北京化工大学：《有机化学》课程教学资源（英文讲义）Chapter 3 Reactions of Alkanes：Bond-Dissociation Energies, Radical Halogenation, and Reactivity.pdf_大学文库

2 3-2 Structure of Alkyl Radicals: Hyperconjugation The relative stabilities of the alkyl radicals can be explained by the overlap between the orbital containing the unpaired electron (p orbital) and a hydrogen bonding orbital on the adjacent carbon atom (sp3 orbital). This overlap is called hyperconjugation. No hyperconjugation Increasing Hyperconjugation Æ Hyperconjugation allows the bonding pair of electrons in the σ bond to delocalize into the partially empty p lobe. The stabilization of radicals by resonance, another type of delocalization involving π orbitals, is considerably stronger. Another factor in the stabilization of secondary and tertiary radicals is the relief of steric crowding as the radical carbon assumes sp2 hybridization. 3-3 Conversion of Petroleum: Pyrolysis High temperatures cause bond homolyis. Both C-H and C-C bonds are ruptured at high temperature in a process called pyrolysis. The resulting radicals can combine to form higher or lower molecular weight compounds, or form alkenes by further hydrogen extraction: Catalysts are often used to accelerate and control pyrolysis reactions: o Zeolite,482 C,2min 3 456 17% 31% 23% 18% 11% Dodecane C + C + C + C + other products ⎯⎯⎯⎯⎯⎯→ Catalysts function by lowering the energy of activation of the reaction, often by changing the reaction mechanism. In addition, catalysts also frequently increase the selectivity of the reaction, increasing the amounts of certain products selectively, compared to the uncatalyzed reaction. Petroleum is an important source of alkanes. Breaking an alkane down into smaller fragments is called cracking. Cracking is used in the processing of crude oil in order to alter the natural hydrocarbon composition to a more useful set of products. The higher boiling point components of crude petroleum are cracked by pyrolysis. Cracking the residual oil fraction yields approximately: 30% gas, 50% gasoline, 20% higher molecular weight oils, and coke. Chlorination of Methane: The Radical Chain Mechanism 3-4 Chlorine converts methane into chloromethane. Chlorine and methane gas do not react unless irradiated using UV light, or heated to a temperature above 300oC. During the chemical reaction, the first product formed is chloromethane, CH3Cl (and HCl). If sufficient chlorine is present, further substitution may occur, forming CH2Cl2, CHCl3, and finally CCl4. The chlorination of methane can be shown to be exothermic: Since this reaction does not occur at room temperature, the activation energy must be high. The mechanism explains the experimental conditions required for reaction. A mechanism is a detailed, step-by-step description of all of the changes in bonding that occur in a reaction. The mechanism for the chlorination of methane involves three stages: 1. Initiation 2. Propagation 3. Termination

5 If chlorine atoms extracted and replaced primary and secondary hydrogens with equal ease, the product mixture would contain 3 times as much 1-chloropropane as 2-chloropropane. This would be termed a statistical product ratio. In actuality, secondary C-H bonds (DH0=98.5 kcal mol-1) are weaker than primary C-H bonds DH0 = 101 kcal mol-1). At 25oC the observed product ratio is 43:57 rather than 3:1. Secondary C-H bonds are more reactive than primary ones. In the propane molecule, there are 6 primary and 2 secondary hydrogen atoms. The relative reactivity of secondary and primary hydrogens in chlorinations can be calculated: relative reactivity of yield of product from number of / a secondary hydrogen secondary hydrogen abstraction secondary hydrogens = Relative reactivity of yield of product from a primary hydrogen prim ⎛ ⎞⎛ ⎞ ⎜ ⎟⎜ ⎟ ⎝ ⎠⎝ ⎠ 57/2 = 4 number of 43/6 / ary hydrogen abstraction primary hydrogens ≈ ⎛ ⎞⎛ ⎞ ⎜ ⎟⎜ ⎟ ⎝ ⎠⎝ ⎠ Chlorine exhibits a selectivity of 4:1 in the removal of secondary hydrogen atoms over primary hydrogen atoms at 25oC. The relative reactivity of secondary C-H hydrogens to primary C-H hydrogens depends both on the nature of the extracting species, X·, and the temperature. At 600oC, the chlorination of propane exhibits a statistical distribution of products. Every collision between a chlorine atom and a propane molecule has sufficient energy to lead to reaction. Chlorination is unselective at this temperature and leads to a product ratio governed by statistical factors. Tertiary C-H bonds are more reactive than secondary ones. 2-Methylpropane contains 1 tertiary and 9 primary hydrogen atoms. When chlorinated at 25oC, two products are formed, 2- chloro-2-methylpropane and 1-chloro-2-methylpropane, with yields in the ratio of 36:64% respectively. The relative reactivities of tertiary to primary hydrogen atoms in the reaction can be calculated: relative reactivity of yield of product from number of / a tertiary hydrogen tertiary hydrogen abstraction tertiary hydrogens = Relative reactivity of yield of product from a primary hydrogen primary ⎛ ⎞⎛ ⎞ ⎜ ⎟⎜ ⎟ ⎝ ⎠⎝ ⎠ 36/1 = 5 number of 64/9 / hydrogen abstraction primary hydrogens ≈ ⎛ ⎞⎛ ⎞ ⎜ ⎟⎜ ⎟ ⎝ ⎠⎝ ⎠ The selectivity decreases with increasing temperature, as in the secondary case. The relative reactivities of C-H bonds in chlorinations are roughly: Tertiary:Secondary:Primary = 5:4:1 Selectivity in Radical Halogenation with Fluorine and Bromine 3-7 Consider the reaction of fluorine with 2-methylpropane. At 25oC two products are formed: 2-Fluoro-2-methylpropane : 1-Fluoro-2-methylpropane Observed: 14:86 (~1:6.1) Expected: 1:9 (statistical)

