When understanding the synthetic reaction mechanism of organic chemistry, an important factor is the stability of the carbocation. Since carbocation is an unstable substance, it reacts quickly. However, even for the same carbocation, there are differences in stability depending on what kind of molecule it is.
It is very important to understand these differences. When the stability of the carbocation changes, the reactions that occur are different.
This is related to a phenomenon called hyperconjugation. Understanding this phenomenon also allows us to predict the stability of radicals and carbanions.
Carbocation is one of the intermediates in synthetic reactions. Here, we will learn about the stability of carbocation, and then we will go on to explain the stability of other intermediates such as radicals, carbanions and aryl cations.
Table of Contents
- 1 Tertiary, Secondary and Primary Carbocation Related to Stability
- 2 Radical Stability Is the Same As Carbocation
- 3 Stability with Double Bonds (Aryl Cations) and Aromatic Rings (Benzyl Cations)
- 4 Stability of Intermediates Is Involved in Synthetic Reactions in Organic Chemistry
Tertiary, Secondary and Primary Carbocation Related to Stability
An important intermediate is the carbocation. Carbocation is often found in organic chemistry and is easy to understand among the intermediates. Therefore, we will first discuss the properties of carbocation.
The stability of a carbocation depends on the shape of the molecule. They are as follows.
- Tertiary > Secondary > Primary
The stability of a carbocation depends on how many alkyl chains (or hydrogen atoms) are attached to a positively charged carbocation.
Many people look at this order and try hard to remember it. Without understanding why, they move on to the next step.
But it’s important to understand the nature of the intermediates and the resonant structure of the carbocation; it’s also involved in major organic synthesis reactions, including the SN1 reaction. So make sure you learn why they are in this order.
Carbocation Is Planar in sp2 Hybrid Orbitals
Why does the stability of the carbocation differ depending on the molecular structure? This is related to a phenomenon called hyperconjugation. Hyperconjugation in carbocations can be expressed in simple terms as follows.
- A phenomenon in which the electrons involved in a bond give electrons to a positively charged carbon atom.
What form of molecule does a carbocation take? The carbocation is an sp2 hybrid orbital. All the compounds whose bond angles are 120 degrees to each other are sp2 hybrid orbitals. It looks like the following.
In carbocation, there are three bonds to a carbon atom. Each of these bonds is located at the farthest away from each other. This results in a bond angle of 120°.
At the same time, in the sp2 hybrid orbital, each atom is located in the same plane. In a carbocation, all the atoms bound to the carbon of the cation are in the plane.
This structure is important for understanding hyperconjugation. The carbocation also has an empty orbital. Since there are an empty orbital (p-orbital) that can hold two electrons, it is common to draw orbital above and below the positively charged carbon atoms to explain hyperconjugation.
Hyperconjugation Causes Electrons to Delocalization
What if it is not a hydrogen atom that is attached to the carbon atom, but a methyl group (CH3)? In this case, the result is as follows.
A vertical empty p-orbital extends from the carbocation, as mentioned above. This p-orbital interacts with the C-H bond of the adjacent methyl group. This is hyperconjugation.
By sharing electrons, the carbon and hydrogen are bonded together. The single bond in this case is called a σ bond (sigma bond). However, if there is an atom lacking electrons, such as a carbocation, the electrons forming the σ-bond will share electrons with the carbocation.
The p-orbital of the carbocation and the C-H bond of the methyl group are parallel. As a result, the electrons forming the σ bond (C-H bond) share an electron with a positively charged carbon atom.
The carbon atom is responsible for pushing out electrons. The alkyl chain functions as an electron donating group. Therefore, although the force is weak, the influence of the carbon atom next to the carbocation stabilizes the cation.
On the other hand, what if there is no alkyl chain (such as a methyl group) next to it? In the case of a carbocation in which a hydrogen atom is directly bonded to a carbon atom, there are no orbitals that overlap parallel to the p-orbital. Therefore, there is no hyperconjugation and the cation is not stabilized.
Also, the more alkyl chains there are next to each other, the more electrons forming the C-H bond (electrons in the σ-bond) share electrons with the carbon atoms of the carbocation. As a result, the carbocation becomes more stable. This is why they are most stable in tertiary carbocations.
