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Alkanes L.O. 1. To know the general formula of alkanes. 2. To be able to construct simple mechanisms for alkane reactions. 3. To find out how alkanes are extracted from crude oil for use as fuels. Starter: Name your compound, draw its full structural formula and skeletal formula, and identify all functional groups present Alkanes – some key facts Alkanes CnH2n+2 (n = 1, 2, 3 etc) Alkanes are generally very because the molecules are since C and H have Alkanes are also described as because they contain Types of bond breaking Homolytic bond breaking Alkane s Homo – same Covalent bond breaks equally resulting in the production of FREE RADICALS Homolytic fission Heterolytic bond breaking Hetero – different Covalent bond breaks and one atom takes both electrons. Ionic intermediates result Heterolytic fission Alkanes with halogens, Cl2 or Br2 Initiated by energy from UV light successive substitution (replacement) of H atoms in the alkane by Cl / Br atoms. mixture of chloro- / bromoalkanes eg Methane + Cl2 mono-, di-, tri- and tetrachloromethanes Excess CH4 CH3Cl MORE likely Excess Cl2 CCl4 MORE likely Halogen-methane substitution : mechanism = a FREE RADICAL SUBSTITUTION mechanism Also a CHAIN REACTION a product from one stage of process is used in the next stage. THREE stages : 1. INITIATION (Start) 2. PROPOGATION (Products Grow) 3. TERMINATION (Finish) 1. Initiation Caused by energy input from UV light covalent bond in a few Cl2 molecules broken HOMOLYSIS - one e- from bond pair to each atom neutral halogen atoms = FREE RADICALS Cl Cl Cl + Cl UV energy 2. Propogation steps . (a) Cl free radical from initiation step collides with a molecule of CH4 and removes a H atom. H H C H H H + Cl H C + H first substitution product (HCl) . + a methyl free radical (H C ). 3 H Cl (b) . CH free radical from step (a) collides with a 3 NEW molecule of Cl2 and removes a Cl atom. H H C H H Cl Cl H C Cl + H second substitution product (CH3Cl) . + a NEW chlorine free radical (Cl ). Cl Propagation steps (a) and (b) ALTERNATE HCl + CH3Cl each repetition of (a) and (b) 3. Termination steps propagation steps stopped by consumption of free radicals combine to form neutral molecules Free Radicals in Mixture CH3 Cl . . Possible termination products CH3Cl Cl2 CH3CH3 H H C H + U.V. Cl2 H H + Cl2 Cl C Cl Cl + HCl H chloromethane H Cl Cl C + H H HCl Cl + HCl Cl dichloromethane tetrachloromethane + Cl2 Similarly for Br2 C + Cl2 Cl H C Cl Cl + HCl trichloromethane Your turn…. Draw out a full mechanism to show the free radical substitution reaction of bromine with ethane to form 1,1-dibromoethane. Reacting particles 1. Free radicals : . Neutral particle with an unpaired electron . eg Cl , H3C 2. Nucleophiles : “Nucleus loving” particle - electron rich - can provide a lone pair to form a covalent bond eg :NH3,:OH2, :OH-, :CN3. Electrophiles : (NOT metal ions!) “Electron loving” particle - electron deficient - can accept a lone pair to form a covalent bond eg H+, Br+, +CH3 Can you…….? 1. Confidently name organic compounds? 2. Confidently identify and name functional groups? 3. Confidently identify types of structural isomerism in simple organic compounds? 4. Identify a free radical? 5. Draw the free radical substitution mechanism for the halogenation of alkanes? Homework: Prepare an A4 poster explaining how fuels (alkane fuels) are made from crude oil. Your poster must contain information on the following: Fractional distillation, Cracking, Catalytic cracking, Reforming, Adding ethanol and ethers Due: Thursday 14th Jan