When learning chemistry in college, there are two things that many people don’t understand when studying chemistry: the σ bond (sigma bond) and the π bond (pi bond). Because they don’t understand the meaning of these two bonds, they don’t understand what the professor is saying in class.
Essentially, the concepts of σ and π bonds are very simple. When two substances combine, the only difference is whether they are tightly or loosely bonded together. If you learn this concept, you will be able to understand σ- and π-bonds.
The understanding of σ- and π-bonds will also enable us to understand the reactivity of chemicals. We will also be able to understand the reactivity of double and triple bonds in covalent bonds.
In this section, we will explain the differences between σ- and π-bonds, their properties and characteristics in an easy-to-understand manner.
Table of Contents
- 1 We Should Forget the Image of Double Bonds in High School Chemistry
- 2 The sp3 Hybrid Orbitals Have Four Hands in s and p Orbitals
- 3 π-Bonds (pi-Bonds) Are loosely coupled to the Bond Axis
- 4 Learn About the Types of Covalent Bonds and the Differences Between Them
We Should Forget the Image of Double Bonds in High School Chemistry
What image do you have in mind when you understand the double bond of a compound? The majority of people who have learned high school chemistry, including when assembling a model of a molecule, have the following image of a bond.
Just forget about this molecular image. As long as this image is in your head, you will never understand σ and π bonds in chemistry.
When molecules are bonded together, they are actually bonded together not in these simple forms, but by special forms. There are two types of bonds between molecules: σ- and π-bonds; σ-bonds and π-bonds must be clearly distinguished.
The sp3 Hybrid Orbitals Have Four Hands in s and p Orbitals
When atoms combine, they need to give themselves a hand. An atomic hand is an electron orbital.
Each atom or molecule has orbitals. These orbitals are called s orbital and p orbital. If we focus on the single-bonded carbon atom, the carbon atom has one s-orbital and three p-orbitals, which means that there are four hands. In other words, a carbon atom can be bonded in four places.
In ethane, one carbon atom is bonded to four atoms. It should be understandable that there are four hands in a carbon atom; if there are four hands due to an s orbital or p orbital, this is called on sp3 hybridization.
In the case of sp3 hybrid orbitals, there are hands in various directions. The four hands are not free to move, and they are pointing in different directions.
Electrons have a negative charge. Therefore, each hand is repelled from each other, and as a result, each hand is oriented in a different direction.
However, the content becomes very complicated when we get terms like s-orbital, p-orbital and sp3 hybridization. So first, let’s just understand that a carbon atom has four hands and can bond with other atoms and molecules.
σ-Bond (Sigma Bond) Forms a Covalent Bond and the Bond Energy Is High
A carbon atom can bind to other atoms and molecules using four different hands. So how does a carbon atom bond with another atom or molecule in a single bond (bond to one place)? Naturally, the atom chooses the easiest way. The atom puts its hand out to its partner and makes a single bond.
This is the sigma bond.
When a human being shakes hands with another person, he or she can point out one arm to shake hands without any difficulty. You can shake hands with your partner with a strong force, and this is the image of σ-bond.
Since the handshake is so strong, the bonds between the molecules are not easily broken and they do not separate. The σ-bond has very high binding energy, and the bonds between atoms are very strong. When electron orbitals overlap with each other, they create a σ-bond.
It may seem obvious, but without understanding this fact, you can’t understand π-bonds.
In any case, the σ bond is what holds hands with other molecules without the molecule assuming a strange body position. Among the covalent bonds, the σ bond has a very strong bonding energy and the state is stable. This is because the atoms are able to reach out their own hands and bond strongly with their counterparts.
The σ-Bond of a Single Bond Can Be Rotated: the Example of Ethane
If the molecule is made up of only σ-bonds (sigma bonds), it is a single bond; the C-C and C-H bonds are single bonds and are connected by only one hand.
In the case of a single bond, the σ-bond can be rotated. For example, in ethane, they are all single bonds and they are all σ-bonds. Therefore, in ethane, all bonds in ethane can freely rotate on their axes. The following is the structural formula for ethane.
When a carbon atom bonds with another atom or molecule, it always starts out as a sigma bond. Any single bond is a sigma bond and is a very strong bond.
Ethane is known for its low reactivity. In order for an organic compound to react to form another compound, the bond must be broken. However, the σ-bond has a strong binding energy and the molecules are strongly bonded together, making it difficult for organic compounds to react with each other.
The reason why organic chemical reactions require very strong energy to react with ethane is because ethane is composed entirely of single bonds (σ-bonds).
π-Bonds (pi-Bonds) Are loosely coupled to the Bond Axis
On the other hand, there are not only σ bonds but also π bonds (pi bonds) in covalent bonds. Even though they are the same covalent bond, there are different types; σ bonds and pi bonds must be considered separately.
We have just discussed single bonds. You can recognize a single bond as a σ bond. On the other hand, there are some organic compounds that have double or triple bonds. In addition to single bonds, compounds with double and triple bonds will also have pi bonds.
It is important to understand that compounds with double and triple bonds have π-bonds.
