Presentation on theme: "Synthesis of Alkenes Major approaches to the synthesis of alkenes:"— Presentation transcript:
1 Synthesis of Alkenes Major approaches to the synthesis of alkenes: Dehydrohalogenation of Alkyl HalidesE2 mechanism – most usefulE1 mechanismDehalogenation of Vicinal DibromidesDehydration of Alcohols
2 Synthesis of AlkenesDehydrohalogenation can occur via either an E2 or E1 mechanism.Loss of H+ and X- ions from adjacent carbons, forming a new pi bondNaOHD
3 Synthesis of AlkenesThe most synthetically useful dehydrohalogenation reactions occur under E2 reaction conditions.3o or bulky 2o alkyl halidestrong basesstrong bulky bases are best when using 2o alkyl halidesless likely to undergo substitution reactions
4 Synthesis of Alkenes Common strong bulky bases triethylamine diisopropylaminet-butoxide ion2,6-dimethylpyridine
5 Synthesis of Alkenes Mechanism of E2 Dehydrohalogenation concerted reactionanti-coplanar transition state
6 Synthesis of AlkenesE2 elimination reactions can take place in cyclohexanes only when proton and leaving group can get into a trans-diaxial arrangementcorresponds to anti-coplanar
7 Synthesis of AlkenesStrong, less hindered bases (MeO-, EtO-, etc) generally give the most substituted alkene (Saytzeff’s rule) as the major product.
8 Synthesis of AlkenesStrong, bulky bases usually give the Hoffmann product (least highly substituted alkene) as the major productbulky bases often abstract a proton from a less hindered carbon
9 Synthesis of AlkenesExample: Predict all elimination product(s) of the following reactions. Which one is the major product?
10 Synthesis of AlkenesExample: Predict all possible elimination products for the following reaction. Which one will be the major product?
11 Synthesis of Alkenes Dehalogenation of Vicinal Dibromides two possible reagentsNaI (E2 mechanism)Zn/HOAc (redox reaction)
12 Anti-coplanar conformation required Synthesis of AlkenesDehalogenation using I- takes place via a concerted, stereospecific E2 mechanismAnti-coplanar conformation requiredTrans-diaxial conformation required for cycloalkanes
13 Synthesis of AlkenesExample: Predict the major elimination product formed in the following reactions.
14 Synthesis of Alkenes Dehydration of Alcohols removal of water equilibrium processdrive reaction to completion by removing alkene as formed (LeChatelier’s Principle)H2SO4D
15 Synthesis of Alkenes Typical reaction conditions alcohol substrate Order of reactivity:3o > 2o > 1o alcoholacid catalystconc. H2SO4conc. H3PO4heat
16 Synthesis of Alkenes Mechanism of Dehydration (E1) Step 1: Protonation of the hydroxyl group (fast)Step 2: Ionization (RDS)+
17 Synthesis of Alkenes Step 3: Proton abstraction (fast) Rearrangements to form more stable carbonium ions are common in dehydration reactions.Saytzeff’s product preferred.
18 Synthesis of AlkenesExample: Propose a mechanism for the following reaction.C
19 Synthesis of Alkenes Step 1: Protonation of OH group Step 2: Ionization with Methyl Shift
20 Synthesis of AlkenesStep 3: Abstraction of proton
21 Synthesis of AlkenesExample: Predict the major product formed in the following reaction.
22 Reactions of AlkenesThe most common reactions of alkenes are addition reactions:the addition of a reagent to the pi bond with subsequent formation of new sigma bondsnumber of elements of unsaturation decreases
23 Reactions of AlkenesThe electrons in the p bond of C=C are delocalized above and below the sigma bondmore loosely heldIn the presence of a strong electrophile, the double bond acts as a nucleophile, donating the p electrons to the electrophile and forming a new s bond.
24 Reactions of AlkenesMost reactions of alkenes are electrophilic addition reactions.Step 1: Attack of electrophile on pi bond forming a carbonium ion:Step 2: Nucleophile attacks carbonium ion giving product.
26 Reactions of AlkenesIn the previous example, the proton added to the secondary carbon, forming the most stable carbonium ion.Markovnikov’s Rule:Asymmetric reagents such as H-X add to a C=C so that the proton adds to the carbon (in the double bond) that already has the greater number of hydrogen atoms.“The rich get richer”
27 Reactions of Alkenes Markovnikov’s Rule (extended): In an electrophilic addition to an alkene, the electrophile adds in such a way as to give the most stable intermediate.
28 Reactions of AlkenesExample: Predict the product formed in each of the following reactions.
29 Reactions of AlkenesAnti-Markovnikov Addition of HBrIn the presence of peroxides, HBr adds to C=C via a free radical mechanism giving the “Anti-Markovnikov” product.Works only with HBr (not HCl or HI) due to relative bond strengths.
30 Reactions of Alkenes Benzoyl peroxide Acetyl peroxide Some common peroxides:Benzoyl peroxideAcetyl peroxideDi-t-butyl peroxideDiethyl peroxide
31 Reactions of Alkenes HBr Example: Predict the product of the following reaction.HBr