CHEMICAL REACTIVITY AND MECHANISMS, AND SUBSTITUTION REACTIONS

Transcription

1 EMIAL REATIVITY AND MEANISMS, AND SUBSTITUTION REATIONS A STUDENT SOULD BE ABLE TO: 1. Understand the concepts of: enthalpy, entropy, free energy, equilibrium and kinetics. Given a reaction coordinate diagram, identify transition states and reactive intermediates, and calculate activation energies and enthalpies of reaction. Understand the ammond Postulate as applied to exothermic and endothermic reactions. 2. Propose a reaction mechanism for a simple transformation. The mechanism will include a step-by-step explanation of bond-breaking and -making events that lead to product formation; and includes intermediates (if any) and mechanistic arrows. 3. Define, recognize, and give examples of each of the following terms: Nucleophile (electron pair donor) and electrophile (electron pair acceptor) Substrate; leaving group Methyl halide; 1 o RX, 2 o RX, 3 o RX, allylic RX, benzylic RX S N 1, S N 2 reactions; rate-determining step Retention, inversion, racemization (reaction stereochemistry) Polar and nonpolar, protic and aprotic solvents 4. Predict the stability of carbocations, and predict when a carbocation will rearrange. Stability: 3 o > 2 o > 1 o > 3 + > vinyl +, phenyl +. Also, delocalization stabilizes cations. 5. Predict the product or products of nucleophilic substitution reactions, including the stereochemistry where appropriate. Note that the statement of the problem will usually not say which of the reaction types is occurring in any individual case. 6. From your knowledge of the mechanism, predict and interpret experimental results for S N 1 and S N 2 reactions. Be prepared to draw mechanisms with arrows that show electron movement. For a summary of important factors that influence reaction rates, see the study guide on the next page. 7. Propose syntheses using these reactions and those learned earlier. Use cyanide and acetylide anions in S N 2 reactions to make - bonds.

2 Factors Affecting The Rates Of Substitution Reactions: Substrate: S N 2 reactions are fastest for 3 substrates; rates are: 3 > 1 > 2 >> 3 (this is a steric effect; larger groups interfere with the approaching nucleophile). S N 1 reactions are faster for 3 substrates (because the more stable the carbocation, the faster the reaction; this means 3 > 2 >> 1 > 3 ). Vinylic (R 2 =R ) and aromatic substrates are unreactive in either reaction type. Allylic and benzylic substrates can react via either process. Leaving group: Both reactions are faster when the leaving group is a weak base. For the alkyl halides as leaving groups, the resulting order is I > Br > l >> F. Other commonly used leaving groups are RSO 3 - and 2 O. Nucleophile: S N 2 reactions are usually faster when a strong nucleophile is used (there are some solvent effects on this). Nucleophile identity and concentration have no effect on the rate of S N 1 reactions. Solvent: Polar protic solvents give faster S N 1 reactions; polar aprotic solvents such as DMSO give faster S N 2 reactions. Temperature: igh temperatures increase the rates of all reaction types by increasing both the collision frequency and the energy factor. Reactant concentration: The reaction rate is directly proportional to substrate concentration for all reaction types. For S N 2 reactions, the rate is also proportional to the nucleophile concentration. For S N 1 reactions, nucleophile concentration has no effect on reaction rate at all.

3 To best prepare for this module, please work hapter 6 and 7 Skill Builder problems in the textbook. A STUDENT WO AS MASTERED TE OBJETIVES ON TE PREVIOUS PAGES SOULD BE ABLE TO SOLVE TE FOLLOWING PROBLEMS AND RELATED ONES: 1.1 Use the Bond Dissociation Energies (BDE, kj/mol) in your book to calculate the rxn. Then, state whether the reaction is exothermic or endothermic, and state if the enthalpy is favorable or unfavorable. a) ( 3 ) 2 Br + 2 O ( 3 ) 2 O + Br b) 3 O + l 3 l + 2 O 1.2 Predict whether the S rxn will be favorable, unfavorable, or about zero, and explain why. a) b) c)

