Question

Predict shapes for the organic molecules chloroform, \(\mathrm{CHCl}_{3},\) and 1,1 -dichloroethene, \(\mathrm{Cl}_{2} \mathrm{C}=\mathrm{CH}_{2}\).

Short answer

Chloroform (\( \mathrm{CHCl}_3 \)) is tetrahedral, and 1,1-dichloroethene (\( \mathrm{Cl}_2\mathrm{C} = \mathrm{CH}_2 \)) is trigonal planar.

Step-by-step solution

Understand the Molecules

First, identify the structural components of the molecules. Chloroform, \( \mathrm{CHCl}_3 \), consists of one carbon atom bonded to one hydrogen and three chlorine atoms. 1,1-Dichloroethene, \( \mathrm{Cl}_2\mathrm{C} = \mathrm{CH}_2 \), comprises two carbon atoms double-bonded to each other, where one carbon is bonded to two chlorine atoms and the other carbon to two hydrogen atoms.

Apply VSEPR Theory to Chloroform

The Valence Shell Electron Pair Repulsion (VSEPR) theory helps predict the shape of \( \mathrm{CHCl}_3 \). The carbon atom forms four single bonds (sp³ hybridization) with no lone pairs, creating a tetrahedral shape.

Analyze the Shape of 1,1-Dichloroethene

For \( \mathrm{Cl}_2\mathrm{C} = \mathrm{CH}_2 \), consider the carbon-carbon double bond. Each carbon forms three sigma bonds, resulting in an sp² hybridization and a trigonal planar shape around each carbon atom.

Confirm the Shapes

In \( \mathrm{CHCl}_3 \), all bond angles are approximately 109.5°, characteristic of a tetrahedral geometry. For \( \mathrm{Cl}_2\mathrm{C} = \mathrm{CH}_2 \), each carbon forms 120° bond angles, affirming the trigonal planar structure.

Key concepts

Molecular Geometry

Molecular geometry is the three-dimensional arrangement of atoms in a molecule. It determines how a molecule interacts with other molecules. Understanding molecular geometry helps in predicting the behavior and reactivity of chemical substances.

• VSEPR theory provides a systematic way to determine molecular shape based on the repulsion between electron pairs.
• Shapes include linear, bent, tetrahedral, trigonal planar, and more.
• The geometrical shape directly affects the molecule's polarity and biological activity.

A molecule’s geometry is essential for determining its physical properties, such as boiling and melting points, solubility, and how it interacts with light. For example, in the molecule of chloroform (\(\mathrm{CHCl}_3\)), the molecular geometry is tetrahedral despite having different atoms bonded to the central carbon atom. This arrangement allows for a common bond angle, indicating the geometric consistency predicted by VSEPR theory.

sp³ Hybridization

Hybridization is a key concept in molecular geometry. It involves mixing atomic orbitals into new hybrid orbitals suitable for the pairing of electrons to form chemical bonds. In sp³ hybridization:

• An atom's s-orbital mixes with three p-orbitals to form four equivalent sp³ hybrid orbitals.
• These orbitals are oriented in a tetrahedral geometry.

sp³ hybridization typically occurs when a carbon atom forms four sigma bonds, as in chloroform, \(\mathrm{CHCl}_3\). Each sigma bond extends from the sp³ hybrid orbitals, providing a framework for the structure's tetrahedral shape. This type of bonding allows for the distinct angle of 109.5° between bonds, which minimizes electron pair repulsion. Understanding sp³ hybridization helps in visualizing molecular shapes and predicting how they might form bonds in chemical reactions.

Tetrahedral Shape

A tetrahedral shape is a specific three-dimensional configuration where a central atom is surrounded by four equivalent bonds positioned at equal angles of 109.5°. This shape is derived from sp³ hybridization of the central atom.

• A perfect example is the molecule of chloroform, \(\mathrm{CHCl}_3\), where the central carbon atom forms four bonds.
• The tetrahedral configuration results in symmetry and balance in molecular geometry.

This shape ensures that electron pairs, whether bonding or non-bonding, maintain maximum separation from each other, which is a principle foundation in VSEPR theory. The tetrahedral shape is fundamental in explaining many molecular properties, including reactivity and interaction with other molecules.

Trigonal Planar Shape

The trigonal planar shape is another geometric form achieved in molecules that possess sp² hybridization.

• In this geometry, a central atom is surrounded by three groups of atoms in a plane, forming a \(120\degree\) angle between each pair of bonds.
• This configuration typically arises from the presence of double bonds, as seen in alkenes.

In the compound 1,1-dichloroethene, \(\mathrm{Cl}_2\mathrm{C} = \mathrm{CH}_2\), each carbon atom adopts a trigonal planar geometry due to the double bonding that restricts rotation and ensures the \(sp²\) hybridization. The uniform 120° angles in this shape help predict the orientation of substituents around the double bond. The planar geometry influences how these molecules stack and interact in larger structures, significantly impacting their physical and chemical properties.