11. Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations

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1 11. Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations Based on McMurry s Organic Chemistry, 6 th edition 2003 Ronald Kluger Department of Chemistry University of Toronto Alkyl Halides React with Nucleophiles and Bases Alkyl halides are polarized at the carbon-halide bond, making the carbon electrophilic Nucleophiles will replace the halide in C-X bonds of many alkyl halides(reaction as Lewis base) Nucleophiles that are Brønsted bases produce elimination

2 11.1 The Discovery of the Walden Inversion In 1896, Walden showed that (-)-malic acid could be converted to (+)-malic acid by a series of chemical steps with achiral reagents This established that optical rotation was directly related to chirality and that it changes with chemical alteration Reaction of (-)-malic acid with PCl 5 gives (+)- chlorosuccinic acid Further reaction with wet silver oxide gives (+)-malic acid The reaction series starting with (+) malic acid gives (-) acid Reactions of the Walden Inversion

3 Significance of the Walden Inversion The reactions alter the array at the chirality center The reactions involve substitution at that center Therefore, nucleophilic substitution can invert the configuration at a chirality center The presence of carboxyl groups in malic acid led to some dispute as to the nature of the reactions in Walden s cycle 11.2 Stereochemistry of Nucleophilic Substitution Isolate step so we know what occurred (Kenyon and Phillips, 1929) using 1- phenyl-2-propanol Only the second and fifth steps are reactions at carbon So inversion certainly occurs in the substitution step

4 Hughes Proof of Inversion React S-2-iodo-octane with radioactive iodide Observe, initially that racemization of mixture is twice as fast as incorporation of label so it must be with inversion Racemization in one reaction step would occur at same rate as incorporation The Nature of Substitution Substitution, by definition, requires that a "leaving group", which is also a Lewis base, departs from the reacting molecule. A nucleophile is a reactant that can be expected to participate effectively in a substitution reaction.

5 Substitution Mechanisms S N 1 Two steps with carbocation intermediate Occurs in 3, allyl, benzyl S N 2 Two steps combine - without intermediate Occurs in primary, secondary Reactant and Transition-state Energy Levels Higher reactant energy level (red curve) = faster reaction (smaller ΔG ). Higher transitionstate energy level (red curve) = slower reaction (larger ΔG ).

6 Two Stereochemical Modes of Substitution Substitution with inversion: Substitution with retention: 11.3 Kinetics of Nucleophilic Substitution Rate (V) is change in concentration with time Depends on concentration(s), temperature, inherent nature of reaction (barrier on energy surface) A rate law describes relationship between the concentration of reactants and conversion to products A rate constant (k) is the proportionality factor between concentration and rate Example: for S converting to P V = d[s]/dt = k [S]

7 Reaction Kinetics The study of rates of reactions is called kinetics Rates decrease as concentrations decrease but the rate constant does not Rate units: [concentration]/time such as L/(mol x s) The rate law is a result of the mechanism The order of a reaction is sum of the exponents of the concentrations in the rate law the example is second order 11.4 The S N 2 Reaction Reaction is with inversion at reacting center Follows second order reaction kinetics Ingold nomenclature to describe characteristic step: S=substitution N (subscript) = nucleophilic 2 = both nucleophile and substrate in characteristic step (bimolecular)

8 S N 2 Process The reaction involves a transition state in which both reactants are together S N 2 Transition State The transition state of an S N 2 reaction has a planar arrangement of the carbon atom and the remaining three groups

9 11.5 Characteristics of the S N 2 Reaction Sensitive to steric effects Methyl halides are most reactive Primary are next most reactive Secondary might react Tertiary are unreactive by this path No reaction at C=C (vinyl halides) Reactant and Transition-state Energy Levels Affect Rate Higher reactant energy level (red curve) = faster reaction (smaller ΔG ). Higher transitionstate energy level (red curve) = slower reaction (larger ΔG ).

10 Steric Effects on S N 2 Reactions The carbon atom in (a) bromomethane is readily accessible resulting in a fast S N 2 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 S N 2 reactions. Steric Hindrance Raises Transition State Energy Very hindered Steric effects destabilize transition states Severe steric effects can also destabilize ground state

11 Order of Reactivity in S N 2 The more alkyl groups connected to the reacting carbon, the slower the reaction The Nucleophile Neutral or negatively charged Lewis base Reaction increases coordination at nucleophile Neutral nucleophile acquires positive charge Anionic nucleophile becomes neutral See Table 11-1 for an illustrative list

12 Relative Reactivity of Nucleophiles Depends on reaction and conditions More basic nucleophiles react faster (for similar structures. See Table 11-2) Better nucleophiles are lower in a column of the periodic table Anions are usually more reactive than neutrals The Leaving Group A good leaving group reduces the barrier to a reaction Stable anions that are weak bases are usually excellent leaving groups and can delocalize charge

13 Poor Leaving Groups If a group is very basic or very small, it is prevents reaction The Solvent Solvents that can donate hydrogen bonds (-OH or NH) slow S N 2 reactions by associating with reactants Energy is required to break interactions between reactant and solvent Polar aprotic solvents (no NH, OH, SH) form weaker interactions with substrate and permit faster reaction

