Problem 24 - Chemistry Matter and Change

Question

Predict What geometric arrangement would you expect from the bonds surrounding the carbon atom in alkanes, alkenes, and allynes? (Hint: VSEPR theory can be used to predict the shape.)

Step-by-Step Solution

Answer

Alkanes have a tetrahedral geometry, alkenes have a trigonal planar geometry, and alkynes have a linear geometry.

Step 1: Understand the VSEPR Theory

VSEPR stands for Valence Shell Electron Pair Repulsion theory, which is used to predict the geometry of molecules. This theory states that electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsion between them.

Step 2: Analyze the Carbon Atom in Alkanes

In alkanes, each carbon atom forms four single bonds. According to VSEPR theory, with four regions of electron density, the molecular geometry is tetrahedral. Thus, carbon in alkanes has a tetrahedral geometry with bond angles of approximately 109.5 degrees.

Step 3: Analyze the Carbon Atom in Alkenes

In alkenes, carbon forms three single bonds and one double bond. This results in three regions of electron density. According to VSEPR theory, this leads to a trigonal planar arrangement, with bond angles of approximately 120 degrees around the carbon.

Step 4: Analyze the Carbon Atom in Alkynes

In alkynes, carbon forms one triple bond and one single bond. This leads to two regions of electron density. VSEPR theory predicts a linear geometry in this case, with bond angles of 180 degrees around the carbon atom.

Key Concepts

Understanding Molecular Geometry

Molecular geometry encompasses the three-dimensional arrangement of atoms around a central atom in a molecule. A key theory to understand this concept is the Valence Shell Electron Pair Repulsion (VSEPR) theory. This theory helps predict molecular shapes by focusing on how electron pairs, both bond and lone pairs, arrange themselves around a central atom. They do this to minimize repulsion, leading to a specific geometrical arrangement.
The most common shapes include:

• Tetrahedral: Four regions of electron density with bond angles of 109.5 degrees.
• Trigonal Planar: Three regions of electron density with bond angles of 120 degrees.
• Linear: Two regions of electron density, forming a straight line with bond angles of 180 degrees.

These arrangements help in predicting how different atoms are spatially organized, which is crucial for understanding the properties and reactivity of molecules.

Molecular Geometry of Alkanes

Alkanes are the simplest form of hydrocarbons with only single bonds between carbon atoms. In alkanes, each carbon atom forms four covalent bonds, generally with hydrogen. According to VSEPR theory, four regions of electron density around each carbon cause them to adopt a tetrahedral geometry.
In a tetrahedral arrangement, each bond angle measures approximately 109.5 degrees. This creates a three-dimensional shape which is stable and allows minimal repulsion between electron clouds. Alkanes are known for their saturated nature, meaning they contain the maximum number of hydrogen atoms per carbon atom, making them relatively non-reactive compared to other hydrocarbons.

Exploring Alkenes and Their Geometry

Alkenes are characterized by the presence of at least one carbon-carbon double bond. This double bond is crucial in determining their molecular geometry. With one double bond and two single bonds around each carbon in the double-bonded pair, there are three regions of electron density.
This configuration leads to a trigonal planar geometry around the carbon atoms involved in the double bond. In this geometry, bond angles are approximately 120 degrees. The planar nature of alkenes gives them distinctive chemical properties, including reactivity compared to alkanes. Alkenes are considered unsaturated hydrocarbons, which means they have fewer hydrogen atoms attached to the carbon chain than alkanes.

Alkynes and Their Linear Arrangement

Alkynes stand out among hydrocarbons due to their carbon-carbon triple bond. This triple bond results in a unique linear molecular geometry for the carbons involved. Around each carbon involved in the triple bond, there are only two regions of electron density.
According to VSEPR theory, the best arrangement to minimize repulsion in this situation is linear. This results in bond angles of 180 degrees, creating the straight and elongated shape characteristic of alkynes. Unlike alkanes, alkynes are unsaturated, containing fewer hydrogen atoms, which also contributes to their higher reactivity and utility in various chemical reactions.