Question

Draw a line-bond structure for 1,3 -butadiene, \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}_{2} .\) Indicate the hybridization of the orbitals on each carbon, and predict the value of each bond angle.

Short answer

All carbons are \( sp^2 \) hybridized with bond angles of 120°; draw the line-bond: \( \mathrm{H}_2\mathrm{C}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}_2 \).

Step-by-step solution

Identify the Structure

1,3-butadiene consists of a total of four carbon atoms in a chain. The molecular formula is given as \( \mathrm{H}_2\mathrm{C}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}_2 \), indicating two double bonds between pairs of carbon atoms at positions 1-2 and 3-4.

Draw the Skeleton Structure

Illustrate the basic carbon chain for the molecule as follows: \( \mathrm{C}-\mathrm{C}-\mathrm{C}-\mathrm{C} \). Add the double bonds to reflect the given molecular structure: \( \mathrm{C}=\mathrm{C}-\mathrm{C}=\mathrm{C} \). This is the line-bond representation of 1,3-butadiene.

Add Hydrogen Atoms

Attach hydrogens to the carbon atoms to complete their valence. The terminal carbon atoms (1 and 4) each have two hydrogen atoms to complete their valence (\( \mathrm{CH}_2 \)), while the inner carbon atoms (2 and 3) have one hydrogen each (\( \mathrm{CH} \)).

Determine Hybridization of Carbon Atoms

All carbon atoms involved in double bonds use \( sp^2 \) hybridization due to the presence of one double bond. Thus, each carbon atom in 1,3-butadiene is \( sp^2 \) hybridized.

Predict Bond Angles

An \( sp^2 \) hybridized carbon forms bond angles of approximately 120° due to the trigonal planar arrangement. Therefore, all bond angles around each carbon atom in 1,3-butadiene will be approximately 120°.

Key concepts

Hybridization in Organic Molecules

Hybridization in organic molecules is a fundamental concept to understand when predicting structure and reactivity. It involves the mixing of atomic orbitals to form new hybrid orbitals, which can form covalent bonds. Each carbon atom's role within a molecule dictates the type of hybridization it undergoes. For instance, carbon atoms in alkanes are typically sp3 hybridized, while those in alkenes, like 1,3-butadiene, are often sp2 hybridized. The type of hybridization affects the molecule's geometry and bonding properties, so recognizing which orbitals are involved is crucial for understanding organic molecular structures.

Bond Angles in Organic Chemistry

Bond angles in organic chemistry are determined by the type of hybridization present in the molecule. Each type of hybridization results in a unique geometrical shape. In 1,3-butadiene, each carbon is sp2 hybridized, which leads to bond angles of approximately 120 degrees. These angles result from the trigonal planar geometry around each sp2 hybridized carbon. Understanding bond angles helps predict how molecules will interact in three-dimensional space, which is essential for determining molecular behavior and reactivity.

sp2 Hybridization

sp2 hybridization occurs when one s orbital and two p orbitals mix to form three equivalent hybrid orbitals. These orbitals are oriented in a trigonal planar arrangement with bond angles of about 120 degrees. In 1,3-butadiene, all carbon atoms are sp2 hybridized due to the presence of double bonds. This hybridization allows each carbon atom to form three sigma bonds—two with neighboring carbon atoms and one with hydrogen—and one pi bond, which is essential for double bond formation. Understanding sp2 hybridization helps explain the geometry and the stiff, planar nature of compounds like 1,3-butadiene.

Drawing Organic Structures

Drawing organic structures involves several steps to ensure accurate representation. First, identify the carbon backbone, which serves as the skeleton of the molecule. Then, add the necessary double or triple bonds to reflect the molecule's specific features. In 1,3-butadiene, you must draw two double bonds between carbon 1-2 and 3-4. Once the backbone is established, complete the valence of each carbon by adding hydrogen atoms. For line-bond structures, it is essential to correctly represent bonds and hybridization, as these provide insight into the molecule's geometry and chemical behavior. This visual representation aids in predicting how the molecule functions chemically and physically.