Designating Alkene Diastereomers Cis-Trans System • Useful for 1,2-disubstituted alkenes • Examples: 4The (E)-(Z) System for Designating Alkene Diastereomers • Difficulties encountered for tri-substituted and tetra-substituted alkenes • Solution: The Cahn-Ingold-Prelog convention is used to assign the groups of highest priority on each carbon. ClBrCH3Hcis or trans?Cl is cis to CH3 and trans to Br 5The (E)-(Z) System for Designating Alkene Diastereomers (Cahn-Ingold-Prelog convention) 1. Assign the groups of highest priority on each carbon (higher atomic number, the higher priority) 2. (E) configuration – the highest priority groups are on the opposite side of the double bond • “E ” stands for “entgegen”; it means “opposite” in German 3. (Z) configuration – the highest priority groups are on the same side of the double bond • “Z ” stands for “zusammen”; it means “together” in German On carbon 2: Priority of Br > COn carbon 1: Priority of Cl > Hhighest priority groups are Br (on carbon 2) and Cl (on carbon 1)⇒ClBrCH3H216Examples 7In Class Problem: Using the (E)-(Z) and the (R)-(S) designations give the IUPAC names for each of the following: CCH BrClH2C CCH3OHH(A)CH CH3CH3CCCBrH3CHOCH3CH3(B)8 (1) E (2) Z (1) R (2) SRelative Stabilities of Alkenes Generally cis alkenes are less stable than trans alkenes because of steric hinderance or steric interactions. 9Heat of Hydrogenation The relative stabilities of alkenes can be measured using the exothermic heats of hydrogenation. The same alkane product must be obtained to get accurate results. 10 Heats of hydrogenation of three butene isomers: CH3H3CCH3H3CHHH HHHH>>Disubstituted E Disubstituted Z Mono- substitutedOverall Relative Stabilities of Alkenes The greater the number of attached alkyl groups (i.e. the more highly substituted the carbon atoms of the double bond), the greater the alkene’s stability 11In Class Problem: Arrange the following alkenes in order of decreasing stability: 1-pentene; (E)-2-pentene; (Z)-2-pentene; 2-methyl-2-butene. 12Cycloalkenes 13 Cycloalkenes containing 5 carbon atoms or fewer exist only in the cis form Trans – cyclohexene and trans – cycloheptene have a very short lifetime and have not been isolated cyclohexenetrans - cyclohexene(highly strained=> does NOT exist at r.t.)Cycloalkenes 14 Trans – cyclooctene has been isolated. The molecule is chiral and exists as a pair of enantiomers. cis - cyclooctenetrans - cyclooctenesSynthesis of Alkenes via Elimination Reactions Two most important means for synthesizing alkenes. 15 Dehydrohalogenation of Alkyl Halides C CHXHHHHHHHHbase-HXDehydration of Alcohols C CHOHHHHHHHHHH+, heat-HOHDehydrohalogenation 16 The best reaction conditions to use when synthesizing an alkene by dehydrohalogenation are those that promote an E2 mechanism C CHXB:C CE2B:H+X+How to Favor an E2 Mechanism The following conditions favor reaction by an E2 mechanism: (1) A secondary or tertiary halide should be used- steric hindrance will inhibit substitution. (2) When a primary halides are employed, a bulky base should be used- steric hindrance will inhibit substitution. (3) Strong and non-polarizable bases such as alkoxides should be used- will reduce SN1 and E1 competition. (4) Sodium ethoxide in ethanol and potassium tert-butoxide in tert-butyl alcohol are typically used to promote E2 reactions. (5) Elevated temperature generally favor elimination over substitution. 17In the following examples of E2 elimination processes only one alkene product is possible. What is the major reaction pathway when two or more possible alkene products may be formed? Zaitsev’s Rule 18Rate = H3CHCBrCH3EtOkBrHbHaB2-methyl-2-butene2-methyl-1-butene Ha  Hb (2nd order overall) ⇒ bimolecular 19When a small base is used (e.g. EtO⊖ or HO⊖) the major product will be the more highly substituted alkene (the more stable alkene). Examples: BrHbHaEtONaEtOH70oC+69% 31%(eliminate Ha) (eliminate Hb)(1)(2)BrEtOKEtOH51%18%31%+ +69%70oC20Zaitsev’s Rule ● In elimination reactions, the more highly substituted alkene product predominates Stability of alkenes C CMeMeMeMe>C CMeMeHMe>C CHMeMeH>C CHMeHMe>C CHMeHH2122Announcements • 127 students (160). 5 pts added to exam 3. Still today! • Survey Trends – More practice problem – Review/problem practice session before exams – More accessibility to instructor/TA – Access to solutions to HW/practice problems • Wiley Homework solutions will be available after due date • Problem Practice Sessions: – Thursdays 3-4 pm (ES&T, 2354) • Review Sessions: – Friday Nov 1st 3-5 pm (ES&T, 2312) – Thursday