Chemistry
Chemistry
10th Edition
ISBN: 9781305957404
Author: Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
Publisher: Cengage Learning
Bartleby Related Questions Icon

Related questions

Q: 2. -CH3 1. OH 2. + H3C-Mgl a Proton transfer b = Lewis acid/base c = Electrophilic addition aqueous…
A:
Q: Analyze this reaction mechanism and identify all species that act as nucleophiles in this process: B…
A:
Q: Show the detailed mechanism between SO3 and toluene. Please show the ortho and para products and use…
A: Sulfonation using SO3/conc. H2SO4 produces sulfonic acid, predominantly the 4-sulfonic acid product;…
Q: H CH3 CH3 9-BBN В H3C. (1 eq.) 3 + 1 9-borabicyclo[3.3.1]nonane (9-BBN)
A: According to the Lewis , the acids are that species which can accept a lone pair of electrons or…
Q: Grignard reagents undergo a general and very useful reaction with ketones. What is the structure of…
A:
Q: Please give a reasonable mechanism for the following reaction using a multiple component reactions.…
A: In the given reaction Ti(NMe2)2(dpma) acts as catalyst.
Q: A wittig reaction experiment: Week 1 -》 preparation of phosphonium salt  Materials :…
A:
Q: help please answer in text form with proper workings and explanation for each and every part and…
A: The reaction shown involves the substitution of a bromine atom on a benzyl bromide with a cyanide…
Q: Cannizzaro reaction has carried out using 3.1 mL benzaldehyde and strong base. What is the yield of…
A: The balanced equation is  2 C6H5CHO + KOH → C6H5CH2OH + C6H5COOK. here the reduction product is…
Q: From the table of available reagents select the one(s) you would use to convert 1-phenylethanol to…
A: The conversion given is : 1-phenylethanol i.e PhCH(OH)CH3 to acetophenone i.e PhCOCH3.
Q: Weak nucleophiles (H-Nu) react with aldehydes and ketones under acid-catalyzed conditions. Let HA…
A: Below are the 3 steps of the complete mechanism: Step-1: Reaction of HA to the nucleophilic site of…
Q: 2. Give a detailed mechanisms for the following reaction. Be sure to include all steps! Separately…
A:
Q: . In the synthesis of 1-bromobutane, what is the inorganic by-product left in the reaction flask…
A: Explained as follows
Q: Measurement of the reaction kinetics (kx/kH) for the following transformation gave the value of p =…
A: We have to write the reaction mechanism for the given organic reaction:
Q: I really need to get the right answer and chegg might not have the right answer please can I get the…
A:
Q: Both nucleophiles (I and II) have two negative charges on either end of the molecule. Which is the…
A:
Q: Examine the reaction/mechanism provided for the acid-catalyzed dehydration of 2-methyl-2-butanol.…
A: Given is reaction mechanism for dehydration reaction of alcohol. The given mechanism is two step…
Q: Treatment of benzene with tetramethyl tetrahydrofuran in H2SO4 produces tetramethyl…
A: Treatment of benzene with tetramethyl tetrahydrofuran in H2SO4 produces tetramethyl…
Q: Can you do 1 and 2 and explain it? State the mechanism in which through reaction proceeds sn2 or sn1
A:
Q: e. HC=CCH3 7. Write the mechanism for the following reactions: OH H₂SO4 H₂O 18 18
A:
Q: 20. Which reaction sequence would be used to prepare I 1 Co II III IV :0 + ta H MgBr + 2 CH3MgBr 1.…
A: The given problem is based on the functional interconversions in organic chemistry using various…
Q: Using 2-methyloxirane and 1-bromopropane as our sources of carbon, which of the reagents below will…
A: In the given reaction 2-methyloxirane reacts with 1-bromopropane to forms a ketone.
Q: Nucleophilic substitution reactions of Alkyl Halides experiment 1: In SN2 experiment with 12%…
A: We have to compare the SN1 and SN2 reaction rates of the above compounds.
Q: Was this experiment green and were the solvents safe? Could other have alternatives been used…
A: Guaifenesin is synthesized by Guaicol in an SN2 reaction with NaOH which provides conjugate base of…
Q: Wolff-Kishner reduction of 7-methyl-2-cycloheptenone produces 3-methylcycloheptene, not…
A: An arrow always depicts from a region of high electron density to low electron density ; that is…
Q: Write the Sn2 reaction mechanism between iodobutane with sodium hydroxide, NaOH. Predict the…
A:
Q: Solve all of them they are part of same question. Thank you
A: Applying concept of organic synthesis and reagent.
Q: Rank the relative reactivity of nucleophiles. The shortest time means fastest reaction, which gets…
A: To arrange the nucleophile in order of reactivity.,
Q: Draw the structure of all the alkyl halides and nucleophiles/bases used in the above published…
A:
Q: 4. The success of each reaction in a Radical Fluorination of an Ibuprofen Derivative is determined…
A: In order to find the mass of the dibromomethane we have to first write down the given data: We take…
Q: сно N H 3 Н 2. Ni А -Н 20 60 °с, 90 atm B
A:
Question

