Sigma and Pi Bonds

Sigma and Pi bonds are the two types of covalent bonds found in molecules and compounds. Sigma and Pi Bonds play a crucial role in understanding the structure, stability, and reactivity of a wide range of chemical species. Sigma bonds are characterized by their head-on overlap, greater electron density along the bond axis, and the ability to rotate freely. Pi bonds, on the other hand, involve parallel p-orbital overlap, electron density above and below the internuclear axis, and restrict rotation to some degree.

In this article, we will discuss the concept of sigma and pi bonds including their various examples, characteristics, and key differences between both the bonds. By the end of this article, you will have a solid understanding of these essential covalent bonds i.e., Sigma and Pi Bonds; and their significance in the world of chemistry.

What is Sigma Bond?

Sigma bond is formed by end-to-end overlapping of bonding orbitals along the internuclear axis. This is called head-on overlap or axial overlap. The overlap of s orbitals, as well as the overlap of p orbitals in a single bond, results in sigma bonds. Sigma bonds allow for free rotation around the bond axis because the electron density is concentrated along the bond axis.

Characteristics of Sigma Bonds

Key characteristics of sigma bonds are:

• Sigma bond is a strong bond with a well-defined direction.
• The electron density in a sigma bond is concentrated along the internuclear axis.
• Sigma bonds allow for free rotation around the bond axis.
• Sigma bonds can exist in single, double, or triple bonds.
• Sigma bonds exhibit cylindrical symmetry along the bond axis.

Examples of Sigma Bonds

There are various examples of sigma bonds as all single bonds are simaga bonds only. Some common examples are:

• In methane (CH₄), the carbon-hydrogen single bonds are sigma bonds.
• In ethene (C₂H₄), the carbon-carbon double bond includes one sigma bond and one pi bond.
• In a water molecule (H₂O), there are two sigma bonds: one between each hydrogen atom and the oxygen atom.
• In ammonia(NH₃), there are three sigma bonds, one for each hydrogen atom bonded to the nitrogen atom.

Sigma Bonds in Molecular Orbital Theory

• In molecular orbital theory, sigma bonds are explained in terms of the interaction between atomic orbitals to form molecular orbitals.
• In molecular orbital theory, the starting point is the consideration of atomic orbitals of the individual atoms in a molecule.
• The formation of sigma bonds involves the overlap of atomic orbitals from two atoms.
• When two atomic orbitals overlap, they combine to form molecular orbitals.
• In the case of a sigma bond, the constructive interference of the wave functions of the two atomic orbitals results in a sigma molecular orbital (σ MO).
• Molecular orbital theory predicts the formation of both bonding and antibonding molecular orbitals.
• The bonding MO (σ bonding) has lower energy and is associated with electron density between the nuclei, which stabilizes the molecule.
• The antibonding MO (σ* antibonding) has higher energy and contains electron density outside the internuclear region.

Types of Sigma Bond

Sigma bonds can be categorized into different types based on the nature of the atomic orbitals involved and the way they overlap. The main types of sigma bonds include:

s-s Overlapping

In ss overlapping, two s orbitals from two atoms overlap directly along the internuclear axis (head-on overlap).

For example, in the hydrogen molecule (H2), two hydrogen atoms form a sigma bond through ss overlapping.

In this case, there is overlap of two half filled s-orbitals along the internuclear axis as shown below:

s-p Overlapping

In this case, there is overlap between half filled s-orbital of one atom and half filled p-orbitals of another atom. In sp overlapping, one s orbital and one p orbital from two different atoms overlap directly along the internuclear axis.

A classic example of sp overlapping is found in the carbon-hydrogen (C-H) bonds in methane (CH₄), where the carbon atom’s 2s orbital overlaps with the 1s orbital of the hydrogen atom to form sigma bonds.

p-p Overlapping

This type of overlapping takes place between half filled p-orbitals of the two approaching atoms. In pp overlapping, two parallel p orbitals from two atoms overlap side by side above and below the internuclear axis.

For instance, in a molecule like ethene (C₂H₄), the carbon-carbon double bond consists of both a sigma bond and a pi bond formed through pp overlapping.

What are Pi Bonds?

