Question

2-Butene \(\left(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{3}\right)\) is a starting material in the manufacture of lubricating oils and many other compounds. Draw two different structures for 2 -butene, indicating the \(\sigma\) and \(\pi\) bonds in each.

Short answer

Cis and trans isomers of 2-butene differ in the positions of the H atoms around the double bond, each having one \( \sigma \) and one \(\text{pi} \) bond in the C=C.

Step-by-step solution

- Identify the structure of 2-butene

2-Butene has the chemical structure \(\text{CH}_3-\text{CH}=\text{CH}-\text{CH}_3\). It consists of a double bond between the second and third carbon atoms in a chain of four carbon atoms.

- Draw the cis isomer

For the cis isomer, the hydrogen atoms attached to the double-bonded carbon atoms are on the same side. The structure can be represented as: \(\text{H}_3\text{C}-\text{CH}=\text{CH}-\text{CH}_3\). Indicate the \( \sigma \) and \( \pi \) bonds: Each carbon-carbon single bond (C-C) and carbon-hydrogen bond (C-H) represents a \(\text{sigma} \) bond. The double bond consists of one \(\text{sigma} \) bond and one \(\text{pi}\) bond.

- Draw the trans isomer

For the trans isomer, the hydrogen atoms attached to the double-bonded carbon atoms are on opposite sides. The structure can be represented as: \(\text{H}_3\text{C}-\text{CH}=\text{CH}-\text{CH}_3\). Likewise, indicate the \( \text{sigma} \) and \( \pi \) bonds in this isomer: Each carbon-carbon single bond (C-C) and carbon-hydrogen bond (C-H) represents a \(\text{sigma} \) bond. The double bond consists of one \(\text{sigma} \) bond and one \(\text{pi}\) bond.

- Summary of bonds

Both isomers of 2-butene (\text{cis} and \text{trans}) contain: - Four \( \sigma \) bonds from C-H in each carbon-hydrogen link. - Three \(\text{sigma} \) bonds from C-C in carbon-carbon single bonds. - One \(\text{sigma} \) bond and one \(\text{pi} \) bond in the C=C double bond.

Key concepts

Cis and Trans Isomers

Cis and trans isomers, also known as geometric isomers, are forms of a molecule that contain the same sequence of atoms but differ in the arrangement around a double bond or a ring structure. This difference comes from the restricted rotation around the double bonds, as altering the arrangement would involve breaking and reforming the \(\text{pi} \) bond.

sigma and pi bonds

Understanding sigma and pi bonds is crucial in the study of chemical structures. A sigma (\(\text{sigma} \)) bond is the first bond formed between two atoms. It involves the head-to-head overlap of atomic orbitals. These bonds are strong and are present in all single bonds.
Conversely, a pi (\(\text{pi} \)) bond is formed from the side-to-side overlap of two parallel p-orbitals and is found in double and triple bonds. The pi bond is generally weaker than a sigma bond due to the less effective overlap.
In a double bond, there is one sigma bond and one pi bond.

Chemical Structure Identification

Chemical structure identification involves recognizing and representing the arrangement of atoms within a molecule. For 2-butene, you must distinguish between its cis and trans isomers.
* Cis isomer: The hydrogen atoms on the double-bonded carbons are on the same side, giving the molecule a bent shape around the double bond.
* Trans isomer: The hydrogens on the double-bonded carbons are on opposite sides, making the molecule more linear around the double bond.
Correctly identifying these structures helps to understand the molecule's properties and interactions.