← Back

Chapter 14: Conjugated Compounds and Ultraviolet Spectroscopy

Every problem in the chapter beside the book’s answer, a chapter at a time as you scroll.

Chapter 14: Conjugated Compounds and Ultraviolet Spectroscopy

15 problems · 21 figures

14-1 §14.1 source ↗

Allene, H2C = C = CH2, has a heat of hydrogenation of –298 kJ/mol (–71.3 kcal/mol). Rank a conjugated diene, a nonconjugated diene, and an allene in order of stability.

Answer

Expected ΔH°hydrog for allene is −252 kJ/mol. Allene is less stable than a nonconjugated diene, which is less stable than a conjugated diene.

14-2 §14.2 source ↗

Give the structures of both 1,2 and 1,4 adducts resulting from reaction of 1 equivalent of HCl with 1,3-pentadiene.

Answer

1-Chloro-2-pentene, 3-chloro-1-pentene, 4-chloro-2-pentene

14-3 §14.2 source ↗

Look at the possible carbocation intermediates produced during addition of HCl to 1,3-pentadiene (Problem 14-2), and predict which 1,2 adduct predominates. Which 1,4 adduct predominates?

Answer

4-Chloro-2-pentene predominates in both.

14-4 §14.2 source ↗

Give the structures of both 1,2 and 1,4 adducts resulting from reaction of 1 equivalent of HBr with the following compound:

Answer

1,2 Addition: 6-bromo-1,6-dimethylcyclohexene 1,4 Addition: 3-bromo-1,2-dimethylcyclohexene

14-5 §14.3 source ↗

The 1,2 adduct and the 1,4 adduct formed by reaction of HBr with 1,3-butadiene are in equilibrium at 40 °C. Propose a mechanism by which the interconversion of products takes place.

Answer

Interconversion occurs by SN1 dissociation to a common intermediate cation.

14-6 §14.3 source ↗

Why do you suppose 1,4 adducts of 1,3-butadiene are generally more stable than 1,2 adducts?

Answer

The double bond is more highly substituted.

14-7 §14.5 source ↗

Predict the product of the following Diels–Alder reaction:

Answer

14-8 §14.5 source ↗

Which of the following alkenes would you expect to be good Diels–Alder dienophiles?
(a)
(b)
(c)
(d)
(e)

Answer

Good dienophiles: (a), (d)

14-9 §14.5 source ↗

Which of the following dienes have an s-cis conformation, and which have an s-trans conformation? Of the s-trans dienes, which can readily rotate to s-cis?
(a)
(b)
(c)

Answer

Compound (a) is s-cis. Compound (c) can rotate to s-cis.

14-10 §14.5 source ↗

Predict the product of the following Diels–Alder reaction:

Answer

14-11 §14.6 source ↗

Draw a segment of the polymer that might be prepared from 2-phenyl-1,3-butadiene.

Answer

14-12 §14.6 source ↗

Show the mechanism of the acid-catalyzed polymerization of 1,3-butadiene.

Answer

14-13 §14.7 source ↗

Calculate the energy range of electromagnetic radiation in the UV region of the spectrum from 200 to 400 nm (see Section 12.5). How does this value compare with the values calculated previously for IR and NMR spectroscopy?

Answer

300–600 kJ/mol; UV energy is greater than IR or NMR energy.

14-14 §14.7 source ↗

If pure vitamin A has λmax = 325 (ε = 50,100), what is the vitamin A concentration in a sample whose absorbance at 325 nm is A = 0.735 in a cell with a pathlength of 1.00 cm?

Answer

1.46 × 10−5 M

14-15 §14.8 source ↗

Which of the following compounds would you expect to show ultraviolet absorptions in the 200 to 400 nm range?
(a)
(b)
(c)
(d)
(e)
(f)

Answer

All except (a) have UV absorptions.

Loading chapter 15…

Problems and answers are adapted from Organic Chemistry: A Tenth Edition by John McMurry, OpenStax, and are used under the CC BY-NC-SA 4.0 licence. Access for free at https://openstax.org/books/organic-chemistry/pages/1-why-this-chapter. Illustrations: Copyright Rice University, OpenStax, under CC BY-NC-SA 4.0 license. This adaptation is released under the same licence, is non-commercial, and is neither affiliated with nor endorsed by OpenStax or Rice University.