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Chapter 20: Q7. (page 1052)
The IR spectrum of trans-oct-2-enoic acid is shown. Point out the spectral characteristics that allow you to tell that this is a carboxylic acid, and show which features lead you to conclude that the acid is unsaturated and conjugated.
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Name the following carboxylic acids (when possible, give both a common name and a systematic name).
(a)
(b)
(c)
(d)
(e)
(f)
In the presence of a trace of acid, δ-hydroxyvaleric acid forms a cyclic ester (lactone).
HO-CH2CH2CH2CH2-COOH
δ-hydroxyvaleric acid
(a)Give the structure of the lactone, called δ-valerolactone.
(b) Propose a mechanism for the formation of δ-valerolactone.
Tributylin hydride (Bu3SnH)is used synthetically to reduce alkyl halides, replacing a halogen atom with hydrogen. Free-radical initiators promote this reaction, and free-radical inhibitors are known to slow or stop it. You job is to develop a mechanism, using the following reaction as an example.
The following bond-dissociation enthalpies may be helpful:
(a) Propose initiation and propagation steps to account for this reaction.
(b) Calculate values of ΔH for your proposed steps to show that they are energetically feasible. (Hint: A trace of Br2 and light suggests it’s there only as an initiator, to create Brradicals. Then decide which atom can be extracted most favorably from the starting materials by the Brradical. That should complete the initiation. Finally, decide what energetically favored propagation steps will accomplish the reaction).
(a) Propose a mechanism for the following reaction:
(b) Use the bond-dissociation enthalpies given in Table 4-2 (page 203) to calculate the value of ∆H° for each step shown in your mechanism. (The BDE for is about 280 kJ/mol, or 67 kcal/mol.) Calculate the overall value of ∆H° for the reaction. Are these values consistent with a rapid free-radical chain reaction?
The 2000 Nobel Prize in Chemistry was awarded for work on polyacetylenes. Acetylene can be polymerized using a Ziegler-Natta catalyst. The cis or trans stereochemistry of the products can be controlled by careful selection and preparation of the catalyst. The resulting polyacetylene is an electrical semiconductor with a metallic appearance. cis-Polyacetylene has a copper color, and trans-polyacetylene is silver.
(a) Draw the structures of cis- and trans-polyacetylene.
(b) Use your structures to show why these polymers conduct electricity.
(c) It is possible to prepare polyacetylene films whose electrical conductivity is anisotropic. That is, the conductivity is higher in some directions than in others. Explain how this unusual behavior is possible.
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