In this study, the researcher compared S N 2 and E2reaction rates for four substrates. Three of the substrates had a second halogen on the bposition in the molecule. This work also compared the behavior of two nucleophiles:dianion I and II. You should read the abstract and look at Scheme 1 (p. 3082) and Table 4(p. 3086).Abstract: The gas-phase reactions of benzoate and phenolate containing dianions with a series of ‚-substitutedalkyl bromides (X-CH2CH2Br, X ) H, F, Cl, Br) have been studied in a quadrupole ion trap mass spectrometer.Branching ratios between SN 2 and E2 products were measured and rate constants were determined. The‚-halogens increase both the S N 2 and E2 rates, but the effect is greater for the latter process and thereforethese substituents lead to an increase in the amount of elimination. The kinetic data for the SN 2 reactions canbe analyzed via a two-parameter, linear free-energy relationship and the results indicate that field-effects (i.e.,electron-withdrawing groups) strongly favor the reaction (FF ) 1.83). In contrast, analysis of the availablecondensed phase data for these substrates indicates that halogens strongly retard the reaction (FF ) -2.04).The dramatic reversal in substituent effects can be explained by a simple electrostatic model which suggeststhat solvation causes the system to shift to a more highly ionized SN2 transition state.3a) Both nucleophiles (I and II) have two negative charges on either end of the molecule.Which is the reactive side for substitution or elimination? Explain why that is the caseshowing all relevant structures. 3b) Are the results from the study in agreement with theory in terms of getting moresubstitution or more elimination given the identity of the alkyl halide and nucleophile?Explain your answer. Scheme 1c=c-O-co₂"+ CH₂CH₂BrSN2=c-Q-caCH,CH, (3)+ BrⒸC=CO-CO₂H+ CH₂=CH₂ + Br(4)reactions of two dianion nucleophiles with a series of simplehalides and representative reactions are outlined in Scheme 1.It can be seen that by starting with a doubly charged nucleophile,each pathway leads to two ionic products. The alkylated (SN2)and protonated (E2) nucleophiles still retain charges andtherefore can be used to identify the reaction mechanism (eqs3 and 4).A key assumption of the method is that the second chargedoes not radically distort the reactivity of the nucleophilic centerso that the dianion can provide a good model for the propertiesof a singly charged nucleophile. The potential problem with adianion is that it contains a significant amount of internalelectrostatic repulsion that will be released in the course of areaction with an alkyl halide as the charged species separate.This issue has been explored by computational methods, andfor dianions with fairly large initial charge separations (~15 Åor more), the second charge has only a modest effect on thepotential energy surface of a nucleophilic reaction such as anSN2 substitution.22 The effect of the second charge is limitedbecause in reaching the transition state of an SN2 reaction, thereis a relatively small increase in the charge separation so only asmall amount of the internal electrostatic repulsion is released.The majority of the internal electrostatic repulsion is releasedafter the transition state and has no effect on the kinetics orproduct distribution of the reaction. As a result, dianionnucleophiles can provide realistic models of singly chargedanalogues in these reactions.23In a previous communication, we demonstrated the utilityof the method and applied it to a series of alkyl bromides withvarying substitution patterns at the a-carbon (i.e., 1°, 2°, and3º). Here, we describe an investigation of the effect of placingelectron-withdrawing groups at the ß-carbon of the substrate.Specifically, the reactions of dianions I and II with a series ofO-co₂2-haloethyl bromides (fluoro, chloro, and bromo) have beenexamined. Both of these dianions were used in our previousstudy and the diphenylacetylene framework provides a reason-able charge separation (~14 Å) that limits the effects of internalelectrostatic repulsion. It is well-known that electron-withdraw-ing groups at the B-carbon should enhance E2 rates becausethey stabilize the incipient negative charge that develops on theß-carbon in the E2 transition state.2.24 Much