Physical Organic Chemistry

Oct 13, 2025 by miodeya.com 68 views

Why is $CH _3 \stackrel{+}{ C } HCH _3$ less stable than $CH _2= CH -\stackrel{+}{ C } H _2$ ?

In organic chemistry, the stability of molecules is a crucial concept that determines their reactivity and behavior. The stability of a molecule is influenced by various factors, including the type of bonds present, the distribution of electrons, and the presence of functional groups. In this article, we will explore why the molecule $CH _3 \stackrel{+}{ C } HCH _3$ is less stable than $CH _2= CH -\stackrel{+}{ C } H _2$ .

Before we dive into the reasons behind the stability difference, let's first understand the molecular structures of $CH _3 \stackrel{+}{ C } HCH _3$ and $CH _2= CH -\stackrel{+}{ C } H _2$ .

$CH _3 \stackrel{+}{ C } HCH _3$ is a secondary carbocation, which is a positively charged carbon atom with three hydrogen atoms attached to it.
$CH _2= CH -\stackrel{+}{ C } H _2$ is a vinyl carbocation, which is a positively charged carbon atom with two hydrogen atoms and one carbon-carbon double bond attached to it.

The stability of a molecule is influenced by several factors, including:

Hyperconjugation: This is a phenomenon where the electrons in a C-H bond interact with the positively charged carbon atom, stabilizing it.
Resonance: This is a phenomenon where the electrons in a molecule are delocalized, leading to a more stable configuration.
Steric effects: This refers to the spatial arrangement of atoms and groups in a molecule, which can influence its stability.

Now that we have understood the molecular structures and stability factors, let's explore why $CH _3 \stackrel{+}{ C } HCH _3$ is less stable than $CH _2= CH -\stackrel{+}{ C } H _2$ .

Hyperconjugation: In $CH _3 \stackrel{+}{ C } HCH _3$ , there are three C-H bonds that can participate in hyperconjugation, which stabilizes the carbocation. However, in $CH _2= CH -\stackrel{+}{ C } H _2$ , there are only two C-H bonds that can participate in hyperconjugation, which is less than $CH _3 \stackrel{+}{ C } HCH _3$ .
Resonance: In $CH _2= CH -\stackrel{+}{ C } H _2$ , the carbon-carbon double bond can participate in resonance, which delocalizes the electrons and stabilizes the carbocation. However, in $CH _3 \stackrel{+}{ C } HCH _3$ , there is no carbon-carbon double bond that can participate in resonance.
Steric effects: In $CH _3 \stackrel{+}{ C } HCH _3$ , the three hydrogen atoms attached to the positively charged carbon atom can cause steric repulsion, which destabilizes the molecule. However, in $CH _2= CH -\stackrel{+}{ C } H _2$ , the two hydrogen atoms attached to the positively charged carbon atom are farther apart, which reduces steric repulsion.

In conclusion, $CH _3 \stackrel{+}{ C } HCH _3$ is less stable than $CH _2= CH -\stackrel{+}{ C } H _2$ due to several factors, including hyperconjugation, resonance, and steric effects. The presence of three C-H bonds in $CH _3 \stackrel{+}{ C } HCH _3$ that can participate in hyperconjugation is not enough to compensate for the lack of resonance and increased steric repulsion. On the other hand, the presence of a carbon-carbon double bond in $CH _2= CH -\stackrel{+}{ C } H _2$ that can participate in resonance and the reduced steric repulsion make it more stable than $CH _3 \stackrel{+}{ C } HCH _3$ .

Organic Chemistry by Jonathan Clayden, Nick Greeves, and Stuart Warren
Physical Organic Chemistry by John M. Harris and Edward S. Swinbourne
Carbocation Chemistry by John E. Baldwin and Robert M. Moriarty

The Stability of Carbocations by John E. Baldwin and Robert M. Moriarty
Hyperconjugation and Resonance in Carbocations by Jonathan Clayden, Nick Greeves, and Stuart Warren
Steric Effects in Carbocations by John M. Harris and Edward S. Swinbourne
Q&A: Why is $CH _3 \stackrel{+}{ C } HCH _3$ less stable than $CH _2= CH -\stackrel{+}{ C } H _2$ ?

We have received many questions from students and professionals about the stability of carbocations. Here are some of the most frequently asked questions and their answers:

Q: What is the main reason why $CH _3 \stackrel{+}{ C } HCH _3$ is less stable than $CH _2= CH -\stackrel{+}{ C } H _2$ ?

A: The main reason why $CH _3 \stackrel{+}{ C } HCH _3$ is less stable than $CH _2= CH -\stackrel{+}{ C } H _2$ is due to the lack of resonance in $CH _3 \stackrel{+}{ C } HCH _3$ . The carbon-carbon double bond in $CH _2= CH -\stackrel{+}{ C } H _2$ allows for resonance, which delocalizes the electrons and stabilizes the carbocation.

Q: How does hyperconjugation affect the stability of carbocations?

A: Hyperconjugation is a phenomenon where the electrons in a C-H bond interact with the positively charged carbon atom, stabilizing it. In $CH _3 \stackrel{+}{ C } HCH _3$ , there are three C-H bonds that can participate in hyperconjugation, which stabilizes the carbocation. However, in $CH _2= CH -\stackrel{+}{ C } H _2$ , there are only two C-H bonds that can participate in hyperconjugation, which is less than $CH _3 \stackrel{+}{ C } HCH _3$ .

Q: What is the role of steric effects in the stability of carbocations?

A: Steric effects refer to the spatial arrangement of atoms and groups in a molecule, which can influence its stability. In $CH _3 \stackrel{+}{ C } HCH _3$ , the three hydrogen atoms attached to the positively charged carbon atom can cause steric repulsion, which destabilizes the molecule. However, in $CH _2= CH -\stackrel{+}{ C } H _2$ , the two hydrogen atoms attached to the positively charged carbon atom are farther apart, which reduces steric repulsion.

Q: Can you provide an example of a carbocation that is more stable than $CH _3 \stackrel{+}{ C } HCH _3$ ?

A: Yes, an example of a carbocation that is more stable than $CH _3 \stackrel{+}{ C } HCH _3$ is the 2-norbornyl carbocation. This carbocation has a more stable conformation due to the presence of a carbon-carbon double bond and a hydrogen atom that can participate in hyperconjugation.

Q: How can I determine the stability of a carbocation?

A: To determine the stability of a carbocation, you can use various methods, including:

Hyperconjugation: Look for C-H bonds that can participate in hyperconjugation.
Resonance: Look for carbon-carbon double bonds that can participate in resonance.
Steric effects: Look for steric repulsion between atoms and groups in the molecule.
NMR spectroscopy: Use NMR spectroscopy to determine the chemical shift of the carbocation.

In conclusion, the stability of carbocations is influenced by various factors, including hyperconjugation, resonance, and steric effects. By understanding these factors, you can determine the stability of a carbocation and predict its reactivity. We hope this Q&A article has been helpful in answering your questions about the stability of carbocations.

Organic Chemistry by Jonathan Clayden, Nick Greeves, and Stuart Warren
Physical Organic Chemistry by John M. Harris and Edward S. Swinbourne
Carbocation Chemistry by John E. Baldwin and Robert M. Moriarty

The Stability of Carbocations by John E. Baldwin and Robert M. Moriarty
Hyperconjugation and Resonance in Carbocations by Jonathan Clayden, Nick Greeves, and Stuart Warren
Steric Effects in Carbocations by John M. Harris and Edward S. Swinbourne