CONJUGATED PI SYSTEMS AND PERICYCLIC REACTIONS

Transcription

1 CNJUGATED PI SYSTEMS AND PERICYCLIC REACTINS A STUDENT SULD BE ABLE T: 1. Recognize and give examples of conjugated, isolated, and cumulated dienes, and of allylic intermediates. Also, name given the structure, and draw the structure given the name, of polyenes (review). Distinguish conjgated dienes as s-cis or s-trans. 2. Distinguish resonance structures from other types, draw resonance structures of given species, and predict the relative stabilities of such structures (review). 3. Predict both the orders of diene and carbocation stabilities, and the relative reactivities of compounds in reactions proceeding by way of these intermediates. Diene stabilities: conjugated > isolated > cumulated Carbocations: allylic > 3 > allyl > 2 > 1 > C 3 > vinylic 4. Propose mechanisms for reactions involving conjugated systems, and explain and predict experimental results in such reactions from knowledge of the mechanisms involved. Important reactions are: Additions to conjugated dienes involving carbocations (Markovnikov) and free radicals (anti-markovnikov addition of Br). Factors contributing to the 1,2- vs. 1,4- addition conflict are important. The Diels-Alder reaction. Endo product is favored over exo. The starting diene must be conjugated and in the s-cis conformation. These reactions are faster when: 1) The diene has electron-releasing (usually alkyl) groups attached to it, and the dienophile has electron-withdrawing groups attached to it, and 2) vice versa. ther pericyclic reactions 5. Given the starting material and reaction conditions, predict the product(s) of reactions involving allylic intermediates and diene starting materials. Important reactions include: Free radical substitution at allylic positions (review) Addition reactions to dienes (1,2 and 1,4 additions) The Diels-Alder reaction, and other pericyclic reactions 6. Devise syntheses of, or syntheses employing, dienes. 7. Identify and explain the processes involved in UV-Visible spectroscopy, including: The importance of the ighest ccupied and Lowest Unoccupied Molecular rbitals The energy, frequency, and wavelength relationships between ultraviolet, visible, and other radiation. Alkenes and nonconjugated dienes have absoption maxima at wavelengths less than 200 nm, while conjugated systems have maxima at wavelengths greater than 200 nm. More conjugation gives absorption maxima at longer wavelengths.

2 To best prepare for this module, please work Chapter 17 Skill Builder problems in the textbook. A STUDENT W AS MASTERED TE BJECTIVES N TE PREVIUS PAGE SULD BE ABLE T SLVE TE FLLWING PRBLEMS AND RELATED NES: 1.1 Draw an example of each of the following. a) an allylic carbocation b) a cumulated diene 1.2 Name each of the following. a) C 3 b) C 3 c) \ / \ / C=C C 3 C=C C 2 C 3 / \ / / \ / C=C C=C / \ / \ C Draw each of the following. a) 1,3-cyclohexadiene b) allyl chloride c) trans-1,3-pentadiene Is this s-cis or s-trans? 2.1 Draw resonance structures of the species indicated. a) b) C2 total: five forms 2.2 Which of the structures shown contributes more to the resonance hybrid? C 3 C 3 A. C 3 -C-C=C 2 B. C 3 -C=C-C 2

3 2.3 Which of these is a valid resonance structure of? A B C. D 3.1 a) Which of the following dienes is the most stable? b) Which is the least stable? c)which one has an absorption maximum above 200 nm in its UV-Vis spectrum? A. C=C2 B. C=C2 C. C=C2 D. =C=C2 3.2 a) Which of the following carbocations is the most stable? b) Which is the least stable? A. C 2 CC=C 2 B. C 3 C=CC 2 C. 2 C=CC 2 C 2 D. C 3 C=CC 3 C 3 C 3 C 3 C a) Which of the following free radicals is the most stable? b) Which is the least stable? A. C2 B. C3 C. C3 D. C3 4.1 Propose a mechanism for each of the following reactions. light a) 2 C=C-CD 3 Cl > DCl 2 C=C-CD 2 Cl D 2 C=C-C 2 Cl 3 b) 2 C=C-C=C > 3 C-C=C-C Arrange these dienes based on their reactivity as a Diels-Alder diene in a reaction with maleic anhydride, fastest first. a) =C2 b) C3 c) C3 d) C3 4.3 Arrange these dienophiles based on their reactivity as Diels-Alder dieneophiles reacting with cyclopentadiene, fastest first. a) 2C=C2 b) c) d) = =

4 4.4 Draw mechanistic arrows for these pericyclic reactions. a) b) c) d) 5. Predict the product or products of each of the following reactions. Indicate stereoisomers where they are produced. high temperature a) Br > b) CCl4, dark Br > c) 2 C=C-C=C 2 I(one mole) Which is major at low temperature? high temp? d) C 2=C-C=C > e) 2C=C-C N --- f) > g)

5 6. Propose a synthesis of each of the compounds shown, from the given starting material and any other needed reagent. a) from b) from c) C3 from C3C CC 3 C3 d) C2 from 7. a) Circle the chromophore in each of these molecules. b) Which has the largest difference in energy between the M and LUM? c) Which has the largest max in the UV/Vis range?

