Question

Sketch the structures for (a) methane, (b) ethane, and (c) propane.

Short answer

Methane is a tetrahedral molecule with one carbon atom and four hydrogen atoms. Ethane consists of two carbon atoms bonded to each other and to three hydrogen atoms each. Propane has a chain of three carbon atoms with a total of eight hydrogen atoms bonded to them in a zig-zag shape.

Step-by-step solution

Understand Methane Structure

Methane (CH4) is the simplest alkane, consisting of one carbon atom bonded to four hydrogen atoms. It has a tetrahedral shape, with the carbon atom in the center and hydrogen atoms at the corners of the tetrahedron.

Understand Ethane Structure

Ethane (C2H6) is made up of two carbon atoms single-bonded to each other, with each carbon atom also being bonded to three hydrogen atoms. The shape of ethane is also tetrahedral at each carbon, but the molecule in its entirety is longer due to the two carbon atoms.

Understand Propane Structure

Propane (C3H8) consists of three carbon atoms in a chain. Each end carbon atom is bonded to three hydrogen atoms, and the middle carbon atom is bonded to two hydrogen atoms. The molecule is also tetrahedral around each carbon atom, resulting in a zig-zag shape for the carbon chain.

Key concepts

Methane Structure

Methane, with its chemical formula of CH4, is the most basic example of an alkane. In terms of geometry, the methane molecule adopts a tetrahedral structure. This means that if you picture the carbon atom at the center of a geometric solid known as a tetrahedron, each of the four hydrogen atoms would sit at the points, or vertices, of this shape.

The carbon-hydrogen bonds spread out uniformly, making angles of approximately 109.5 degrees with each other. This uniform distribution of bond angles results in a three-dimensional figure renowned for its symmetry and stability. In essence, methane is like a perfectly balanced ball, where each hydrogen atom is equidistant from the others. It's this spatial arrangement that grants methane its characteristically nonpolar properties and gaseous state at room temperature. Understanding the structure of methane is foundational when delving into more complex alkanes.

Ethane Structure

Now, if we step up just slightly on the complexity ladder, we encounter ethane, which is represented by C2H6. The structure of ethane is an elongated version of methane. There are now two central carbon atoms each making three bonds with hydrogen atoms, and they also share a bond with each other.

When we examine the molecule more closely, we notice that the bonded carbon atoms form a central axis with the hydrogen atoms fanning out in a 3D arrangement around each carbon. Each individual carbon is still tetrahedral in shape, much like with methane, but together they form a sort of 'extended tetrahedron'. This single bond between the two carbon atoms allows for rotation, making ethane a dynamic molecule. A great exercise for students is to use molecular models to visualize how ethane's structure allows for this rotation, improving their understanding of molecular geometry and alkane properties.

Propane Structure

Building further on this concept brings us to propane, the third member of the alkane family, with its formula C3H8. Here, we have three carbon atoms linked in a linear fashion like a chain. Yet, similar to the preceding alkanes, each carbon atom maintains a tetrahedral geometry with its associated hydrogen atoms.

The two end carbons bond with three hydrogen atoms respectively, while the central carbon atom bonds with two hydrogen atoms. This central placement makes the middle carbon atom unique, as it is bonded to two other carbons rather than three hydrogen atoms. The shape of the molecule somewhat resembles a zig-zag when viewed from certain angles, a characteristic pattern that will become more pronounced with larger alkanes. Propane's molecular arrangement not only serves as the first introduction to carbon chain isomerism for many students, which is a critical concept as they delve deeper into organic chemistry, but also illustrates the beginning of structural diversity in hydrocarbon chains.