The wider the area of electrons, the more stable they are. This is called delocalization of electrons. The more electrons in the C-H bond interact with the carbocation through hyperconjugation, the more electrons are delocalized and the more stable the structure becomes.
In fact, there are no methyl cations. Nor do primary methyl cations form spontaneously. On the other hand, secondary carbocations do produce a few. Tertiary carbocations are more likely to be formed due to their higher stability.
One of the most important synthetic reactions in organic chemistry is the SN1 reaction, in which the carbocation is formed first and then the chemical reaction takes place; in the SN1 reaction, the formation of the tertiary carbocation (or secondary carbocation) is important. The reason for this is the high stability of the tertiary carbocation.
Interacting with Empty p-orbital in the Plane
Since the carbocation is hyperconjugated, the more alkyl chains are attached to the carbocation, the more stable the cation becomes. However, there is a condition for this. It must be able to interact with an empty p-orbital.
Empty p-orbital are always created in carbocation. When the alkyl chains are bound, the p-orbital of the carbocation and the C-H bond can overlap in parallel, resulting in hyperconjugation. In contrast, if the alkyl chains cannot interact with the empty p-orbital in parallel, the carbocation is not stable, even if it is a tertiary carbocation.
For example, no carbocation is formed in the following compounds.
Why doesn’t this compound form despite being a tertiary carbocation? In the case of this compound, even if it becomes a carbocation, the empty p-orbital and the adjacent C-H bond are not parallel. Therefore, the carbocation is not stable.
Just as methyl cations are not produced, even tertiary carbocations will not produce a cation if no hyperconjugation occurs. As for why carbocations stabilize, understand that the carbocation shares electrons weakly with the electrons in the single bond (σ bond).
Radical Stability Is the Same As Carbocation
In the same way, we can understand the stability of other intermediates as well. In organic chemical reactions, radicals may be produced. What is the three-dimensional structure of radicals? In radicals, there are three bonds, like carbocation, which are sp2 hybrid orbitals.
In contrast to carbocation, radicals have one unpaired electron. However, although their properties are different from those of carbocation, the stability of radicals is the same as that of carbocation. The stability of radicals is as follows.
- Tertiary > Secondary > Primary
It is the effect of hyperconjugation that leads to this stability of the radicals. In other words, the stability of the radicals can be explained by exactly the same reasons as the stability of the carbocation.
Radicals, like carbocation, are also lacking in electrons. As a result, their order of stability is the same as that of the carbocation.
The Stability of the Carbanion Is Reversed
If we consider the delocalization of electrons, we can understand the stability of carbocation and radicals. On the other hand, what is the stability of carbanions?
Carbanion has a negative charge. Therefore, their properties are completely different from those of carbocation and radicals, and they are not hyperconjugated. In addition, compared to carbocation and radicals, the stability of carbanions is reversed as shown below.
The same reason for this can be explained by considering the delocalization of electrons.
As mentioned above, carbon acts as an electron donating group. When the electrons of a carbanion spread out, they cannot disperse the anion’s electrons if there is a carbon atom bonded next to it. The electrons are pushed out by the neighboring carbon atom and the electron density is rather high. As a result, the molecule becomes unstable.
Comparing methyl anion and tertiary carbanion, the electrons in methyl anion are more widely distributed and can be delocalized. On the other hand, tertiary carbanions have a higher electron density.
-Inductive Effect Occurs and Stabilizes Depending on the Degree of Electronegativity
For reference, what if the hydrogen atoms in the methyl anion are halogens? In this case, the electrons of the methyl anion are more widely dispersed by the highly electronegativity halogen. This delocalization makes the methyl anion more stable.
Electrically charge differences within the same molecule caused by electronegativity are known as the inductive effect. The inductive effect results in different stability of the anion intermediate.
This is different in principle from stabilization by hyperconjugation. However, the principle is the same as for carbocation, where the stability is changed by delocalization of electrons.
Stability with Double Bonds (Aryl Cations) and Aromatic Rings (Benzyl Cations)
Once you have learned about the stability of these intermediates, you should then try to understand the stability of compounds with a conjugated structure. Once you learn that the delocalization of electrons is important to the stability of the intermediates, it is easier to understand the stability of allyl cations and benzyl cations.
