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John E. McMurry http://www.cengage.com/chemistry/mcmurry Chapter 12 Organohalides: Nucleophilic Substitutions and Eliminations Richard Morrison • University of Georgia, Athens Alkyl Halides Alkyl halide or haloalkanes • Compounds with a halogen atom bonded to a saturated sp3hybridized carbon atom • Widespread in nature • • Chloromethane is released in large amounts by ocean kelp, as well as by forest fires and volcanoes Vast array of industrial applications • Use as inhaled anesthetics, refrigerants, pesticides, and solvents Alkyl Halides Other halo-substituted compounds are providing important leads to new medicines • Pentahalogenated alkene Halomon, isolated from the red alga Portieria hornemannii, exhibits anticancer activity against several human tumor cell lines Alkyl Halides Alkyl halides are not often involved in the biochemical pathways of terrestrial organisms • The kinds of reactions they undergo – nucleophilic substitutions and eliminations – are frequently involved • Alkyl halide chemistry acts as a relatively simple model for many mechanistically similar but structurally more complex reactions found in biomolecules 12.1 Naming Alkyl Halides Haloalkanes • • Commonly called alkyl halides The halogen is treated as a substituent on a parent alkane chain Alkyl halides can be named by following three steps 1. Find the longest chain, and name it as the parent • If a double or triple bond is present, it must be included in the parent chain Naming Alkyl Halides Number the carbons of the parent chain beginning at the end nearer the first substituent, whether alkyl or halo 2. • 3. Assign each substituent a number according to its position on the chain If the parent chain can be properly numbered from either end by step 2, begin at the end nearer the substituent that has alphabetical precedence Naming Alkyl Halides Many simple alkyl halides are also named by identifying first the alkyl group and then the halogen • CH3I can either be called iodomethane or methyl iodide • Names shown in parentheses will not be used in this textbook Naming Alkyl Halides The carbon-halogen bond of alkyl halides is polarized with the carbon atom bearing a slight positive charge (d+) and the halogen a slight negative charge (d-) • The alkyl halide C-X carbon atom behaves as an electrophile in polar reactions Naming Alkyl Halides Halogens increases in size down the periodic table • Lengths of corresponding carbon-halogen bonds increase accordingly The C-X carbon bond strengths decrease going down the periodic table 12.2 Preparing Alkyl Halides from Alkenes: Allylic Bromination Methods of preparing alkyl halides • Reactions of HX and X2 with alkenes in electrophilic addition reactions • • The hydrogen halides HCl, HBr, and HI react with alkenes by a polar mechanism to give the product of Markovnikov addition Bromine and chlorine undergo anti addition through a halonium ion intermediate to give 1,2-dihalogenated products Preparing Alkyl Halides from Alkenes: Allylic Bromination Alkyl halides can also be prepared from alkenes by reaction with Nbromosuccinimide (NBS) in the presence of light • Bromine is substituted for hydrogen at the position next to the double bond – the allylic position Preparing Alkyl Halides from Alkenes: Allylic Bromination Mechanism of allylic bromination • Br. radical abstracts an allylic hydrogen atom of the alkene forming an allylic radical plus HBr • The allylic radical reacts with Br2 to yield the product and a Br. radical which cycles back to carry on the chain • Br2 results from reaction of NBS with the HBr formed in the first step Preparing Alkyl Halides from Alkenes: Allylic Bromination Allylic radicals are more stable than related alkyl radicals by about 40 kJ/mol (9 kcal/mol) and are more stable than vinylic radicals by about 85 kJ/mol (19 kcal/mol) • According to the Hammond postulate, allylic radicals should form faster Preparing Alkyl Halides from Alkenes: Allylic Bromination • Allylic radicals are stable due to resonance Preparing Alkyl Halides from Alkenes: Allylic Bromination • Allylic bromination of an unsymmetrical alkene often leads to a mixture of products • Products are not usually formed in equal amounts because the intermediate allylic radical is not symmetrical and