Between the -SH and -OH groups in 2-mercaptoethanol, which acts as the dominant nucleophile?
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Correct answer
Option analysis
This option correctly identifies the thiol as the more nucleophilic center performing conjugate addition onto acrylonitrile. The thiol group (-SH) is significantly more nucleophilic than the hydroxyl group (-OH), adding via conjugate addition to the β-position of the α,β-unsaturated nitrile.
Assuming that the hydroxyl oxygen is a stronger nucleophile than the thiol sulfur, resulting in ether formation instead of thioether formation. Sulfur is larger and more polarizable than oxygen, making the thiol group substantially more nucleophilic and faster to add to Michael acceptors than the alcohol.
Believing nucleophilic attack occurs directly on the cyano carbon (1,2-addition) rather than conjugate addition at the β-alkene carbon (1,4-addition). Acrylonitrile is an activated Michael acceptor; soft nucleophiles like thiols preferentially attack the β-carbon via conjugate addition rather than the hard nitrile carbon.
Believing the hydroxyl oxygen attacks the nitrile carbon directly via 1,2-addition while leaving the alkene intact. Conjugate addition (Michael addition) by the softer sulfur atom at the β-carbon dominates over 1,2-addition of oxygen to the nitrile group.
The substrate is acrylonitrile (), an -unsaturated nitrile (Michael acceptor). The reagent is 2-mercaptoethanol (), which has two nucleophilic heteroatoms: oxygen (-OH) and sulfur (-SH).
Compare nucleophilicity of -SH vs -OH: Sulfur is larger, more polarizable, and less electronegative than oxygen. This makes the thiol group a significantly stronger and softer nucleophile than the hydroxyl group. Thus, the lone pair on sulfur preferentially attacks the electrophilic -carbon of acrylonitrile.
Conjugate 1,4-addition: Nucleophilic attack of -SH at the -carbon () pushes the -electrons to form a resonance-stabilized carbanion/ketenimine intermediate: . Subsequent proton transfer yields the neutral conjugate addition product .
The resulting major product is 3-((2-hydroxyethyl)thio)propanenitrile, , which corresponds to Option A.
Direct 1,2-addition across the nitrile triple bond gives C=NH products like C and D. This pathway is kinetically and thermodynamically disfavored. Instead, 1,4-conjugate addition across the activated double bond is favored. Attack by oxygen rather than sulfur (Option B) is disfavored because -OH has lower nucleophilicity.
Between the -SH and -OH groups in 2-mercaptoethanol, which acts as the dominant nucleophile?
The thiol group (-SH), because sulfur is more polarizable and softer than oxygen.Where does the thiol nucleophile attack on acrylonitrile (CH2=CH-CN)?
At the β-alkene carbon via conjugate (Michael) addition.Quick checks
Sulfur is in period 3 with larger 3p orbitals. Its electron cloud is held less tightly and is much more polarizable than oxygen's. Therefore, thiols act as soft nucleophiles. They react much better with soft Michael acceptors like α,β-unsaturated nitriles.
In -unsaturated nitriles or carbonyls, soft nucleophiles like thiolates or thiols prefer conjugate addition. They attack the soft -carbon. They do not attack the hard -hybridized cyano carbon in a direct 1,2-addition.
