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Transcript
Chapter 4 Alcohols and Alkyl Halides Overview of Chapter This chapter introduces chemical reactions and their mechanisms by focusing on two reactions that yield alkyl halides. (1) alcohol + hydrogen halide ROH + HX RX + H2O (2) alkane + halogen RH + X2 RX + HX Both are substitution reactions. 4.1 Functional Groups Functional Group A structural unit in a molecule responsible for its characteristic behavior under a particular set of reaction conditions Families of Organic Compounds and their Functional Groups Alcohol Alkyl halide ROH RX (X = F, Cl, Br, I) Amine primary amine: RNH2 secondary amine: R2NH tertiary amine: R3N Families of Organic Compounds and their Functional Groups Epoxide C C O Ether Nitrile ROR' RCN Nitroalkane Sulfide Thiol RNO2 RSR' RSH Many Classes of Organic Compounds Contain a Carbonyl Group O O C C Carbonyl group R Acyl group Many Classes of Organic Compounds Contain a Carbonyl Group O O C C Carbonyl group R H Aldehyde Many Classes of Organic Compounds Contain a Carbonyl Group O O C C Carbonyl group R Ketone R' Many Classes of Organic Compounds Contain a Carbonyl Group O O C C Carbonyl group R OH Carboxylic acid Many Classes of Organic Compounds Contain a Carbonyl Group O O C C Carbonyl group R Ester OR' Many Classes of Organic Compounds Contain a Carbonyl Group O O C C Carbonyl group R Amide NH2 4.2 IUPAC Nomenclature of Alkyl Halides IUPAC Nomenclature There are several kinds of IUPAC nomenclature. The two that are most widely used are: functional class nomenclature substitutive nomenclature Both types can be applied to alcohols and alkyl halides. Functional Class Nomenclature of Alkyl Halides Name the alkyl group and the halogen as separate words (alkyl + halide). CH3F CH3CH2CHCH2CH2CH3 Br CH3CH2CH2CH2CH2Cl H I Functional Class Nomenclature of Alkyl Halides Name the alkyl group and the halogen as separate words (alkyl + halide). CH3F CH3CH2CH2CH2CH2Cl Methyl fluoride Pentyl chloride CH3CH2CHCH2CH2CH3 Br 1-Ethylhexyl bromide H I Cyclohexyl iodide Substitutive Nomenclature of Alkyl Halides Name as halo-substituted alkanes. Number the longest chain containing the halogen in the direction that gives the lowest number to the substituted carbon. CH3CH2CH2CH2CH2F CH3CHCH2CH2CH3 Br CH3CH2CHCH2CH3 I Substitutive Nomenclature of Alkyl Halides Name as halo-substituted alkanes. Number the longest chain containing the halogen in the direction that gives the lowest number to the substituted carbon. CH3CH2CH2CH2CH2F 1-Fluoropentane CH3CH2CHCH2CH3 I 3-Iodopentane CH3CHCH2CH2CH3 Br 2-Bromopentane Substitutive Nomenclature of Alkyl Halides Cl CH3 CH3 Cl Halogen and alkyl groups are of equal rank when it comes to numbering the chain. Number the chain in the direction that gives the lowest number to the group (halogen or alkyl) that appears first. Substitutive Nomenclature of Alkyl Halides Cl 5-Chloro-2-methylheptane CH3 CH3 2-Chloro-5-methylheptane Cl 4.3 IUPAC Nomenclature of Alcohols Functional Class Nomenclature of Alcohols Name the alkyl group and add "alcohol" as a separate word. CH3CH2OH CH3 CH3CCH2CH2CH3 CH3CHCH2CH2CH2CH3 OH OH Functional Class Nomenclature of Alcohols Name the alkyl group and add "alcohol" as a separate word. CH3CH2OH Ethyl alcohol CH3CHCH2CH2CH2CH3 OH 1-Methylpentyl alcohol CH3 CH3CCH2CH2CH3 OH 1,1-Dimethylbutyl alcohol Substitutive Nomenclature of Alcohols Name as "alkanols." Replace -e ending of alkane name with -ol. Number chain in direction that gives lowest number to the carbon that bears the —OH group. CH3CH2OH CH3 CH3CCH2CH2CH3 CH3CHCH2CH2CH2CH3 OH OH Substitutive Nomenclature of Alcohols Name as "alkanols." Replace -e ending of alkane name with -ol. Number chain in direction that gives lowest number to the carbon that bears the —OH group. CH3CH2OH Ethanol CH3CHCH2CH2CH2CH3 OH 2-Hexanol or Hexan-2-ol CH3 CH3CCH2CH2CH3 OH 2-Methyl-2-pentanol or 2-Methylpentan-2-ol Substitutive Nomenclature of Alcohols OH CH3 CH3 OH Hydroxyl groups outrank alkyl groups when it comes to numbering the chain. Number the chain in the direction that gives the lowest number to the carbon that bears the OH group Substitutive Nomenclature of Alcohols OH 6-Methyl-3-heptanol CH3 CH3 5-Methyl-2-heptanol OH 4.4 Classes of Alcohols and Alkyl Halides Classification Alcohols and alkyl halides are classified as primary secondary tertiary according to their "degree of substitution." Degree of substitution is determined by counting the number of carbon atoms directly attached to the carbon that bears the halogen or hydroxyl group. Classification H CH3CH2CH2CH2CH2F OH primary alkyl halide secondary alcohol CH3 CH3CHCH2CH2CH3 Br secondary alkyl halide CH3CCH2CH2CH3 OH tertiary alcohol 4.5 Bonding in Alcohols and Alkyl Halides Dipole Moments alcohols and alkyl halides are polar H H H + C + H H + O – H C Cl H = 1.7 D = 1.9 D – Dipole Moments alcohols and alkyl halides are polar = 1.7 D = 1.9 D Dipole-Dipole Attractive Forces + – + – – + + – + – Dipole-Dipole Attractive Forces + – + – – + + – + – 4.6 Physical Properties of Alcohols and Alkyl Halides: Intermolecular Forces Boiling point Solubility in water Density Effect of Structure on Boiling Point CH3CH2CH3 CH3CH2F CH3CH2OH Molecular weight 44 48 46 Boiling point, °C -42 -32 +78 Dipole moment, D 0 1.9 1.7 Effect of Structure on Boiling Point CH3CH2CH3 Molecular weight 44 Boiling point, °C -42 Dipole moment, D 0 Intermolecular forces are weak. Only intermolecular forces are induced dipole-induced dipole attractions. Effect of Structure on Boiling Point CH3CH2F Molecular weight 48 Boiling point, °C -32 Dipole moment, D 1.9 A polar molecule; therefore dipole-dipole and dipole-induced dipole forces contribute to intermolecular attractions. Effect of Structure on Boiling Point CH3CH2OH Molecular weight 46 Boiling point, °C +78 Dipole moment, D 1.7 Highest boiling point; strongest intermolecular attractive forces. Hydrogen bonding is stronger than other dipole-dipole attractions. Figure 4.4 Hydrogen bonding in ethanol – – + + Figure 4.4 Hydrogen bonding in ethanol Boiling point increases with increasing number of halogens Compound CH3Cl CH2Cl2 CHCl3 CCl4 Boiling Point -24°C 40°C 61°C 77°C Even though CCl4 is the only compound in this list without a dipole moment, it has the highest boiling point. Induced dipole-induced dipole forces are greatest in CCl4 because it has the greatest number of Cl atoms. Cl is more polarizable than H. But trend is not followed when halogen is fluorine Compound CH3CH2F CH3CHF2 CH3CF3 CF3CF3 Boiling Point -32°C -25°C -47°C -78°C But trend is not followed when halogen is fluorine Compound CH3CH2F CH3CHF2 CH3CF3 CF3CF3 Boiling Point -32°C -25°C -47°C -78°C Fluorine is not very polarizable and induced dipoleinduced dipole forces decrease with increasing fluorine substitution. Solubility in water Alkyl halides are