Alkenes Part PDF

Alkenes continuation  2- Dehydration of Alcohols Heating alcohols with strong protonic acids like H2SO4, H3PO4 will generate alkenes. The role of acid is to catalyze the removal of (-OH) group in the form of H2O molecule. The (-OH) anion is Poor leaving group, because it is...

Alkenes continuation  2- Dehydration of Alcohols Heating alcohols with strong protonic acids like H2SO4, H3PO4 will generate alkenes. The role of acid is to catalyze the removal of (-OH) group in the form of H2O molecule. The (-OH) anion is Poor leaving group, because it is highly basic anion, while H2O is not basic and hence can be easily removed (Good leaving group). # The experimental conditions are related to the alcohol structure. H H H H concd H C C H C C + H2O H2SO4 H O H 180oC H H Ethanol (a 1o alcohol) OH 85% H3PO4 o + H2O 165-170 C Cyclohexanol Cyclohexene (80%) CH3 CH2 20% aq. H2SO4 H3C C OH o C + H2O 85 C H3C CH3 CH3 tert-Butyl alcohol 2-Methylpropene (84%) Acid-Catalyzed Alcohol Dehydration occurs by E1 mechanism E1 is a unimolecular reaction (involves one species = alkyloxonium ion), and it is a two steps reaction. The leaving group first leaves, generating a carbocation intermediate, which then loses a proton in a separate step.  Why the 3ry alcohol gives alkene easier than 2ry, than 1ry? The order of the stability of the carbocation intermediate is as follows: R R H H C+ > C+ > C+ > C+ R R R H R H H H 3o > 2o > 1o > Methyl ??? Due to the electron donation properties of the alkyl group, 3ry (3 alkyl = 3 +I), 2ry (2 alkyl = 2 +I) & 1ry (1 alkyl = 1 +I). Due to the lower stability of primary carbocations, primary alcohols dehydration proceed via E2 mechanism. Primary alcohol dehydration via E2 mechanism  What if more than one product can be formed? In alcohol dehydration the major product will always be Zaitsev product (the more substituted alkene). Alkene synthesis by debromination of vicinal dibromides. X C C C C X X X A vic-dihalide A gem-dihalide Mechanism of debromination is E2 To Summarize:  Quiz Draw structural formulas for the alkenes formed on dehydrohalogenation of the following haloalkanes. Predict which alkene is the major product.  Each of the following alcohols has been subjected to acid- catalyzed dehydration & yields a mixture of two isomeric alkenes. Identify the two alkenes & predict which one is the major?  Reactions Of Alkenes 1] Addition to the C=C (Electrophilic addition). 2] Miscellaneous reactions.  1- Addition to the C=C. Addition reactions (opposite of elimination) are characterized by two groups adding across a double bond. In the process, the double bond is destroyed, and we say that the two groups (X and Y) have “added” across the double bond.  Important Concepts & Terminology Characteristics of the double bond: 1) π bond is weaker than σ bond so the addition is energetically favorable. 2) The C=C bond acts as a nucleophile, so it can attack any electrophile. C C + X Y C C X Y  bond  bond 2 bonds Bonds broken Bond formed  Regiochemistry: Regiochemistry refers to the positioning of the H and the Br in the product: which side gets the H and which side gets the Br? Regiochemistry becomes irrelevant when:  Stereochemistry: Suppose that we have an anti-Markovnikov addition of H and OH across this alkene: How do these group add on the double bond? (from above, from below? In the same direction or opposite?), in order to show that we need first to show what is above & what is below?  If both groups added on the same side of the plane (either from above the plane or from below the plane), we will call this a syn addition: If each group added on opposite side of the plane, we would call this an anti addition:  We see that there are two products that arise from a syn addition, and two products that arise from an anti addition:  1- Hydrogenation of alkenes (H-H addition) A] General Notes: The process of adding hydrogen to an alkene is a reduction. Without a catalyst the reaction does not take place at an appreciable rate (Catalytic hydrogenation). The most commonly used catalysts for hydrogenation (finely divided platinum, nickel, palladium) apparently serve to adsorb hydrogen molecules on their surface. B] Mechanism: H H H H H3C C C H H CH3 H3C C C CH H3C C 3 C CH H H H H Syn H H 3 adsorbtion T.S addn Alkane(Butane) Ni crystal Ni crystal C] Regiochemistry: Irrelevant as we are adding 2 H. D] Stereochemistry: Syn addition.  2- Addition of H-X to the double bond (Hydrohalogenation) A] General Notes: Hydrogen halides (HI, HBr, HCl, and HF) add to the double bond of alkenes. The order of reactivity of the HX is HI > HBr > HCl > HF (??) B] Mechanism: C] Regiochemistry: Since we are adding two different atoms, regioselectivity arises if we have an asymmetrical alkene. The regiochemistry will be determined by the preference for the reaction to proceed via the more stable carbocation intermediate. Addition of HX gives Markovnikov product. D] Hydrohalogenation of alkenes to give anti-Markovnikov product: This anti-Markovnikov addition occurs only when HBr is used in the presence of peroxides and does not occur significantly with HF, HCl, and HI even when peroxides are present. HBr addition between Markovnikov & anti- Markovnikov  Quiz Draw the product(s) for each of the following reactions (Try the mechanism at home):