In this study, the researcher compared S N 2 and E2
reaction rates for four substrates. Three of the substrates had a second halogen on the b
position 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 ‚-substituted
alkyl 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 therefore
these substituents lead to an increase in the amount of elimination. The kinetic data for the SN 2 reactions can
be 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 available
condensed 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 suggests
that solvation causes the system to shift to a more highly ionized SN2 transition state.

4a) Consider Table 4 and suggest why FCH 2 CH 2 Br (5.7 vs. 1) has a higher S N 2 reaction rate
than EtBr. 

4b) Consider Table 4 and suggest why BrCH 2 CH 2 Br (35.8 vs. 5.7) has a higher S N 2 reaction
rate than FCH 2 CH 2 Br. 
 
 
Scheme 1 c=c-O-co₂" + CH₂CH₂Br SN2 =c-Q-caCH,CH, (3) + Br ⒸC=CO-CO₂H + CH₂=CH₂ + Br (4) reactions of two dianion nucleophiles with a series of simple halides 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 and therefore can be used to identify the reaction mechanism (eqs 3 and 4). A key assumption of the method is that the second charge does not radically distort the reactivity of the nucleophilic center so that the dianion can provide a good model for the properties of a singly charged nucleophile. The potential problem with a dianion is that it contains a significant amount of internal electrostatic repulsion that will be released in the course of a reaction with an alkyl halide as the charged species separate. This issue has been explored by computational methods, and for dianions with fairly large initial charge separations (~15 Å or more), the second charge has only a modest effect on the potential energy surface of a nucleophilic reaction such as an SN2 substitution.22 The effect of the second charge is limited because in reaching the transition state of an SN2 reaction, there is a relatively small increase in the charge separation so only a small amount of the internal electrostatic repulsion is released. The majority of the internal electrostatic repulsion is released after the transition state and has no effect on the kinetics or product distribution of the reaction. As a result, dianion nucleophiles can provide realistic models of singly charged analogues in these reactions.23 In a previous communication, we demonstrated the utility of the method and applied it to a series of alkyl bromides with varying substitution patterns at the a-carbon (i.e., 1°, 2°, and 3º). Here, we describe an investigation of the effect of placing electron-withdrawing groups at the ß-carbon of the substrate. Specifically, the reactions of dianions I and II with a series of O-co₂ 2-haloethyl bromides (fluoro, chloro, and bromo) have been examined. Both of these dianions were used in our previous study and the diphenylacetylene framework provides a reason- able charge separation (~14 Å) that limits the effects of internal electrostatic repulsion. It is well-known that electron-withdraw- ing groups at the B-carbon should enhance E2 rates because they stabilize the incipient negative charge that develops on the ß-carbon in the E2 transition state.2.24 Much of the previous condensed phase work has focused on systems with either a substituted aryl group or a powerful electron-withdrawing group such as nitro or cyano at the ß-carbon.25-29 Less has been reported on the effects of simple electron-withdrawing groups such as halogens. In a very early study, Olivier and Weber showed that 1,2-dibromoethane was over 100 times more reactive than 1,1-dibromoethane under elimination conditions.30 Goering and Espy³1 also found that halogens and other groups Energy Rectants Reactant Complex Transition State (8) Products (5) v Product Complex Reaction Coordinate Figure 1. Double-well potential for gas-phase ion-molecule reac- tions: (a) positive activation energy (solid line) and (b) negative activation energy (dashed line). at the B-carbon increase the E2 rate. In contrast, Okamoto and co-workers³2 found that alkoxy and chloro substituents at the ß-carbon caused a reduction in E2 rates in aqueous solution. They rationalized this result as a steric effect. As for SN2 reactions, there is evidence from condensed phase studies that electron-withdrawing groups at the ß-carbon (including halo- gens) lead to rate reductions. 32-35 In the present study, we test the validity of the generalization that electron-withdrawing groups at the ß-carbon increase E2 rates and decrease SN2 rates by determining the SN2 and E2 rate constants of the reactions of the 2-haloethyl bromides with I and II. In addition, we evaluate the relative abilities of the various halogens to stabilize/ destabilize the transition states. To support the experimental work, ab initio calculations have been completed. Given the size of the dianions, it is impractical to pursue calculations on the actual experimental systems. Instead, we have used two types of model systems. The reaction of acetate with the 2-haloethyl bromides provides a reasonable model of the reactions of the benzoate dianion, I. The other model system uses methoxide as the nucleophile. Of course, methoxide does not provide a realistic model of the phenolate nucleophile, II, because it is a much stronger gas-phase base, but it does offer insight into the effects of base strength and charge localization on the competition between SN2 and E2 reactions in the B-substituted systems. The SN2 and E2 reactions involve a single barrier and lead to a double-well potential energy surface originally proposed by Brauman (Figure 1). For each reaction, we have computationally characterized four stationary points on the surface: the reactants, the reactant ion- dipole complex, the transition state, and the products. On this type of gas-phase potential energy surface, the activation energy is defined as the energy difference between the transition state and the separated reactants. As a result, positive (Figure la) and negative (Figure 1b) activation energies are possible. Experimental Section Mass Spectrometry. All experiments were completed in a modified Finnigan 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.
expand button
