7. Nucleophilic attack will be most facile on
a. CH2=CF2
b. CH2=CCl2
c. CF2=CF2
d. CH2=CH2
Nucleophilic Attack on Alkenes: CH2=CF2 Most Facile
Nucleophilic attack occurs most readily on CH2=CF2 among the given options due to fluorine’s strong electron-withdrawing inductive effect, which activates the double bond toward nucleophiles.
Option Analysis
a. CH2=CF2: Fluorine, the most electronegative halogen, withdraws electrons strongly through sigma bonds, polarizing the pi electrons and making the beta carbon (CH2) highly electrophilic for nucleophilic attack. Fluoroalkenes like this undergo facile nucleophilic addition or substitution, unlike typical alkenes.
b. CH2=CCl2: Chlorine withdraws electrons inductively but less effectively than fluorine and donates via resonance (+R effect), partially stabilizing the pi system and reducing electrophilicity compared to fluoro derivatives. Thus, slower nucleophilic reactivity.
c. CF2=CF2: Both carbons bear fluorines, symmetrically deactivating the double bond; the extreme electron withdrawal raises the LUMO too high, hindering nucleophilic approach despite high polarity. Least reactive fluoroalkene here.
d. CH2=CH2: Unsubstituted ethylene lacks electron-withdrawing groups, so its pi electrons repel nucleophiles; it prefers electrophilic addition exclusively.
Nucleophilic attack on alkenes like CH2=CF2 becomes highly favorable when electron-withdrawing groups polarize the double bond.
Reactivity Factors
Fluorine’s superior inductive withdrawal (-I effect) exceeds chlorine’s, as F’s electronegativity (4.0) dwarfs Cl’s (3.0), enhancing electrophilicity at the unsubstituted carbon.
-
CH2=CF2: Optimal asymmetry for regioselective attack.
-
Others: Reduced activation from resonance or symmetry.
Mechanism Insight
Nucleophile adds to beta carbon, forming a carbanion stabilized by F; elimination may follow in substitution. Fluoroalkenes show regioselectivity with nucleophiles attaching to CF2 side.
CSIR NET Relevance
This MCQ tests vinyl halide/alkene reactivity distinctions; answer is a. CH2=CF2. Vinyl chlorides resist substitution due to sp2-hybrid C-X strength, but fluoroalkenes uniquely enable addition.
