Question

Indicate the \(\sigma\) and \(\pi\) bonds in the following molecules: \(\mathrm{C}_{6} \mathrm{H}_{6}, \mathrm{C}_{6} \mathrm{H}_{12}, \mathrm{CH}_{2} \mathrm{Cl}_{2}, \mathrm{CH}_{2}=\mathrm{C}=\mathrm{CH}_{2}, \mathrm{CH}_{3} \mathrm{NO}_{2}, \mathrm{HCONHCH}_{3}\)

Short answer

Benzene: 12 σ, 3 π; Cyclohexane: 18 σ, 0 π; Dichloromethane: 4 σ, 0 π; Propadiene: 4 σ, 2 π; Nitromethane: 7 σ, 1 π; Acetamide: 11 σ, 1 π.

Step-by-step solution

Identify Bond Types in Benzene

Benzene \((\text{C}_6\text{H}_6)\) is an aromatic compound with a ring structure. Each carbon is connected to another carbon by both a single \(\sigma\) bond and a \(\pi\) bond, forming a conjugated system. Thus, benzene contains 12 \(\sigma\) bonds and 3 \(\pi\) bonds.

Determine Bonds in Cyclohexane

Cyclohexane \((\text{C}_6\text{H}_{12})\) is a saturated compound. Each carbon atom forms two \(\sigma\) bonds with neighboring carbon atoms and one \(\sigma\) bond with a hydrogen atom. This means there are 18 \(\sigma\) bonds and no \(\pi\) bonds.

Analyze Dichloromethane Bonds

Dichloromethane \((\text{CH}_{2}\text{Cl}_2)\) contains carbon bonded to two hydrogen atoms and two chlorine atoms. All these are single \(\sigma\) bonds, totaling 4 \(\sigma\) bonds and no \(\pi\) bonds.

Bond Types in Propadiene

Propadiene \(\text{(CH}_{2}=\text{C}=\text{CH}_{2})\) consists of a linear structure with the central carbon making two double bonds with the terminal carbons. This results in 4 \(\sigma\) bonds and 2 double \(\pi\) bonds (one for each double bond), hence 2 \(\pi\) bonds.

Examine Bonds in Nitromethane

Nitromethane \((\text{CH}_3\text{NO}_2)\) consists of a methyl group \(\text{(CH}_3)\) connected to a nitrogen bound to two oxygen atoms. It contains 7 \(\sigma\) bonds, and in the \(\text{NO}_2\) group, there is one \(\pi\) bond between nitrogen and one oxygen due to resonance.

Determine Bonds in Acetamide

Acetamide \((\text{HCONHCH}_3)\) includes an amide group consisting of a carbonyl group \(\text{(C=O)}\) and a nitrogen giving another possible \(\pi\) bond. The molecule contains 11 \(\sigma\) bonds and one \(\pi\) bond (from the carbonyl group).

Key concepts

Sigma bonds

Sigma bonds are the fundamental type of covalent bonds found in molecules. These bonds result from the head-on overlap of atomic orbitals, which allows for a strong connection between atoms. Sigma bonds are typically represented in single bonds, but each double and triple bond also contains a sigma bond. Because of their head-on overlap, sigma bonds are very strong and provide significant stability to a molecule's structure.
In most organic molecules, sigma bonds are found between carbon atoms as well as between carbon and other atoms like hydrogen, nitrogen, oxygen, or chlorine.

• They form the framework or backbone of a molecule.
• Every bond in a molecule, whether single, double, or triple, includes exactly one sigma bond.
• Sigma bonds allow for the free rotation of bonded atoms around the bond axis.

Pi bonds

Pi bonds occur when the parallel orbitals overlap sideways. Unlike sigma bonds, pi bonds do not allow for the free rotation of bonded atoms, which affects the overall geometry and rigidity of a molecule. Pi bonds are typically found in double and triple bonds, where they supplement the sigma bonds already present.
Pi bonds arise from the overlapping of p orbitals and are crucial in the formation of conjugated systems, where they help to create delocalized electron clouds.

• Each double bond in organic compounds includes one pi bond, while each triple bond includes two pi bonds.
• Pi bonds prevent the free rotation of atoms because of the parallel nature of p-orbital overlap.
• They are less strong than sigma bonds and provide additional electron sharing abilities.

Molecular structure

The molecular structure of a compound defines how atoms are arranged in a molecule and determines many of its physical and chemical properties. This intricate arrangement is influenced by the type and number of bonding interactions, such as sigma and pi bonds, which dictate the overall shape and functionality.
Understanding molecular structure involves looking at the hybridization of orbitals, bond angles, and the spatial arrangement of atoms.

• The specific connectivity of atoms influences boiling and melting points, reactivity, and solubility.
• Bond angles and lengths determine the three-dimensional shape.
• Molecules with similar structures often exhibit similar properties.

Organic compounds

Organic compounds are the diverse class of carbon-containing molecules, often associated with living organisms. These compounds are characterized by a backbone of carbon atoms bonded primarily with hydrogen atoms and sometimes other elements like oxygen, nitrogen, sulfur, and halogens.
The versatility of carbon forms the basis for the enormous variety of organic compounds. Carbon's ability to form strong covalent sigma bonds and engage in multiple bonding as seen with pi bonds allows for complex molecular architectures.

• They range from simple molecules like methane to complex polymers and biomolecules.
• Organic chemistry studies these compounds and their reactions.
• They are fundamental to biological processes and industrial applications.

Conjugated system

A conjugated system is a structural feature in molecules where alternating single and multiple bonds create a delocalized electron system. These systems are typically found in compounds with pi bonds, where the p orbitals overlap to allow electron delocalization over several adjacent atoms.
This electron delocalization imparts unique properties to conjugated systems, such as altered absorption of light, which can be observed in molecules like chlorophyll and DNA bases.

• They provide chemical stability through electron delocalization.
• Conjugated systems are integral in colored compounds due to their interaction with light.
• They contribute significantly to the reactivity and electronic properties of a molecule.