Which of the following is a Lewis acid? a.) carbocation b.) carbon free radical c.) answers a and b d.) carbanion 4. Which of the following is a poor leaving group? a.) fluoride b.) chloride c.) bromide d.) iodide 5. Which of the following can never act as a nucleophile? a.) Alcohols b.) ethers c.) carboxylic acids d.) halides
The Correct Answer and Explanation is:
1. Which of the following is a Lewis acid?
Correct answer: a) carbocation
A Lewis acid is a species that accepts an electron pair. Among the options, a carbocation is the only one that can act as a Lewis acid. This is because it has an electron-deficient carbon atom that seeks to accept an electron pair to complete its octet. In contrast:
- A carbon free radical (b) has an unpaired electron and is not electron-deficient in the same way as a carbocation, so it does not act as a Lewis acid.
- A carbanion (d) has a negatively charged carbon, which donates electrons, making it a Lewis base rather than an acid.
Thus, the correct answer is a carbocation, as it seeks to accept an electron pair to stabilize itself.
2. Which of the following is a poor leaving group?
Correct answer: a) fluoride
A leaving group is an atom or group of atoms that departs with an electron pair during a reaction. The better the leaving group, the more stable it is once it leaves. Among the options, fluoride is the poorest leaving group. This is because the fluoride ion (F-) is highly basic and holds onto its electrons tightly, making it less likely to leave. In contrast:
- Chloride (Cl-) and bromide (Br-) are better leaving groups than fluoride because they are larger and more stable when they acquire the negative charge.
- Iodide (I-) is the best leaving group of the halides due to its size and the ability to stabilize the negative charge effectively.
3. Which of the following can never act as a nucleophile?
Correct answer: c) carboxylic acids
A nucleophile is a species that donates an electron pair to form a new bond with an electrophile. Among the options:
- Alcohols (a) have a lone pair of electrons on oxygen, making them good nucleophiles.
- Ethers (b), while not as strong nucleophiles as alcohols, still have lone pairs on oxygen, enabling them to act as nucleophiles.
- Carboxylic acids (c), however, are poor nucleophiles because the oxygen in the carbonyl group is already highly electronegative, and the structure is more stabilized in a way that reduces the nucleophilicity of the oxygen atoms.
- Halides (d) are typical nucleophiles because halogens like chloride, bromide, and iodide have lone pairs that can be donated to an electrophile.
So, the correct answer is carboxylic acids, as they are not nucleophilic in this context.