4 1.3 In the reaction coordinate diagram shown: (a) What point(s) represent(s) transition state(s)? (b) What point(s) is (are) reactive intermediate(s)? B D Energy A E Reaction coordinate 1.4 Answer the following questions using the reaction coordinate diagram shown. a) What is E act of A B? 50 b) What is Eact for B A? c) What is for A B? d) What is for B A? Potential Energy, kj / mole e) For the reaction A B, the transition state comes on the reaction coordinate, and is structurally similar to the A Reaction coordinate B A. early, reactant B. early, product. late, reactant D. late, product 2. Draw mechanistic arrows. INT: include lone pairs and implied as necessary. a) b)

5 2. c) d) 3.1 Identify the substrate, the nucleophile, and the leaving group in each of the following reactions. lassify the substrate as methyl, primary, secondary or tertiary. a) 3 2 Br + 3 S 3 2 S 3 + Br b) ( 3 ) 2 OSO 2 F + N 3 ( 3 ) 2 N FSO haracterize each of the following reactions as proceeding with retention, inversion, or racemization. lassify the substrate as methyl, primary, etc. What reaction type is each (S N 1, S N 2)? a) + N3 ---> Br + 3N Br ()2 ()2 ()2 O b) I + 2O ---> I + O c) + O- ---> l l 222O 3.3 Which of the following is a protic solvent? Which is polar, aprotic? O a) b) 3 SO 3 c) 3 2 O d) -N( 3 ) 2

6 4.1 Arrange these carbocations based on stability, #1 = most stable. 4.2 Which of these carbocations is likely to rearrange? Draw the expected product. 5. Predict the major organic product of each of the following reactions. a) Br + 3 N 2 b) 3 I + ( 6 5 ) 3 P c) d) 6.1 Provide curved mechanism arrows for each of the following reactions. a) Br SN2 + O > O + Br

7 6.1 Mechanisms, continued b) Br SN > 2 + O ---> O + O ---> O 22 O + Br O2 water onsider the reaction 3 Br 3 + N > 3 (N 3 ) 3 + Br ethanol What is the effect on the rate of the reaction if each of the following changes is made? Possible answers: increases, decreases, remains the same. IF TE RXN. IF TE RXN. ANGE IS S N 1 IS S N 2 Use 3 2 Br instead of 3 Br 3 Use 3 I 3 instead of 3 Br 3 Use N 2 instead of N 3 Increase the concentration of 3 Br 3 Decrease the concentration of N 3 7. Propose syntheses of each of the compounds shown, from the given starting materials and any other needed reagents. a) 3 2 N 2 from ethyl bromide b) O2 from 2Br

8 7. Synthesis, continued c) racemic 3 (O) from (R)-2-bromopentane d) 2-hexyne from 1-chloropropane e) 3 2 OSO 2 Ph from 3 2 Br

9 ANSWERS TO TE PROBLEMS: 1.1 a) ( 3 ) 2 Br + 2 O ( 3 ) 2 O + Br break: 2 o -Br -O make: 2 o -O -Br rxn = + 34 kj/mol endothermic, unfavorable b) 3 O + l 3 l + 2 O break: 3 -O -l make: 3 -l -O rxn = -37 kj/mol exothermic, favorable 1.2 a) S rxn ~ 0 : no great change in order/disorder b) S rxn < 0: large decrease in disorder (2 reactants make 1 product); unfavorable entropy c) S rxn > 0: large increase in disorder (1 reactant makes 2 products); favorable entropy 1.3 a) B and D are transition states. b) is a reactive intermediate. 1.4 a) E act for A B = 20 kj/mole b) E act for B A = 30 kj/mole c) for A B = -10 kj/mole d) for B A = +10 kj/mole e) A (based on ammond Postulate) 2. a) proton transfer: 2 arrows b) nucleophilic attack: 1 arrow c) loss of leaving group: 1 arrow

10 2. d) carbocation rearrangement (1,2-hydride shift): 1 arrow 3.1 a) 3 2 Br is a primary substrate, 3 S is the nucleophile, Br is the leaving group b) ( 3 ) 2 -OSO 2 F is a secondary substrate, N 3 is the nucleophile, FSO 3 is the leaving group. 3.2 a) Inversion (at the reacting, retention at the other stereocenter); 2 o RX; S N 2 b) Racemization; 3 o RX; S N 1 c) Retention (at the stereocenters, reacting carbon inverts); 1 o RX; S N ompound (c) is a protic solvent: has O or N. The rest only have, are aprotic. ompounds (b) and (d) are polar, aprotic #2, secondary #4, vinylic #1, tertiary and allylic #3, primary + 5. a) Br + 3 N N Br (S N 2) primary RX, good nucleophile + + b) 3 I + ( 6 5 ) 3 P ( 6 5 ) 3 P 3 + I (S N 2) methyl RX, good nucleophile