The allyl cation has a double bond. In benzyl cations, there is a benzene ring. Compounds with the following structures are allyl cations and benzyl cations.
Double and triple bonds involve π bonds; in π bonds, the orbits are known to extend up and down. The first single bond is always a σ bond. In addition, when making a double or triple bond, the electron orbitals are stretched up and down, and the bond is formed by the overlap of the electron orbitals.
Because the electron orbitals are forced to make the bond, the π bond has a weak bonding force.
However, the presence of π orbitals above and below the carbocation allows it to share electrons with the empty orbitals of the carbocation.
You can also write resonance structures for allyl and benzyl compounds. For example, for a benzyl cation, it looks like the following.
A double bond (alkene) or an aromatic ring (benzene ring) allows us to write a resonance structure. As a result, both aryl cations and benzyl cations can be stabilized. The cation is not present in only one place, but the delocalization of electrons carries a positive charge in many places. As a result, the resonance stabilizes the molecule.
Primary carbocation does not form spontaneously. However, even if they are primary carbocation, they are exceptionally stable if you can write a resonance structure. Therefore, aryl cations and benzyl cations are formed.
For example, when comparing a secondary carbocation with a primary aryl cation, the primary aryl cation is more stable.
Primary allyl cations are less affected by hyperconjugation than primary carbocations because there are fewer alkyl chains bound to them. However, because of the effects of resonance, the structure is more easily stabilized.
The Allyl and Benzyl Radicals Are Equally Stable
Exactly the same thing can be said for radicals. With allyl and benzyl radicals, they are more stable. Here are the structural formulas for allyl and benzyl radicals.
It resonates with allyl and benzyl radicals. For example, here is the resonance structure of the benzyl radical.
The presence of a conjugated structure due to double bonds allows them to be resonance structure and take on a stable structure. As a result, the delocalization of electrons stabilizes the allyl and benzyl radicals.
Of course, even though they are stable, carbocation and radicals are still highly unstable and are highly reactive intermediates. However, when considering the ease of formation of intermediates, it is important to understand their stability.
In any case, it is important to understand that radicals have a certain order of stability for the same reason as carbocation.
In the case of radicals, the order of stability is as follows
- Benzyl radicals > Allyl radicals > Tertiary radicals > Secondary radicals > Primary radicals
In the case of radicals, the delocalization of electrons through resonance is strongly involved. Also, benzyl radicals with aromatic rings can write more resonance structures than allyl radicals. The electrons are delocalized to that extent and the stability is increased.
As a reminder, this order is not true for carbocation. Just understand that with radicals, the stability will change depending on this order.
-Allyl and Benzyl Anions Are Also Stable
For reference, for allyl and benzyl compounds, the anions are also stable. When a negatively charged carbon is generated, it is more likely to be stable if there is a double bond (alkene) or benzene ring next to it.
The reason for this is the same as for cations and radicals: they can make resonance structures. It is important to understand that the resonance structure allows electrons to exist in many different places, which increases the stability of the molecule.
Stability of Intermediates Is Involved in Synthetic Reactions in Organic Chemistry
When predicting how a compound will react, you need to understand how stable the intermediate products are. The more stable the intermediates are, the easier they are to form. This means that organic synthesis reactions are more likely to proceed.
Although there are several intermediates, typical intermediates are carbocation, carboanion and radicals. In particular, carbocation is involved in the SN1 reaction, an important synthetic reaction.
When understanding the stability of carbocations and radicals, it is important to understand the following
- The tertiary is stable.
- Allyl compounds (with a double bond next to it) are stable.
- Benzyl compounds (with a benzene ring next to it) are stable.
In carbanions, the tertiary is unstable. Therefore, the order of stability must be reversed. However, in carbanions, it is common for allyl compounds, which are alkenes, and benzyl compounds, which have an aromatic ring next to them, to be more stable.
We have explained as clearly as possible why these intermediates are stable in a particular order. Let’s learn about these reasons and understand the reaction mechanism of organic synthesis.