So what is a π-bond? I explained earlier that the σ-bond is when you put out your hand to shake hands. In π-bonds, on the other hand, instead of extending your hand to another person, you should extend your hand directly upward. You must somehow shake hands with the other person in this state.
It can seem very difficult to shake hands with someone while your hands are extended upward. Moreover, it is impossible to hold hands when you are at a distance from your partner. In any case, you will find it difficult to reach straight up and shake hands with your hand outstretched.
When a carbon atom binds with another molecule and holds hands, as mentioned above, it always starts out as a sigma bond. But what if the molecule makes a double or triple bond instead of a single bond?
As explained in the sp3 hybridization, the four hands coming out of the carbon are in different directions. They can’t move their arms freely like humans, and the direction in which they can reach out is already fixed. So the position of the arms is fixed.
Double and Triple Bonds Involve pi Bonds
When making a double bond, the atom must somehow reach out and shake the other atom’s hand in this state. That is, the atoms must somehow bond together with their arms extended upwards. As a result, the electrons bond as follows.
A π bond is a state in which atoms work hard to bond together after they put out their hands perpendicular to the bond axis. The reason why the atoms cannot reach out to other atoms like the σ bond is because, as already mentioned, they cannot move their arms freely like a human being. The location of the arm is fixed.
Because the atoms hold their hands in a bad body position, the π bond is weaker than the σ bond. This means that the binding energy is low, and the hand cannot be grasped strongly. In a double bond, there is one σ bond and one π bond.
On the other hand, what about the triple bond? In a triple bond, atoms are bound to each other by a π bond as well as a σ bond.
The same is true for reaching out to the side with respect to the axis of bond of the σ bond. From this state, the atoms do their best to reach out and try to hold their hands. In a triple bond, there is one σ bond and two π bonds.
In a σ bond, the electron orbitals overlap to form a bond. On the other hand, in a π bond, you can imagine a thinly overlapping electron cloud (where electrons exist like a cloud) rather than overlapping electron orbitals.
Highly Reactive Double- and Triple-Bonded of π-Bonds: Examples of Ethylene and Acetylene
Molecules are strongly bound to each other, and the σ-bond is the one with the strongest binding energy. On the other hand, the π bond (pi bond) is not strongly bonded and has a weak handhold. For this reason, they are more reactive in organic synthesis.
When organic compounds react with each other, the following process takes place
- Molecules collide with each other.
- The molecule becomes a high energy state.
- The bond breaks and a new one is made.
The bond must be broken. However, since the σ-bond is a strong bond, it cannot be broken easily. I noted that ethane, which is made up of only single bonds, is not very reactive because all the bonds are σ-bonds.
On the other hand, each of the π bonds is loose, and because the binding energy of the π bond is low, the bonds break just by applying a little energy, and the compounds react with each other.
Ethylene (ethene) and acetylene are frequently used to explain pi bonds. Ethane is a compound with only a single bond, while ethylene (ethene) has a double bond. Acetylene has a triple bond.
Due to their double and triple bonds, ethylene and acetylene have π bonds; compared to σ bonds, π bonds are looser. Therefore, while ethane is less reactive, ethylene and acetylene are known for their highly reactive compounds.
Even though single-bonded compounds may be stable, double and triple bonds tend to be unstable. This is because, among covalent bonds, the pi bond is not the strongest bond.
Not All pi Bonds, Such As Benzene Rings and Carbon Dioxide, Are Weak Bonds
In general, you can think of π bonds as weak bonds. The presence of double or triple bonds makes them more reactive.
However, not all compounds with double bonds (compounds with π bonds) are weak bonds. For example, the benzene ring is connected by a double bond. In other words, it has a pi bond.
However, with benzene, electrons will be present in every part of the benzene ring and it takes on a stable structure. Therefore, it is not as reactive as ethylene or acetylene.
Also, carbon dioxide has the structure O=C=O. Because of its double bonds, it is a molecule with π bonds as well as σ bonds. However, carbon dioxide is a stable molecule and it takes a lot of energy to make carbon dioxide react chemically.
Not all compounds with π-bonds are highly reactive. However, when you understand the nature of pi bonds, you can consider compounds with pi bonds (organic compounds with double or triple bonds) to be highly reactive.
Learn About the Types of Covalent Bonds and the Differences Between Them
When molecules combine, they are often linked together by covalent bonds. There are two types of these covalent bonds, σ bonds (sigma bonds) and π bonds (pi bonds).
Any atom will initially make a σ-bond by overlapping electron orbitals each other. Whenever you shake hands with someone, you point your hand out to them. There is no other choice, and this is the same for molecular bonds. Any single bond should be understood as a σ-bond.
On the other hand, when it comes to making double or triple bonds, it’s a lot harder. Unlike humans, the hands of an atom cannot move their arms freely. Therefore, they must extend their arms perpendicular to the bond axis and do their best to hold their hands together. As a result, the bond is weaker than the σ bond. This is a π bond, and ethylene and acetylene are frequently used as examples.
Here we have explained σ and π bonds in an easy-to-understand manner. Let’s understand that there are different types of covalent bonds and learn the characteristics of σ and π bonds.