Problem 25 - Chemistry and Chemical Reactivity

Question

Give approximate values for the indicated bond angles. (a) \(\mathrm{O}-\mathrm{S}-\mathrm{O}\) in \(\mathrm{SO}_{2}\) (b) \(\mathrm{F}-\mathrm{B}-\mathrm{F}\) angle in \(\mathrm{BF}_{3}\) (c) \(\mathrm{Cl}-\mathrm{C}-\mathrm{Cl}\) angle in \(\mathrm{Cl}_{2} \mathrm{CO}\) (c) \(\mathrm{H}-\mathrm{C}-\mathrm{H}\) (angle 1 ) and \(\mathrm{C}-\mathrm{C} \equiv \mathrm{N}\) (angle 2 ) in acetonitrile. (EQUATION CAN'T COPY)

Step-by-Step Solution

Answer

(a) ~117°; (b) 120°; (c) ~120°; (d) ~109.5° and 180°.

Step 1: Determine hybridization and geometry for SO2

The molecule \( \text{SO}_2 \) is known to have a bent shape due to the hybridization of the sulfur atom. Sulfur has two bonded oxygens and one lone pair, leading to an atomic hybridization of \( sp^2 \). This hybridization typically corresponds to a trigonal planar arrangement, but the lone pair causes a deviation from a perfect 120° angle.

Step 2: Set bond angles for BF3

\( \text{BF}_3 \) is a molecule where boron is the central atom. Boron forms three bonds with fluorine atoms and has no lone pairs; thus, it adopts an \( sp^2 \) hybridization with a trigonal planar geometry. This results in \( ext{F}- ext{B}- ext{F} \) bond angles of approximately 120°.

Step 3: Analyze geometry of Cl2CO

The \( \text{C} \) atom in \( \text{Cl}_2\text{CO} \) is bonded to two chlorine atoms and an oxygen, with no lone pairs. This configuration leads to an \( sp^2 \) hybridization, resulting again in a structure with approximately 120° bond angles around carbon. However, considering the effect of the double bond with oxygen, the actual \( \text{Cl}-\text{C}-\text{Cl} \) angle might be slightly less, due to increased electron density in the \( \text{C}=\text{O} \) bond.

Step 4: Examine acetonitrile for H-C-H and C-C≡N angles

In acetonitrile (CH3CN), the \( ext{C} \) atom bonded to hydrogens shows \( sp^3 \) hybridization, meaning tetrahedral geometry, resulting in \( ext{H}- ext{C}- ext{H} \) bond angles close to 109.5°. However, the \( ext{C}- ext{C}\equiv ext{N} \) segment has linear geometry due to \( sp \) hybridization, as the carbon triples bond with nitrogen, leading to a \( 180° \) bond angle for \( ext{C}- ext{C}- ext{N} \).

Key Concepts

Hybridization

Hybridization is a concept in chemistry where atomic orbitals mix to form new hybrid orbitals. These hybrid orbitals influence the shape and bond angles of molecules.
For instance, when understanding a molecule like sulfur dioxide ( SO_2 ), sulfur exhibits sp^2 hybridization. This involves mixing one s orbital with two p orbitals, resulting in three hybrid orbitals.

• This type of hybridization generally forms a trigonal planar geometry, with ideal bond angles of 120°.


• However, due to the presence of a lone pair on sulfur, there is a deviation to a bent shape, adjusting the bond angles slightly.

Understanding hybridization helps predict molecular shapes and bond angles.

Trigonal Planar Geometry

Trigonal planar geometry is a molecular shape resulting from sp^2 hybridization. In this configuration, three atoms are symmetrically arranged around a central atom.
Boron trifluoride ( BF_3 ) is a classic example of a molecule with trigonal planar geometry. Here, boron forms three bonds with fluorine atoms without any lone pairs.

• The geometry leads to bonds radiating at approximately 120° apart, forming a flat, triangular plane.


• This equal distribution minimizes electron repulsion among the bonded electron pairs, maintaining stability in the molecule.

Molecules with trigonal planar geometry are often easy to spot due to their characteristic planar structure.

Bent Molecular Shape

The bent molecular shape occurs when there are lone pairs on the central atom, altering the bond angles from their original geometric arrangement.
In the example of sulfur dioxide ( SO_2 ), the presence of a lone pair on the sulfur atom results in a bent shape rather than the typical trigonal planar structure.

• Lone pairs exert greater repulsion compared to bonding pairs, thus compressing the bond angle below 120°.


• In a bent molecular shape like SO_2 , typical bond angles are often around 119° to 104.5°, depending on the atoms involved and the lone pairs present.

The bent shape significantly influences the molecule's physical properties, such as polarity, due to uneven charge distribution.

Linear Geometry

Linear geometry is observed when a central atom is bonded to two other atoms with no lone pairs occupying its space. This geometry often results from sp hybridization.
In acetonitrile (CH_3CN), the C-C≡N segment demonstrates linear geometry due to the triple bond between the carbon and nitrogen atoms.

• Linear geometry results in a bond angle of 180°, meaning the bonded atoms are directly opposite each other in a straight line.


• This linear arrangement is common in various molecules with either sp hybridization or linear organizational symmetry.

For molecules like CH_3CN , the linear structure plays a crucial role in defining its chemical reactivity and bonding characteristics.