Problem 56 - Chemistry An Atoms First Approach

Question

Many important compounds in the chemical industry are derivatives of ethylene \(\left(\mathrm{C}_{2} \mathrm{H}_{4}\right) .\) Two of them are acrylonitrile and methyl methacrylate. Complete the Lewis structures, showing all lone pairs. Give approximate values for bond angles \(a\) through \(f\). Give the hybridization of all carbon atoms. In acrylonitrile, how many of the atoms in the molecule must lie in the same plane? How many \(\sigma\) bonds and how many \(\pi\) bonds are there in methyl methacrylate and acrylonitrile?

Step-by-Step Solution

Answer

The Lewis structures for acrylonitrile and methyl methacrylate are H2C=CH-C≡N and CH2=C(CH3)CO2CH3, respectively. Approximate bond angles are: in acrylonitrile, a. H-C-H = 120° and b. C-C-N = 180°; in methyl methacrylate, c. H-C-H = 120°, d. H-C-C = 120°, e. C-C-O = 120°, and f. O-C-O = 180°. The hybridization of carbon atoms in acrylonitrile are: C1 and C2 = sp2, C3 = sp; and in methyl methacrylate: C1 and C2 = sp2, C3 = sp3, C4 and C5 = sp2. In acrylonitrile, 6 atoms lie in the same plane. The number of σ and π bonds in acrylonitrile are 6 and 3, respectively, while in methyl methacrylate, there are 11 σ bonds and 2 π bonds.

Step 1: Molecular Formulas

First, we need to write the molecular formulas for both acrylonitrile and methyl methacrylate: Acrylonitrile (C3H3N): Carbon (C) = 3 Hydrogen (H) = 3 Nitrogen (N) = 1 Methyl Methacrylate (C5H8O2): Carbon (C) = 5 Hydrogen (H) = 8 Oxygen (O) = 2

Step 2: Lewis Structures

Next, we need to draw the Lewis structures of both compounds, showing all lone pairs: Acrylonitrile: H2C=CH-C≡N Methyl Methacrylate: CH2=C(CH3)CO2CH3

Step 3: Bond Angles

Now we will determine the approximate values for bond angles a through f: Acrylonitrile: a. H-C-H: 120° (sp2 hybridized C) b. C-C-N: 180° Methyl Methacrylate: c. H-C-H: 120° (sp2 hybridized C) d. H-C-C: 120° (sp2 hybridized C) e. C-C-O: 120° (sp2 hybridized C) f. O-C-O: 180° (sp hybridized C)

Step 4: Hybridization of Carbon Atoms

Now, we will find the hybridization of all the carbon atoms: Acrylonitrile: C1 and C2 = sp2 C3 = sp Methyl Methacrylate: C1 and C2 = sp2 C3 = sp3 C4 and C5 = sp2

Step 5: Atoms in Same Plane for Acrylonitrile

In acrylonitrile, there are two sp2 hybridized carbon atoms (C1 and C2), and one sp hybridized carbon atom (C3). These carbon atoms and the atoms bonded to them all lie in the same plane. Thus, there are 6 atoms (C1, C2, C3, H1, H2, and N) that lie in the same plane.

Step 6: Counting σ and π Bonds

Finally, we will count the number of sigma (σ) bonds and pi (π) bonds in both compounds: Acrylonitrile: σ bonds: 6 (3 C-H, 1 C-C, 1 C-N) π bonds: 3 (1 C=C, 1 C≡N) Methyl Methacrylate: σ bonds: 11 (5 C-H, 2 C-C, 2 C-O, 1 O-C) π bonds: 2 (1 C=C)

Key Concepts

Hybridization of Carbon Atoms

Understanding the hybridization of carbon atoms is crucial in predicting the structure and properties of organic molecules. In simple terms, hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals, suitable for the pairing of electrons to form chemical bonds. Carbon’s ground state configuration has two unhybridized 'p' orbitals and two 'sp^3' hybridized orbitals, but depending on the molecule, carbon can undergo different hybridizations such as 'sp3', 'sp2', or 'sp'.

For instance, in acrylonitrile, carbons C1 and C2 are 'sp^2' hybridized because they form a double bond (C=C) and single bonds with hydrogen and/or another carbon. The third carbon, C3, is 'sp' hybridized due to the presence of a triple bond (C≡N), indicating more 's' character and thus a linear arrangement. In methyl methacrylate, carbons C1 and C2 are also 'sp^2' hybridized, but C3 is 'sp^3' due to single bonds only, leading to a tetrahedral geometry. C4 and C5 revert to 'sp^2' hybridization because of a double bond with oxygen (C=O).

This concept is pivotal because the hybridization of an atom determines the atom’s geometry and the bond angles, which can significantly influence the molecule's physical and chemical properties.

Bond Angles in Organic Molecules

Most organic molecules are composed of bonds that are not restricted to a linear arrangement, creating a three-dimensional structure. These spatial arrangements are described by bond angles, which are the angles between adjacent lines representing bonds.

For 'sp^3' hybridized carbon atoms, like in methane (CH₄), the bond angle is approximately 109.5°, reflecting a tetrahedral structure. When the carbon is 'sp^2' hybridized, as seen in ethylene (C2H4) or in parts of acrylonitrile and methyl methacrylate, the bond angles are about 120°, indicative of a trigonal planar configuration. Finally, in 'sp' hybridized atoms, such as the terminal carbon in acrylonitrile, the bond angles reach 180°, showcasing a linear arrangement.

These bond angles are not just theoretical predictions; they are observed repeatedly in many organic molecules and play a significant role in the biological activity and reactivity of organic compounds. Understanding these angles assists chemists in predicting how molecules will interact and the types of reactions they may undergo.

Sigma and Pi Bonds

Sigma (σ) and pi (π) bonds are the types of covalent chemical bonds formed between atoms. A sigma bond, the strongest type of covalent bond, is the result of the head-on overlap of atomic orbitals. Pi bonds, on the other hand, stem from the side-by-side overlap of p-orbitals and are generally weaker than sigma bonds.

In molecules like acrylonitrile and methyl methacrylate, both σ and π bonds are present. Single bonds (C-H or C-C) are always σ bonds, which allow for free rotation around the bond axis. However, when carbon atoms are involved in double (C=C) or triple (C≡N) bonds, they include both σ and π bonds. The double bond has one σ and one π bond, while the triple bond has one σ and two π bonds.

The presence of these π bonds plays a significant role in the chemical reactivity of a molecule, particularly in reactions like addition and polymerization. The π bonds are also responsible for the absorption of UV light, which can result in coloration in some organic molecules. By recognizing the count and location of σ and π bonds, chemists can predict and explain the reactivity, stability, and other properties of organic molecules effectively.