Complete the molecular orbital description for the ground state of cyclopentadiene shown

Consider the allylic bromination of cyclohexene labeled at C3 with practice Problem 13.1 13C. Neglecting stereoisomers, what products would you expect from this reaction?

(a) Draw resonance structures for the carbocation that could be formed from (E)-2- practice Problem 13.2 butenyl trifluoromethanesulfonate. OTf (b) One of the resonance structures for this carbocation should be a more important contributor to the resonance hybrid than the other. Which resonance structure would be the greater contributor? (c) What products would you expect if this carbocation reacted with a chloride ion?

Write the important resonance structures for each of the following:

From each set of resonance structures that follow, designate the one that would contrib- practice Problem 13.4 ute most to the hybrid and explain your choice:

The following enol (an alkene-alcohol) and keto practice Problem 13.5 (a ketone) forms of C2H4O differ in the positions for their electrons, but they are not resonance structures. Explain why they are not.

(a) Which other compounds in Section 13.5 are conjugated dienes? (b) Which other compounds are isolated dienes? (c) Which compound is an isolated enyne?

Two compounds, A and B, have the same molecular formula, C6H8. Both A and B practice Problem 13.7 react with two molar equivalents of hydrogen in the presence of platinum to yield cyclohexane. Compound A shows three signals in its broadband decoupled 13C NMR spectrum. Compound B shows only two 13C NMR signals. Compound A shows an absorption maximum at 256 nm, whereas B shows no absorption maximum at wavelengths longer than 200 nm. What are the structures of A and B?

Propose structures for D, E, and F. IR: no peak near 3300 cm1 UV: max ~ 230 nm IR: ~3300 cm1, sharp UV: max ~ 230 nm IR: ~3300 cm1, sharp UV: max < 200 nm

Predict the products of the following reactions.

(a) Suggest a structural explanation for the fact that the 1,2-addition reaction of practice Problem 13.10 1,3-butadiene and hydrogen bromide occurs faster than 1,4-addition? (Hint: Consider the relative contributions that the two forms and make to the resonance hybrid of the allylic cation.) (b) How can you account for the fact that the 1,4-addition product is more stable?

Draw transition states for the endo and exo DielsAlder reaction of maleic anhydride with cyclopentadiene. The transition state leading to only one enantiomer need be shown in each case.

If we were to write a chemical equation for the reaction of (2E,4E)-hexa-2,4-diene and Practice Problem 13.12 methyl propenoate we might write it as shown below. (a) Explain why we can predict that the endo and exo products will each be formed as a pair of enantiomers. (b) What is the stereochemical relationship between either one of the endo enantiomers and either of the exo enantiomers?

If (2Z,4Z)-hexa-2,4-diene were able to undergo a DielsAlder reaction with methyl propenoate, what would be the products? (Hint: There are four products comprised of two pairs of enantiomers. One enantiomer pair would predominate.) In reality, this Diels Alder reaction is impractical because the s-cis conformation of the diene required for the reaction is of high energy due to steric hindrance between the methyl groups.

What products would you expect from the following reactions?

Which diene and dienophile would you employ to synthesize the following compounds?

DielsAlder reactions also take place with triple-bonded (acetylenic) dienophiles. Which diene and which dienophile would you use to prepare the following?

1,3-Butadiene and the dienophile shown below were used by A. Eschenmoser in his synthesis of vitamin B12 with R. B. Woodward. Draw the structure of the enantiomeric DielsAlder adducts that would form in this reaction and the two transition states that lead to them.

Provide the reagents needed to synthesize 1,3-butadiene starting from (a) 1,4-Dibromobutane (b) HO OH (c) OH (d) Cl (e) Cl (f) OH (g

What product would you expect from the following reaction? Cl Cl 2 NaOEt EtOH, heat

What products would you expect from the reaction of 1 mol of 1,3-butadiene and each of the following reagents? (If no reaction would occur, you should indicate that as well.) (a) 1 mol of Cl2 (b) 2 mol of Cl2 (c) 2 mol of Br2 (d) 2 mol of H2, Ni (e) 1 mol of Cl2 in H2O (f) Hot KMnO4 (excess) (g) H2O, cat. H2SO4

Provide the reagents necessary to transform 2,3-dimethyl-1,3-butadiene into each of the following compounds.

Provide the reagents necessary for each of the following transformations. In some cases several steps may be necessary. (a) 1-Butene 9: 1,3-butadiene (b) 1-Pentene 9: 1,3-pentadiene (c) OH Br Br (d) Br

Conjugated dienes react with radicals by both 1,2- and 1,4-addition. Write a detailed mechanism to account for this fact using the peroxide-promoted addition of one molar equivalent of HBr to 1,3-butadiene as an illustration.

UVVis, IR, NMR, and mass spectrometry are spectroscopic tools we use to obtain structural information about compounds. For each pair of compounds below, describe at least one aspect from each of two spectroscopic methods (UVVis, IR, NMR, or mass spectrometry) that would distinguish one compound in a pair from the other. (a) 1,3-Butadiene and 1-butyne (b) 1,3-Butadiene and butane

When 2-methyl-1,3-butadiene (isoprene) undergoes a 1,4-addition of hydrogen chloride, the major product that is formed is 1-chloro-3-methyl-2-butene. Little or no 1-chloro-2-methyl-2-butene is formed. How can you explain this?

When 1-pentene reacts with N-bromosuccinimide (NBS), two products with the formula C5H9Br are obtained. What are these products and how are they formed?

