Methane - Organic Chemistry I - Lecture Slides, Slides of Organic Chemistry

Download Methane - Organic Chemistry I - Lecture Slides and more Slides Organic Chemistry in PDF only on Docsity! Methane docsity.com Hydrocarbons – compounds containing only carbon and hydrogen. hydrocarbons aliphatic aromatic alkanes alkenes alkynes docsity.com Chemistry of methane (reactions)? CH4 + H2O  CH4 + conc. H2SO4  CH4 + conc. NaOH  CH4 + sodium metal  CH4 + KMnO4  CH4 + H2/Ni  CH4 + Cl2  NR (no reaction) NR NR NR NR NR NR docsity.com Methane is typically unreactive. It does not react with water, acids, bases, active metals, oxidizing agents, reducing agents, or halogens. Reactions of methane: 1. Combustion (oxidation;complete & partial) 2. Halogenation docsity.com Reactions of Methane 1. Combustion (oxidation) a) complete oxidation CH4 + 2 O2 , flame or spark  CO2 + H2O + energy b) partial oxidation 6 CH4 + O2 , 1500 o  CO + H2 + H2C2 (acetylene) CH4 + H2O , Ni, 850 o  CO + H2 docsity.com CH4 + Cl2, hv  CH3Cl + HCl methyl chloride chloromethane CH3Cl + Cl2, hv  CH2Cl2 + HCl methylene chloride dichloromethane CH2Cl2 + Cl2, hv  CCl3H + HCl chloroform trichloromethane CCl3H + Cl2, hv  CCl4 + HCl carbon tetrachloride tetrachloromethane docsity.com CH4 + Br2, hv  CH3Br + HBr methyl bromide bromomethane CH3Br + Br2, hv  CH2Br2 + HBr methylene bromide dibromomethane CH2Br2 + Br2, hv  CBr3H + HBr bromoform tribromomethane CBr3H + Br2, hv  CBr4 + HBr carbon tetrabromide tetrabromomethane docsity.com CH3I CH2I2 iodomethane diiodomethane methyl iodide methylene iodide CHI3 CI4 triiodomethane tetraiodomethane iodoform carbon tetraiodide docsity.com Mechanism for the monochlorination of methane initiating step: 1) Cl2  2 Cl• propagating steps: 2) Cl• + CH4  HCl + CH3• 3) CH3• + Cl2  CH3Cl + Cl• then 2), then 3), then 2), etc. terminating steps: 4) Cl• + Cl•  Cl2 5) Cl• + CH3•  CH3Cl 6) CH3• + CH3•  CH3CH3 docsity.com Energy Changes? ΔH Homolytic bond dissociation energies (see inside the front cover of M&B) H—Cl 103 Kcal/mole Cl—Cl 58 Kcal/mole CH3—H 104 Kcal/mole CH3—Cl 84 Kcal/mole docsity.com We need only consider those bonds that are broken or formed in the reaction. CH3—H + Cl—Cl  CH3—Cl + H—Cl +104 +58 -84 -103 PE: +162 -187 ΔH = +162 –187 = -25 Kcal/mole (exothermic, gives off heat energy) docsity.com Rates of chemical reactions depend on three factors: Collision frequency (collision per unit time) Probability factor (fraction of collisions with correct geometry) Energy factor (fraction of collisions with sufficient energy) ―sufficient energy‖ = Energy of activation, minimum energy required for a collision to go to the product. docsity.com FE] Collisions with energy > Ei Collisions with energy > Es —> —> Number of molecules of particular energy Number of molecules of particular energy Average Energy —> Energy —> Figure 2.5. Distribution of kinetic energy Figure 2.6. Distribution of kinetic energy among molecules. among collisions, Wh Alntethutinn AC anMicinn anacniane ne tn mioht awnant be dacerihed hu a ® docsity.com Eact/RTePZrate  ** Z = collision frequency P = probability factor e-Eact/RT = fraction of collisions with E > Eact Note: rate decreases exponentially as the Eact increases! docsity.com Rate determining step (RDS) = the step in the mechanism that determines the overall rate of a reaction. In a ―chain reaction‖ this will be the slowest propagating step. For chlorination of methane, which propagating step is slower? Step 2) ΔH = +1 Kcal/mole Eact > +1 Kcal (estimated) Step 3) ΔH = -26 Kcal/mole Eact > 0 Kcal (estimated) Step 2 is estimated to be slower than step 3 and is the RDS docsity.com An ―alternate mechanism: 2) Cl• + CH4  CH3Cl + H• 3) H• + Cl2  HCl + Cl• Why not this mechanism? Step 2: ΔH = +104-84 = +20 Kcal/mole; Eact > +20 Kcal Step 3: ΔH = +58-103 = -45 Kcal/mole; Eact > 0 Kcal RDS for this mechanism is step 2 and requires a minimum of 20Kcal/mole! Unlikely compared to our mechanism where the RDS only requires an estimated minimum of 1 Kcal! docsity.com 2. Halogenation Δ or hυ CH4 + X2  CH3X + HX requires heat or light X2: Cl2 > Br2  I2 why?…how?…mechanism docsity.com ΔH for each step in the mechanism by halogen: F Cl Br I 1 +38 +58 +46 +36 2 -32 +1 +16 +33 3 -70 -26 -24 -20 4 -38 -58 -46 -36 5 -108 -84 -70 -56 6 -88 -88 -88 -88 docsity.com Estimation of Eact for the propagating steps: Eact (est.) F Cl Br I 2 >0 >+1 >+16 >+33 3 >0 >0 >0 >0 Step 2 is the RDS Rate Cl2 > Br2 because in the RDS Eact(Cl2) < Eact(Br2) NR with I2 because RDS Eact(I2) > +33 Kcal/mole only 1/1012 collisions would have E > +33 at 275o docsity.com Potential energy ——> CH, + Cl: —> CH; + HCI Progress of reaction —> 2.3 Potential energy changes during the progress of reaction: the methane-chlorine atom reaction. docsity.com Hammond’s Postulate: the higher the Eact of a step in a mechanism, the later the transition state is reached and the more the transition state will look like the products. In step 2 of the mechanism for the bromination of methane, the Eact is estimated to be > +16 Kcal/mole. Since the Eact is high, the transition state is reached later in this step than it is in chlorination and will look more like the products: [ Br----H-----------CH3 ] ‡ δ• δ• docsity.com Reactions of Methane 1. Combustion (oxidation) a) complete oxidation CH4 + 2 O2 , flame or spark  CO2 + H2O + heat b) partial oxidation 6 CH4 + O2, 1500 oC  CO + H2 + H2C2 CH4 + H2O, 850 o, Ni  CO + H2 2. Halogenation CH4 + X2, heat or hv  CH3X + HX requires heat or light Cl2 > Br2 NR with I2 docsity.com