6 SLUTINS T SAMPLE PRBLEMS: 1.1 a) an allylic carbocation b) a cumulated diene C 3 C=C-C 2 2 C=C=C 2 (There are many other possible examples here.) 1.2 a) 2,4-dimethyl-1,3-pentadiene b) trans,cis-2,4-heptadiene or (2E,4Z)-2,4-heptadiene c) 1,3,5,7-cyclooctatetraene 1.3 a) 1,3-cyclohexadiene (s-cis) b) allyl chloride c) trans-1,3-pentadiene C 3 \ / 2C=CC 2Cl C=C / \ 2C=C 2.1 Resonance structures: a) b) C2 C2 C2 C2 C2 2.2 A: 3 o allylic carbocation > 1 o allylic. Carbocation stability is more important to factor in, vs alkene stability. 2.3 C 3.1 a) Most stable C; b) least stable D; c) absorption maximum above 200 nm, C. 3.2 a) Most stable B (allylic); b) least stable D (vinylic) 3.3 a) Most stable B (allylic and 3 o ) ; b) least stable C (vinylic) 4.1 light a) 2 C=C-CD 3 Cl > DCl 2 C=C-CD 2 Cl D 2 C=C-C 2 Cl light Initiation: Cl > 2 Cl Propagation: Cl 2 C=CCD 3 ---> DCl 2 C=C-CD 2 <---> C 2 -C=CD 2 then 2 C=C-CD 2 Cl 2 ---> Cl 2 C=C-CD 2 Cl or C 2 -C=CD 2 Cl 2 ---> Cl ClC 2 -C=CD 2 (The termination steps have been omitted).

7 3 4.1 b) 2 C=C-C=C > 3 C-C=C-C C 4.3 D 2 C=C-C=C > 2 3 C-C C=C 2 <---> 3 C-C=C-C 2 3 C-C=C-C > 3 C-C=C-C C-C=C-C > 3 3 C-C=C-C 2 (Note: C 3 CC=C 2 also forms) 4.4 arrows can go clockwise or counterclockwise, as long as they re all in the same direction. a) b) c) d) 5. The product or products: a) high temperature Br > Br enantiomer Br b) CCl4, dark Br > Br Br and its enantiomer peroxides c) 2 C=C-C=C 2 I (one mole) 2 C=C-CI-C 3 (R and S) C 3 -C=C-C 2 I (cis and trans) At low temp, 2 C=C-CI-C 3 is major; 1,2 product forms fastest At high temp, C 3 C=C-C 2 I is major; this 1,4 product is more stable

8 d) C2=C-C=C > e) 2C=C-C N ----> same as CN endo CN f) -----> same as g) 6. Synthesis problems: a) Br > 2 KtBu, heat > Br Br Also K: 1. NBS, RR (or Br 2 with heat or light), then KtBu b) C2=C-C=C2 ----> c) C3C CC > C3 C3 d) 2C=CC2 ----> C2 same as C2

9 7. a) b) A: largest energy gap for least conjugation c) C: max is inversely related to energy

10 Name Third Drill Test, (Sample A) rganic Chemistry 2220DR Answer All Questions 1) Name: C 3C2 \ / 2) Draw 2-isopropyl-1,3-cycloheptadiene C=C / \ C 3 / Br C=C / \ C 2 C 3 3) a) Which of the compounds shown is the MST stable? b) Which one has an absorption maximum > 200 nm in its UV-visible spectrum? A. C C2 B. C=C2 C. CC3 D. C=C2 4) Predict the product or products (if any) of each of the following reactions. a) -C-C=C 2 3 b) C 2=C C=C > c) C2 Cl2 light 5) Propose a Diels-Alder synthesis of the compound shown. 6) Draw a resonance structure for:.. C2 7) Which of the following dienes gives the SLWEST Diels-Alder reaction? C2 C3 C2 C2 C3 C3 C3 C3 8) Propose a mechanism for: C 3 C=C-C=C 2 Cl C 3 C 2 -C=C-C 2 Cl

11 Name Third Drill Test (Sample B) rganic Chemistry 2220DR Answer All Questions 1. a) Give the complete IUPAC name of each compound. b) Classify each of the dienes as conjugated, cumulated, or isolated. c) Which of these absorbs UV-visible light at a longer wavelength? A. B. 2. Draw the structure(s) of the organic product(s) of each of the following reactions. a) Cl > light b) > c) C 3 C 3 =C C= -----> 3. Propose a synthesis of each of the compounds shown from the indicated starting materials and any other needed reagents. a) C from

12 b) C 3 C=CC 2 Br from C 3 C 2 CC 3 4. Which of the following cations is the most stable? Which is least stable? A. B. C. 5. Propose a mechanism for the following reaction > 6. Draw a resonance structure of the one shown. C 3 C 2 C=C C=C C 2 C 3 C 2