Chemistry Genius - Properties and Reactions of Alkenes lyrics

Nomenclature Physical Properties The Double Bond Alkenes are unsaturated moleules. Catalytic Hydrogenation of Alkenes When hydrogen gas reacts with alkene in the presence of catalysts, hydrogen is distributed across the double bond. Hydrogenation produces heat. As the stability of the alkene increases, the heat of hydrogenation decreases. Alkenes become more stable with more carbon bonded to the double-bonded carbons. Oddly enough, cis-cycloalkanes are much more strained than their trans counterparts. Preparation of Alkenes from Haloalkanes and Alkyl Sulfonates Bimolecular elimination. Leaving group and beta H must be anti to each other, thus E2 is stereospecific. Favors trans over cis. Complete trans preference is rare in E2. by Dehydration of Alcohols The OH group and a neighboring hydrogen are removed. They bond together to form water. Primary alcohols are the most difficult to dehydrate, thus the requisite high temperatures. Dehydrations occur with rearrangment. Dehydration of alcohols occurs with a strong nonnucleophilic acid, such as sulfuric acid and elevated temperatures. Primary alcohols become alkenes from 170°C-180°C via the E2 mechanism. Secondary alcohols become alkenes from 100°C-140°C typically via the E1 mechanism. Tertiary alcohols become alkenes from 25°C-80°C via the E1 mechanism. Reactions of Alkenes Catalytic Hydrogenation The hydrogenation of an alkene to an alkane needs to take place over insoluble materials like palladium, platinum, and nickel. The hydrogen-hydrogen bond disa**ociates and the hydrogens bond to the surface of the metal. This makes it easier for the hydrogen to bond to the alkene. Hydrogenation is stereospecific. When hydrogen atoms are added to the same face of the double bond, it is a syn addition. However, when there is steric hindering, hydrogen will add itself to the less hindered face, allowing for enantioselective hydrogenation. Addition of Hydrogen Halides On asimmetrical alkenes, Markovnikov's rule predicts that the hydrogen will attach to the less substituted side of the double bond, and the halide at the more substituted end. However, if instead of a halide, we choose to add a molecule that is not a good halide, carbocation shifts may occur. Alcohol Synthesis by Hydration An alkene in a aqueous solution of sulfuric acid, which is poorly nucleophilic, water acts as the nucleophile. Water follows Markovnikovs rule. Howevver, carbocation rearrangement may occur. Addition of Halogens Halogen addition works best for chlorine and bromine. Added on opposite sides of the double bond, considered an anti addition. Another ion, such as water, could compete before the second halide could be added. This results in the anti addition of Bromide and OH. HBr is also formed. The OH can close with a neighboring hydrogen to form an oxacyclopropan.