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MCAT O-Chem
Ch. 5
| Question | Answer |
|---|---|
| physical properties of alkenes | mp and bp increase with increasing molecular weight. Trans-alkenes generally have higher mp than cis because they are more symmetric which allows for better packing. trans tend to have lower bp than cis because they are less polar |
| polarity | result of an asymmetric distribution in a molecule. has partially positive and partially negative region. dipole points from electropositive alkyl to electronegative alkene |
| common method of synthesizing alkenes | elimination reaction of either alcohols or alkyl halides |
| E1 | 2-step process and the reaction rate depends on the concentration of only the substrate. |
| Step 1 of E1 rxn | departure of leaving group and formation of carbocation intermediate |
| Step 2 of E1 rxn | a proton on an adjacent carbon (the B-carbon) is removed by a weak base, and the double bond is formed with electrons from the now broken carbon-hydrogen bond |
| factors that favor E1 | polar protic solvents, the ability to form a stable carbocation, highly branched carbon chains, good leaving groups, and the absence of a good nucleophile. |
| E2 | rate depends on the substrate and nucleophile. a strong base removes a proton, a halide ion anti to the proton leaves, resulting in the formation of a double bond. produces either cis or trans |
| ways to control E2 vs Sn2 | 1. steric hindrance is important in E2 rxns. 2. a stronger base favors E2 over Sn2, Sn2 is favored over E2 by weak lewis bases (strong nucleophiles) |
| controlling Sn1 vs E1 | done by controlling conditions, specifically polarity of solvent and temperature. |
| redox rxn and H | if a species is reduced it will be getting more hydrogen, if it is oxydized it will be losing hydrogen, and thus gaining double bonds and, often oxygen |
| catalytic hydrogenation | reducing an alkene by adding molecular hydrogen to the double bond with the aid of a metal catalyst. Added on same face of double bond, syn addition |
| Markovnikov's rule | the addition of a group to the more substituted carbon of a double bond. Does this because the more stable carbocation intermediate(the more highly substituted) will form in the slowest step and the nucleophile then attack +charge in fast rxn 2nd step |
| addition of water | double bond is protonated according to Markovnikov's rule, forming the most stable carbocation. Carbocation reacts with water, yielding a protonated alcholol, which then loses a proton to become and alcohol. must be performed at low temp |
| Free radical additions | Occurs in the presence of peroxides, oxygen, or UV light. disobeys Markovnikov rule because X adds first to the double bond, producing most stable free radical. Halogen ends up on least substituted carbon |
| what characterizes a mild or weak reaction | cold, basic, dilute (such as adding alcohols to a double bond) |
| what characterizes rigorous or strong reactions | hot, acidic (breaking the double bond altogether and forming carboxylic acids) |
| peroxycarboxylic acids | are strong oxidizing agents that can oxidize alkenes. products are epoxides (also called oxiranes) syn addition |
| polymerization | creation of long, high-molecular weight chains(polymers) composed of repeating subunits(monomers). usually occurs through a radical mechanism, but anionic and even cationic polymerization commonly observed. |
| alkynes | hydrocarbons that possess one or more C-C triple bonds. all triple bonds form straight lines with 180 between carbons as a result of the sp hybridization |
Created by:
adam87
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