Question

In the organic compound ${\mathrm{CH}}_2=\mathrm{CH}-{\mathrm{CH}}_2-{\mathrm{CH}}_2-\mathrm{C}\equiv \mathrm{CH}$, the pair of hydridised orbitals involved in the formation of: ${\mathrm{C}}_2-{\mathrm{C}}_3$ bond is: (a) $sp-s{p}^2$ (b) $sp-s{p}^3$ (c) $s{p}^2-s{p}^3$ (d) $s{p}^3-s{p}^3$

Short answer

The pair of hybridised orbitals is (c) $s{p}^2-s{p}^3$.

Step-by-step solution

Identify the Atoms Involved

The bond in question is between the second carbon (C_2) and the third carbon (C_3) in the molecule given, which is ${\mathrm{CH}}_2=\mathrm{CH}-{\mathrm{CH}}_2-{\mathrm{CH}}_2-\mathrm{C}\equiv \mathrm{CH}$. C_2 and C_3 are identified as the carbons in the sections ${\mathrm{CH}}_2=\mathrm{CH}$ and ${\mathrm{CH}}_2-{\mathrm{CH}}_2$, respectively.

Determine Hybridization of C_2

C_2 is part of a double bond (${\mathrm{CH}}_2=\mathrm{CH}$), meaning it forms two sigma bonds and engages in a pi bond. A carbon atom forming a double bond is $s{p}^2$ hybridized.

Determine Hybridization of C_3

C_3 is in the ${\mathrm{CH}}_2-{\mathrm{CH}}_2$ section with only single bonds, meaning it forms four sigma bonds. A carbon atom forming only single bonds is $s{p}^3$ hybridized.

Evaluate the Pairing of Hybrid Orbitals

Finally, assess the types of hybridized orbitals involved in the ${\mathrm{C}}_2-{\mathrm{C}}_3$ bond. The bond is formed by the overlap of $s{p}^2$ orbitals from C_2 and $s{p}^3$ orbitals from C_3.

Key concepts

Hybridization

In organic chemistry, hybridization is a key concept that describes the mixing of atomic orbitals to form new hybrid orbitals. These hybrid orbitals are essential for explaining molecular geometry and bonding properties. When atoms bond, their electrons interact, and hybrid orbitals are formed, allowing effective overlap for bond formation.
Hybridization defines the type of hybrid orbitals an atom uses during bonding. Different hybridizations explain the formation of single, double, and triple bonds in organic molecules. Understanding hybridization helps to visualize how molecules are structured and how atoms within a molecule are connected.
In carbon atoms, common types of hybridization include **sp**, **sp2**, and **sp3**, each having distinct geometrical arrangements and bonding capabilities. Recognizing these types allows predicting the molecule’s shape, its reactivity, and the type of bonds present.

Sigma Bonds

Sigma bonds, denoted by the symbol $\sigma$, are the strongest type of covalent chemical bonds in molecules. They are formed by the direct overlap of two atomic orbitals. In sigma bonds, the electron density is concentrated along the axis connecting the two nuclei of the bonding atoms.
This overlap can occur between:

• Two s orbitals
• One s and one p orbital
• Two p orbitals
• Hybrid orbitals

Most single bonds in organic compounds are sigma bonds. These bonds allow for free rotation around the bond axis, contributing to the flexibility of single-bonded carbon chains. Identifying sigma bonds is crucial in understanding the basic framework of a molecule.

Pi Bonds

Pi bonds, symbolized as $\pi$, are a type of covalent bond that is formed from the side-to-side overlap of two atomic orbitals. Unlike sigma bonds, pi bonds do not have electron density along the internuclear axis but rather above and below this axis.
Pi bonds are typically found in double and triple bonds, where they accompany sigma bonds to add strength and shorten the bond length. In a double bond, for example, one bond is a sigma bond, and the other is a pi bond.
These bonds restrict the free rotation around the bond axis, resulting in rigid structures. Understanding pi bonds is vital for recognizing the kinds of bonds present in unsaturated hydrocarbons, such as alkenes and alkynes, as they dictate the chemical properties and reactivity of the compound.

sp2 Hybrid Orbitals

The **sp2 hybrid orbitals** are formed when one s orbital and two p orbitals mix to produce three equivalent hybrid orbitals. This type of hybridization occurs in atoms that form double bonds and is key in the structure of alkenes.
In the sp2 hybridization, the sp2 orbitals arrange themselves in a planar, trigonal shape with a bond angle of approximately 120 degrees to minimize repulsion. The third unhybridized p orbital forms a pi bond with another p orbital.
In the exercise example, the carbon involved in the double bond (C_2) is sp2 hybridized. This allows for the formation one sigma bond and one pi bond in the double-bonded structure, creating a stable yet reactive conformation.

sp3 Hybrid Orbitals

The **sp3 hybrid orbitals** come from the combination of one s orbital with three p orbitals. This hybridization results in four equivalent orbitals oriented tetrahedrally, at about 109.5-degree angles.
The sp3 hybridization is typical for carbon atoms forming single bonds, as seen in alkanes. It explains the tetrahedral geometry, allowing for four sigma bonds to be formed around the carbon atom.
In the given exercise, the carbon atom (C_3) with sp3 hybridization is involved in forming single bonds. This hybridization results in a more flexible and less reactive structure compared to those with double or triple bonds. Understanding sp3 hybridization aids in visualizing the molecule's 3D arrangement and predicting its interactions in chemical reactions.