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Alkene reactions

Organic Chemistry MCAT

QuestionAnswer
What is the characteristic that distinguish alkenes from alkanes when ranking boiling point and melting point? The stereochemistry. Transk alkenes have a higher melting point that cis alkenes because more symmetry. But, trans alkenes have low boiling point because they are less polar (intermolecular forces) because the vectors cancel each other out.
How alkenes can be synthesized? Elimination of alcohols and alkyl halides. i.e-dehydration and dehydrohalogenation (hydrogen and a halide)
What are the two types of elimination reactions? E1 and E2
What is the E1 reaction? Exactly the same as SN1 reaction. Characterized by the formation of the carbocation intermediate.
What distingushes E1 from SN1? Higher temperatures favor E1 pathway
What is the E2 reaction? Depends on a strong base to eliminate the proton. Forms two products and the trans product is favored as it is sterically unhindered.
What are the features that distinguish E2 from SN2? Highly substituted carbon chains favor E2 over SN2 because E2 favors the more stable alkene. Also, only 1" and 2" carbon chains can be attacked by the nucleophile during SN2. A strong base prefers E2 while SN2 prefers an excellent nucleophile (weak base)
What are the reactions that an alkene undergo? Reduction, electrophilic additions (addition of HX, X2, and water), free radical additions, hydroboration, oxidation (KMN04, ozonolysis, percarboxylic acids) and polymerization.
What is the alkene reduction? Platinum, Palladium and Raney nickel are used to add hydrogens to alkenes and make them alkanes. The two hydrogens should end up in the same face.
What is electrophilic addition of alkenes? The double bonds of alkenes act as a nucelophile while the lewis acids will act as electrophiles.
What is the Markovnikov rule? The carbocation on the carbon that is most substituted will react with the electrophiles.
What happens when HX and X2 is added to alkenes? The "X" of the HX goes to the most stable carbocation intermediate while the "H" goes to the least substituted carbon forming an alkyl halide. The alkene reaction with dihalide initially forms a cyclic halonium ion followed by a dihalo compound.
What happens when water is added to an alkene? Adding water in presence of an acid (sulfuric acid) yields an alcohol
What is the free radical addition of alkenes? When a bromine radical is added to an alkene, it adds to the least substituted carbon (anti-markovnikov). It occurs in presence of peroxides and UV light.
What is the hydroboration of alkenes? The borane in presence of hydrogen peroxide will result in an anti-markovnikov reaction resulting in the formation of an alcohol. The -OH group is on the least substituted carbon.
Explain the oxidation of nonterminal alkenes using KMNO4? An alkene reacts with cold KMNO4 and results in a 1,2 vicinal diols (on the same side). When reacted with hot KMNO4, nonterminal alkene result in two moles of carboxylic acids. But, if the nonterminal alkenes are disubstituted, a ketone is formed.
What would happen when a terminal alkene react with hot KMNO4? A terminal alkene reacts with hot KMNO4, and results in a carboxylic acid and carbon dioxide. If terminal alkene is disubstituted, then a ketone and a carbon dioxide is formed.
Explain the ozonolysis of an alkene? The ozonolysis of alkene in the presence of zinc and acid will result in a aldehyde, and a ketone if disubstituted.
What would happen when the aldehyde/ketone that is formed due to ozonolysis is reduced with sodium borohydride or lithium aluminum hydride? A primary alcohol for an aldehyde and a secondary alcohol for a ketone.
Explain the reaction of a peroxycarboxylic acid (-COOOH) with an alkene? The reaction of an alkene with MCPBA or peroxyacetic acid results in an epoxide
Explain the polymerization of alkenes? In the presence of high temperatures and high pressures, polymers can be formed.
Created by: prijuvarghese