Q: A solution of isoprene has an absorbance of 0.56 at its $$\lambda_{max} in a 1 cm $$cell at a concentration of $$4.0 \times 10$$^{-5}$$ M. What is the molar absorptivity ($$\varepsilon$$)? Use A$$ = \varepsilon c l$$.

$$\varepsilon = 1.4 \times 10^4$ $$\mathrm{L\,$$mol^{-1}$$\,$$cm^{-1}$$}$$

Question 1

Which of the following best explains why β-carotene, the pigment found in carrots, appears orange to the human eye?

Accepted Answer

It absorbs light in the blue region of the visible spectrum due to its long conjugated system, reflecting orange light.

Question 2

Which of the following statements about the stability of conjugated dienes is correct?

Accepted Answer

Conjugated dienes are more stable than isolated dienes due to delocalization of π electrons, which is reflected in a lower heat of hydrogenation.

Question 3

Given the following compounds, which would have the lowest heat of hydrogenation?

(a) hexa-1,2-diene
(b) hexa-1,3-diene
(c) hexa-1,4-diene
(d) hexa-1,3,5-triene

Accepted Answer

hexa-1,3,5-triene

Question 4

Which of the following best describes the bond lengths in 1,3-butadiene?

Accepted Answer

The central C–C bond is shorter than a typical single bond but longer than a double bond due to partial double-bond character from π-electron delocalization.

Question 5

Which molecular orbital in 1,3-butadiene is the highest in energy?

Accepted Answer

$$\pi^*_4$$

Question 6

Which conformation of 1,3-butadiene is more stable and why?

Accepted Answer

The s-trans conformation is more stable due to less steric hindrance between the hydrogen atoms.

Question 7

Which of the following best explains the difference between kinetic and thermodynamic control in the addition of HBr to 1,3-butadiene?

Accepted Answer

Kinetic control at low temperature favors the product that forms faster (1,2-addition), while thermodynamic control at higher temperature favors the more stable product (1,4-addition).

Question 8

Which of the following is true about the Diels–Alder reaction?

Accepted Answer

It is a [4+2] cycloaddition between a conjugated diene and a dienophile, forming a cyclohexene ring.

Question 9

Which of the following best describes the effect of conjugation on the UV absorption of organic molecules?

Accepted Answer

Conjugation lowers the energy gap between the HOMO and LUMO, causing absorption at longer wavelengths.

Question 10

A solution of isoprene has an absorbance of 0.56 at its $$\lambda_{max} in a 1 cm $$cell at a concentration of $$4.0 	imes 10$$^{-5}$$ M. What is the molar absorptivity ($$\varepsilon$$)?

Use A$$ = \varepsilon c l$$.

Accepted Answer

$$\varepsilon = 1.4 	imes 10^4$ $$\mathrm{L\,$$mol^{-1}$$\,$$cm^{-1}$$}$$

Question 11

Which of the following best describes the molecular orbital diagram for ethylene ($$\mathrm{C_2H_4}$$)?

Accepted Answer

It has one bonding $$\pi$$ molecular orbital and one antibonding $$\pi^*$$ molecular orbital, each formed from the overlap of two p orbitals.

Question 12

Which of the following statements about allylic carbocations is correct?

Accepted Answer

Allylic carbocations are stabilized by resonance with the adjacent double bond, delocalizing the positive charge.

Question 13

Which of the following is the correct order of stability for the following species: methyl radical, 1° radical, 2° radical, 3° radical, allyl radical, benzyl radical?

Accepted Answer

methyl \u003c 1° \u003c 2° \u003c 3° \u003c allyl \u003c benzyl

Question 14

Which of the following best explains why N-bromosuccinimide (NBS) is used in allylic bromination reactions?

Accepted Answer

NBS provides a low, constant concentration of $$\mathrm{Br_2$}$$, preventing addition across the double bond and favoring allylic bromination.

Question 15

Which of the following is a requirement for a diene to participate in a Diels–Alder reaction?

Accepted Answer

The diene must be in the s-cis conformation.

Question 16

Which of the following compounds would you expect to have the longest wavelength ($$\lambda_{max}) of UV $$absorption?

Accepted Answer

A conjugated tetraene

Question 17

Which of the following best describes the transition responsible for UV absorption in conjugated systems?

Accepted Answer

$$\pi \rightarrow \pi^*$$ electronic transition

Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy: Study Notes

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