(a) The hydrogen atoms attached to C3 of 1,4-pentadiene are unusually susceptible to abstraction by radicals. How can you account for this? (b) Can you provide an explanation for the fact that the protons attached to C3 of 1,4-pentadiene are more acidic than the methyl hydrogen atoms of propene?

Provide a mechanism that explains formation of the following products. Include all intermediates, formal charges, and arrows showing electron flow.

Provide a mechanism for the following reaction. Draw a reaction energy coordinate diagram that illustrates the kinetic and thermodynamic pathways for this reaction.

Predict the products of the following reactions.

Provide a mechanism that explains formation of the following products.

Provide a mechanism that explains formation of the following products.

Treating either 1-chloro-3-methyl-2-butene or 3-chloro-3-methyl-1-butene with Ag2O in water gives (in addition to AgCl) the following mixture of alcohol products. Cl Cl OH HO Ag2O Ag2O 15% 85% (a) Write a mechanism that accounts for the formation of these products. (b) What might explain the relative proportions of the two alkenes that are formed?

Dehydrohalogenation of 1,2-dihalides (with the elimination of two molar equivalents of HX) normally leads to an alkyne rather than to a conjugated diene. However, when 1,2-dibromocyclohexane is dehydrohalogenated, 1,3-cyclohexadiene is produced and not cyclohexyne. What factor accounts for this?

The heat of hydrogenation of allene is 298 kJ mol-1 , whereas that of propyne is 290 kJ mol-1 . (a) Which compound is more stable? (b) Treating allene with a strong base causes it to isomerize to propyne. Explain.

Although both 1-bromobutane and 4-bromo-1-butene are primary halides, the latter undergoes elimination more rapidly. How can this behavior be explained?

Complete the molecular orbital description for the ground state of cyclopentadiene shown at right. Shade the appropriate lobes to indicate phase signs in each molecular orbital according to increasing energy of the molecular orbitals. Label the HOMO and LUMO orbitals, and place the appropriate number of electrons in each level, using a straight single-barbed arrow to represent each electron.

Why does the molecule shown below, although a conjugated diene, fail to undergo a DielsAlder reaction?

Rank the following dienes in order of increasing reactivity in a DielsAlder reaction (1 = least reactive, 4 = most reactive). Briefly explain your ranking

Give the structures of the products that would be formed when 1,3-butadiene reacts with each of the following:

Cyclopentadiene undergoes a DielsAlder reaction with ethene at 160180 8C. Write the structure of the product of this reaction.

Acetylenic compounds may be used as dienophiles in the DielsAlder reaction (see Practice Problem 13.16). Write structures for the adducts that you expect from the reaction of 1,3-butadiene with

Predict the products of the following reactions.

Which diene and dienophile would you employ in a synthesis of each of the following?

When furan and maleimide undergo a DielsAlder reaction at 25 8C, the major product is the endo adduct G. When the reaction is carried out at 90 8C, however, the major product is the exo isomer H. The endo adduct isomerizes to the exo adduct when it is heated to 90 8C. Propose an explanation that will account for these results.

Two controversial hard insecticides are aldrin and dieldrin. [The Environmental Protection Agency (EPA) halted the use of these insecticides because of possible harmful side effects and because they are not biodegradable.] The commercial synthesis of aldrin began with hexachlorocyclopentadiene and norbornadiene. Dieldrin was synthesized from aldrin. Show how these syntheses might have been carried out.

a) Norbornadiene for the aldrin synthesis (Problem 13.46) can be prepared from cyclopentadiene and acetylene. Show the reaction involved. (b) It can also be prepared by allowing cyclopentadiene to react with vinyl chloride and treating the product with a base. Outline this synthesis.

Two other hard insecticides (see Problem 13.46) are chlordan and heptachlor. Show how they could be synthesized from cyclopentadiene and hexachlorocyclopentadiene.

Isodrin, an isomer of aldrin, is obtained when cyclopentadiene reacts with the hexachloronorbornadiene, shown here. Propose a structure for isodrin.

Provide the reagents necessary to achieve the following synthetic transformations. More than one step may be required.

Explain the product distribution below based on the polarity of the diene and dienophile, as predicted by contributing resonance structures for each.

Mixing furan (Problem 13.45) with maleic anhydride in diethyl ether yields a crystalline solid with a melting point of 1258C. When melting of this compound takes place, however, one can notice that the melt evolves a gas. If the melt is allowed to resolidify, one finds that it no longer melts at 125 8C but instead it melts at 56 8C. Consult an appropriate chemistry handbook and provide an explanation for what is taking place.

Draw the structure of the product from the following reaction (formed during a synthesis of one of the endiandric acids by K. C. Nicolaou):

Draw all of the contributing resonance structures and the resonance hybrid for the carbocation that would result from ionization of bromine from 5-bromo-1,3-pentadiene. Open the computer molecular model at the books website depicting a map of electrostatic potential for the pentadienyl carbocation. Based on the model, which is the most important contributing resonance structure for this cation? Is this consistent with what you would have predicted based on your knowledge of relative carbocation stabilities? Why or why not?

Elucidate the structures of compounds A through I in the following road map problem. Specify any missing reagents.

a) Write reactions to show how you could convert 2-methyl-2-butene into 2-methyl-1,3-butadiene. (b) Write reactions to show how you could convert ethylbenzene into the following compound: CN C6H5 (c) Write structures for the various DielsAlder adduct(s) that could result on reaction of 2-methyl-1,3-butadiene with the compound shown in part (b).