Question

Methyl isocyanate, \(\mathrm{CH}_{3} \mathrm{NCO}\), was made infamous in 1984 when an accidental leakage of this compound from a storage tank in Bhopal, India, resulted in the deaths of about 3,800 people and severe and lasting injury to many thousands more. (a) Draw a Lewis structure for methyl isocyanate. (b) Draw a ball-and-stick model of the structure, including estimates of all the bond angles in the compound. (c) Predict all the bond distances in the molecule. (d) Do you predict that the molecule will have a dipole moment? Explain.

Short answer

The Lewis structure of methyl isocyanate (CH3NCO) consists of a central carbon atom double-bonded to nitrogen and single-bonded to an oxygen and three hydrogen atoms. In the ball-and-stick model, C-N and C-O bond angles are around 120°, while the C-H bond angles are around 109.5°. The bond distances are approximately C-N ≈ 1.25 Å, C-O ≈ 1.43 Å, and C-H ≈ 1.09 Å. The molecule has a net dipole moment due to the differences in electronegativity between C-O and C-N bonds and their angular arrangement, making it polar.

Step-by-step solution

(Step 1: Draw the Lewis structure of methyl isocyanate)

(First, we need to count the total number of electrons for each atom in the molecule. Carbon (C) has 4 valence electrons, hydrogen (H) has 1, nitrogen (N) has 5, and oxygen (O) has 6. The CH3NCO has a total of 4 + (3 × 1) + 5 + 6 = 18 electrons. Arrange the atoms so that carbon is in the center, with nitrogen on one side and the three hydrogen atoms on the other side. Connect each hydrogen atom to carbon with a single bond and connect the nitrogen and oxygen atoms to carbon with a double bond. This arrangement will result in 3 single bonds, 1 double bond, and 1 lone pair of electrons on nitrogen, and two lone pairs on oxygen.)

(Step 2: Draw a ball-and-stick model with bond angles estimates)

(In the ball-and-stick model, use spheres to represent the atoms and sticks to represent bonds. Place the carbon at the center, nitrogen on one side at an angle of around 120°, and the three hydrogen atoms on the opposite side at angles of around 109.5° (tetrahedral). Label each bond angle for clarity.)

(Step 3: Predict bond distances)

(Using information from bond distance tables and knowing that C-N, C-O, C-H, and N-O involve single and double bonds, the predicted bond distances are as follows: C-N (double bond) ≈ 1.25 Å, C-O (single bond) ≈ 1.43 Å, C-H (single bond) ≈ 1.09 Å, and N-O (single bond) ≈ 1.45 Å.)

(Step 4: Predicting the dipole moment and explanation)

(Dipole moments are caused by differences in electronegativity between atoms in a bond. Using the electronegativity scale, we find that oxygen (3.44) is more electronegative than carbon (2.55) and nitrogen (3.04), and nitrogen is more electronegative than carbon. Therefore, there will be a dipole moment between C-O and C-N bonds. However, due to the angular arrangement of the molecule, the overall dipole moment won't be zero, as these bond dipoles will not cancel out. The molecule will have a net dipole moment, making it polar.)

Key concepts

Lewis Structure

The process of drawing a Lewis structure begins with arranging electrons to represent shared and lone electron pairs. For methyl isocyanate ((CH_3NCO)), we start by calculating the total number of valence electrons from carbon (C), hydrogen (H), nitrogen (N), and oxygen (O). Each element contributes a specific number based on its group in the periodic table: C has 4, H has 1, N has 5, and O has 6.

By adding these up, we get 18 valence electrons to distribute. In the structure, C is central, bonded to H atoms and to N, which in turn is bonded to O. We create bonds by pairing electrons between atoms: single bonds for C-H and likely a double bond between C-N, and C-O to fill the valence shells. N also carries a lone pair, and O has two, as they follow the octet rule, except for H, which follows the duet rule.

In our final structure, bonds are visually represented by lines, and lone pairs by dots around the respective atoms, showcasing how the 18 valence electrons are distributed among the atoms to give a stable structure.

Ball-and-Stick Model

Visualizing molecules in three dimensions is effectively achieved through a ball-and-stick model, which provides clear insights into spatial arrangement. For methyl isocyanate, the carbon (C) atom is depicted as the central 'ball', with 'sticks' - representing chemical bonds - extending towards other 'balls', which represent hydrogen (H), nitrogen (N), and oxygen (O) atoms.

The bond angles in this model illustrate the molecule's geometry: the three hydrogen atoms are positioned around the central carbon atom, each approximating the tetrahedral angle of 109.5°, while the bond angle between nitrogen and carbon is around 120°. These bond angles help us to understand the molecule’s shape, crucial for predicting how it interacts with other molecules.

Bond Angles

Bond angles are crucial in determining the shape and reactivity of a molecule. For methyl isocyanate, the bond angles are estimated based on the types of hybridization at atomic centers. The H-C-H bond angle tends towards the ideal tetrahedral angle of 109.5°. However, the C-N-C bond angle, involving sp2 hybridized orbitals, is closer to 120° owing to the trigonal planar shape around the sp2 hybridized carbon atom.

These angles contribute to the molecule's overall geometry, affecting its ability to interact with other substances, a factor of lethal consequence in the Bhopal tragedy.

Bond Distances

Predicting bond distances involves understanding covalent bond lengths between different types of atoms. In methyl isocyanate, bond distances can be estimated using known average values for similar bond types.

The C-N bond, being a double bond, is shorter and stronger than a single bond, at approximately 1.25 Å. The C-O and N-O bonds, being single bonds, have estimated distances of 1.43 Å and 1.45 Å, respectively. Lastly, the C-H bond is about 1.09 Å. These distances give an idea of how compact or spread out the molecular structure can be, which aids in modeling interactions at a molecular level.

Dipole Moment

Dipole moments arise from differences in electronegativity between bonded atoms, leading to an uneven distribution of electron density. In methyl isocyanate, electronegativity differences between C-O and C-N create partial charges at these bonds, contributing to individual bond dipole moments.

While each bond has its dipole moment, it's the molecule's shape that determines if these individual dipoles cancel out or result in a net dipole moment. Due to its angular structure, methyl isocyanate has a net dipole moment, affirming its molecular polarity. This property plays a significant role in determining the molecule's interactions with other polar substances, solvents, and in its overall chemical behavior.

Molecular Polarity

Molecular polarity is shaped by both the individual dipole moments of bonds and the molecule's overall geometry. In methyl isocyanate, due to the presence of polar C-N and C-O bonds and the non-linear arrangement of atoms, the molecule exhibits a net dipole moment.

This polarity affects how the molecule interacts with its environment, including how it behaves in different solvents and its reactivity. It's the polar nature of methyl isocyanate that makes it soluble in water and polar solvents, which unfortunately contributed to its rapid dispersal following the Bhopal incident, exacerbating the disaster.