reaction at the two ends is not equally likely Preparing Alkyl Halides from Alkenes: Allylic Bromination • Products of allylic bromination reactions are useful for conversion into conjugated dienes by dehydrohalogenation with base Worked Example 12.1 Predicting the Product of an Allylic Bromination Reaction What products would you expect from the reaction of 4,4-dimethylcyclohexene with NBS? Worked Example 12.1 Predicting the Product of an Allylic Bromination Reaction Strategy Draw the alkene reactant and identify the allylic positions. Label the two different allylic positions A and B. Now abstract an allylic hydrogen from each position to generate the two corresponding allylic radicals. Each of the two allylic radicals can add a Br atom at either end (A or a; B or b) to give a mixture of up to four products. Draw and name the products Worked Example 12.1 Predicting the Product of an Allylic Bromination Reaction Solution Reaction at positions B and b are identical so a total of only three products are formed 12.3 Preparing Alkyl Halides from Alcohols Many common methods have been developed to transform alcohols into alkyl halides • Treat the alcohol with HCl, HBr, or HI • Simplest method • The reaction works best with tertiary alcohols, R3COH • Primary and secondary alcohols react slowly and at higher reaction temperatures Preparing Alkyl Halides from Alcohols The reaction of HX with a tertiary alcohol is so rapid that it is often carried our simply by bubbling the pure HCl or HBr gas into a cold ether solution of the alcohol • Conversion of 1-methylcyclohexanol into 1-chloro-1methylcyclohexane Preparing Alkyl Halides from Alcohols Primary and secondary alcohols are best converted into alkyl halides by treatment with either thionyl chloride (SOCl2) or phosphorus tribromide (PBr3) • Reactions normally take place readily under mild conditions • Reactions are less acidic and less likely to cause acid-catalyzed rearrangements than the HX method 12.4 Reactions of Alkyl Halides: Grignard Reagents Grignard Reagents: • Named after discoverer, Victor Grignard • Alkylmagnesium halide, RMgX, produced from reaction of alkyl halides, RX, with magnesium metal in ether or tetrahydrofuran (THF) solvent • Examples of organometallic compounds because they contain a carbon-metal bond • They can also be made from alkenyl (vinylic) and aryl (aromatic) halide Halogens Reactions of Alkyl Halides: Grignard Reagents The carbon-magnesium bond within the Grignard reagent is polarized • The carbon atom is both nucleophilic and basic Reactions of Alkyl Halides: Grignard Reagents Grignard Reagents are magnesium salts, R3C- +MgX, of a carbon acid, R3C-H • They react with weak acids such as H2O, ROH, RCO2H, and RNH2 to abstract a proton and yield hydrocarbons • Since hydrocarbons are weak acids, with pKa’s in the range of 44 to 60, carbon anions are very strong bases Alkyl halide • • Grignard Reagent Hydrocarbon They have no role in biochemistry They are useful carbon-based nucleophiles in many laboratory reactions • They act as models for more complex carbon-based nucleophiles that are important in biological chemistry 12.5 Discovery of the Nucleophilic Substitution Reaction Alkyl halides are electrophiles that react with nucleophiles/bases in one of two ways • Undergo substitution of the X group by the nucleophile • Undergo elimination of HX to yield an alkene 12.5 Discovery of the Nucleophilic Substitution Reaction In 1896, the German chemist Paul Walden found that the pure enantiomers (+) and (-)-malic acids, could be interconverted by a series of simple substitution reactions • Walden’s cycle of reactions interconverting (+) and (-)malic acids • Because (-)-malic acid was converted into (+)malic acid, some reactions in the cycle must have occurred with a change, or inversion, in configuration at the chirality center Discovery of the Nucleophilic Substitution Reaction Nucleophilic substitution reactions • One of the most common and versatile reaction types in organic chemistry • Explain the transformations taking place in Walden’s cycle • Each step involves the substitution of one nucleophile (in Walden’s cycle: chloride ion, Cl-, or hydroxide ion, HO-) by another Discovery of the Nucleophilic Substitution Reaction • Series of investigations undertaken during the 