of the previouscondensed phase work has focused on systems with either asubstituted aryl group or a powerful electron-withdrawing groupsuch as nitro or cyano at the ß-carbon.25-29 Less has beenreported on the effects of simple electron-withdrawing groupssuch as halogens. In a very early study, Olivier and Webershowed that 1,2-dibromoethane was over 100 times morereactive than 1,1-dibromoethane under elimination conditions.30Goering and Espy³1 also found that halogens and other groupsEnergyRectantsReactantComplexTransitionState(8)Products(5)vProductComplexReaction CoordinateFigure 1. Double-well potential for gas-phase ion-molecule reac-tions: (a) positive activation energy (solid line) and (b) negativeactivation energy (dashed line).at the B-carbon increase the E2 rate. In contrast, Okamoto andco-workers³2 found that alkoxy and chloro substituents at theß-carbon caused a reduction in E2 rates in aqueous solution.They rationalized this result as a steric effect. As for SN2reactions, there is evidence from condensed phase studies thatelectron-withdrawing groups at the ß-carbon (including halo-gens) lead to rate reductions. 32-35 In the present study, we testthe validity of the generalization that electron-withdrawinggroups at the ß-carbon increase E2 rates and decrease SN2 ratesby determining the SN2 and E2 rate constants of the reactionsof the 2-haloethyl bromides with I and II. In addition, weevaluate the relative abilities of the various halogens to stabilize/destabilize the transition states.To support the experimental work, ab initio calculations havebeen completed. Given the size of the dianions, it is impracticalto pursue calculations on the actual experimental systems.Instead, we have used two types of model systems. The reactionof acetate with the 2-haloethyl bromides provides a reasonablemodel of the reactions of the benzoate dianion, I. The othermodel system uses methoxide as the nucleophile. Of course,methoxide does not provide a realistic model of the phenolatenucleophile, II, because it is a much stronger gas-phase base,but it does offer insight into the effects of base strength andcharge localization on the competition between SN2 and E2reactions in the B-substituted systems. The SN2 and E2 reactionsinvolve a single barrier and lead to a double-well potentialenergy surface originally proposed by Brauman (Figure 1). Foreach reaction, we have computationally characterized fourstationary points on the surface: the reactants, the reactant ion-dipole complex, the transition state, and the products. On thistype of gas-phase potential energy surface, the activation energyis defined as the energy difference between the transition stateand the separated reactants. As a result, positive (Figure la)and negative (Figure 1b) activation energies are possible.Experimental SectionMass Spectrometry. All experiments were completed in a modifiedFinnigan LCQ quadrupole ion trap mass spectrometer equipped with(25) Bunnett, J. F.; Sridharan, S.; Cavin, W. P. J. Org. Chem. 1979, 44,1463.(26) Crosby, J.; Stirling, C. J. M. J. Am. Chem. Soc. 1968, 90, 6869.(27) Gandler, J. R.; Yokoyama, T. J. Am. Chem. Soc. 1984, 106, 130.(28) Marshall, D. R.; Thomas, P. J.; Stirling, C. J. M. J. Chem. Soc.,Perkin Trans. 2 1977, 1914.(29) Saunders: W. H., Jr. Acc. Chem. Res. 1976, 9, 19.(30) Olivier, S. C. J.; Weber, A. P. Recl. Trav. Chim. 1934, 53, 1087.(31) Goering, H. L.; Espy, H. H. J. Am. Chem. Soc. 1956, 78, 1454.(32) Okamoto, K.; Kita, T.; Araki, K.; Shingu, H. Bull. Chem. Soc. Jpn.1967, 40, 1913. Table 4. Relative Rates for the SN2 and E2 Reactions of Dianionswith Alkyl BromidesICH3CH₂BrFCH₂CH₂BrE2SN21.79.30.00.03.517.10.4SN21.05.7CICH₂CH₂Br13.7BrCH₂CH₂Br35.8EtCH₂CH₂Br2.8"The yield was too low for accurate partial rate measurements.IIaaE20.14.7530780

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Answered: Both nucleophiles (I and II) have two…

Chemistry

10th Edition

ISBN:9781305957404

Author:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste

Publisher:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste

Chapter1: Chemical Foundations

Section: Chapter Questions

Problem 1RQ: Define and explain the differences between the following terms. a. law and theory b. theory and...

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