insoluble in water. Methanol, ethanol, isopropyl alcohol are completely miscible with water. The solubility of an alcohol in water decreases with increasing number of carbons (compound becomes more hydrocarbon-like). Figure 4.5 Hydrogen Bonding Between Ethanol and Water Density Alkyl fluorides and alkyl chlorides are less dense than water. Alkyl bromides and alkyl iodides are more dense than water. All liquid alcohols have densities of about 0.8 g/mL. 4.7 Preparation of Alkyl Halides from Alcohols and Hydrogen Halides ROH + HX RX + H2O Reaction of Alcohols with Hydrogen Halides ROH + HX RX + HOH Hydrogen halide reactivity HF least reactive HCl HBr HI most reactive Reaction of Alcohols with Hydrogen Halides ROH + HX RX + HOH Alcohol reactivity CH3OH Methanol RCH2OH R2CHOH Primary Secondary least reactive R3COH Tertiary most reactive Preparation of Alkyl Halides 25°C (CH3)3COH + HCl (CH3)3CCl + H2O 78-88% OH + HBr 80-100°C Br + H2O 73% CH3(CH2)5CH2OH + HBr 120°C CH3(CH2)5CH2Br + H2O 87-90% Preparation of Alkyl Halides A mixture of sodium bromide and sulfuric acid may be used in place of HBr. CH3CH2CH2CH2OH NaBr H2SO4 heat CH3CH2CH2CH2Br 70-83% 4.8 Mechanism of the Reaction of Alcohols with Hydrogen Halides: Hammond’s Postulate About mechanisms A mechanism describes how reactants are converted to products. Mechanisms are often written as a series of chemical equations showing the elementary steps. An elementary step is a reaction that proceeds by way of a single transition state. Mechanisms can be shown likely to be correct, but cannot be proven correct. About mechanisms For the reaction: (CH3)3COH + HCl tert-Butyl alcohol 25°C (CH3)3CCl + H2O tert-Butyl chloride the generally accepted mechanism involves three elementary steps. Step 1 is a Brønsted acid-base reaction. Step 1: Proton Transfer (CH3)3C .. O: + H .. : Cl .. H Like proton transfer from a strong acid to water, proton transfer from a strong acid to an alcohol is normally very fast. fast, bimolecular H + (CH3)3C O : + H tert-Butyloxonium ion .. – : Cl: .. Step 1: Proton Transfer (CH3)3C .. O: + H .. : Cl .. H Two molecules react in this elementary step; therefore it is bimolecular. fast, bimolecular H + (CH3)3C O : + H tert-Butyloxonium ion .. – : Cl: .. Step 1: Proton Transfer (CH3)3C .. O: + H .. : Cl .. H fast, bimolecular H + (CH3)3C O : + H tert-Butyloxonium ion .. – : Cl: .. Species formed in this step (tertbutyloxonium ion) is an intermediate in the overall reaction. Potential Energy Diagram for Step 1 (CH3)3CO Potential energy H Cl H (CH3)3COH + H—Cl Reaction coordinate + (CH3)3CO H H + Cl – Hammond's Postulate If two succeeding states (such as a transition state and an unstable intermediate) are similar in energy, they are similar in structure. Hammond's postulate permits us to infer the structure of something we can't study (transition state) from something we can study (reactive intermediate). Step 2: Carbocation Formation Dissociation of the alkyloxonium ion involves H bond-breaking, without + any bond-making to (CH3)3C O : compensate for it. It has a high activation H energy and is slow. slow, unimolecular H + + :O: tert-Butyl cation H (CH3)3C Step 2: Carbocation Formation H A single molecule reacts in this step; therefore, it is unimolecular. + (CH3)3C O : H slow, unimolecular H + + :O: tert-Butyl cation H (CH3)3C Step 2: Carbocation Formation H + (CH3)3C O : H The product of this step is a carbocation. It