1920s and 1930s to clarify the mechanism of nucleophilic substitution reactions and to find out how inversions of configuration occur • Walden cycle interconverts two enantiomers of 1-pheylpropan2-ol • • • At least one step must involve inversion of configuration of chirality center Chirality centers are marked by asterisks Bonds broken in each reaction are indicated by red wavy lines Discovery of the Nucleophilic Substitution Reaction Nucleophilic substitution reaction of a primary or secondary alkyl halide or tosylate always proceeds with inversion of configuration • The inversion of stereochemical configuration must take place in the second step, the nucleophilic substitution of tosylate ion by acetate Worked Example 12.2 Predicting the Stereochemistry of a Nucleophilic Substitution Reaction What product would you expect from a nucleophilic substitution reaction of (R)-1-bromo-1-phenylethane with cyanide ion, -C≡N as nucleophile? Show the stereochemistry of both reactant and product, assuming that inversion of configuration occurs. Worked Example 12.2 Predicting the Stereochemistry of a Nucleophilic Substitution Reaction Strategy • Draw the R enantiomer of the reactant and then change the configuration of the chirality center while replacing the –Br with a -CN Worked Example 12.2 Predicting the Stereochemistry of a Nucleophilic Substitution Reaction Solution 12.6 The SN2 Reaction Kinetics • Study of reaction rates • In every chemical reaction there is a direct relationship between the rate at which the reaction occurs and the concentrations of the reactants • Kinetics of a simple nucleophilic substitution: • • • At a given temperature and concentration of reactants, the substitution occurs at a certain rate If concentration of OH- doubles, the frequency of encounter between the reaction partners double and the reaction rate also doubles If concentration of CH3Br doubles, the reaction rate again doubles The SN2 Reaction Second-order reaction • A reaction in which the rate is linearly dependent on the concentrations of two species • Mathematically expressed using the rate equation Reaction rate = rate of disappearance of reactant = k x [CH3Br] x [-OH] where [CH3Br] = CH3Br concentration in molarity [-OH] = -OH concentration in molarity k = a constant value (the rate constant) The SN2 Reaction SN2 reaction • Short for substitution, nucleophilic, bimolecular • A mechanism that accounts for both the inversion of configuration and the second-order kinetics that are observed with nucleophilic substitution reactions • Suggested in 1937 by E. D. Hughes and Christopher Ingold • Essential features of the SN2 Reaction • • Takes place in a single step without intermediates Incoming nucleophile reacts with the alkyl halide or tosylate (the substrate) from a direction opposite the group that is displaced (the leaving group) The SN2 Reaction The mechanism of the SN2 reaction when (S)-2-bromobutane reacts with –OH to give (R)-butan-2-ol • The reaction takes place in a single step • Incoming nucleophile approaches from a direction 180º away from the leaving halide ion, thereby inverting the stereochemistry at carbon The SN2 Reaction • The SN2 reaction occurs when an electron pair on the nucleophile Nu:forces out the group X:-, which takes with it the electron pair from the former C-X bond • The transition state of an SN2 reaction has a planar arrangement of the carbon atom and the remaining three groups 12.7 Characteristics of the SN2 Reaction Rate of a chemical reaction is determined by ∆G‡, the energy difference between reactant ground state and transition state • Lowering the reactant energy or raising the transition-state energy increases ∆G‡ and decreases the reaction rate • Raising the reactant energy or decreasing the transitionstate energy decreases ∆G‡ and increases the reaction rate Characteristics of the SN2 Reaction The Substrate: Steric Effects in the SN2 Reaction • The SN2 transition state involves partial bond formation between the incoming nucleophile and the alkyl halide carbon atom • Bromomethane is readily accessible resulting in a fast SN2 reaction Characteristics of the SN2 Reaction Steric hindrance to the SN2 reaction. The carbon atom in (a) is readily accessible, resulting in a fast SN2 reaction. The carbon atoms in (b) bromoethane (primary), (c) 2-bromopropane (secondary), and (d) 2-bromo-2-methylpropane (tertiary) are successively more hindered, resulting in successively slower SN2 reactions Characteristics of the SN2 Reaction SN2 reactions occur only at relatively unhindered sites • Relative reactivities for some different substrates are as follows: Characteristics of the SN2 Reaction • Vinylic halides (R2C=CRX) and aryl halides are unreactive toward SN2 reaction • The incoming nucleophile would have to approach in the plane of the carbon-carbon double bond to carry out a backside displacement Characteristics of the SN2 Reaction The Nucleophile • A Lewis base • Any species either neutral or negatively charged that has an unshared pair of electrons • If negatively charged, the product is neutral • If neutral, the product is positively charged Characteristics of the SN2 Reaction Wide array of substances prepared using SN2 reactions Characteristics of the SN2 Reaction Trends found in SN2 reactions: • Nucleophilicity roughly parallels basicity when comparing nucleophiles that have the same reacting atom • • • OH- is more basic and more nucleophilic than acetate ion, CH3CO2-, which in turn is more basic and more nucleophilic than H2O “Nucleophilicity” is the affinity of a Lewis base for a carbon atom in the SN2 reaction “Basicity” is the affinity of a base for a proton Characteristics of the SN2 Reaction • Nucleophilicity usually increases going down a column of the periodic table • • • HS- is more nucleophilic than HOHalide reactivity order is I-> Br-> ClGoing down the periodic table, elements have their valence electrons in successively larger shells, where they are successively farther from the nucleus, less tightly held, and consequently more reactive • Negatively charged nucleophiles are usually more reactive than neutral ones • SN2 reactions are often carried out under basic conditions rather than neutral or acidic conditions Characteristics of the SN2 Reaction The Leaving Group • Best leaving groups are those that best stabilize the negative charge in the transition state • • The greater the extent of the charge stabilization by the leaving group, the lower the energy of the transition state and the more rapid the reaction Weak bases such as Cl– and tosylate ion make good leaving groups, while strong bases such as OH– and NH2– make poor leaving groups Characteristics of the SN2 Reaction Alkyl fluorides, alcohols, ethers, and amines do not typically undergo SN2 reactions • To carry out an SN2 reaction with an alcohol, the HO– must be converted into a better leaving group • • A primary or secondary alcohol is converted into an alkyl chloride by reaction with SOCl2 A primary or secondary alcohol is converted into an alkyl bromide by reaction with PBr3 Characteristics of the SN2 Reaction • An alcohol can be made more reactive toward nucleophilic substitution by treating it with p-toluenesulfonyl cholride to form a tosylate. Characteristics of the SN2 Reaction Epoxides • Three-membered cyclic ethers • Much more reactive than other ethers because of angle strain in the three-membered ring • • React with aqueous acid to give 1,2-diols React readily with many other nucleophiles • Propene oxide reacts with HCl to give 1-chloropropan-2-ol by SN2 backside attack on the less hindered primary carbon atom Characteristics of the SN2 Reaction The Solvent • Protic solvents – those that contain an –OH or –NH group – are generally the worst for SN2 reactions • Decrease the rates of SN2 reactions by lowering the ground-state energy of the nucleophile • Methanol and ethanol slow down SN2 reactions by solvation of the reactant nucleophile • Solvent molecules hydrogen bond to the nucleophile and form “a cage” around it Characteristics of the SN2 Reaction • Polar aprotic solvents, which are polar but do not have –OH or – NH group, are the best solvents for SN2 reactions • • • Increase the rates of SN2 reactions by raising the ground-state energy of the nucleophile Solvents can dissolve many salts because of their high polarity, but they solvate metal cations rather than nucleophilic anions • Bare unsolvated anions have greater nucleophilicity and SN2 reactions take place at correspondingly faster rates Example: Reaction of azide ion with 1-bromobutane Characteristics of the SN2 Reaction