In the formation of pi bond, the atomic orbitals overlap in such a way that their axes remain parallel to each other and perpendicular to the internuclear axes. Pi bonds usually form in addition to sigma bonds in double or triple bonds (like in alkynes or alkynes) and involve the overlap of unhybridized p orbitals.Pi bonds restrict rotation around the bond axis to a certain extent because the electron density is above and below the internuclear axis.

Characteristics of Pi Bonds

• Pi bonds limit the rotation between atoms in a molecule.
• In a pi bond, the electron density is concentrated above and below the internuclear axis.
• Pi bonds are generally weaker than sigma bonds due to their side-to-side overlap.
• In pi bonds, the electron density is distributed over a larger area.
• Pi bonds are commonly found in double and triple bonds.

Examples of Pi Bonds

• Ethene (also known as ethylene) contains a double bond between two carbon atoms. In this bond, there is one sigma bond (σ) and one pi bond (π) formed by the overlap of p-orbitals.
• Benzene is a six-membered ring structure with alternating single and double bonds. It features three sigma bonds (C-C) and three pi bonds (C=C).
• In the oxygen molecule (O₂), a double bond exists between the two oxygen atoms. This double bond contains one sigma bond and one pi bond. The pi bond forms when the p-orbitals of the oxygen atoms overlap side by side.
• In the nitrogen molecule (N₂), there is a triple bond between the two nitrogen atoms, consisting of one sigma bond (σ) and two pi bonds.

Sigma and Pi Bonds Differences

Differences between sigma and pi bond are as follows:

Characteristic	Sigma (σ) Bond	Pi (π) Bond
Bond Formation	Formed by head-on or end-to-end overlap of atomic orbitals.	Formed by the side-to-side overlap of atomic orbitals.
Number of Bonds in a Single Bond	A single sigma bond is always present in a single covalent bond.	A single pi bond is typically accompanied by a sigma bond in a single bond.
Electron Distribution	Electrons are concentrated along the axis between the two nuclei.	Electrons are distributed above and below the bond axis, creating a “cloud” of electrons.
Bond Strength	Sigma bonds are generally stronger and more stable than pi bonds.	Pi bonds are weaker and more susceptible to disruption than sigma bonds.
Rotation	Sigma bonds allow free rotation around the bond axis.	Pi bonds restrict rotation and create a double bond or triple bond character.
Hybridization	Sigma bonds can form with s and p orbitals and involve sp, sp2, or sp3 hybridization.	Pi bonds typically involve p-p overlap and may require the use of unhybridized p orbitals.
Location in Multiple Bonds	Sigma bonds are found in single bonds and the first bond in multiple bonds (e.g., in a double bond or a triple bond).	Pi bonds are found in multiple bonds, such as the second and third bonds in a double bond or a triple bond.
Overlap Type	Head-to-head overlap of orbitals.	Side-to-side overlap of orbitals.
Examples	C-C Single Bond, C-H Bond, C=C Double Bond, C≡C Triple Bond	C=C Double Bond, C≡C Triple Bond, N=N Triple Bond
Strength	Generally stronger	Generally Weaker
Number in Multiple Bonds	One sigma bond in a single bond; one sigma bond in double bond (plus one pi bond); one sigma bond in triple bond (plus two pi bonds)	One pi bond in double bond; two pi bonds in triple bond
Electron Density	Concentrated along the Internuclear Axis	Concentrated above and below the Internuclear Axis
Rotation	Allows free rotation around the Bond Axis	Restricts rotation due to side-to-side overlap
Geometry of Orbitals	Sigma orbitals are Cylindrically Symmetric.	Pi orbitals have two lobes above and below the bond axis.
Occurrence	Found in all covalent bonds, including single, double, and triple bonds	Found in double and triple bonds

Examples of Sigma and Pi Bonds

There are various examples of sigma and pi bonds. Let’s discuss some examples as follows:

Sigma and Pi Bonds in Ethene (C₂H₄)

In molecules with double (π) or triple (σ) bonds, sigma bonds also exist in addition to the pi bonds. For example, in ethene (C₂H₄), the carbon-carbon bond contains one sigma bond and one pi bond.

The sigma bond is the one directly between the two carbon atoms (C-C), and the pi bond forms above and below the sigma bond in the p-orbitals of the carbon atoms.

