The constructive overlap of the $$\\pi_1$ and $$\\$$pi_2$$ molecular orbitals in 1,3-butadiene.

Question 1

Which of the following best describes the difference between conjugated and non-conjugated dienes?

Accepted Answer

Conjugated dienes have alternating single and double bonds, allowing for delocalization of electrons.

Question 2

Which molecular orbital interaction is responsible for the increased stability of 1,3-butadiene compared to ethylene?

Accepted Answer

The constructive overlap of the $$\pi_1$ and $$\$$pi_2$$ molecular orbitals in 1,3-butadiene.

Question 3

Which conformation of a conjugated diene is most stable and why?

Accepted Answer

s-trans, because it minimizes van der Waals repulsions.

Question 4

Which statement about the heat of hydrogenation of conjugated dienes is correct?

Accepted Answer

Conjugated dienes release less heat during hydrogenation than isolated dienes, indicating greater stability.

Question 5

Which of the following best explains why allylic cations are more stable than typical carbocations?

Accepted Answer

Allylic cations are stabilized by resonance, allowing charge delocalization.

Question 6

In the addition of HBr to buta-1,3-diene at low temperature, which product predominates and why?

Accepted Answer

The 1,2-addition product predominates because it forms faster (kinetic control).

Question 7

Which reagent and conditions are used to favor allylic bromination over addition to the double bond?

Accepted Answer

N-bromosuccinimide (NBS) with light or heat to initiate free radical formation.

Question 8

Why do allylic halides undergo SN2 reactions faster than non-allylic halides?

Accepted Answer

The transition state is stabilized by conjugation with the pi bond, lowering activation energy.

Question 9

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 10

What is the main stereochemical outcome of the Diels-Alder reaction?

Accepted Answer

Syn addition of the diene to the dienophile, forming a six-membered ring.

Question 11

According to the Endo Rule in Diels-Alder reactions, which product is favored?

Accepted Answer

The product where the electron-withdrawing groups on the dienophile are oriented towards the diene (endo product).

Question 12

Which statement about the conservation of orbital symmetry in pericyclic reactions is correct?

Accepted Answer

Constructive overlap of the diene HOMO and dienophile LUMO stabilizes the transition state.

Question 13

Which cycloaddition is thermally forbidden but photochemically allowed?

Accepted Answer

[2+2] cycloaddition of two ethylenes to form cyclobutene.

Question 14

Which equation describes the relationship between energy and wavelength in UV absorption?

Accepted Answer

$$\Delta E = h \nu = \frac{hc}{\lambda}$$

Question 15

Which factor increases the $$\lambda_{max} of a $$conjugated diene in UV-VIS spectroscopy?

Accepted Answer

Increasing the number of conjugated double bonds.

Question 16

How does the addition of alkyl groups to a conjugated diene affect its $$\lambda_{max} in UV-$$VIS spectroscopy?

Accepted Answer

Each alkyl group increases $$\lambda_{max} by $$about 5 nm.

Question 17

Which of the following compounds would have the highest $$\lambda_{max} in UV-$$VIS spectroscopy?

Accepted Answer

Octa-1,3,5,7-tetraene

Question 18

What is the effect of locking a diene into the s-cis conformation on its reactivity in the Diels-Alder reaction?

Accepted Answer

It increases the reactivity because the s-cis conformation is required for the reaction.

Question 19

Which term describes the structural feature responsible for UV-VIS absorption in a molecule?

Accepted Answer

Chromophore

Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy

Study Guide - Practice Questions