Transcribed Image Text:Scheme 1 c=c-O-co₂" + CH₂CH₂Br SN2 =c-Q-caCH,CH, (3) + Br ⒸC=CO-CO₂H + CH₂=CH₂ + Br (4) reactions of two dianion nucleophiles with a series of simple halides 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 and therefore can be used to identify the reaction mechanism (eqs 3 and 4). A key assumption of the method is that the second charge does not radically distort the reactivity of the nucleophilic center so that the dianion can provide a good model for the properties of a singly charged nucleophile. The potential problem with a dianion is that it contains a significant amount of internal electrostatic repulsion that will be released in the course of a reaction with an alkyl halide as the charged species separate. This issue has been explored by computational methods, and for dianions with fairly large initial charge separations (~15 Å or more), the second charge has only a modest effect on the potential energy surface of a nucleophilic reaction such as an SN2 substitution.22 The effect of the second charge is limited because in reaching the transition state of an SN2 reaction, there is a relatively small increase in the charge separation so only a small amount of the internal electrostatic repulsion is released. The majority of the internal electrostatic repulsion is released after the transition state and has no effect on the kinetics or product distribution of the reaction. As a result, dianion nucleophiles can provide realistic models of singly charged analogues in these reactions.23 In a previous communication, we demonstrated the utility of the method and applied it to a series of alkyl bromides with varying substitution patterns at the a-carbon (i.e., 1°, 2°, and 3º). Here, we describe an investigation of the effect of placing electron-withdrawing groups at the ß-carbon of the substrate. Specifically, the reactions of dianions I and II with a series of O-co₂ 2-haloethyl bromides (fluoro, chloro, and bromo) have been examined. Both of these dianions were used in our previous study and the diphenylacetylene framework provides a reason- able charge separation (~14 Å) that limits the effects of internal electrostatic repulsion. It is well-known that electron-withdraw- ing groups at the B-carbon should enhance E2 rates because they stabilize the incipient negative charge that develops on the ß-carbon in the E2 transition state.2.24 Much of the previous condensed phase work has focused on systems with either a substituted aryl group or a powerful electron-withdrawing group such as nitro or cyano at the ß-carbon.25-29 Less has been reported on the effects of simple electron-withdrawing groups such as halogens. In a very early study, Olivier and Weber showed that 1,2-dibromoethane was over 100 times more reactive than 1,1-dibromoethane under elimination conditions.30 Goering and Espy³1 also found that halogens and other groups Energy Rectants Reactant Complex Transition State (8) Products (5) v Product Complex Reaction Coordinate Figure 1. Double-well potential for gas-phase ion-molecule reac- tions: (a) positive activation energy (solid line) and (b) negative activation energy (dashed line). at the B-carbon increase the E2 rate. In contrast, Okamoto and co-workers³2 found that alkoxy and chloro substituents at the ß-carbon caused a reduction in E2 rates in aqueous solution. They rationalized this result as a steric effect. As for SN2 reactions, there is evidence from condensed phase studies that electron-withdrawing groups at the ß-carbon (including halo- gens) lead to rate reductions. 32-35 In the present study, we test the validity of the generalization that electron-withdrawing groups at the ß-carbon increase E2 rates and decrease SN2 rates by determining the SN2 and E2 rate constants of the reactions of the 2-haloethyl bromides with I and II. In addition, we evaluate the relative abilities of the various halogens to stabilize/ destabilize the transition states. To support the experimental work, ab initio calculations have been completed. Given the size of the dianions, it is impractical to pursue calculations on the actual experimental systems. Instead, we have used two types of model systems. The reaction of acetate with the 2-haloethyl bromides provides a reasonable model of the reactions of the benzoate dianion, I. The other model system uses methoxide as the nucleophile. Of course, methoxide does not provide a realistic model of the phenolate nucleophile, II, because it is a much stronger gas-phase base, but it does offer insight into the effects of base strength and charge localization on the competition between SN2 and E2 reactions in the B-substituted systems. The SN2 and E2 reactions involve a single barrier and lead to a double-well potential energy surface originally proposed by Brauman (Figure 1). For each reaction, we have computationally characterized four stationary points on the surface: the reactants, the reactant ion- dipole complex, the transition state, and the products. On this type of gas-phase potential energy surface, the activation energy is defined as the energy difference between the transition state and the separated reactants. As a result, positive (Figure la) and negative (Figure 1b) activation energies are possible. Experimental Section Mass Spectrometry. All experiments were completed in a modified Finnigan 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 Dianions with Alkyl Bromides I CH3CH₂Br FCH₂CH₂Br E2 SN2 1.7 9.3 0.0 0.0 3.5 17.1 0.4 SN2 1.0 5.7 CICH₂CH₂Br 13.7 BrCH₂CH₂Br 35.8 EtCH₂CH₂Br 2.8 "The yield was too low for accurate partial rate measurements. II a a E2 0.1 4.7 530 780
expand button
Transcribed Image Text:Table 4. Relative Rates for the SN2 and E2 Reactions of Dianions with Alkyl Bromides I CH3CH₂Br FCH₂CH₂Br E2 SN2 1.7 9.3 0.0 0.0 3.5 17.1 0.4 SN2 1.0 5.7 CICH₂CH₂Br 13.7 BrCH₂CH₂Br 35.8 EtCH₂CH₂Br 2.8 "The yield was too low for accurate partial rate measurements. II a a E2 0.1 4.7 530 780
Expert Solution
Check Mark
This question has been solved!
Explore an expertly crafted, step-by-step solution for a thorough understanding of key concepts.
bartleby
This is a popular solution
See solutionCheck out a sample Q&A here
Step 1: part 1
VIEW
Step 2: step 2
VIEW
Solution
VIEW
bartleby