A Summary of SN2 Reaction Characteristics Substrate Steric hindrance raises the energy of the SN2 transition state, increasing ∆G‡ and decreasing the reaction rate. As a result, SN2 reactions are best for methyl and primary substrates Characteristics of the SN2 Reaction Nucleophile Basic, negatively charged nucleophiles are less stable and have a higher ground-state energy than neutral ones, decreasing ∆G‡ and increasing SN2 reaction rate Characteristics of the SN2 Reaction Leaving group Good leaving groups (more stable anions) lower the energy of the transition state decreasing ∆G‡ and increasing SN2 reaction rate Characteristics of the SN2 Reaction Solvent Protic solvents solvate the nucleophile, thereby lowering its ground-state energy, increasing ∆G‡, and decreasing SN2 reaction rate Polar aprotic solvents surround the accompanying cation but not the nucleophilic anion, thereby raising the groundstate energy of the nucleophile, decreasing ∆G‡ and increasing SN2 reaction rate 12.8 The SN1 Reaction SN1 reaction • Unimolecular nucleophilic substitution reaction The SN1 Reaction • Rate of reaction depends only on the alkyl halide concentration and is independent of the H2O concentration • Reaction is a first-order process • The concentration of the nucleophile does not appear in the rate equation Reacton rate = Rate of disappearance of alkyl halide = k × RX • Rate-limiting step or rate-determining step • Slowest step of a multi-step chemical reaction The SN1 Reaction • Mechanism of the SN1 reaction of 2-bromo-2methylpropane with H2O involves three steps • The first step – spontaneous, unimolecular dissociation of the alkyl bromide to yield a carbocation – is rate-limiting The SN1 Reaction SN1 reaction • Rate-limiting step is a spontaneous dissociation of the alkyl halide to give carbocation intermediate The SN1 Reaction • • If an SN1 reaction is carried out on one enantiomer of a chiral reactant and proceeds through an achiral carbocation intermediate, the product will be optically inactive The symmetrical intermediate carbocation can react with a nucleophile equally well from either side, leading to a racemic 50 : 50 mixture of enantiomers The SN1 Reaction • SN1 reactions on enantiomerically pure substrates do not occur with complete racemization • Most give minor (0-20%) excess of inversion • Reaction of (R)-6-chloro-2,6-dimethyloctane with H2O • Product is approximately 80% racemized and 20% inverted (80% R,S + 20% S is equivalent to 40% R and 60% S). The SN1 Reaction Ion pairs in an SN1 reaction • The leaving group shields one side of the carbocation intermediate from reaction with the nucleophile, thereby leading to some inversion of configuration rather than complete racemization 12.9 Characteristics of the SN1 Reaction SN1 Reaction • Factors that lower ∆G‡ , either by lowering the energy level of the transition state or by raising the energy level of the ground state, favor faster SN1 reaction The Substrate • The more stable the carbocation intermediate, the faster the SN1 reaction • • • According to the Hammond postulate any factor that stabilizes a high-energy intermediate also stabilizes the transition state leading to that intermediate Stability of carbocations 3º > 2 º > 1º > –CH3 Reaction is favored for more highly stabilized carbocation intermediates Characteristics of the SN1 Reaction The resonance-stabilized allylic and benzylic cations also favor reaction Characteristics of the SN1 Reaction Due to resonance stabilization: • Primary allylic and primary benzylic carbocations are about as stable as secondary alkyl carbocations • Secondary allylic and secondary benzylic carbocations are about as stable as a tertiary alkyl carbocations Characteristics of the SN1 Reaction Allylic and benzylic substrates are particularly reactive in SN2 and SN1 reactions • Allylic and benzylic C-X bonds are about 50 kJ/mol (12 kcal/mol) weaker than the corresponding saturated bonds Characteristics of the SN1 Reaction The Leaving group • An identical reactivity order is found for the SN1 reaction as for the SN2 reaction Characteristics of the SN1 Reaction For SN1 reactions carried out under acidic conditions neutral water is sometimes the leaving group • Mechanism of the SN1 reaction of a tertiary alcohol with HBr to yield an alkyl halide • Leaving