Alkynes

2/16/09 9:10 PM Alkynes: C=_C triple bond General formula: CnH2n-2 Example: HC=_CH, common name: acetylene (ethyne) Other alkynes: CH3C=_CH, methylacetylene (propyne); CH3CH2C=_CH, ethylacetylene (1-butyne); CH3C=_CCH3, dimethylacetylene (2-butyne) Properties of Alkenes and Alkynes: Colorless, like alkanes. Low-molar-mass compounds are gases, whereas compounds with higher molecular weights are liquids or solids. Oxidized by O2 to give CO2 and H2O. Called unsaturated compounds Addition reactions: molecules with the general formula X?Y (such as hydrogen, halogens, hydrogen halides, and water) add across the carbon-carbon double bond in order to increase the number of groups attached to each carbon. The result is a compound with four groups bonded to each carbon. The products of addition reactions are often substituted alkanes. For example, the addition of bromine to ethylene forms 1,2-dibromoethane; the addition of 2 mol of chlorine to acetylene gives 1,1,2,2-tetrachloroethane. Markovnikov?s rule: when a reagent HX adds to an unsymmetrical alkene, the hydrogen atom in the reagent becomes attached to the carbon that already has the largest number of hydrogens. Nomenclature of substituted alkanes: The substituent groups in substituted alkanes are identified by the name and position of the substituent on the alkane chain. Hydrogenation: A reaction in which the reagent added to the double bond is hydrogen (H2). Often speeded up in the presence of a catalyst like platinum, palladium, and rhodium. Makes foods less susceptible to spoilage; converts it from a liquid to a solid. Alkynes 2/16/09 9:10 PM 2/16/09 9:10 PM