Alkenes and Alkynes

Microbiology department KASU/16/MCB/ CHEM 104 Assignment Assignment Questions: what are the physical and chemical properties of Alkenes, Alkynes and Aromatic hydrocarbons? Alkenes Physical properties 1. Alkenes are colorless gas with faint sweetish smell 2. It is sparingly soluble in water. 3. It is slightly less dense than air. 4. It has no action on litmus. Chemical properties Combustion 1. Alkenes burns readily in air or oxygen with a smoky and luminous flame because of its high proportion by mass (about 86%) of carbon. In the process, it oxidizes completely to form water and carbon(IV)oxide. 2. Alkenes forms explosive mixtures when mixed with air. Addition reactions An addition reaction involves the direct addition of an attacking reagent across the double or triple bond of an unsaturated compound to yield a saturated product or at least one in which the degree of saturation is increased. 1. Alkenes undergoes addition reactions with hydrogen, halogens, hydrogen halides, and many other compounds. With hydrogen: If a mixture of hydrogen and ethane is passed over a finely divided nickel, platinum or palladium catalyst, ethane is formed as an addition compound. H2C=CH2(g) CH3CH3 or C2H6. This reaction, which is also known as hydrogenation, is very important in the hardening of oils. With halogen: Clorine and bromine add on to ethane readily at room temperature to form the halogenated product, 1,2 – dicloroethene and 1,2 – dibromoethane respectively. CH2=CH2(g) + Cl2(g) CH2ClCH2CL orC2H4Cl2(I) CH2=CH2(g) + Br2(I) CH2BrCH2Br orC2H4Br2(I) Similalarly, with iodine solution (iodine dissolved in ethanol), 1, 2 – diiodoethane is formed at room temperature. With hydrogen halides: Ethene combines readily with hydrogen iodide vapour at room temperature to form iodoethane. Hydrogen chloride and hydrogen bromide reacts in a similar manner but more slowly. CH2=CH2(g)+HI(g) CH3CH2I C2H4I(I) Polymerization Polymerization is a process whereby two or more simple molecules are linked to form a much larger molecule. Ethane and its derivatives undergo polymerizations to form important compounds such as poly(ethane) and poly (chloroethene) Alkynes Physical properties 1. Ethyne is a colorless gas with a characteristics sweet smell when pure. 2. It is only sparingly soluble in water. 3. It is slightly less dens than air. 4. It is unstable and may explode on compression to a liquid. For storage purposes, it is usually dissolved in propanone (acetone) and kept in steel cylinders at about 12 atmospheres pressure. Chemical properties Combustion ethyne burns in air to give a very smoky and luminous flame, owing to its high carbon content(about 92% by mass). A mixture of ethyne with air or oxygen may explode violently on ignition. In pure oxygen, ethyne undergoes complete combustion and burns with non-luminous, very hot flame of about 3000 C. This flame is made use of in the oxy-Ethyne(oxyacetylene) touch which is used in welding and cutting metal scraps. 2C2H2(g) + 5O2(g) 2H2O(g) + 4CO2(g) Additional reaction Ethyne is highly unsaturated, containing a carbon carbon triple bond in its structure. It can readily form addition products combining with a maximum of four univalent atoms or radicals per molecule. These reactions take place in two stages: 1. The first stage yields a product with a carbon-carbon double bond. 2. The second stage converts this into a fully saturated compound with only carbon-carbon single bonds. With hydrogen Ethyne reacts with twice its volume of hydrogen in the presence of a nickel catalyst at about 200 C to form ethane, and then ethane by successive additions of hydrogen atoms across the carbon-carbon triple bond. With the halogen Ethyne reacts with chlorine and bromine in the presence of metallic halide catalyst at room temperature to yield halogenated compounds. In the case of chlorine, the reaction is very different if pure Ethyne and chlorine are used in the absence of a catalyst. Instead of an addition compound, carbon and hydrogen chloride will be formed with a violent explosion. With hydrogen halide Ethyne combines readily with hydrogeniodide at room temperature, and with hydrogen bromide at 100 C. The reaction with hydrogen chloride, however is very slow. Polymerization Ethyne polymerizes to form the aromatic hydrocarbon, benzene, when it is passed through a hot tube containing a complex organo-nickel catalyst. 3C2H2(g) C6H6(I) AROMATIC HYDROCARBONS-BENZENE Physical properties 1. Benzene is a colorless liquid with a sweet smell. 2. It is insoluble in water. 3. It has a boiling point of 80 C. Chemical properties Benzene undergoes substitution reactions, e .g, chlorobenzene is formed when benzene reacts with chlorine. Cl (I) + Cl2(g) Fe (I) + HCl(g) FeCl3 Reference: Chemistry textbook for secondary school