Fill in the blancks for the following statement:

Alkyl halides are excellent _________ for substitution reactions because the halogen atom _________ electron density via __________ and the halogen can also serve as a good leaving group.

Question

Fill in the blancks for the following statement:

Alkyl halides are excellent _________ for substitution reactions because the halogen atom _________ electron density via __________ and the halogen can also serve as a good leaving group.

BlackTom AI · Accepted Answer

Alkyl halides are electrophiles because the carbon bonded to a halogen bears a partial positive charge as a result of the halogen withdrawing electron density through the inductive effect.

1) Relevant knowledge points and concepts
- Electronegativity difference: halogens (F, Cl, Br, I) are more electronegative than carbon, so the C–X sigma bond is polarized (C is δ+, X is δ–).
- Inductive effect (–I): this is the sigma-bond electron withdrawal transmitted through the bond framework; it lowers electron density at the carbon.
- Leaving-group ability: halide ions are relatively stable anions (especially I–, Br–) and can depart, making substitution (SN1 and SN2) favorable.
- Mechanisms: SN2 involves nucleophilic attack at the electrophilic carbon with simultaneous expulsion of the halide; SN1 proceeds by loss of the halide to form a carbocation that is then attacked by a nucleophile.

2) Step-by-step reasoning
- Compare electronegativities: C ≈ 2.5, Cl ≈ 3.0, Br ≈ 2.8, I ≈ 2.5–2.6; the more electronegative halogen pulls electron density toward itself.
- This polarization produces a δ+ center on the carbon (site susceptible to nucleophilic attack) — hence the molecule functions as an electrophile in substitution reactions.
- The halogen can leave as X–; the stability of X– (greater for larger, more polarizable halides) makes it a good leaving group, facilitating substitution.

3) Calculations/intermediate steps
- No numerical calculation required; the key intermediate concept is the generation of an electrophilic carbon (Cδ+) and a leaving anion X– during the reaction coordinate for SN1/SN2.

4) Why the correct answer is correct
- “Electrophiles” fits because the halogen’s inductive withdrawal creates an electron-poor carbon. “Withdraws” correctly describes the direction of electron flow (toward X). “Induction” specifies the mechanism (sigma-bond, through-bond effect) rather than resonance or donation.

5) Why other plausible answers are incorrect
- “Nucleophiles” would be wrong because alkyl halides do not donate electron density to attack electrophiles; they are attacked. “Donates” is wrong because halogens do not increase electron density at carbon in this context. “Resonance” is incorrect because the halogen’s effect here is through the sigma-bond inductive effect, not pi-resonance (except in special conjugated systems).

6) Conclusion
This reasoning confirms the correct completion: “electrophiles; withdraws; induction.”

Fill in the blancks for the following statement: Alkyl halides are excellent _____ for substitution reactions because the halogen atom _ electron density via ______ and the halogen can also serve as a good leaving group.单项选择题

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Fill in the blancks for the following statement: Alkyl halides are excellent _________ for substitution reactions because the halogen atom _________ electron density via __________ and the halogen can also serve as a good leaving group.单项选择题