14 11.6 The S N 1 Reaction Tertiary alkyl halides react rapidly in protic solvents by a mechanism that involves departure of the leaving group prior to addition of the nucleophile Called an S N 1 reaction occurs in two distinct steps while S N 2 occurs with both events in same step If nucleophile is present in reasonable concentration (or it is the solvent), then ionization is the slowest step S N 1 Energy Diagram k 1 k -1 k 2 Step through highest energy point is rate-limiting (k 1 in forward direction) V = k[rx] Rate-determining step is formation of carbocation

15 Rate-Limiting Step The overall rate of a reaction is controlled by the rate of the slowest step The rate depends on the concentration of the species and the rate constant of the step The highest energy transition state point on the diagram is that for the rate determining step (which is not always the highest barrier) This is the not the greatest difference but the absolute highest point (Figures 11.8 and 11.9 the same step is rate-determining in both directions) Stereochemistry of S N 1 Reaction The planar intermediate leads to loss of chirality A free carbocation is achiral Product is racemic or has some inversion

16 S N 1 in Reality Carbocation is biased to react on side opposite leaving group Suggests reaction occurs with carbocation loosely associated with leaving group during nucleophilic addition Alternative that S N 2 is also occurring is unlikely Effects of Ion Pair Formation If leaving group remains associated, then product has more inversion than retention Product is only partially racemic with more inversion than retention Associated carbocation and leaving group is an ion pair

17 Delocalized Carbocations Delocalization of cationic charge enhances stability Primary allyl is more stable than primary alkyl Primary benzyl is more stable than allyl 11.9 Characteristics of the S N 1 Reaction Tertiary alkyl halide is most reactive by this mechanism Controlled by stability of carbocation

18 Allylic and Benzylic Halides Allylic and benzylic intermediates stabilized by delocalization of charge (See Figure 11-13) Primary allylic and benzylic are also more reactive in the S N 2 mechanism Effect of Leaving Group on S N 1 Critically dependent on leaving group Reactivity: the larger halides ions are better leaving groups In acid, OH of an alcohol is protonated and leaving group is H 2 O, which is still less reactive than halide p-toluensulfonate (TosO - ) is excellent leaving group

19 Nucleophiles in S N 1 Since nucleophilic addition occurs after formation of carbocation, reaction rate is not affected normally affected by nature or concentration of nucleophile Solvent Is Critical in S N 1 Stabilizing carbocation also stabilizes associated transition state and controls rate Solvation of a carbocation by water

20 Polar Solvents Promote Ionization Polar, protic and unreactive Lewis base solvents facilitate formation of R + Solvent polarity is measured as dielectric polarization (P) (Table 11-3) Nonpolar solvents have low P Polar SOLVENT have high P values Effects of Solvent on Energies Polar solvent stabilizes transition state and intermediate more than reactant and product

21 11.10 Alkyl Halides: Elimination Elimination is an alternative pathway to substitution Opposite of addition Generates an alkene Can compete with substitution and decrease yield, especially for S N 1 processes Zaitsev s Rule for Elimination Reactions (1875) In the elimination of HX from an alkyl halide, the more highly substituted alkene product predominates

22 Mechanisms of Elimination Reactions Ingold nomenclature: E elimination E1: X - leaves first to generate a carbocation a base abstracts a proton from the carbocation E2: Concerted transfer of a proton to a base and departure of leaving group The E2 Reaction Mechanism A proton is transferred to base as leaving group begins to depart Transition state combines leaving of X and transfer of H Product alkene forms stereospecifically

23 E2 Reaction Kinetics One step rate law has base and alkyl halide Transition state bears no resemblance to reactant or product V=k[R-X][B] Reaction goes faster with stronger base, better leaving group Geometry of Elimination E2 Antiperiplanar allows orbital overlap and minimizes steric interactions

24 E2 Stereochemistry Overlap of the developing π orbital in the transition state requires periplanar geometry, anti arrangement Allows orbital overlap Predicting Product E2 is stereospecific Meso-1,2-dibromo-1,2-diphenylethane with base gives cis 1,2-diphenyl RR or SS 1,2-dibromo-1,2-diphenylethane gives trans 1,2-diphenyl

25 11.12 Elimination From Cyclohexanes Abstracted proton and leaving group should align trans-diaxial to be anti periplanar (app) in approaching transition state (see Figures and 11-20) Equatorial groups are not in proper alignment Kinetic Isotope Effect Substitute deuterium for hydrogen at α position Effect on rate is kinetic isotope effect (k H /k D = deuterium isotope effect) Rate is reduced in E2 reaction Heavier isotope bond is slower to break Shows C-H bond is broken in or before ratelimiting step

26 11.14 The E1 Reaction Competes with S N 1 and E2 at 3 centers V = k [RX] Stereochemistry of E1 Reactions E1 is not stereospecific and there is no requirement for alignment Product has Zaitsev orientation because step that controls product is loss of proton after formation of carbocation

27 Comparing E1 and E2 Strong base is needed for E2 but not for E1 E2 is stereospecifc, E1 is not E1 gives Zaitsev orientation Summary of Reactivity: S N 1, S N 2, E 1, E 2 Alkyl halides undergo different reactions in competition, depending on the reacting molecule and the conditions Based on patterns, we can predict likely outcomes (See Table 11.4)