Synthesis of Alkenes Major approaches to the synthesis of alkenes: Dehydrohalogenation of Alkyl Halides E2 mechanism most useful 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 H3C H3C H C C CH3 Synthesis of Alkenes The most synthetically useful
dehydrohalogenation reactions occur under E2 reaction conditions. 3o or bulky 2o 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 N or H CH3CH )2CH t-BuO CCH
3 3 O - t-butoxide ion (CH3)2CH CH3 N or H t-Bu (CH3CH )2CH CCH 3 3 diisopropylamine
O (CH3)2CH CH3 (CH3CH2)3N or E - H3C H C CCH3 CHCH H3CH3CCH NCCH 3 3 or t-B 3 3
- O 2,6-dimethylpyridine Synthesis of Alkenes Mechanism of E2 Dehydrohalogenation concerted reaction B H anti-coplanar transition state C C
C C + B H + X X H Ph C H3C C Ph -
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 (Saytzeffs rule) as the major product. Synthesis of Alkenes Strong, bulky bases usually give the Hoffmann product (least highly substituted
alkene) as the major product bulky bases often abstract a proton from a less hindered carbon Synthesis of Alkenes Example: Predict all elimination product(s) of the following reactions. Which one is the major product? KOH KOH CH CHCH CH 3 2 3 CH3CHCH2CH3
C H OH, 2 5 C2H5OH, BrBr H CH3 Br H DH NaOCH3 CH3OH, Synthesis of Alkenes
Example: Predict all possible elimination products for the following reaction. Which one will be the major product? Br CH3 Et3N Synthesis of Alkenes Dehalogenation of Vicinal Dibromides two possible reagents NaI (E2 mechanism) Zn/HOAc (redox reaction) +
Na I - Br C C Br C C + I Br + NaBr
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. H Br H3C
C H C NaI CH3 Br Br Br acetone NaI acetone
Synthesis of Alkenes Dehydration of Alcohols removal of water C C CH3 CH 3equilibrium C OH process H2SO4 CH2 C CH3 CH3 + H2O
drive reaction to completion by removing alkene as formed CH (LeChateliers Principle) 3 Synthesis of Alkenes Typical reaction conditions alcohol substrate Order of reactivity: 3o > 2o > 1o 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. Saytzeffs product preferred.
Synthesis of Alkenes Example: Propose a mechanism for the following reaction. CH3 CH3CCH2OH CH3 H2SO4 o 150 C H3C H3C C=CHCH3
Synthesis of Alkenes Step 1: Protonation of OH group Step 2: Ionization with Methyl Shift Synthesis of Alkenes Step 3: Abstraction of proton Synthesis of Alkenes Example: Predict the major product formed in the following reaction. OH conc. H2SO4 heat 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 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 electrons to the electrophile and forming a
new 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. Markovnikovs 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 Markovnikovs 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 COOC Reactions of Alkenes Some common peroxides: O O HI COOC CH CO-OCCH
Benzoyl peroxide 3 3 HI HI O O CH CO-OCCH CH3CO-OCCH 3 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
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