16. Conjugated Systems

c. How many are isolated dienes?

d. How many are cumulated dienes?

North American termite soldiers, when encountering enemy insects, contract their mandibular muscles, expelling a mixture of chemicals that essentially trap their enemies in a glue-like substance. This weapon, built into the face of the termite, is called the fontanellar gun. It releases a mixture of pinene (62%), myrcene (27%), and limonene (11%). pinene myrcene limonene 62% 27% 11%

(a) Identify the isoprene units in pinene, myrcene, and limonene.

North American termite soldiers, when encountering enemy insects, contract their mandibular muscles, expelling a mixture of chemicals that essentially trap their enemies in a glue-like substance. This weapon, built into the face of the termite, is called the fontanellar gun. It releases a mixture of pinene (62%), myrcene (27%), and limonene (11%).

(b) Suggest an acid-catalyzed mechanism by which pinene could be produced from limonene.

The UV spectrum of 1-phenylprop-2-en-1-ol shows an intense absorption at 220 nm and a weaker absorption at 258 nm. When this compound is treated with dilute sulfuric acid, it rearranges to an isomer with an intense absorption at 250 nm and a weaker absorption at 290 nm. Suggest a structure for the isomeric product and propose a mechanism for its formation.

Determine whether each structure is likely to be colored or not. For those that you predict to be colored, indicate the extended conjugation by marking the series of continuous sp² hybridized atoms.

(i)

a. How many linear dienes have molecular formula C₆H₁₀? (Disregard cis–trans isomers.)

b. How many of the linear dienes in part a are conjugated dienes?

The ψ₂ molecular orbital for octa-1,3,5,7-tetraene is shown. Draw ψ₃.

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Classify the following dienes and polyenes as isolated, conjugated, cumulated, or some combination of these classifications.

(a) cycloocta-1,4-diene

(b) cycloocta-1,3-diene

(c) cyclodeca-1,2-diene

(d) cycloocta-1,3,5,7-tetraene

(e) cyclohexa-1,3,5-triene (benzene)

(f) penta-1,2,4-triene

Determine whether each structure is likely to be colored or not. For those that you predict to be colored, indicate the extended conjugation by marking the series of continuous sp² hybridized atoms.

(h)

Answer the following questions for the molecular orbitals (MOs) of 1,3,5,7-octatetraene:

a. How many MOs does the compound have?

Phenolphthalein is an acid–base indicator. In solutions of pH < 8.5, it is colorless; in solutions of pH > 8.5, it is deep red-purple. Account for the change in color.

Determine whether each structure is likely to be colored or not. For those that you predict to be colored, indicate the extended conjugation by marking the series of continuous sp² hybridized atoms.

(d)

(e)

(f)

How many 𝝅 electrons are involved in the conjugated system for each of the following molecules?

(a)

Answer the following questions for the molecular orbitals (MOs) of 1,3,5,7-octatetraene:

b. Which are the bonding MOs, and which are the antibonding MOs?