Bài giảng hoá học hữu cơ alkanes

Chapter 2010 ALKANES Outlines Homologous series of alkanes Isomerism Nomenclature Conformation of ethane Laboratory preparation Physical properties Chemical reactions  Alkanes are hydrocarbons, they contain only carbon and hydrogen  The general formula is CnH2n+2 ( n≥1), n is an integer  Alkanes are also called alphatic compounds, meaning “fat”  Consist of only sp3 hybridized C and H atoms connected by sigma bonds  The simplest molecules is methane, CH4 natural gas), ethane (C2H6), propane (C3H8), octane (C8H18 gasoline),…  Methane, ethane form a significant portion of the atmospheres of the outer gas planets  Jupiter, Saturn, Uranus, Neptune (1.5% methane, 1.5 ppm ethane)  Some alkanes form through the slow decomposition of organic substances (wood, lignite and mineral coal)  The most important commercial sources for alkanes are natural gas and oil Homologous series  What is a homologous series?  A homologous series is a series of organic compounds with a similar general formula, possessing similar chemical properties due to the presence of the same functional group, and shows a gradation in physical properties as a result of increase in molecular size and mass Homologous series  If one hydrogen atom from methane (CH4) is replaced by a methyl radical, -CH3 the new hydrocarbon will have the composition CH3CH3 (C2H6) named ethane Homologous series  If one hydrogen atom from ethane is replaced by a methyl radical -CH3, the new hydrocarbon propane CH3-CH2-CH3 (C3H8) will be obtained  If you continue the successive substitution of one H atom from the hydrocarbon with one radical -CH3 you will obtain a series of hydrocarbons, each different from one another with one group of CH2  Alkanes belong to a homologous series of organic compounds in which the members differ by a constant atomic mass of 14 Homologous series Methane CH4 Ethane CH3-CH3 Propane CH3-CH2-CH3 Butane CH3-CH2-CH2-CH3 Pentane CH3-CH2-CH2-CH2-CH3 Hexane CH3-CH2-CH2-CH2-CH2-CH3 Heptane CH3-CH2-CH2-CH2-CH2-CH2-CH3 Octane CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH3 Nonane CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3 Isomerism  What is the isomers ?  Isomers are compounds that have the same numbers and kinds of atoms but differ in the way the atom arranged  Alkanes are generally unaffected by most bases and acids  The C-H bond of alkanes are only slightly polarized (nearly the same electronegavity)  The C-H bond prefers to a homolytic breakage producing free radicals Halogenation  Halogenation is a substitution reaction, in which a C-H bond is broken and a new C-X bond is formed R-H + X2 → R-X + HX  The reaction is usually with Cl2 and Br2  with F2 - explosive; difficult to control  with iodine - generally unreactive  In this reaction a halogen atom replaces one or more of hydrogen atoms of the alkane  The reaction of chlorine with methane produces a mixture of: CH4 + Cl2 → CH3-Cl (methyl chloride) + CH2-Cl2 (diclomethane) + CH-Cl3 (triclomethane, chloroform) + CCl4 (tetraclomethane) + HCl  The reactivity of hydrogen atoms can be ranked in the order: tertiary(3º) >> secondary(2º) > primary(1º)  Chlorination of most higher alkanes gives a mixture of isomeric monochloro products as well as more highly halogenate compounds  Chlorine is relatively unselective  Chlorination of propane at 25 ºC gives CH3-CH2-CH3 + Cl2 → CH3-CH2CH2Cl + CH3-CHCl-CH3 45% 1-chloropropane chloropropane 55% 2-  Chlorination of isobutane: CH3 CH3 CH3 CH CH3 Cl2 light + CH3 CH3 CH CH2Cl CH3 C CH3 Isobutyl chloride (48%) Cl + HCl tert-Butyl chloride (29%) Bromination  Bromine is generally less reactive toward alkanes than chlorine  Bromine is more selective in the site of attack when it does react CH3 CH3 CH CH3  Br2 light 1270 CH3 CH3 CH3 C CH3 + Br tert-Butyl bromide (>99%) CH3 CH CH2Br (trace) The reaction of bromine with isobutane produces almost one product + HBr Radical mechanism of Halogenation  Free radical substitution (SR) occurs in steps  Initiation the halogen radicals form by homolysis  Energy in the form of heat, light or radical initiators (peroxides) is required Cl : Cl heat or light 2Cl  Chain reaction or Propagation  The halogen radical abstracts a hydrogen from the alkane to give an alkyl radical Cl + CH4 HCl + CH3 CH3 CH3Cl + Cl2  This reacts further + Cl  Chain termination step where the radicals recombine CH3 + Cl CH3Cl  The reactivity of the halogens decreases in the order: F2 > Cl2 > Br2 > I2 Video-1 Nitration reaction  Nitric acid react with alkanes in the gaseous phase at high temperatures produces to nitroalkanes:  R-H + HNO3 → R-NO2 + H2O  CH3-CH2-CH3 + HNO3 → CH3-CH2-CH2NO2 (32%) CH3-CH(NO2)CH3 (33%) CH3-CH2-NO2 (26%) CH3-NO2 (9%)  The Nitration reaction is not only break C-H bond, Oxidation  The combustion of hydrocarbons has been the most important source of heat energy for human civilizations  The mechanism of combustion is complex CnH2n+2 + (1.5n+0.5)O2 → (n+1)H2O + nCO2 CH3-CH2-CH3 + O2 → CO2 + 4H2O  Incomplete oxidation in the presence of a catalyst leads to various products R-CH3 + O2 → R-CH2-OH (alcohol) → R-CH=O (aldehyde) → RCOOH (acid) Thermic decomposition  Cracking breaks larger molecules into smaller ones  Thermal (Pyroliz): temperature > 6500C  Catalytic method : temperature < 6500C  The thermal cracking process follows a homolytic mechanism with formation of free-radicals  Acid catalysts are used for cracking (such as silica-alumina and zeolites)  They promote a heterolytic breakage of bonds yielding a carbocation and the very unstable hydride anion  The products of thermic decomposition are  smaller alkanes used as fuel,  alkenes used as starting materials for chemical industry CnH2n+2 CmH2m+2 + CpH2p where n = m + p and p >= [...]...Isomerism  Isomeric Alkanes - Alkanes with more than 3 carbon atoms can be arranged in a multiple number of ways, forming different structural isomers (constitutional isomers)  Constitutional isomers: differ in order of attachment of atoms  The carbon atoms in alkanes can be arranged in a straight chain  This isomer is called the n-isomer... 22,158,734,535,770,411,074,184 isomers, many of which are not stable Nomenclature  The trivial name for alkanes is "paraffins"  Trivial names are usually historical artifacts They’ve been retained due to familiar usage in industry  The first four alkanes with a unbranched chain are called: methane, ethane, propane, butane  Alkanes with five or more carbon atoms are named by adding the ending “-ane” to the appropriate