Question

Ethyl acetate, \(\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}\), is a fragrant substance used bothas a solvent and as an aroma enhancer. Its Lewis structure is (a) What is the hybridization at each of the carbon atoms of the molecule? (b) What is the total number of valence electrons in ethyl acetate? (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?

Short answer

(a) The hybridization of the carbon atoms in ethyl acetate, C4H8O2, are as follows: - sp3 for the carbon in the CH3 group - sp3 for the carbon in the CH2 group - sp2 for the carbon in the O-C=O group (b) The total number of valence electrons in ethyl acetate is 36. (c) There are 8 σ (sigma) bonds in the molecule. (d) There is 1 π (pi) bond in the molecule. (e) There are 18 nonbonding electrons, or 9 nonbonding pairs of electrons, in the molecule.

Step-by-step solution

1. Draw the Lewis Structure of Ethyl Acetate

Ethyl acetate (C4H8O2) has the following molecular structure: H-CH2-CH3-O-C=O. To draw the Lewis structure, start by placing the atoms according to the molecular formula, connect them with single bonds, and then add the remaining electrons.

2. Hybridization of the Carbon Atoms

Now, let's analyze the hybridization of each carbon atom: - The carbon (C) atom in the CH3 group is connected to three hydrogens and one carbon. This carbon is sp3 hybridized. - The carbon (C) atom in the CH2 group is connected to two hydrogens and one carbon. This carbon is also sp3 hybridized. - The carbon (C) atom in the O-C=O group is connected to two oxygens with a double bond to one oxygen and a single bond to the other. This carbon atom is sp2 hybridized.

3. Total Number of Valence Electrons

To calculate the total number of valence electrons, count the valence electrons for each atom in the molecule. For ethyl acetate, the total number of valence electrons is: \[ (4 \times 4) + (8 \times 1) + (2 \times 6) = 16 + 8 + 12 = 36 \]

4. Count the σ (Sigma) Bonds

In the ethyl acetate molecule, we have the following σ bonds: - 6 C-H σ bonds (3 in the CH3 group and 3 in the CH2 group) - 1 C-O σ bond (in the O-C=O group) - 1 C-C σ bond (between the CH2 and CH3 groups) So, the total number of σ bonds in the molecule is 6 + 1 + 1 = 8 σ bonds.

5. Count the π (Pi) Bonds

To identify and count the π bonds, look for double or triple bonds in the molecule. In ethyl acetate, we have only one double bond, which is between the carbon (C) and oxygen (O) in the O-C=O group. This double bond constitutes 1 π bond.

6. Nonbonding Valence Electrons

To find the number of nonbonding pairs of valence electrons in the molecule, we'll first count the number of valence electrons used in bonding (σ and π bonds): - σ bonds use 2 electrons each: 8 σ bonds × 2 = 16 electrons. - π bonds use 2 electrons each: 1 π bond × 2 = 2 electrons. So, a total of 16 + 2 = 18 electrons are used in bonding. Since there are 36 valence electrons in the molecule, the number of nonbonding pairs of electrons is: \( 36 - 18 = 18 \) nonbonding electrons, or 9 nonbonding pairs of electrons.

Key concepts

Hybridization

Understanding hybridization is crucial to grasp the molecular geometry of molecules like ethyl acetate. Hybridization is the process of mixing atomic orbitals into new hybrid orbitals that can form sigma bonds. It helps in determining the arrangement of atoms around a central atom. In ethyl acetate, the carbon atoms exhibit two kinds of hybridization:

• sp³ Hybridization: The carbon atoms in the CH₃ and CH₂ groups are sp³ hybridized. This means that one s orbital and three p orbitals mix to form four sp³ hybrid orbitals. Each of these carbon atoms forms three sigma bonds with hydrogen and one sigma bond with carbon, leading to a tetrahedral shape.
• sp² Hybridization: The carbon atom in the O-C=O group is sp² hybridized. Here, one s orbital and two p orbitals combine to form three sp² hybrid orbitals. The carbon forms sigma bonds with two oxygens and a pi bond with one oxygen due to the leftover unhybridized p orbital, resulting in a trigonal planar shape.

These hybridizations provide insight into the molecular structure and bonding patterns of ethyl acetate.

Valence Electrons

Valence electrons are the outermost electrons of an atom that participate in chemical bonding. Knowing the number of valence electrons in a compound helps predict how atoms will connect. For ethyl acetate, we calculate the total number of valence electrons by summing the valence electrons of each individual atom:

• Carbon (C): Each carbon atom has 4 valence electrons. With four carbon atoms, the contribution is 4 × 4 = 16 electrons.
• Hydrogen (H): Each hydrogen atom has 1 valence electron. For eight hydrogen atoms, the total is 8 × 1 = 8 electrons.
• Oxygen (O): Each oxygen atom has 6 valence electrons. For two oxygens, the total is 2 × 6 = 12 electrons.

Adding these up gives a total of 36 valence electrons for ethyl acetate. These electrons are utilized in forming bonds and filling nonbonding areas.

Sigma Bonds

Sigma (\(\sigma\)) bonds are the strongest type of covalent bonds formed by the head-on overlapping of atomic orbitals. They are essential in holding the molecular structure together. In ethyl acetate, we identify these sigma bonds to understand how atoms are interconnected:

• C-H \(\sigma\) bonds: There are six bonds, three in each of the CH₃ and CH₂ groups.
• C-C \(\sigma\) bond: A single bond connects the CH₂ and CH₃ groups.
• C-O \(\sigma\) bonds: There are two such bonds, one in the O-C=O part and another connecting the ethyl and acetate groups.

In total, ethyl acetate has 8 sigma bonds (6 C-H + 1 C-C + 1 C-O), each formed using two valence electrons, which contribute to the stability and overall structure of the molecule.

Pi Bonds

Pi (\(\pi\)) bonds are a type of covalent bond between atoms where there is a lateral overlapping of unhybridized p orbitals. These bonds share electron density above and below the bond axis, supplementing the sigma bonds in multiple bonds. In ethyl acetate, pi bonds give additional stability to the structure by forming double bonds. We find pi bonds in:

• C=O double bond: The carbon to oxygen double bond in the O-C=O group includes one pi bond. This bond consists of additional overlapping above and below the plane of the involved atoms.

This presence of one pi bond in ethyl acetate highlights the significance of double bonds, which are key features in determining the chemical reactivity and properties of the molecule.