11 5. c) d) 6.1 urved mechanism arrows: a) Br SN2 + O > O + Br b) Br SN > 2 + O ---> O + O ---> O 22 O + Br O2 6.2 IF TE RXN. IF TE RXN. ANGE IS S N 1 IS S N 2 Use 3 2 Br instead of 3 Br 3 decreases increases Use 3 I 3 instead of 3 Br 3 increases increases Use N 2 instead of N 3 remains the same increases Note: in truth, this would change the mechanism to E2 (next module)

12 6.2 (continued) IF TE RXN. IF TE RXN. ANGE IS S N 1 IS S N 2 Increase the concentration of 3 Br 3 increases increases Decrease the concentration of N 3 remains the same decreases 7. a) 3 2 N 2 from 3 2 Br Simple way, on paper: 3 2 Br + N N 2 + Br (S N 2) Better way in the lab: 3 2 Br + N N Br (S N 2) then: 3 2 N O 3 2 N O (acid-base) b) O2 from 2Br c) 2Br + O > O2 + Br- will have counterion, such as Na +, K + etc. d) e)

13 Name Seventh Drill Test (Sample A) Organic hemistry 2210DR Answer All Questions 1. onsider the reaction ( 3 2 ) 2 l + 3 O ( 3 2 ) 2 O 3 + l What is the effect of each of the given changes on the rate of the reaction? Possible answers: increases, decreases, remains the same. hange Effect if rxn is S N 2 Effect if rxn is S N 1 Use 3 O, not 3 O Use ( 3 2 ) 2 O, not ( 3 2 ) 2 l Increase the concentration of 3 O Use ( 3 2 ) 3 l, not ( 3 2 ) 2 l [RBr] [NaS 3 ] relative rate S N 1 S N 2 (circle one) ) What are the values of a) and b) E AT for A B? Use the reaction diagram shown. a) b) Energy, kj/mole 0 4) Predict the major organic product or products of each of the following reactions A B 5) Propose a synthesis of ( 3 ) 2 O 2 3 from 3 2 Br and any other needed reagents. 6) Propose a mechanism for the reaction of ( 3 ) 3 l + 2 O ( 3 ) 3 O + l

14 Name Seventh Drill Test (Sample B) Organic hemistry 2210 DR Answer All Questions 1. Draw the structure of the product or products of each of the following reactions. You must show the three-dimensional structure where appropriate. a) I 2 N > b) ()3 l 2 ()2O > 3 2 O c) I > 2. For each of the following sets, predict which reaction will be faster. Set 1 a) ( ) 3 l + O ) 3 O + l b) ( ) 3 Br + O ) 3 O + Br Set 2 a) Br + 2 S S + Br b) Br + S S + Br Set 3 DMSO a) OSO 2 Ph + O > O + PhSO 3 3 O b) OSO 2 Ph + O > O + PhSO 3

15 3. The reaction shown below was run several times using different concentrations of reagents and the rate of the reaction was determined each time. The results are shown. From this information, deduce the mechanism of the reaction. Note: OTs is shorthand for OSO the p-toluenesulfonate or tosylate group, a leaving group. ( ) 2 OTs + Br ----> ( ) 2 Br + TsO [ROTs] [Br ] Relative Rate ircle the correct mechanism SN1 SN Sketch a reaction coordinate diagram for an S N 2 reaction. Label the positions for the reactants, the products, and the transition state, and indicate where the energy of activation would be measured. 5. Which of the following is a polar aprotic solvent? O A. l 4 A. O 2 2 O. --N( 3 ) 2 D. 2 O 6. haracterize both the stereochemistry and the mechanism of the following reaction. 2Br + 2O ---> 2O + Br Stereochemistry at stereocenters: retention racemization inversion (circle one) Mechanism: S N 1 S N 2 (circle one)