7 CFCs release chlorine atoms upon ultraviolet irradiation. Many common refrigerants are chlorofluorocarbons which can react with UV light and release chlorine atoms. These refrigerants are otherwise quite inert and slowly diffuse through the troposphere up into the stratosphere. Once in the stratosphere, the chlorine atoms formed can catalytically convert ozone back into molecular oxygen, thus depleting the ozone layer. Initiation Step: hν F C Cl F C + Cl 3 3 − ⎯⎯→⋅ ⋅ Propagation Steps: Stratospheric ozone has decreased by about 6% since 1978. •As early as 1978 large reductions in the ozone concentration above Antarctica were noticeable. •Each year since 1995 more than 85% of the ozone in the lower stratosphere over Antarctica is destroyed. •The area affected is 2.5 times the area of Europe and stretches far enough north to affect the southern time of South America. •A reduction of 60% in the ozone concentration above the Arctic region was measured during the winter of 2000. •These reductions have been correlated to the concentration of ClO in the upper atmosphere. The source of the ClO (at least 75%) has been shown to be CFC’s. Other sources of atmospheric chlorine, such as sea spray and volcanoes, have been shown to be minor contributors. The world is searching for CFC substitutes. December 31, 1995 marked the end of the production of CFC’s in the industrial world. Replacing the CFC’s are hydrochlorofluorocarbons, HCFC’s, and hydrofluorocarbons, HFC’s. HCFC’s are more chemically reactive than CFC’s and decompose in the troposphere before reaching the stratosphere. HFC’s have been demonstrated to be safe for the ozone layer and are currently displacing the HCFC’s, some of which could escape decomposition in the troposphere and reach the stratosphere. CFC Substitutes: CH2FCF3 CHClF2 CHCl2CF3 CH3CCl2F CH3CClF2 HFC-134a HCFC-22 HCFC-123 HCFC-141a HCFC-142b 3-10 Combustion and the Relative Stabilities of Alkanes Hydrocarbons, when burned in oxygen, produce CO2 and H2O and large amounts of heat energy. This energy is termed the heat of combustion. 2 CnH2n+2 + (3n+1) O2 Æ 2n CO2 + (2n+2) H2O + heat Measurement of the heats of combustion of hydrocarbons yields information about their bond energies. ΔH0 comb of alkanes increases with chain length (there are more C and H atoms to oxidize). ΔH0 comb of isomeric alkanes is generally not the same, even though they have the same numbers of carbon and hydrogen atoms. Butane -687.4 kcal mol-1 2-Methylpropane -685.4 kcal mol-1 Butane is said to be less thermodynamically stable than its isomer. This difference is due to the difference in bond energies between the contained C-H bonds and C-C bonds present in the two molecules. 3 Important Concepts 1. Bond Homolysis – Yields radicals or free atoms. ΔHo for this process is the bond dissociation energy, DHo. 2. For Alkanes, the C-H Bond Strength: CH3—H > RCH2—H > R2CH—H > R3C—H This is due to the order of increasing hyperconjugative stabilization of the corresponding radicals: CH3· < RCH2 · < R2CH · < R3C · 3. Catalysts – Speed up the establishment of the equilibrium between reactants and products. Catalysts do not change the equilibrium position. 4. Alkanes React with Halogens – (Except iodine) by a radical chain mechanism to give haloalkanes. This involves an initiation step, two propagation steps, and various termination steps