3-1 Strength of Alkane Bonds:Radicals CHAPTER 3 vtic cleavage Reactions of Alkanes: Bond-Dissociation Energies B一AB Radical Halogenation,and Reactivity morethan e tomndpred a说gede2eano ic cleavage results in the formation of ions,rathe AB→A+ B A-B ete vtic Cleavage port ion formatio DHHF>HCI≥HBr>HI of ra adicals deter mines the C-H bond (A Sin r trend exists for C-C bonds.) ciation ener Methane>Primary>Secondary>Tertiary C-H Bo CHCH,CH,CH+-101 kcal mot H.+. 8.5 kcal mo C-H Bond WeakerMore Stable Radical CH.CH.CR: 1
1 CHAPTER 3 Reactions of Alkanes: Bond-Dissociation Energies, Radical Halogenation, and Reactivity 3-1 Strength of Alkane Bonds: Radicals Breaking a bond requires heat. This energy is called the bonddissociation energy, DH0, or bond strength. H-H Æ H· + H· ΔH0 = DH0 = 104 kcal mol-1 Radicals are formed by homolytic cleavage. When a bond breaks leaving the bonding electrons equally divided between the atoms, the process is called homolytic cleavage or homolysis: Species containing more than one atom and an unpaired electron are called radicals. Free atoms and radicals exist as intermediates in small concentrations during the course of many reactions but cannot usually be isolated. Heterolyltic cleavage results in the formation of ions, rather than radicals: Homolytic Cleavage: Nonpolar solvents Gas phase Heterolytic Cleavage: Polar solvents (stabilize ions) Electronegativities of atoms support ion formation Dissociation energies, DH0, refer only to homolytic cleavages: Bonds are strongest when overlapping orbitals are of similar energy and size: DH0: HF > HCl > HBr > HI The stability of radicals determines the C-H bond strengths. The bond dissociation energies in alkanes generally decreases with the progression: Methane > Primary > Secondary > Tertiary C-H Bond Stronger Less Stable Radical CH3-H Æ CH3· + H· DH0 = 105 kcal mol-1 CH3CH2-H Æ CH3CH2 · + H· DH0 = 101 kcal mol-1 (CH3)2CH-H Æ (CH3)2CH· + H· DH0 = 98.5 kcal mol-1 (CH3)3C-H Æ (CH3)3C· + H· DH0 = 96.5 kcal mol-1 C-H Bond WeakerMore Stable Radical (A similar trend exists for C-C bonds.) Radical stability increases (and thus the energy required to create them decreases) in the progression: CH3· < primary < secondary < tertiary
3-2 Structure of Alkyl Radicals:Hyperconjugation ohctaSdfe2 ◆CH,·+CHIOLCH.CH ”。g88阳 +CH-+-CH0CC国 elthe of theb eocolnomerer oleum is an important source of alkanes. to alter th on of the C.C. 三 3-4Chlorination of Methane:The Radical Chain ine convert ion of all of th ho heated ttrc sm for the chlorin n of methane involves thre 3.Termination SaCao5aenaeiaotgratromtempenare,the 2
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
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3 The chlorination of methane can be studied step by step. Initiation: The first step in the reaction is the heat- or light-induced homolytic cleavage of a molecule of chlorine (the weakest bond in the mixture). Only a relatively small number of initiation events are required to convert all of the reactants into products. Two subsequent self-sustaining propagation steps occur repeatedly without additional homolysis of Cl2. Propagation Step 1: One of the chlorine atoms abstracts a hydrogen atom from a methane molecule: This abstraction is an endothermic process and the equilibrium is slightly unfavorable. In this case, the activation energy is not high and there is enough heat to overcome the barrier: Propagation Step 2: The methyl radical abstracts a chlorine atom from another Cl2 molecule yielding chloromethane and a new chlorine atom. This step is exothermic and supplies the driving force for the overall reaction. 43 3 Slightly Very Favorable Unfavorable Drives First Reaction CH + Cl CH + HCl CH Cl + Cl + HCl ⋅⋅ ⋅ ˆˆˆˆ†ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ† ‡ ˆ ˆ ˆ ˆ ˆˆ ‡ ˆ ˆ ˆ ˆ ˆ ˆ ˆ The overall enthalpy change for the two propagation steps is: Chain Termination When two radicals find each other and combine to form a covalent bond they are no longer available to participate in propagating the reaction. The chlorination of methane is an example of a radical chain mechanism. To minimize the production of di- and more highly substituted chloromethane, a large CH4/Cl2 concentration ratio is used
3-5 Other Radical Halogenations of Methane or the irstao the F d Br I TRBLE 3-4 +一+ -51+1+18+3 CH,+飞→OH,:+元 -n-27-24-2 CH+一茶+： of6teotnempcaghereacionmjaeoenchaacterist states (iodine reac These two rules the c he second propagation step is exothermic. -6chlor 正3a产yaa +CH--C:+ -31+1+1w --14 L+→0，X:+H或 -1g-5-6 +B The propagation steps are: CHCH,+.c.-CHCH+.-2 al md H+→CHCH+G=-26km 4
4 3-5 Other Radical Halogenations of Methane Fluorine is most reactive, iodine least reactive. The dissociation energies of F2, Br2, and I2 are all lower than that of Cl2 so each can easily initiate a radical chain. The enthalpies for the first and second propagation steps for the four halogens are: In the first propagation step, the very strong H-F bond results in a strong exothermic reaction for fluorine. The remaining values for Cl, Br, and I reflect the decreasing bond strengths of the HCl, HBr, and HI molecules. Comparing fluorine to iodine: The fluorine reaction has a negligible activation barrier. In the transition state, the fluorine atom is relatively far from the hydrogen and the hydrogen is still very close to its attached carbon atom. The converse is true for the iodine reaction. There, the transition state occurs only when the H-I bond is nearly made and the C-H bond is nearly broken. Early transition states (fluorine reaction) are often characteristic of fast exothermic processes. Late transition states (iodine reaction) are often characteristic of slow endothermic processes. These two rules are known as the Hammond postulate. The second propagation step is exothermic. Fluorine has the most and iodine the least exothermic second propagation step. Notice that the overall enthalpy change for iodination is positive. There is not enough energy released in the second step to make up for the large absorption of energy in the first step. Iodine does not react with methane to form methyl iodide and hydrogen iodide. Chlorination of Higher Alkanes: Relative Reactivity and Selectivity 3-6 The chlorination of ethane proceeds by a radical chain process analogous to that of methane. There is only one product: chloroethane. Δ or hν 0 1 CH CH + Cl CH CH Cl + HCl H 28 kcal mol 33 2 32 ⎯⎯⎯→ Δ =− − The propagation steps are:
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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)