is an intermediate in the overall process. slow, unimolecular H + + :O: tert-Butyl cation H (CH3)3C Potential Energy Diagram for Step 2 (CH3)3C H O H Potential energy (CH3)3C + (CH3)3CO H H Reaction coordinate + + H 2O Transition State for Carbocation Formation Note the “carbocation character” in the transition state: (CH3)3C H O H Whatever stabilizes carbocations will stabilize the transition state leading to them. Carbocation R + C R R The key intermediate in reaction of secondary and tertiary alcohols with hydrogen halides is a carbocation. A carbocation is a cation in which carbon has 6 valence electrons and a positive charge. Figure 4.8 Structure of tert-Butyl Cation CH3 + H3C C CH3 Positively charged carbon is sp2 hybridized. All four carbons lie in same plane. Unhybridized p orbital is perpendicular to plane of four carbons. Step 3: Carbocation Capture Bond formation between the positively charged .. – + carbocation and the (CH3)3C + : Cl: .. negatively charged chloride ion is fast. fast, bimolecular (CH3)3C .. Cl .. : tert-Butyl chloride Two species are involved in this step. Therefore, this step is bimolecular. Step 3: Carbocation Capture (CH3)3C + + .. – : Cl: .. This is a Lewis acidLewis base reaction. The carbocation is the Lewis acid; chloride ion is the Lewis base. fast, bimolecular (CH3)3C .. Cl .. : tert-Butyl chloride The carbocation is an electrophile. Chloride ion is a nucleophile. Step 3: Carbocation Capture + H3C CH3 C+ + Cl – CH3 Lewis acid Lewis base Electrophile Nucleophile H3C CH3 C H3C Cl Potential Energy Diagram for Step 3 (CH3)3C Cl Potential energy + (CH3)3C + Cl– (CH3)3C—Cl Reaction coordinate 4.9 Potential Energy Diagrams for Multistep Reactions: The SN1 Mechanism Potential Energy Diagram - Overall The potential energy diagram for a multistep mechanism is simply a collection of the potential energy diagrams for the individual steps. Consider the three-step mechanism for the reaction of tert-butyl alcohol with HCl. (CH3)3COH + HCl 25°C (CH3)3CCl + H2O carbocation formation carbocation capture R+ proton transfer ROH + ROH2 RX carbocation formation – + (CH3)3C O H Cl carbocation capture ‡ R+ H proton ‡ transfer ROH + ROH2 RX carbocation formation H + (CH3)3C O ‡ carbocation capture ‡ + R+ H proton transfer ROH + ROH2 RX carbocation formation ‡ carbocation capture + (CH3)3C R+ proton transfer ROH + ROH2 RX – Cl ‡ Mechanistic Notation The mechanism just described is an example of an SN1 process. SN1 stands for substitution-nucleophilicunimolecular. The molecularity of the rate-determining step defines the molecularity of the overall reaction. Mechanistic Notation The molecularity of the rate-determining step defines the molecularity of the overall reaction. + (CH3)3C H O + H Rate-determining step is unimolecular dissociation of alkyloxonium ion. 4.10 Structure, Bonding, and Stability of Carbocations Carbocations R + C R R Most carbocations are too unstable to be isolated, but occur as reactive intermediates in a number of reactions. When R is an alkyl group, the carbocation is stabilized compared to R = H. Carbocations H + C H H Methyl cation least stable Carbocations H3C + C H H Ethyl cation (a primary carbocation) is more stable than CH3+ Carbocations H3C + C CH3 H Isopropyl cation (a secondary carbocation) is more stable than CH3CH2+ Carbocations H3C + C CH3 CH3 tert-Butyl cation (a tertiary carbocation) is more stable than (CH3)2CH+ Figure 4.14 