Problem 99 - Chemistry The Molecular Science

Question

Halothane, which had been used as an anesthetic, has the molecular formula \(\mathrm{CHBrClCF}_{3}\). (a) Write the Lewis structure for halothane. (b) Is halothane a polar molecule? Explain your answer. (c) Can hydrogen bonding occur in halothane? Explain.

Step-by-Step Solution

Answer

(a) Lewis structure includes single bonds; (b) Polar due to asymmetry; (c) No hydrogen bonding.

Step 1: Count Valence Electrons

To draw the Lewis structure of halothane \( \text{CHBrClCF}_3 \), first count the total number of valence electrons. Carbon (C) has 4 valence electrons, hydrogen (H) has 1, bromine (Br) has 7, chlorine (Cl) has 7, and fluorine (F) has 7 each. The total number of valence electrons is \( 1 \times 4 + 1 \times 1 + 1 \times 7 + 1 \times 7 + 3 \times 7 = 42 \) electrons.

Step 2: Arrange Atoms and Connect with Bonds

In halothane, carbon atoms serve as central atoms due to their ability to form four bonds. Arrange the hydrogen, bromine, chlorine, and fluorine atoms around the two carbon atoms. Connect atoms with single bonds: H-C-Br, C-Cl, C-C, and C-(three F). This arrangement accounts for 8 single bonds or 16 electrons.

Step 3: Distribute Remaining Electrons

Distribute the remaining electrons to complete the octets for bromine, chlorine, and fluorine atoms. Each fluorine will have 6 non-bonding electrons since 1 pair is used for bonding. Similarly, distribute electrons to complete the octets for bromine and chlorine.

Step 4: Determine Molecular Geometry

Analyze the shape around each carbon atom. The first carbon (attached to H, Br, and Cl) forms a tetrahedral geometry due to four single bonds. The second carbon (attached to the first carbon and three fluorines) also forms a tetrahedral geometry.

Step 5: Assess Molecular Polarity

For polarity, evaluate the electronegativity differences and the symmetry of the molecule. Large differences are found between C-F and C-Cl bonds causing dipoles. The molecule's asymmetry, due to different atoms attached to the central carbons, results in an overall dipole moment, thus halothane is polar.

Step 6: Assess Potential for Hydrogen Bonding

Hydrogen bonding requires a hydrogen atom directly attached to a highly electronegative atom like oxygen, nitrogen, or fluorine. In halothane, hydrogen is bound to carbon, which is not sufficiently electronegative, so hydrogen bonding is not possible.

Key Concepts

Lewis structure of Halothane

The Lewis structure is a way to visually represent the arrangement of atoms and electrons in a molecule. To draw the Lewis structure for halothane \(\mathrm{CHBrClCF}_{3}\), we start by counting the total valence electrons. - Carbon (C) has 4 valence electrons. - Hydrogen (H) has 1 valence electron. - Bromine (Br), chlorine (Cl), and each fluorine (F) have 7 valence electrons.
Combining these, the molecule has a total of 42 valence electrons. As we construct the structure, the carbon atoms become central due to their ability to bond with multiple atoms. Each carbon atom connects to other atoms with single bonds. So, the architecture becomes: H-C (attached with Br and Cl), C-C (connected with three Fs).
After connecting these atoms, complete the octets by distributing the remaining electrons around bromine, chlorine, and fluorine. This ensures each non-hydrogen atom has 8 electrons surrounding it, accounting for stable configuration. This basic setup ensures that each element achieves a noble gas electron configuration, which is the primary goal of the Lewis structure method.

Molecular Polarity of Halothane

A molecule's polarity is determined by the distribution of electric charge across its atoms. For halothane, assessing molecular polarity involves looking at both the electronegativity differences and the molecule's shape. - **Electronegativity** refers to an atom's ability to attract bonding electrons. For halothane, the significant differences in electronegativity occur particularly between carbon and atoms like fluorine and chlorine.
The bonds like C-F and C-Cl are "polar bonds," meaning there is an unequal sharing of electrons. These differences create "dipoles"—regions with positive and negative charge.
- **Molecular Shape** affects how these dipoles add up across the molecule. Although carbon forms tetrahedral geometry, the presence of different atoms (H, Br, Cl, F) arranged asymmetrically around the carbon atoms leads to an uneven distribution of charge. As a result, the molecule avoids any cancellation of these dipole effects, leading to an overall dipole moment. Therefore, halothane is polar because the molecule's asymmetrical shape and various electronegative atoms attached result in a net polarity.

Hydrogen Bonding in Halothane

Hydrogen bonding is a special type of dipole-dipole interaction, which happens when hydrogen is bonded directly to a highly electronegative atom like oxygen (O), nitrogen (N), or fluorine (F). These atoms pull the hydrogen atom's electron cloud strongly towards themselves, creating a strong partial positive charge on hydrogen. This allows the positively charged hydrogen to engage in efficient intermolecular bonding with nearby electron pairs on other electronegative atoms.
In halothane, hydrogen is only bonded to carbon. Carbon, unlike oxygen, nitrogen, or fluorine, is not sufficiently electronegative to produce the strong partial positive charge on hydrogen needed for hydrogen bonds.
Thus, despite fluorine's high electronegativity, hydrogen bonding does not occur here because hydrogen would need to be directly bonded to fluorine to participate in hydrogen bonding. Therefore, halothane is incapable of forming hydrogen bonds.