Problem 96 - Chemistry and Chemical Reactivity

Question

Acrylamide, \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CHC}(=\mathrm{O}) \mathrm{NH}_{2},\) is a known neuro- toxin and possible carcinogen. It was a shock to all consumers of potato chips and french fries a few years ago when it was found to occur in those products. (a) Sketch the molecular structure of acrylamide and identify all bond angles. (b) Indicate which carbon-carbon bond is the stronger of the two. (c) Is the molecule polar or nonpolar? (d) The amount of acrylamide found in potato chips is \(1.7 \mathrm{mg} / \mathrm{kg} .\) If a serving of potato chips is \(28 \mathrm{g},\) how many moles of acrylamide are you consuming?

Step-by-Step Solution

Answer

(a) See structure above. (b) The stronger C-C bond is the double bond. (c) The molecule is polar. (d) You consume \(6.70 \times 10^{-7}\) moles.

Step 1: Sketch the molecular structure

Acrylamide ( H2C=CHC(=O)NH2 ) has the following structure: The molecule has a CH2 group connected to a CH group, which is double-bonded to a carbonyl group (C=O) that is attached to an amine group (NH2). The structure can be represented as: H2C=CH-C(=O)-NH2.

Step 2: Identify bond angles

In acrylamide, the central carbon of the carbonyl group (C=O) tends to have bond angles of about 120 degrees due to the trigonal planar geometry. The carbon in the (CH2) group has bond angles of approximately 109.5 degrees because of its tetrahedral geometry.

Step 3: Determine the stronger C-C bond

The C-C bond between the (CH2) and (CH) group is a double bond, while the bond between (CH) group and the carbonyl carbon (C=O) is a single bond. A double bond is generally stronger than a single bond. Therefore, the stronger C-C bond is H2C=CH-.

Step 4: Assess molecular polarity

Acrylamide is polar due to the presence of a carbonyl group (C=O) and an amine group (NH2) , both of which create a dipole moment. The overall structure creates an asymmetric distribution of charge, making the molecule polar.

Step 5: Calculate moles of acrylamide

First, find the mass of acrylamide in 28 g of potato chips: \( \frac{1.7 \text{ mg}}{1 \text{ kg}} \times 28 \text{ g} = 0.0476 \text{ mg} \). Convert mg to g: \( 0.0476 \text{ mg} = 0.0000476 \text{ g} \). The molar mass of acrylamide is approximately \( 71.08 \text{ g/mol} \). Thus, the moles of acrylamide are \( \frac{0.0000476 \text{ g}}{71.08 \text{ g/mol}} = 6.70 \times 10^{-7} \text{ moles} \).

Key Concepts

Molecular Structure

The molecular structure of acrylamide, which is denoted chemically as \( \mathrm{H}_2\mathrm{C}=\mathrm{CHC}(=\mathrm{O})\mathrm{NH}_2 \), can be visualized as a sequence of interconnected groups. It consists of a \( \mathrm{CH}_2 \) group which is bonded to a \( \mathrm{CH} \) group. This \( \mathrm{CH} \) group is double-bonded to a carbonyl group \( (\mathrm{C}=\mathrm{O}) \), linking further to an amine group \( (\mathrm{NH}_2) \).

The resulting structure can be summarized as \( \mathrm{H}_2\mathrm{C}=\mathrm{CH-C}(=\mathrm{O})-\mathrm{NH}_2 \). Understanding this arrangement is crucial because it tells us about the types of chemical bonds present. Double bonds, like those found between \( \mathrm{H}_2\mathrm{C}=\mathrm{CH} \), generally imply stronger connectivity as compared to single bonds, which impact the molecule's overall shape and properties.

Bond Angles

In acrylamide, different parts of the molecule feature distinct geometries, influencing the bond angles.

• The carbonyl group, characterized by the double bond \( \mathrm{C}=\mathrm{O} \) and thus a trigonal planar structure, presents bond angles of approximately 120 degrees. This is typical for geometries with \( sp^2 \) hybridization.
• In contrast, the \( \mathrm{CH}_2 \) group exhibits bond angles around 109.5 degrees. This angle arises from its tetrahedral configuration, commonly associated with \( sp^3 \) hybridization.

Together, these varied angles reflect the complexity of acrylamide's molecular structure, making it essential to properly interpret chemical properties and reactions involving this compound.

Molecular Polarity

The polarity of a molecule is determined by its symmetry and the distribution of electrical charges across it. Acrylamide is considered a polar molecule due to its asymmetric shape and the presence of two significant functional groups that inherently create electric dipoles:

• The carbonyl group \( (\mathrm{C}=\mathrm{O}) \) contributes to polarity due to the electronegativity difference between carbon and oxygen, causing a dipole moment towards the oxygen.
• The amine group \( (\mathrm{NH}_2) \) also induces polarity, as the nitrogen's lone pair exerts an additional dipole effect.

These individual dipole moments do not cancel out thanks to the molecule's shape, resulting in an uneven distribution of charge that makes acrylamide polar. This characteristic affects how it interacts with other substances, influencing its solubility and reactivity.

Molar Mass Calculation

Calculating the molar mass of acrylamide provides insight into its composition and enables us to perform conversions between mass and moles. To determine the molar mass, each element in the molecular formula \( \mathrm{H}_2\mathrm{C}=\mathrm{CHC}(=\mathrm{O})\mathrm{NH}_2 \) is accounted for:

• Carbon (C): 3 atoms × 12.01 g/mol = 36.03 g/mol
• Hydrogen (H): 5 atoms × 1.008 g/mol = 5.04 g/mol
• Nitrogen (N): 1 atom × 14.01 g/mol = 14.01 g/mol
• Oxygen (O): 1 atom × 16.00 g/mol = 16.00 g/mol

Adding these values gives acrylamide a total molar mass of approximately 71.08 g/mol.

With this figure, you can convert between the mass of acrylamide in a sample (e.g., in potato chips) and the number of moles, using the formula: \( \text{moles} = \frac{\text{mass in grams}}{\text{molar mass in g/mol}} \). Understanding how to perform this conversion is crucial for quantitatively assessing the presence and impact of acrylamide in various products.