Question

Ethyl acetate, \(\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}\), is a fragrant substance used both as 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) Carbon 1 is sp2 hybridized; Carbon 2 is sp3 hybridized; Carbon 3 is sp2 hybridized, and Carbon 4 is sp3 hybridized. (b) There are 36 total valence electrons. (c) There are 18 valence electrons used to make σ bonds. (d) There are 2 valence electrons used to make π bonds. (e) There are 16 valence electrons remaining in nonbonding pairs.

Step-by-step solution

Identify the hybridization at each carbon atom

Inspect the Lewis structure of ethyl acetate to determine the hybridization of each carbon atom. Count the electron domains around each carbon atom and remember that double bonds count as a single domain since they share a pair of electrons. Carbon 1: 3 electron domains (2 single bonds + 1 double bond) = sp2 hybridized Carbon 2: 4 electron domains (3 single bonds + 1 nonbonding pair) = sp3 hybridized Carbon 3: 3 electron domains (3 single bonds) = sp2 hybridized Carbon 4: 4 electron domains (4 single bonds) = sp3 hybridized

Calculate the total number of valence electrons

Determine the number of valence electrons in ethyl acetate (C4H8O2) by adding the valence electrons for each atom in the molecule: - Carbon (C) has 4 valence electrons - Hydrogen (H) has 1 valence electron - Oxygen (O) has 6 valence electrons Total valence electrons = (4 C atoms × 4 e⁻) + (8 H atoms × 1 e⁻) + (2 O atoms × 6 e⁻) = 16 + 8 + 12 = 36 valence electrons

Calculate the number of electrons used for σ bonds

In ethyl acetate, there are: - 7 single C-H bonds (each having 2 electrons) - 1 single C-C bond (having 2 electrons) - 1 single C-O bond (having 2 electrons) - 1 double C=O bond (count the sigma component having 2 electrons) Total σ electrons = (7 × 2) + 2 + 2 + 2 = 18 electrons in σ bonds

Calculate the number of electrons used for π bonds

In ethyl acetate, there is one double C=O bond. Double bonds have two electron domains - one σ bond and one π bond. We determined earlier in Step 3 that the C=O sigma bond has 2 electrons. Thus, the remaining 2 electrons of the double C=O bond are present in the π bond. Total π electrons = 2 electrons

Calculate the number of nonbonding pair electrons

To find the number of valence electrons remaining in nonbonding pairs, subtract the number of electrons used in σ bonds (Step 3) and π bonds (Step 4) from the total valence electrons calculated in Step 2. Total nonbonding pair electrons = Total valence electrons - (electrons used in σ bonds + electrons used in π bonds) = 36 - (18 + 2) = 36 - 20 = 16 electrons in nonbonding pairs

Key concepts

Hybridization of Carbon Atoms in Ethyl Acetate

Understanding the hybridization of carbon atoms in molecules like ethyl acetate is essential for grasping the molecular geometry and chemical reactivity. Hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals that can give us the molecular geometry that aligns with the observed structure. In the context of ethyl acetate:

Each carbon atom in a molecule can be surrounded by different numbers of atoms or electron pairs. This influences the atom's hybridization state. When a carbon atom is bonded to three other atoms with one double bond, such as carbon 1 in ethyl acetate, it's sp2 hybridized. This hybridization state means carbon 1 uses one s orbital and two p orbitals to form the hybrid orbitals, providing a flat trigonal planar shape to that part of the molecule.

Contrastingly, carbon atoms bonded to four other atoms with no double bonds (like carbons 2 and 4 in ethyl acetate) are sp3 hybridized, using one s orbital and three p orbitals for hybridization, resulting in a tetrahedral shape in those regions of the molecule. Carbon 3, which is also sp2 hybridized, forms part of a double bond contributing to the molecule's reactivity and shape.

Valence Electrons Calculation for Ethyl Acetate

Calculating the valence electrons in a molecule like ethyl acetate is a straightforward process involving a basic understanding of the valence electron count for individual atoms. Valence electrons are the electrons located in the outermost shell of an atom and are significant in chemical reactions as they participate in the formation of bonds.

Each atom contributes a fixed number of valence electrons: carbon contributes 4, hydrogen 1, and oxygen 6. By summing these contributions up for each atom present, we get the total valence electron count for the entire molecule. For ethyl acetate, with the formula \( \mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2} \) we calculate: \( (4 \times 4) + (8 \times 1) + (2 \times 6) = 36 \) valence electrons. This count is critical as it forms the basis for determining how electrons are distributed among bonding and nonbonding pairs in the molecule.

Sigma and Pi Bonds in Ethyl Acetate

In the molecule of ethyl acetate, bonds can be classified into sigma (σ) and pi (π) bonds. Sigma bonds are single covalent bonds formed from head-on overlapping of orbitals, and they form the backbone of all molecular structures. Each bond within the molecule, including single bonds and the single component of double bonds, accounts for two electrons that go into making a σ bond.

Ethyl acetate features several σ bonds including single C-H, C-C, and C-O bonds, as well as the sigma component of the C=O double bond. The π bond, on the other hand, arises from the side-to-side overlap of p orbitals across the two carbons of the C=O double bond and consists of the remaining pair of electrons not accounted for by the σ bond in the double bond. The presence of a π bond alongside a σ bond within the double bond imparts different chemical properties to that region of the molecule as compared to regions with only single σ bonds.

Nonbonding Electron Pairs in Ethyl Acetate

Nonbonding electron pairs, also known as lone pairs, are sets of valence electrons that are not shared between atoms and do not participate in bonding. In ethyl acetate, after accounting for σ and π bonds, the remaining electrons are considered nonbonding.

These nonbonding pairs are found typically on the more electronegative atoms, such as oxygen, in the molecule. They are significant as they contribute to the molecular shape and polarity, which in turn influence the molecule's physical and chemical properties. To ascertain the number of nonbonding electrons in ethyl acetate, we deduct the count of electrons involved in σ and π bonds from the total valence electrons. This resulting number dictates how many electrons are found in these lone pairs and influences the molecular geometry of the substance due to the repulsion between electron pairs as per VSEPR theory.