Problem 75 - General Chemistry: Principles and Modern Applications

Question

Sketch the propyne molecule, \(\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CH}\). Indicate the bond angles in this molecule. What is the maximum number of atoms that can be in the same plane?

Step-by-Step Solution

Answer

The structure of the Propyne molecule is represented as CH3-C≡CH, with bond angles of 109.5 degrees around the first Carbon atom and 180 degrees around the second and third Carbon. All five atoms in the Propyne molecule can lie in the same plane.

Step 1: Sketch the Propyne Molecule

Propyne (\( \mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CH} \)) is a type of alkyne with a Carbon-Carbon triple bond, and its structure should be depicted as CH3-C≡CH. This structure includes a Carbon (C) atom attached to three Hydrogen (H) atoms (forming a methyl group CH3), connected to another Carbon atom which is triple-bonded (≡) to a final Carbon atom bonded to a Hydrogen atom.

Step 2: Indicate the Bond Angles

In the Propyne molecule, the bond angle around the first Carbon atom (in CH3) is 109.5 degrees due to sp3 hybridization. The bond angles around the second and third Carbon atoms (in -C≡C-) are 180 degrees, as these Carbon atoms are linked by triple bonds which result from sp hybridization that produces a linear structure.

Step 3: Identify the maximum number of atoms in the same plane

Regarding the final question, in a linear molecule like Propyne, all atoms lie in the same plane. Therefore, all the atoms in the molecule (five in this case: three Hydrogen atoms and two Carbon atoms) can be in the same plane.

Key Concepts

Bond Angles

Bond angles are the angles formed between a set of bonds originating from a single atom. In propyne, these angles help us understand the geometric arrangement of atoms.

Propyne (\(\text{CH}_3\text{C}\equiv\text{CH}\)) has a methyl group (\(\text{CH}_3\)) where the first carbon atom forms bonds with three hydrogen atoms. Due to its sp3 hybridization, the bond angles here are approximately 109.5 degrees, which is typical for tetrahedral molecules.

However, moving to the carbon-carbon triple bond (\(\equiv\)), formed between the second and third carbon atoms, the bond angles are different. In this part of the molecule, the bond angles are exactly 180 degrees, giving rise to a linear arrangement. This is due to sp hybridization, which straightens the molecule in this region.

sp3 Hybridization

When exploring molecular structures such as propyne, it's helpful to understand how atoms form bonds. This is where the concept of sp3 hybridization comes in.

In propyne, the first carbon atom (terminating in \(\text{CH}_3\)) undergoes sp3 hybridization. In this type of hybridization, one s orbital and three p orbitals combine to form four equivalent sp3 hybrid orbitals.

• These sp3 orbitals arrange themselves in a tetrahedral shape around the central carbon atom, resulting in bond angles close to 109.5 degrees.
• The arrangement minimizes the repulsion between the bonding pairs of electrons, leading to a more stable structure.

This hybridization is typical for single-bonded carbon atoms that connect to four other atoms or groups within organic molecules—like the hydrogen atoms in the methyl group of propyne.

sp Hybridization

In contrast to sp3 hybridization , sp hybridization is crucial in understanding the bonding in the linear segment of propyne. This concept describes how carbon atoms in triple bonds interact with their neighboring atoms.

The second and third carbon atoms in propyne use sp hybridization , where one s orbital and one p orbital merge to form two identical sp hybrid orbitals.

• These sp orbitals align in a linear fashion, making a 180-degree angle between them.
• This linear alignment is crucial for carbon-carbon triple bonds, as it facilitates the close proximity required for their triple bonding.

Such hybridization means these carbon atoms have left unused p orbitals, which overlap to form the pi bonds characterizing the triple bond. This results in the rigid, linear shape observed around the carbon-carbon triple bond in molecules like propyne.