Question 1

Which of the following statements best describes the difference between localized and delocalized electrons in organic molecules?

Accepted Answer

Localized electrons are non-bonding or shared by just two atoms in a bond, while delocalized electrons are shared by three or more atoms and distributed over more than one bond.

Question 2

Which of the following molecules contains delocalized electrons?

Accepted Answer

$$\mathrm{CH_3CO_2^-}$$ \, (acetate \, ion)$

Question 3

Why is the conjugate base of acetic acid ($$\mathrm{CH_3CO_2^-}) $$more stable than the conjugate base of ethanol ($$\mathrm{CH_3CH_2O^-}$$)?

Accepted Answer

Because the negative charge in acetate is delocalized over two oxygen atoms through resonance.

Question 4

Which of the following is a requirement for a molecule to be aromatic?

Accepted Answer

The molecule must have an uninterrupted cyclic cloud of $$\pi$$ electrons and an odd number of pairs of $$\pi$$ electrons.

Question 5

Which of the following carbocations is the most stable?

Accepted Answer

A benzylic carbocation

Question 6

Why is phenol ($$\mathrm{C_6H_5OH}) $$more acidic than cyclohexanol?

Accepted Answer

Because the phenolate ion is stabilized by resonance delocalization of the negative charge over the aromatic ring.

Question 7

Which of the following substituents on benzoic acid will result in the lowest $$pK_a$$ value?

Accepted Answer

$$\mathrm{NO_2$}$$

Question 8

What is the major product when 1 equivalent of HBr is added to 1,3-butadiene?

Accepted Answer

Both 3-bromo-1-butene and 1-bromo-2-butene are formed as major products due to resonance stabilization of the allylic carbocation intermediate.

Question 9

Which of the following best explains why conjugated dienes are more stable than isolated dienes?

Accepted Answer

Conjugated dienes allow for delocalization of $$\pi$$ electrons across multiple adjacent p orbitals, increasing resonance stabilization.

Question 10

Which of the following statements about electrophilic aromatic substitution (EAS) is correct?

Accepted Answer

EAS involves the addition of an electrophile to an aromatic ring, followed by deprotonation to restore aromaticity.

Question 11

Which of the following is NOT a requirement for resonance to occur in a molecule?

Accepted Answer

The molecule must be cyclic.

Question 12

Which of the following best describes the effect of an electron-withdrawing group on the acidity of benzoic acid?

Accepted Answer

It stabilizes the conjugate base by decreasing electron density, making the acid stronger (lower $$pK_a).$$

Question 13

Which of the following statements about the resonance hybrid of benzene is correct?

Accepted Answer

All carbon-carbon bonds in benzene are of equal length due to delocalization of $$\pi$$ electrons.

Question 14

Which of the following best explains why the $$pK_a of $$protonated aniline is much lower than that of protonated cyclohexylamine?

Accepted Answer

Aniline (not the protonated form) is stabilized by resonance, making its conjugate acid (protonated aniline) a stronger acid (lower $$pK_a).$$

Question 15

Which of the following is the correct order of carbocation stability from most to least stable?

Accepted Answer

Benzylic = Allylic \u003e Tertiary \u003e Secondary \u003e Primary \u003e Methyl

Question 16

Which of the following best describes the effect of an electron-donating group (such as $$\mathrm{$$OCH_3$$}) on $$the $$pK_a of $$benzoic acid?

Accepted Answer

It increases the $$pK_a by $$destabilizing the conjugate base through resonance donation.

Question 17

Which of the following is a correct statement about the addition of 1 equivalent of HCl to 2,4-hexadiene?

Accepted Answer

Both 4-chloro-2-hexene (1,2-addition) and 2-chloro-3-hexene (1,4-addition) are formed due to resonance stabilization of the allylic carbocation.

Question 18

Which of the following best explains why the resonance hybrid of the acetate ion is more stable than either of its individual resonance contributors?

Accepted Answer

The resonance hybrid has a lower energy due to delocalization of the negative charge over two oxygen atoms.

Question 19

Which of the following is a characteristic reaction of benzene and other aromatic compounds?

Accepted Answer

Electrophilic aromatic substitution

Delocalization, Stability, Reactivity, and pKa in Organic Chemistry

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