Alkanesalcohols（化学课件）地道英语教学.ppt

Alkyl groups are derived from an alkane molecule by removing a hydrogen atom Alkyl groups are named using the same system as the straight chain alkanes but changing the “-ane ” to “-yl ”. Branched-chain alkanes are named using the longest C chain as the stem. The substituents (in this case, alkyl groups) are named as prefixes. All alkanes having 4 or more carbon atoms show structural isomerism due to the branching of the hydrocarbon chain. Methane, ethane and propane have NO isomers. All hydrocarbons burn in excess O2 to give CO2, H2O and to release heat (complete combustion). Applications: Use of hydrocarbons as fuels for energy combustion of petrol in motor car engines Liquidified Petroleum Gas in cylinders for cooking food When methane reacts with chlorine in the presence of UV light (or sunlight), one of H atoms of methane is substituted by a Cl atom. CH3Cl undergoes further substitution with chlorine to produce a mixture of alkyl chlorides: CH3Cl + Cl2 ???? CH2Cl2 + HCl CH2Cl2 + Cl2 ???? CHCl3 + HCl CHCl3 + Cl2 ???? CCl4 + HCl . (alkane) values indicate which alkane is a better fuel than the other COMBUSTION Alkanes undergo substitution with halogens in the presence of ultraviolet light/sunlight to yield a mixture of alkyl halides. RH + X2 ???? RX (a mixture) + HX R: alkyl group X: halogen UV light SUBSTITUTION CH4 + Cl2 ????? CH3Cl + HCl UV light SUBSTITUTION i.e. CH4 + Cl2 ??? a mixture of CH3Cl + CH2Cl2 + CHCl3 + CCl4 + HCl UV light SUBSTITUTION For mono-substitution by chlorine: Reagent: Cl2 Condition: UV light, excess CH4 CH4 + Cl2 ??? CH3Cl + HCl chloromethane UV light For poly-substitution by chlorine: Cl2 is used in excess, major product is CCl4 (tetrachloromethane). SUBSTITUTION Free radical substitution reactions are very rapid in the presence of light. However, there is no reaction in the dark at room temperature,. SUBSTITUTION Alkanes react with bromine si