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Which of the following is the more stable carbocation?
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+ Biochemistry
+ Carbocation
+ Stability
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Michael Kossin
Which of the following is the more stable carbocation?
Vinyl cations can also be generated by the protonation of acetylenes in strong acid. The vinyl cations are captured by a nucleophile to produce a stable alkene with the nucleophile attached to the double bond.
Which of the following is a more stable carbocation?
This route to vinyl cations has been well studied. As shown in the following diagram, protonation of phenyl acetylene can proceed in two different ways. The acetylenic carbon attached to the phenyl ring may be protonated or the terminal acetylenic carbon may be protonated. As you can see, these two routes produce the two vinyl cations you want to compare.
Upon examining the product(s) found in this reaction, we find that only alkene A2 is produced. Therefore, the reaction proceeds through the intermediacy of vinyl cation A1, not B1. Consequently, the pathway involving A1 must be lower in energy, or said differently, vinyl cation A1 is lower in energy than vinyl cation B1.
Reasons Why Vinyl Cation A1 Is More Stable Than Vinyl Cation B1
The barrier to rotate the phenyl group is very low. Hence, in A1 the phenyl group can readily adopt a conformation where the aromatic p-orbitals are aligned with the empty p-orbital on the adjacent carbocation center. Additional resonance structures can now be drawn with the positive charge delocalized into the aromatic ring. There is no such resonance stabilization with the aromatic ring possible for vinyl cation B1. This suggests that vinyl cation A1 is more stable than vinyl cation B1.
In addition to the resonance structures involving the aromatic ring, as shown at the bottom of the diagram, one can draw 2 more hyperconjugated resonance resonance structures for vinyl cation A1, but only 1 hyperconjugated resonance structure can be drawn for B1; again suggesting that vinyl cation A1 is more stable than vinyl cation B1.
Resonance effects are generally stronger and more controlling than inductive effects. Therefore, the inductive arguments presented in @orthocresol 's answer play only a minor role, being outweighed by the stronger resonance effects described above.
Vinyl cations can also be generated by the protonation of acetylenes in strong acid. The vinyl cations are captured by a nucleophile to produce a stable alkene with the nucleophile attached to the double bond.
Which of the following is a more stable carbocation?
This route to vinyl cations has been well studied. As shown in the following diagram, protonation of phenyl acetylene can proceed in two different ways. The acetylenic carbon attached to the phenyl ring may be protonated or the terminal acetylenic carbon may be protonated. As you can see, these two routes produce the two vinyl cations you want to compare.
Upon examining the product(s) found in this reaction, we find that only alkene A2 is produced. Therefore, the reaction proceeds through the intermediacy of vinyl cation A1, not B1. Consequently, the pathway involving A1 must be lower in energy, or said differently, vinyl cation A1 is lower in energy than vinyl cation B1.
Reasons Why Vinyl Cation A1 Is More Stable Than Vinyl Cation B1
The barrier to rotate the phenyl group is very low. Hence, in A1 the phenyl group can readily adopt a conformation where the aromatic p-orbitals are aligned with the empty p-orbital on the adjacent carbocation center. Additional resonance structures can now be drawn with the positive charge delocalized into the aromatic ring. There is no such resonance stabilization with the aromatic ring possible for vinyl cation B1. This suggests that vinyl cation A1 is more stable than vinyl cation B1.
In addition to the resonance structures involving the aromatic ring, as shown at the bottom of the diagram, one can draw 2 more hyperconjugated resonance resonance structures for vinyl cation A1, but only 1 hyperconjugated resonance structure can be drawn for B1; again suggesting that vinyl cation A1 is more stable than vinyl cation B1.
Resonance effects are generally stronger and more controlling than inductive effects. Therefore, the inductive arguments presented in @orthocresol 's answer play only a minor role, being outweighed by the stronger resonance effects described above.
@ksr Look at this diagram of non-vinylic hypercojugation. The angle between the C−H bond and the p-orbital is 109°, the tetrahedral angle. In the vinylic cation case this angle is increased to 120°. This increase of only ~10° will lessen the effectiveness of hyperconjugation in the vinylic case, but I would not expect it to eliminate vinylic hyperconjugation. Further, note that since a double bond is shorter than a single bond, this slight decrease in overlap due to the increased angle may be more than offset by the increased overlap due to the shorter bond length.More
Aha! do you have a source for the formation of A2 over B2? Not doubting you, I just want to read. I remember I tried searching for protonation of phenylacetylene last night but couldnt quite find anything.More
Vinyl cations can also be generated by the protonation of acetylenes in strong acid. The vinyl cations are captured by a nucleophile to produce a stable alkene with the nucleophile attached to the double bond.
This route to vinyl cations has been well studied. As shown in the following diagram, protonation of phenyl acetylene can proceed in two different ways. The acetylenic carbon attached to the phenyl ring may be protonated or the terminal acetylenic carbon may be protonated. As you can see, these two routes produce the two vinyl cations you want to compare.
Upon examining the product(s) found in this reaction, we find that only alkene A2 is produced. Therefore, the reaction proceeds through the intermediacy of vinyl cation A1, not B1. Consequently, the pathway involving A1 must be lower in energy, or said differently, vinyl cation A1 is lower in energy than vinyl cation B1.
Reasons Why Vinyl Cation A1 Is More Stable Than Vinyl Cation B1
Vinyl cations can also be generated by the protonation of acetylenes in strong acid. The vinyl cations are captured by a nucleophile to produce a stable alkene with the nucleophile attached to the double bond.
This route to vinyl cations has been well studied. As shown in the following diagram, protonation of phenyl acetylene can proceed in two different ways. The acetylenic carbon attached to the phenyl ring may be protonated or the terminal acetylenic carbon may be protonated. As you can see, these two routes produce the two vinyl cations you want to compare.
Upon examining the product(s) found in this reaction, we find that only alkene A2 is produced. Therefore, the reaction proceeds through the intermediacy of vinyl cation A1, not B1. Consequently, the pathway involving A1 must be lower in energy, or said differently, vinyl cation A1 is lower in energy than vinyl cation B1.
Reasons Why Vinyl Cation A1 Is More Stable Than Vinyl Cation B1
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