Stabilization of Carbocations via the Inductive Effect + Positively charged carbon pulls electrons in bonds closer to itself. Figure 4.14 Stabilization of Carbocations via the Inductive Effect Positive charge is "dispersed ", i.e., shared by carbon and the three atoms attached to it. Figure 4.14 Stabilization of Carbocations via the Inductive Effect Electrons in C—C bonds are more polarizable than those in C—H bonds; therefore, alkyl groups stabilize carbocations better than H. Electronic effects transmitted through bonds are called "inductive effects." Figure 4.15 Stabilization of Carbocations via Hyperconjugation + Electrons in this bond can be shared by positively charged carbon because the orbital can overlap with the empty 2p orbital of positively charged carbon Figure 4.15 Stabilization of Carbocations via Hyperconjugation Electrons in this bond can be shared by positively charged carbon because the orbital can overlap with the empty 2p orbital of positively charged carbon Figure 4.15 Stabilization of Carbocations via Hyperconjugation Notice that an occupied orbital of this type is available when sp3 hybridized carbon is attached to C+, but is not available when H is attached to C+. Therefore, alkyl groups stabilize carbocations better than H does. 4.11 Effect of Alcohol Structure on Reaction Rate H Slow step is: + • R O• H + R + •• O •• H H The more stable the carbocation, the faster it is formed. Tertiary carbocations are more stable than secondary, which are more stable than primary, which are more stable than methyl. Tertiary alcohols react faster than secondary, which react faster than primary, which react faster than methanol. High activation energy for formation of methyl cation. + CH3 + H O •• H H Eact + CH3 + •• O •• H H + • CH3 O • H Smaller activation energy for formation of primary carbocation. + + H O •• RCH2 H Eact + RCH2 + •• O •• H H H + • RCH2 O • H Activation energy for formation of secondary carbocation is less than that for formation of primary carbocation. + R2CH + H O •• H Eact H + • R2CH O • H + R2CH + •• O •• H H Activation energy for formation of tertiary carbocation is less than that for formation of secondary carbocation. + R3C + H O •• H H + • R3C O • H Eact + R3C + •• O •• H H 4.12 Reaction of Methyl and Primary Alcohols with Hydrogen Halides. The SN2 Mechanism Preparation of Alkyl Halides 25°C (CH3)3COH + HCl (CH3)3CCl + H2O 78-88% OH + HBr 80-100°C Br + H2O 73% CH3(CH2)5CH2OH + HBr 120°C CH3(CH2)5CH2Br + H2O 87-90% Preparation of Alkyl Halides Primary carbocations are too high in energy to allow SN1 mechanism. Yet, primary alcohols are converted to alkyl halides. Primary alcohols react by a mechanism called SN2 (substitution-nucleophilic-bimolecular). CH3(CH2)5CH2OH + HBr 120°C CH3(CH2)5CH2Br + H2O 87-90% The SN2 Mechanism Two-step mechanism for conversion of alcohols to alkyl halides: (1) proton transfer to alcohol to form alkyloxonium ion (2) bimolecular displacement of water from alkyloxonium ion by halide Example CH3(CH2)5CH2OH + HBr 120°C CH3(CH2)5CH2Br + H2O Mechanism Step 1: Proton transfer from HBr to 1-heptanol CH3(CH2)5CH2 .. O: + H .. : Br .. H fast, bimolecular H + CH3(CH2)5CH2 O : H Heptyloxonium ion + .. – : Br: .. Mechanism Step 2: Reaction of alkyloxonium ion with bromide ion. H .. – + : Br: + CH3(CH2)5CH2 O : .. H slow, bimolecular H CH3(CH2)5CH2 .. Br .. : 1-Bromoheptane + :O: H + – Br CH2 OH2 CH3(CH2)4CH2 ‡ proton transfer ROH + ROH2 RX ‡ 4.13 More on Activation Energy Arrhenius equation A quantitative relationship between the activation energy, the rate constant, and the temperature: k Ae Eact / RT A is the preexponential, or frequency factor (related to the collision frequency and geometry), R=8.314 x 10-3 kJ/Kmol, T is in K. The true activation barrier is known as DG‡ and takes into account the enthalpy and entropy of activation. 