Problem 91 - Chemistry Principles and Practice

Question

Acrolein has the formula \(\mathrm{CH}_{2} \mathrm{C}(\mathrm{H}) \mathrm{C}(\mathrm{O}) \mathrm{H}\). Draw its skeleton structure and Lewis structure.

Step-by-Step Solution

Answer

Create a linear structure: \(\mathrm{H}_2\mathrm{C}=\mathrm{C}( \mathrm{H})-\mathrm{C}(=\mathrm{O})\mathrm{H}\), ensuring correct bonds and octets.

Step 1: Identify the Atoms

First, identify all the atoms in acrolein: \(\mathrm{C}_3\), \(\mathrm{H}_4\), and \(\mathrm{O}_1\). Acrolein consists of three carbon atoms, four hydrogen atoms, and one oxygen atom.

Step 2: Create the Skeleton Structure

In the skeletal structure, represent each carbon atom (\(\mathrm{C}\)) as a point and connect them linearly: \(\mathrm{C}-\mathrm{C}-\mathrm{C}\). Attach hydrogens and oxygen as needed. Acrolein consists of three carbon atoms in a chain. Hydrogen atoms attach to the first and third carbons, as well as the second carbon but only one hydrogen since one bond is reserved for oxygen. Draw an additional double bond between the second carbon and the oxygen: \(\mathrm{H}_2\mathrm{C}=\mathrm{CH}-\mathrm{C}(=\mathrm{O})\mathrm{H}\).

Step 3: Define the Lewis Structure

Allocate electrons around the atoms to reflect the molecule's covalent bonds and valency, emphasizing electron sharing. For acrolein, place electrons to ensure that each hydrogen has 1 bond, each carbon has 4 bonds, and the oxygen has 2 bonds. For the double-bonded carbon and oxygen, ensure that they share two pairs of electrons. Place any remaining electrons as lone pairs on oxygen to fulfill its octet.

Step 4: Double-check Octets and Valency

Verify that all atom valencies are satisfied according to the octet rule, especially for carbon (4 bonds total) and oxygen (2 bonds total plus 2 lone pairs). Hydrogen should always have 1 bond. The bonds should form a linear molecule with double bonds correctly placed between carbon and oxygen.

Key Concepts

Skeletal Structure

When we talk about the skeletal structure in chemistry, we are focusing on simplifying the visualization of how atoms are arranged in a molecule. For a compound like acrolein, the skeletal structure emphasizes the carbon backbone. Carbon atoms are often shown as vertices or line ends, and these atoms form the basic frame.
In drawing acrolein's skeletal structure:

• Begin by arranging the three carbon atoms in a straight line, connecting them with single lines to indicate bonds.
• Next, hydrogen atoms are added, typically at the ends of the molecule, connected to the outer carbon atoms.
• The oxygen is often represented differently, appended to the carbon chain with double bonds.

This skeletal line system simplifies the depiction by not directly showing hydrogen and carbon bonds explicitly but assumes their existence on carbon atoms unless present otherwise.

Molecular Geometry

Molecular geometry discusses the shape formed by atoms in a molecule. It affects the molecule's physical and chemical properties including reactivity, polarity, and color. For a molecule like acrolein, its geometry arises from the double bonds and linear arrangement of carbon atoms.
Each carbon atom in acrolein attempts to achieve an angle of 120° due to the trigonal planar arrangement which is common when dealing with double-bonded carbon.

• For C-C=C in acrolein, this planar configuration is important for understanding electron pair repulsion which dictates that electron clouds spread out as evenly as possible.
• The double bond between carbon and oxygen in acrolein leads to significant rigidity in the molecule's structure, limiting rotational freedom around the bond.

These geometric considerations help predict and explain the function and interaction of acrolein in different chemical environments.

Covalent Bonds

Covalent bonds are what hold atoms together in a molecule through the sharing of electron pairs. In acrolein, these are vital for maintaining the integrity of the molecule.
Each atom seeks to complete its outer electron shell:

• Hydrogen, possessing a single electron, forms bonds by pairing with another electron, usually from carbon.
• Carbon, with four valence electrons, forms four covalent bonds to achieve a stable configuration.
• Oxygen, in acrolein, shares two of its six valence electrons with carbon while maintaining its stability with lone pairs.

Double bonds have a large impact as they allow more electrons to be shared between atoms than single bonds, which is apparent between the carbon and oxygen in acrolein. These bonds influence how molecules interact and react, impacting chemical behavior and properties.