Bonding in carbon compounds Knowledge-Based Download PDF

Questions

1. 1.

   Which of the following statements correctly describes the orbital overlaps that form the carbon–carbon triple bond in an ethyne ($\text{C}_2\text{H}_2$) molecule?

   • A) One $\pi$ bond formed by the coaxial overlap of $sp$ hybrid orbitals and two $\sigma$ bonds formed by the sideways overlap of unhybridized $2p$ orbitals
   • B) Three $\sigma$ bonds formed by the head-on overlap of three equivalent $sp^2$ hybrid orbitals from each carbon atom
   • C) Two $\sigma$ bonds formed by the head-on overlap of $sp$ hybrid orbitals and one $\pi$ bond formed by the sideways overlap of unhybridized $2p$ orbitals
   • D) One $\sigma$ bond formed by the head-on overlap of $sp$ hybrid orbitals and two $\pi$ bonds formed by the sideways overlap of unhybridized $2p$ orbitals
2. 2.

   Which of the following statements correctly explains how the four equivalent carbon–hydrogen ($\text{C}-\text{H}$) bonds in a methane ($\text{CH}_4$) molecule are formed according to valence bond theory?

   • A) Carbon uses its unhybridized $2s$ and three $2p$ valence orbitals directly to overlap with hydrogen $1s$ orbitals, forming bonds with different lengths and energies.
   • B) One $2s$ and three $2p$ orbitals of carbon mix to form four equivalent $sp^3$ hybrid orbitals, each overlapping head-on with a hydrogen $1s$ orbital in a tetrahedral geometry.
   • C) Carbon undergoes $sp^2$ hybridization to establish three coplanar $\sigma$ bonds, while the remaining unhybridized $2p$ orbital forms a delocalized bond with the fourth hydrogen atom.
   • D) An electron is promoted from the $2s$ subshell into a $3d$ orbital to generate four $sp^2d$ hybrid orbitals directed toward the corners of a square planar structure.
3. 3.

   Which of the following statements correctly explains why rotation about the carbon–carbon double bond in alkenes is restricted at ordinary temperatures, unlike rotation about the carbon–carbon single bond in alkanes?

   • A) The two carbon atoms are held together by two collinear $\sigma$ bonds that cannot twist without altering their ideal bond angles.
   • B) The higher electronegativity of $sp^2$-hybridized carbon atoms generates an electrostatic barrier that locks the atoms in place.
   • C) Strong steric hindrance between valence shell nonbonding electron pairs on the two carbon atoms prevents torsional motion.
   • D) Rotating around the carbon–carbon axis would disrupt the parallel, sideways overlap of the unhybridized $2p$ orbitals that form the $\pi$ bond.
4. 4.

   Which of the following statements correctly compares the carbon–carbon bond length and bond strength among ethane ($\text{C}_2\text{H}_6$), ethene ($\text{C}_2\text{H}_4$), and ethyne ($\text{C}_2\text{H}_2$)?

   • A) Ethyne has the shortest carbon–carbon bond length and the highest carbon–carbon bond strength among the three molecules.
   • B) Ethane has the shortest carbon–carbon bond length and the highest carbon–carbon bond strength among the three molecules.
   • C) Ethyne has the longest carbon–carbon bond length and the lowest carbon–carbon bond strength among the three molecules.
   • D) Ethane has the longest carbon–carbon bond length and the highest carbon–carbon bond strength among the three molecules.
5. 5.

   Which of the following correctly identifies the hybridization of each carbon atom and the total number of $\sigma$ (sigma) and $\pi$ (pi) bonds in an ethene ($\text{C}_2\text{H}_4$) molecule?

   • A) $sp^3$ hybridized, with 4 $\sigma$ bonds and 2 $\pi$ bonds
   • B) $sp^3$ hybridized, with 5 $\sigma$ bonds and 1 $\pi$ bond
   • C) $sp^2$ hybridized, with 4 $\sigma$ bonds and 2 $\pi$ bonds
   • D) $sp^2$ hybridized, with 5 $\sigma$ bonds and 1 $\pi$ bond