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Synthesis of Alkenes Major approaches to the synthesis of alkenes: Elimination of Alkyl Halides Dehydrohalogenation E2 mechanism E1 mechanism Dehalogenation of Vicinal Dibromides Dehydration of Alcohols (CH3CH2)3N Synthesis of Alkenes Dehydrohalogenation can occur via either an E2 or E1 mechanism. + Loss of H and X ions from adjacent carbons, forming a new pi bond CH3 CH3CCH2CH3 Br NaOH D H3C H3C H C C CH3 Synthesis of Alkenes The most synthetically useful dehydrohalogenation reactions occur under E2 reaction conditions. o o 3 or bulky 2 alkyl halide strong bases strong bulky bases are best when using 2o alkyl halides less likely to undergo substitution reactions (CH3)2CH Synthesis of Alkenes N H (CH ) CH 3 2 or Et N (CH3CH2)3N 3 Common strong bulky bases (CH3)2CH (CH3)2CH N H (CH3CH2)3N or Et3N triethylamine (CH3)2CH CH3 N or H t-Bu (CH3CH )2CH CCH 3 3 diisopropylamine O (CH3)2CH CH3 N or H CH3CH )2CH t-BuO CCH 3 3 O - t-butoxide ion (CH3CH2)3N or E - H H3C C CCH3 CH CH H3CH3CCH NCCH 3 3 or t-B 3 3 - O 2,6-dimethylpyridine Synthesis of Alkenes Mechanism of E2 Dehydrohalogenation concerted reaction anti-coplanar transition state Synthesis of Alkenes E2 elimination reactions can take place in cyclohexanes only when proton and leaving group can get into a trans-diaxial arrangement corresponds to anti-coplanar Synthesis of Alkenes Strong, less hindered bases (MeO-, EtO-, etc) generally give the most substituted alkene (Saytzeff’s rule) Synthesis of Alkenes Strong, bulky bases give a mixture of Saytzeff’s product (more substituted) and the Hoffmann product (least highly substituted alkene) bulky bases often abstract a proton from a less hindered carbon Synthesis of Alkenes Example: Predict the elimination product(s) of the following reactions. CH CHCH CH 3 2 CH3CHCH2CH3 3 BrBr KOH KOH D C H5OH, D C2H52OH, H CH3 Br H DH NaOCH3 CH3OH, D Synthesis of Alkenes Example: Predict the two possible elimination product(s) for the following reaction. Which one will be the major product? Br CH3 Et3N D Synthesis of Alkenes Dehalogenation of Vicinal Dibromides two possible reagents NaI (E2 mechanism) Zn/HOAc (redox reaction) Synthesis of Alkenes Dehalogenation using I- takes place via a concerted, stereospecific E2 mechanism Anti-coplanar conformation required Trans-diaxial conformation required for cycloalkanes Synthesis of Alkenes Example: Predict the major elimination product formed in the following reactions. Br H H3C C H C NaI CH3 Br Br Br acetone NaI acetone Synthesis of Alkenes of Alcohols C Dehydration C removal of water CH3 CH3 C OH CH3 H2SO4 D CH3 CH2 C CH3 + H2O equilibrium process drive reaction to completion by removing alkene as formed (LeChatelier’s Principle) Synthesis of Alkenes Typical reaction conditions alcohol substrate Order of reactivity: o o o 3 > 2 > 1 alcohol acid catalyst conc. H2SO4 conc. H3PO4 heat Synthesis of Alkenes Mechanism of Dehydration (E1) Step 1: Protonation of the hydroxyl group (fast) Step 2: Ionization (RDS) + Synthesis of Alkenes Step 3: Proton abstraction (fast) Rearrangements to form more stable carbonium ions are common in dehydration reactions. Saytzeff’s product preferred. Synthesis of Alkenes Example: Propose a mechanism for the following reaction. CH3 CH3CCH2OH CH3 H2SO4 H3C o 150 C C=CHCH3 H3C What type of reaction occurs? Dehydration reaction with rearrangement E1 reaction with rearrangment 3 3 Synthesis CH 3 CH3CCH2 + H2SO4 CH3CCH2 O H of Alkenes CH3 + O H H Step 1: Protonation of OH group CH3 CH3 CH3 + H C + CH CCH O H + 2SO H2SO CH3CCH2 O H H 3 3 2 4 4 CH3CCH2OH C=CHCH o 3 H CH CH3 3 150 H3C CH 3 2: Ionization with Methyl Shift Step CHCH 3 3 + CH3CCH2OH CH3CCH2 O H CH3 CH3 H H2SO4 ~CH3 o 150 HSO4- H3C CH3 3 CH3C=CHCH C CH2CH + H2O 3 H3C + Synthesis of Alkenes Step 3: Abstraction of proton CH3 H CH3C C CH3 + H2O + H H C C C H3C + C=CHCH3 + H3O H3C Synthesis of Alkenes Example: Predict the major product formed in the following reaction. CH3 OH H2SO4 D Reactions of Alkenes The most common reactions of alkenes are addition reactions: the addition of a reagent to the pi bond with subsequent formation of new sigma bonds number of elements of unsaturation decreases Reactions of Alkenes The electrons in the p bond of C=C are delocalized above and below the sigma bond more loosely held In 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. Reactions of Alkenes Most 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. Reactions of Alkenes Addition of H-X to Alkenes C C + H X C C H X Reactions of Alkenes In 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” 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. Reactions of Alkenes Example: Predict the product formed in each of the following reactions. CH3 HBr HI Reactions of HI Alkenes Anti-Markovnikov Addition of HBr In the presence of peroxides, HBr adds to C=C via a free radical mechanism giving the “Anti-Markovnikov” product. CH3 H3C H HBr Br CH3CH2O-OCH2CH3 Works only with HBr (not HCl or HI) due to relative bond strengths. O O Reactions COOC of Alkenes Some common peroxides: O O HI COOC CH CO-OCCH Benzoyl peroxide 3 3 HI HI O O CH3CO-OCCH CH3CO-OCCH 3 3 Acetyl peroxide O O OO H3 O O Di-t-butyl peroxide H3C H Br HBr CH3 H H C H H C 3 CH3CH2O-OCH2CH33 Diethyl peroxide Br Br HBr Reactions of Alkenes Example: Predict the product of the following reaction. HI HBr CH3CO-OCCH3 O CH3 O H3C H HBr Br