Trending nowThis is a popular solution!

bartleby

Step by stepSolved in 3 steps with 4 images

See solution
Check out a sample Q&A here
Blurred answer
Knowledge Booster
Background pattern image
Chemistry
Learn more about
Need a deep-dive on the concept behind this application? Look no further. Learn more about this topic, chemistry and related others by exploring similar questions and additional content below.
Similar questions
Recommended textbooks for you
Text book image
Chemistry
Chemistry
ISBN:9781305957404
Author:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
Publisher:Cengage Learning
Text book image
Chemistry
Chemistry
ISBN:9781259911156
Author:Raymond Chang Dr., Jason Overby Professor
Publisher:McGraw-Hill Education
Text book image
Principles of Instrumental Analysis
Chemistry
ISBN:9781305577213
Author:Douglas A. Skoog, F. James Holler, Stanley R. Crouch
Publisher:Cengage Learning
Text book image
Organic Chemistry
Chemistry
ISBN:9780078021558
Author:Janice Gorzynski Smith Dr.
Publisher:McGraw-Hill Education
Text book image
Chemistry: Principles and Reactions
Chemistry
ISBN:9781305079373
Author:William L. Masterton, Cecile N. Hurley
Publisher:Cengage Learning
Text book image
Elementary Principles of Chemical Processes, Bind...
Chemistry
ISBN:9781118431221
Author:Richard M. Felder, Ronald W. Rousseau, Lisa G. Bullard
Publisher:WILEY
SEE MORE TEXTBOOKS