16. Conjugated Systems

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

a. How many MOs does the compound have?

For the following terpenes, identify the isoprene units. In cross-linked or ring-containing terpenes, linkages can be formed between more than just C₁ and C₄ of isoprene

(b)

For the following terpenes, identify the isoprene units. In cross-linked or ring-containing terpenes, linkages can be formed between more than just C₁ and C₄ of isoprene

(c)

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.

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?

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

f. What is the relationship between HOMO and LUMO and symmetric and antisymmetric orbitals?

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

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

(a)

Identify the isoprene units in the terpenes shown. In cross-linked or ring-containing terpenes, linkages can be formed between more than just C₁ and C₄ of isoprene.

(a)

Phenolphthalein is an acid–base indicator that is colorless below pH 8 and red above pH 8. Explain briefly why the first structure is colorless and the second structure is colored.

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.

(g)

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)

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 MOs does 1,3,5,7-octatetraene have?

b. What is the designation of its HOMO (c1, c2, etc.)?

c. How many nodes does its highest energy MO have between the nuclei?

Why is this not a viable representation of the ψ₂, molecular orbital of buta-1,3-diene?