Sigma and Pi Bonds in Acetylene (C₂H₂)

Acetylene (C₂H₂) contains a triple bond between the two carbon atoms. This triple bond consists of one sigma bond and two pi bonds:

In this case, two pi bonds are present above and below the sigma bond. The pi bonds are formed by the lateral overlap of the p-orbitals of the carbon atoms.

Sigma and Pi Bonds in Benzene

In benzene (C₆H₆), there are six sigma (σ) bonds formed by head-on overlap of atomic orbitals, providing structural stability. Additionally, there are three pi (π) bonds associated with the alternating double bonds in the hexagonal ring, contributing to the molecule’s unique stability and reactivity due to the delocalized electron cloud above and below the ring.

Significance of Sigma and Pi Bonds in Chemical Bonding

There are some significance of Sigma and Pi Bonds in chemical bonding, and these are:

• The number and types of sigma and pi bonds in a molecule are crucial in determining its stoichiometry.
• Their importance lies in their contribution to the structure, stability, and reactivity of molecules.
• Sigma bonds allow for free rotation around the bond axis, which is crucial for the study of conformational isomerism in organic chemistry. Pi bonds, on the other hand, restrict rotation, contributing to the rigidity of molecules containing double or triple bonds.

Sample Question on Sigma Bonds and Pi Bonds

Question 1: Discuss Sigma and Pi Bond in detail.

Answer:

Sigma (σ) and pi (π) bonds are two fundamental types of covalent bonds formed between atoms when they share electrons. Sigma bonds are generally stronger than pi bonds because of the more direct overlap of orbitals, resulting in a greater electron density along the bond axis.

Question 2: Explain the Differences between Sigma and Pi Bond.

Answer:

Sigma bonds (σ) are formed by head-on overlap of atomic orbitals, allowing for free rotation along the bond axis. Pi bonds (π) result from the sideways overlap of p orbitals, restricting rotation and forming a double or triple bond. Sigma bonds are stronger and primary, while pi bonds are weaker and secondary in multiple bonds.

Question 3: How does stability of any molecule is determined?

Answer:

The stability of a molecule is primarily determined by the strength of the covalent bonds and the arrangement of those bonds in the molecule’s structure. Sigma bonds provide the primary connection between atoms and are generally stronger than pi bonds. However, pi bonds contribute to the overall bond strength and can influence the molecule’s geometry and reactivity. The combination of sigma and pi bonds allows for the formation of stable molecules with well-defined structures, and their presence is crucial in determining the chemical properties and reactivity of compounds.

Sigma Bonds and Pi Bonds: FAQs

1. What is Sigma and Pi Bonds?

Sigma (σ) bonds result from head-to-head atomic orbital overlap and allow free rotation. Pi (π) bonds form from parallel p orbital overlap, limiting rotation.

2. How Many Sigma Bonds and Pi Bonds Can a Single Bond Have?

A single covalent bond consists of one sigma bond, and there are no pi bonds in a single bond.

3. Can a Double Bond Have Both Sigma and Pi Bonds?

A double bond consists of one sigma (σ) bond and one pi (π) bond.

4. Which Types of Orbitals Can Form Sigma Bonds?

Sigma bonds can form from the overlap of s-s, s-p, p-p, and some d orbitals.

5. What Types of Orbitals Can Form Pi Bonds?

Pi bonds form from the overlap of parallel p-p or d-p orbitals.

6. Can Sigma and Pi Bonds Coexist in the Same Molecule?

Yes, sigma and pi bonds can coexist in the same molecule, as in double and triple bonds.

7. Do All Molecules Have Sigma and Pi Bonds?

Not all molecules have both sigma and pi bonds; some have only sigma bonds.

8. Are Sigma and Pi Bonds Equally Strong?

Sigma bonds are generally stronger than pi bonds due to the greater overlap of orbitals in sigma bonds.

9. How to Figure out Sigma and Pi Bonds?

By drawing the Lewis Structure and identifying Single, Double, and Triple Bonds.

10. What are the Number of Sigma (σ) and Pi (π) Bonds in Benzene?

Sigma (σ) Bonds: There are a total of 12 sigma bonds in benzene. These include the six carbon-carbon single bonds and the six carbon-hydrogen single bonds.

Pi (π) Bonds: There are three pi bonds (π bonds) in benzene, which contribute to its aromaticity.