Problem 60 - Chemistry: The Central Science in SI Units, Global Edition

Question

Benzaldehyde, \(\mathrm{C}_{7} \mathrm{H}_{6} \mathrm{O}\), is a fragrant substance responsible for the aroma of almonds. Its Lewis structure is O=Cc1cccc(C=O)c1 (a) What is the hybridization at each of the carbonatoms of the molecule? (b) What is the total number of valence electrons in benzaldehyde? (c) How many of the valence electrons are used to make \(\sigma\) bonds in the molecule? (d) How many valence electrons are used to make \(\pi\) bonds? (e) How many valence electrons remain in nonbonding pairs in the molecule?

Step-by-Step Solution

Answer

(a) All carbon atoms in benzaldehyde have sp2 hybridization. (b) There are 40 valence electrons in total. (c) 24 valence electrons are used to make σ bonds. (d) 4 valence electrons are used to make π bonds. (e) 12 valence electrons remain in nonbonding pairs.

Step 1: (a) Hybridization at each carbon atom

To determine the hybridization at each carbon atom, we can first count the number of sigma bonds and lone pairs around each carbon atom. - Carbon1 (C1): double bond with O, single bond with C2 => hybridization = sp2 - Carbon2 (C2): single bonds with C1, C3, C6 => hybridization = sp2 - Carbon3 (C3): single bond with C2, C4, H => hybridization = sp2 - Carbon4 (C4): single bond with C3, and C5 => hybridization = sp2 - Carbon5 (C5): single bond with C4, C6, and H => hybridization = sp2 - Carbon6 (C6): single bond with C2, C5, H => hybridization = sp2 So, all carbon atoms have sp2 hybridization in the benzaldehyde molecule.

Step 2: (b) Total number of valence electrons

To find the total number of valence electrons, we need to sum up the valence electrons for every atom in the molecule. For benzaldehyde, we have: 7 Carbon atoms: (7 atoms × 4 valence electrons per carbon) = 28 valence electrons 6 Hydrogen atoms: (6 atoms × 1 valence electron per hydrogen) = 6 valence electrons 1 Oxygen atom: (1 atom × 6 valence electrons per oxygen) = 6 valence electrons Total valence electrons: 28 + 6 + 6 = 40 valence electrons

Step 3: (c) Valence electrons used to make σ bonds

In benzaldehyde, we have the following σ bonds: 6 σ bonds between carbon and hydrogen 5 σ bonds between carbon and carbon 1 σ bond between carbon and oxygen Total σ bonds: 6 + 5 + 1 = 12 σ bonds Every σ bond uses 2 valence electrons, so to calculate the number of valence electrons used for σ bonds, we can multiply the number of σ bonds by 2: 12 σ bonds × 2 valence electrons per σ bond = 24 valence electrons used for σ bonds

Step 4: (d) Valence electrons used to make π bonds

In benzaldehyde, we have the following π bonds: 1 π bond between oxygen and carbon 1 π bond between carbons in the phenyl ring Total π bonds: 1 + 1 = 2 π bond Every π bond uses 2 valence electrons, and hence to calculate the number of valence electrons used for π bonds, we can multiply the number of π bonds by 2: 2 π bonds × 2 valence electrons per π bond = 4 valence electrons used for π bonds

Step 5: (e) Valence electrons remaining in nonbonding pairs

To find the number of valence electrons remaining in nonbonding pairs, we can subtract the valence electrons used for σ bonds and π bonds from the total valence electrons: Total valence electrons - valence electrons used for σ bonds - valence electrons used for π bonds = nonbonding pairs of valence electrons 40 - 24 - 4 = 12 valence electrons in nonbonding pairs

Key Concepts

Valence Electrons

Understanding valence electrons is key to predicting how atoms will bond in a molecule. Valence electrons are the outermost electrons of an atom, involved in forming bonds. These electrons determine the chemical properties of an element.
For benzaldehyde, the total number of valence electrons can be calculated by adding the valence electrons from each atom. Benzaldehyde has:

• 7 carbon atoms, each with 4 valence electrons contributing a total of 28 electrons.
• 6 hydrogen atoms, each with 1 valence electron contributing a total of 6 electrons.
• 1 oxygen atom with 6 valence electrons contributing a total of 6 electrons.

All combined, benzaldehyde has 40 valence electrons.

Sigma Bonds

Sigma bonds (\(\sigma\) bonds) are the strongest type of covalent chemical bonds. They occur when the orbitals of bonding atoms overlap directly on the internuclear axis. In benzaldehyde, these bonds form the staple framework of the molecule.
For benzaldehyde, there are several \(\sigma\) bonds:

• 6 \(\sigma\) bonds between carbon and hydrogen.
• 5 \(\sigma\) bonds between carbon and carbon atoms.
• 1 \(\sigma\) bond between carbon and oxygen.

Each \(\sigma\) bond is formed by sharing a pair of electrons, so 12 \(\sigma\) bonds use 24 valence electrons (12 bonds × 2 electrons per bond). These bonds are essential for maintaining the structural integrity of the molecule.

Pi Bonds

Pi bonds (\(\pi\) bonds) arise when the electrons in the p orbitals overlap above and below the plane of the atoms. They are found in multiple bonds, such as double or triple bonds, providing extra stability and rigidity to the molecule.
In benzaldehyde, \(\pi\) bonds contribute significantly to its double-bonded characteristics:

• 1 \(\pi\) bond between the carbon and oxygen in the carbonyl group (C=O).
• 1 \(\pi\) bond in the phenyl ring.

Thus, these 2 \(\pi\) bonds use a total of 4 valence electrons (2 bonds × 2 electrons per bond). Pi bonds are crucial in defining the reactivity and absorption properties of compounds such as benzaldehyde.

Nonbonding Electrons

Nonbonding electrons, or lone pairs, are valence electrons not involved in chemical bonding. They reside on the atoms and can influence the chemical behavior and interactions of molecules.
For benzaldehyde, after accounting for \(\sigma\) and \(\pi\) bonds, some electrons remain as nonbonding pairs:- The total number of electrons accounted for in bonds is 28 (24 from \(\sigma\) bonds and 4 from \(\pi\) bonds).- This leaves 12 electrons as nonbonding pairs: 40 total valence electrons - 28 used in bonds.These nonbonding electrons play a vital role in dictating the polarity and potential bonding sites for reactions in benzaldehyde.