4.14 Other Methods for Converting Alcohols to Alkyl Halides Reagents for ROH to RX Thionyl chloride SOCl2 + ROH RCl + HCl + SO2 Phosphorus tribromide PBr3 + 3ROH 3RBr + H3PO3 Examples CH3CH(CH2)5CH3 SOCl2 K2CO3 CH3CH(CH2)5CH3 Cl OH (81%) (Pyridine often used instead of K2CO3) (CH3)2CHCH2OH PBr3 (CH3)2CHCH2Br (55-60%) 4.15 Halogenation of Alkanes RH + X2 RX + HX Energetics RH + X2 RX + HX explosive for F2 exothermic for Cl2 and Br2 endothermic for I2 4.16 Chlorination of Methane Chlorination of Methane carried out at high temperature (400 °C) CH4 + Cl2 CH3Cl + HCl CH3Cl + Cl2 CH2Cl2 + HCl CH2Cl2 + Cl2 CHCl3 + HCl CHCl3 + Cl2 CCl4 + HCl 4.17 Structure and Stability of Free Radicals Free Radicals contain unpaired electrons .. .. : O O: Examples: O2 . . .. . : N O: NO Cl .. : Cl . .. Alkyl Radicals R . C R R Most free radicals in which carbon bears the unpaired electron are too unstable to be isolated. Alkyl radicals are classified as primary, secondary, or tertiary in the same way that carbocations are. Figure 4.17 Structure of Methyl Radical Methyl radical is planar, which suggests that carbon is sp2 hybridized and that the unpaired electron is in a p orbital. Alkyl Radicals R . C R R The order of stability of free radicals is the same as for carbocations. Alkyl Radicals H . C H3C H less stable than H Methyl radical H3C less stable than . C CH3 H Isopropyl radical (secondary) . C H H Ethyl radical (primary) H3C less stable than . C CH3 CH3 tert-Butyl radical (tertiary) Alkyl Radicals The order of stability of free radicals can be determined by measuring bond strengths. By "bond strength" we mean the energy required to break a covalent bond. A chemical bond can be broken in two different ways—heterolytically or homolytically. Homolytic In a homolytic bond cleavage, the two electrons in the bond are divided equally between the two atoms. One electron goes with one atom, the second with the other atom. In a heterolytic cleavage, one atom retains both electrons. Heterolytic – + Homolytic The species formed by a homolytic bond cleavage of a neutral molecule are free radicals. Therefore, measure enthalpy cost of homolytic bond cleavage to gain information about stability of free radicals. The more stable the free-radical products, the weaker the bond, and the lower the bond-dissociation energy. Measures of Free Radical Stability Bond-dissociation enthalpy measurements tell us that isopropyl radical is 13 kJ/mol more stable than propyl. . CH3CH2CH2 . + CH3CHCH3 H. + 410 397 H. CH3CH2CH3 Measures of Free Radical Stability Bond-dissociation enthalpy measurements tell us that tert-butyl radical is 30 kJ/mol more stable than isobutyl. . (CH3)2CHCH2 + (CH3)3C . H. + 410 380 H. (CH3)3CH 4.18 Mechanism of Methane Chlorination Mechanism of Chlorination of Methane Free-radical chain mechanism. Initiation step: .. .. .. .. . + . Cl: : Cl: Cl : : Cl .. .. .. .. The initiation step "gets the reaction going" by producing free radicals—chlorine atoms from chlorine molecules in this