group is neutral water Characteristics of the SN1 Reaction The Nucleophile • Does not affect the SN1 reaction rate • SN1 reaction occurs through a rate-limiting step in which nucleophile has no part Characteristics of the SN1 Reaction The Solvent • Solvent effects in the SN1 reaction are due largely to stabilization or destabilization of the transition state • Any factor stabilizing the intermediate carbocation should increase the rate of an SN1 reaction (Hammond postulate) • Carbocation solvation • The electron-rich oxygen atoms of solvent molecules orient around the positively charged carbocation and thereby stabilize it Characteristics of the SN1 Reaction SN1 reactions take place much more rapidly in polar solvents such as water and methanol than in nonpolar solvents such as ether and chloroform Characteristics of the SN1 Reaction A Summary of SN1 Reaction Characteristics Substrate The best substrates yield the most stable carbocations. As a result, SN1 reactions are best for tertiary, allylic, and benzylic, halides Leaving group Good leaving groups increase the reaction rate by lowering the energy level of the transition state for carbocation formation Nucleophile The nucleophile does not affect the reaction rate Solvent Polar solvents stabilize the carbocation intermediate by solvation, thereby increasing the reaction rate Worked Example 12.3 Predicting the Mechanism of a Nucleophilic Substitution Reaction Predict whether each of the following substitution reactions is likely to be SN1 or SN2: Worked Example 12.3 Predicting the Mechanism of a Nucleophilic Substitution Reaction Strategy • Look at the substrate, leaving group, nucleophile, and solvent • Decide from the summaries at the ends of Sections 12.7 and 12.9 whether an SN1 or an SN2 reaction is favored • • SN1 reactions are favored by tertiary, allylic, or benzylic substrates, by good leaving groups, by nonbasic nucleophiles, and by protic solvents SN2 reaction are favored by primary substrates, by good leaving groups, by good nucleophiles, and by polar aprotic solvents Worked Example 12.3 Predicting the Mechanism of a Nucleophilic Substitution Reaction Solution (a) This is likely to be an SN1 reaction because the substrate is secondary and benzylic, the nucleophile is weakly basic, and the solvent is protic (b) The is likely to be an SN2 reaction because the substrate is primary, the nucleophile is a reasonably good one, and the solvent is polar aprotic 12.10 Biological Substitution Reactions SN1 and SN2 reactions are known in biological chemistry • Pathways for biosynthesis of the many thousands of terpenes • The substrate in a biological substitution reaction is often an organodiphosphate rather than an alkyl halide • The leaving group is the diphosphate ion, PPi, rather than a halide ion • Diphosphate group is the “biological equivalent” of a halogen Biological Substitution Reactions • Geraniol Biosynthesis • • A fragrant alcohol found in roses and used in perfumery Two SN1 reactions occur, both with diphosphate ion as the leaving group Biological Substitution Reactions SN2 reactions are involved in almost all biological methylations, which transfer a –CH3 group from an electrophilic donor to a nucleophile • –CH3 donor is S-adenosylmethionine (SAM) • Contains a positively charged sulfur (a sulfonium ion) • Leaving group is the neutral S-adenosylhomocysteine molecule • In the biosynthesis of epinephrine (adrenaline) from norepinephrine, the nucleophilic nitrogen atom of norepinephrine attacks the electrophilic methyl carbon atom of S-adenosylmethionine, displacing Sadenosylhomocysteine • S-adenosylmethionine is a biological equivalent of CH3Cl Biological Substitution Reactions Biosynthesis of epinephrine from norepinephrin • Occurs by an SN2 reaction with S-adenosylmethionine 12.11 Elimination Reactions: Zaitsev’s Rule When a nucleophile/Lewis base reacts with an alkyl halide two kinds of reactions can occur • Substitution where a nucleophile can react at carbon to substitute for the halide • Elimination where a nucleophile can react at a neighboring hydrogen to cause elimination of HX Elimination Reactions: Zaitsev’s Rule Elimination reactions almost always give mixtures of alkene products Zaitsev’s rule • In the elimination of HX from an alkyl halide, the more highly