Question 1

Which statement best explains why conjugated dienes are more stable than isolated dienes?

Accepted Answer

Conjugated dienes have delocalized electrons that provide extra stability through resonance.

Question 2

Given the following heats of hydrogenation: $$\Delta H^\circ$$ for pent-1-ene is $$-126\ \mathrm{kJ}$$ and for trans-pent-2-ene is $$-114\ \mathrm{kJ}$$, what is the predicted heat of hydrogenation for buta-1,3-diene if it behaved as two isolated double bonds?

Accepted Answer

$$-240\ \mathrm{kJ}$$

Question 3

Why is the actual heat of hydrogenation for buta-1,3-diene less negative than the predicted value for two isolated double bonds?

Accepted Answer

Because conjugation provides extra stability, releasing less energy upon hydrogenation.

Question 4

Which bond in buta-1,3-diene is shorter than a typical single bond, and why?

Accepted Answer

The C2—C3 bond, due to partial double bond character from electron delocalization.

Question 5

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

Accepted Answer

s-trans, because it minimizes steric interference between hydrogens.

Question 6

Which of the following best describes the difference between conjugated and isolated double bonds?

Accepted Answer

Conjugated double bonds are separated by one single bond, while isolated double bonds are separated by two or more single bonds.

Question 7

If the heat of hydrogenation for a compound is less negative than expected, what does this indicate about the compound’s stability?

Accepted Answer

The compound is more stable due to resonance or conjugation.

Question 8

Which of the following molecules would you expect to have the lowest heat of hydrogenation?

Accepted Answer

A conjugated diene

Question 9

What is the main reason for the extra stability observed in conjugated systems?

Accepted Answer

Delocalization of pi electrons across adjacent p orbitals.

Question 10

Which of the following statements about the s-cis and s-trans conformations of buta-1,3-diene is correct?

Accepted Answer

The s-trans conformation is more stable by 12 kJ/mol due to reduced steric interference.

Question 11

A student measures the C2—C3 bond length in buta-1,3-diene and finds it to be 1.48 Å. What does this suggest about the bond?

Accepted Answer

It has partial double bond character due to conjugation.

Question 12

Which of the following best describes the electron distribution in a conjugated diene?

Accepted Answer

Electrons are delocalized over three or more adjacent carbons.

Question 13

Why do conjugated dienes release less heat upon hydrogenation compared to isolated dienes?

Accepted Answer

They are more stable due to electron delocalization.

Question 14

Which of the following is true regarding the stability order of hydrocarbons shown in the relative stabilities diagram?

Accepted Answer

Conjugated dienes are more stable than isolated dienes and alkynes.

Question 15

Which experimental technique is commonly used to compare the relative stabilities of alkenes and dienes?

Accepted Answer

Measurement of heats of hydrogenation.

Question 16

A chemist wants to synthesize a compound with maximum stability. Which type of diene should they choose?

Accepted Answer

Conjugated diene

Question 17

Which of the following best explains the partial double bond character of the C2—C3 bond in buta-1,3-diene?

Accepted Answer

Resonance delocalization of pi electrons across the molecule.

Question 18

If a diene is locked in the s-cis conformation, how does this affect its reactivity in cycloaddition reactions?

Accepted Answer

It increases reactivity because the required orbital overlap is always present.

Question 19

Which of the following statements about the interconversion of s-cis and s-trans conformers of buta-1,3-diene is correct?

Accepted Answer

They easily interconvert at room temperature.

Conjugated Systems: Structure, Stability, and Properties of Dienes

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