case. Initiation step is followed by propagation steps. Each propagation step consumes one free radical but generates another one. Mechanism of Chlorination of Methane First propagation step: H3C : H + .. . Cl: .. Second propagation step: .. .. : H3C . + : Cl: Cl .. .. H3C . + .. H : Cl: .. .. .. H3C: Cl: + . Cl: .. .. Mechanism of Chlorination of Methane First propagation step: H3C : H + .. . Cl: .. Second propagation step: .. .. : H3C . + : Cl: Cl .. .. .. .. : Cl : H3C : H + : Cl .. .. H3C . + .. H : Cl: .. .. .. H3C: Cl: + . Cl: .. .. .. .. H3C: Cl: + H : Cl: .. .. Almost all of the product is formed by repetitive cycles of the two propagation steps. First propagation step: H3C : H + .. . Cl: .. Second propagation step: .. .. : H3C . + : Cl: Cl .. .. .. .. : Cl : H3C : H + : Cl .. .. H3C . + .. H : Cl: .. .. .. H3C: Cl: + . Cl: .. .. .. .. H3C: Cl: + H : Cl: .. .. Termination Steps stop chain reaction by consuming free radicals .. H3C . + . Cl: .. .. H3C: Cl: .. Hardly any product is formed by termination step because concentration of free radicals at any instant is extremely low. 4.19 Halogenation of Higher Alkanes Chlorination of Alkanes can be used to prepare alkyl chlorides from alkanes in which all of the hydrogens are equivalent to one another CH3CH3 + Cl2 420°C CH3CH2Cl + HCl (78%) + Cl2 h Cl + HCl (73%) Chlorination of Alkanes Major limitation: Chlorination gives every possible monochloride derived from original carbon skeleton. Not much difference in reactivity of different hydrogens in molecule. Example Chlorination of butane gives a mixture of 1-chlorobutane and 2-chlorobutane. (28%) CH3CH2CH2CH2Cl CH3CH2CH2CH3 Cl2 h CH3CHCH2CH3 (72%) Cl Percentage of Product that Results from Substitution of Indicated Hydrogen if Every Collision with Chlorine Atoms is Productive 10% 10% 10% 10% 10% 10% 10% 10% 10% 10% Percentage of Product that Actually Results from Replacement of Indicated Hydrogen 18% 18% 4.6% 4.6% 4.6% 4.6% 4.6% 4.6% 18% 18% Relative Rates of Hydrogen Atom Abstraction divide by 4.6 18% 4.6% 4.6 =1 4.6 18 4.6 = 3.9 A secondary hydrogen is abstracted 3.9 times faster than a primary hydrogen by a chlorine atom. Similarly, chlorination of 2-methylbutane gives a mixture of isobutyl chloride and tert-butyl chloride. (63%) CH3 CH3CCH2Cl CH3 CH3CCH3 H H Cl2 h CH3 CH3CCH3 (37%) Cl Percentage of Product that Results from Replacement of Indicated Hydrogen 7.0% 37% Relative Rates of Hydrogen Atom Abstraction divide by 7 7.0 7 =1 37 7 = 5.3 A tertiary hydrogen is abstracted 5.3 times faster than a primary hydrogen by a chlorine atom. Selectivity of Free-radical Halogenation R3CH > R2CH2 > RCH3 chlorination: 5 4 1 bromination: 1640 82 1 Chlorination of an alkane gives a mixture of every possible isomer having the same skeleton as the starting alkane. Useful for synthesis only when all hydrogens in a molecule are equivalent. Bromination is highly regioselective for substitution of tertiary hydrogens. Major synthetic application is in synthesis of tertiary alkyl bromides. Synthetic Application of Chlorination of an Alkane Cl Cl2 h (64%) Chlorination is useful for synthesis only when all of the hydrogens in a molecule are equivalent. Synthetic Application of Bromination of an Alkane Br H CH3CCH2CH2CH3 CH3 Br2 h CH3CCH2CH2CH3 CH3 (76%) Bromination is highly selective for substitution of tertiary hydrogens. Major synthetic application is in synthesis of tertiary alkyl bromides.