substituted alkene product predominates • Used to predict major products • Formulated in 1875 by the Russian chemist Alexander Zaitsev Elimination Reactions: Zaitsev’s Rule Elimination reactions can take place by different mechanisms • E1, E2, and E1cB reactions • • • Differ in the timing of C-H and C-H bond breaking All three mechanisms occur in laboratory E1cB predominates in biological pathways Elimination Reactions: Zaitsev’s Rule Elimination Reactions: Zaitsev’s Rule Worked Example 12.4 Predicting the Product of an Elimination Reaction What product would you expect from reaction of 1-chloro1-methylcyclohexane with KOH in ethanol? Worked Example 12.4 Predicting the Product of an Elimination Reaction Strategy • Treatment of an alkyl halide with a strong base such as KOH yields an alkene • To find the products in a specific case, locate the hydrogen atoms on each carbon next to the leaving group and then generate the potential alkene products by removing HX in as many ways as possible • The major product will be the one that has the most highly substituted double bond Worked Example 12.4 Predicting the Product of an Elimination Reaction Solution • The major product is the one that has the most highly substituted double bond • 1-methylcyclohexene 12.12 The E2 Reaction E2 Reaction • • • Most commonly occurring pathway for elimination in the laboratory Occurs when an alkyl halide is treated with a strong base, such as hydroxide ion or alkoxide ion (RO-) Reaction takes place in a single step through a transition state in which the double bond begins to form at the same time the H and X groups are leaving The E2 Reaction Evidence to support E2 reaction mechanisms 1. E2 reactions show second-order kinetics and follow the rate law: • Rate = k × RX × Base Both base and alkyl halide take part in the rate-limiting step The E2 Reaction 2. Deuterium isotope effect (supporting evidence for E2 mechanism) • A carbon-hydrogen bond is weaker than the corresponding carbon-deuterium bond • • A C-H bond is more easily broken than an equivalent C-D bond The rate of C-H bond cleavage is faster The E2 Reaction 3. E2 reactions occur with periplanar geometry • All four reacting atoms – the hydrogen, the two carbons, and the leaving group – lie in the same plane • Anti periplanar geometry occurs when the H and the X are on opposite sides of the molecule • Syn periplanar geometry occurs when the H and the X are on the same side of the molecule The E2 Reaction The sp3 s orbitals in the reactant C-H and C-X bonds must overlap and become p p orbitals in the alkene product • They must overlap in the transition state • Occurs most easily if all orbitals are periplanar The E2 Reaction Anti periplanar geometry for E2 eliminations has specific stereochemical consequences • Meso-1,2-dibromo-1,2-diphenylethane undergoes E2 elimination on treatment with base to give only the E alkene • No Z alkene is formed because the transition state leading to the Z alkene would have to have syn periplanar geometry and thus be higher in energy The E2 Reaction Anti periplanar geometry is particularly important in cyclohexane rings where chair geometry forces a rigid relationship between substituents on adjacent carbon atoms • Only if the hydrogen and the leaving group are trans diaxial can an E2 reaction occur Worked Example 12.5 Predicting the Stereochemistry of an E2 Reaction What stereochemistry do you expect for the alkene obtained by E2 elimination of (1S,2S)-1,2-dibromo1,2-diphenylethane? Worked Example 12.5 Predicting the Stereochemistry of an E2 Reaction Strategy • Draw (1,2-dibromo-1,2-diphenylethane) so that you can see its stereochemistry and so that the –H and –Br groups to be eliminated are anti periplanar • Carry out the elimination while keeping all substituents in approximately their same positions, and see what alkene results Worked Example 12.5 Predicting the Stereochemistry of an E2 Reaction Solution • Anti periplanar elimination of HBr gives (Z)-1-bromo-1,2diphenylethylene 12.13 The E1 and E1cB Reactions The E1 Reaction • A unimolecular elimination reaction in which the C-X bond breaks before the C-H bond, giving a carbocation intermediate • Analogous to the SN1 reaction Two steps are involved in the reaction, the first of which is rate-limiting Carbocation intermediate is present • • The E1 and E1cB Reactions E1 elimination begins with the same unimolecular dissociation as in the SN1 reaction, but dissociation is followed by loss of H+ from the adjacent carbon rather than by the substitution E1 and SN1 reactions normally occur together whenever an alkyl halide is treated in a protic solvent with a nonbasic nucleophile • The best E1 substrates are also the best SN1 substrates and mixtures of substitution and elimination products are usually obtained • When 2-chloro-2-methylpropane is warmed to 65ºC in 80% aqueous ethanol, a mixture results The E1 and E1cB Reactions E1 mechanisms are supported by evidence • E1 reactions show first-order kinetics, consistent with a rate-limiting, unimolecular dissociation process • E1 reactions show no deuterium isotope effect • Rupture of the C-H (or C-D) bond occurs after the rate-limiting step rather than during it • There is no geometric requirement on the E1 reaction • The halide and the hydrogen are lost in separate steps The E1 and E1cB Reactions The E1cB Reaction • A unimolecular elimination reaction in which the C-H bond breaks before the C-X bond, giving a carbanion intermediate • The anion formed expels a leaving group on the adjacent carbon • Common in substrates that have a poor leaving group, such as –OH, that is two carbons removed from a carbonyl group, HO-C-CH-C=O • Poor leaving group disfavors alternative E1 and E2 reactions • The carbonyl group makes the adjacent hydrogen unusually acidic by resonance stabilization of the anion intermediate 12.14 Biological Elimination Reactions All three elimination reactions – E1, E1cB, and E2 – occur in various biological pathways • E1cB mechanism is particularly common • 3-hydroxy carbonyl compounds are frequently converted to conjugated unsaturated carbonyl compounds by elimination reactions • The substrate is usually an alcohol and the H atom is usually adjacent to a carbonyl group, just as in the laboratory Biological Elimination Reactions • Biosynthesis of fats • A 3-hydroxybutyryl thioester is dehydrated to the corresponding unsaturated (crotonyl) thioester 12.15 A Summary of Reactivity: SN1, SN2, E1, E1cB, and E2 SN1, SN2, E1, E1cB, and E2 • Recognizing trends and making generalizations will aid in predicting what will happen Primary alkyl halides • SN2 substitution occurs if a good nucleophile is used • E2 elimination occurs if a strong base is used • E1cB elimination occurs if the leaving group is two carbons away from a carbonyl group A Summary of Reactivity: SN1, SN2, E1, E1cB, and E2 Secondary alkyl halides • SN2 substitution occurs if a weakly basic nucleophile is used in a polar aprotic solvent • E2 elimination predominates if a strong base used • E1cB elimination takes place if the leaving group is two carbons away from a carbonyl group • SN1 and E1 reactions occur if a weakly basic nucleophile is used in a protic solvent Tertiary alkyl halides • • E2 elimination occurs when a base is used SN1 substitution and E1 elimination occur together under neutral conditions, such as in pure ethanol or water • E1cB elimination takes place if the leaving group is two carbons away from a carbonyl group Worked Example 12.6 Predicting the Product and Mechanism of a Reaction Tell whether each of the following reactions is likely to be SN1, SN2, E1, E1cB, or E2, and predict the product of each: Worked Example 12.6 Predicting the Product and Mechanism of a Reaction Strategy • Look carefully in each reaction at the substrate, leaving group, nucleophile, and solvent • Decide from the summary in Section 12.15 which of reaction is likely to be favored Worked Example 12.6 Predicting the Product and Mechanism of a Reaction Solution (a) A secondary, nonallylic substrate can undergo an SN2 reaction with a good nucleophile in a polar aprotic solvent but will undergo and E2 reaction on treatment with a strong base • In this case E2 reaction is likely to predominate Worked Example 12.6 Predicting the Product and Mechanism of a Reaction (b) A secondary benzylic substrate can undergo an SN2 reaction on treatment with a nonbasic nucleophile in a polar aprotic solvent and will undergo an E2 reaction on treatment with a base • Under protic conditions, such as aqueous formic acid (HCO2H), an SN1 reaction is likely, along with some E1 reaction