Question

Draw the two structural isomers of straight-chain pentyne, \(\mathrm{C}_{5} \mathrm{H}_{8}\).

Short answer

The structural isomers are 1-pentyne and 2-pentyne.

Step-by-step solution

Understand the Compound

A pentyne with the formula \( \mathrm{C}_{5}\mathrm{H}_{8} \) is an alkyne, which means it contains a triple bond. It needs to be a straight chain with five carbon atoms.

Locate the Triple Bond for the First Isomer

For the first isomer, place the triple bond between the first and second carbon atoms, counting from one end. This is called 1-pentyne.

Verify the Structure of 1-Pentyne

Check that the molecular structure of 1-pentyne is \( \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{C}\equiv\mathrm{C}-\mathrm{CH}_{3} \). Count the hydrogen atoms: there are eight hydrogen atoms satisfying the formula \( \mathrm{C}_{5}\mathrm{H}_{8} \).

Locate the Triple Bond for the Second Isomer

For the second isomer, move the triple bond to be between the second and third carbon atoms. This is called 2-pentyne.

Verify the Structure of 2-Pentyne

Check that the molecular structure of 2-pentyne is \( \mathrm{CH}_{3}-\mathrm{C}\equiv\mathrm{C}-\mathrm{CH}_{2}-\mathrm{CH}_{3} \). Similarly, verify again that there are eight hydrogen atoms, confirming the formula \( \mathrm{C}_{5}\mathrm{H}_{8} \).

Key concepts

Alkyne

Alkynes are a fascinating group of hydrocarbons that are characterized by the presence of at least one carbon-carbon triple bond. This unique bond is what sets them apart from other hydrocarbons like alkanes and alkenes. Generally, in simple alkynes, this triple bond can be located at various positions along the carbon chain, allowing for the existence of multiple structural isomers.

• The simplest alkyne is acetylene, with the formula \( \mathrm{C}_{2}\mathrm{H}_{2} \). This compound consists of a triple bond connecting two carbon atoms, each bonded to one hydrogen atom.
• Commonly, alkynes are named by adding the suffix '-yne' to the root name of the longest carbon chain that contains the triple bond. For example, with five carbon atoms, we get the name 'pentyne.'

Alkynes are often used in organic synthesis and industrial applications due to their reactivity arising from the presence of this triple bond. Their synthesis, manipulation, and reactions play a significant role in the field of organic chemistry.

Triple Bond

A triple bond is a type of covalent bond where three pairs of electrons are shared between two atoms. In the context of alkynes, this bond involves carbon atoms, which share these electrons to fill their outer shells.

• The triple bond consists of one sigma (\(\sigma\)) bond and two pi (\(\pi\)) bonds. The \(\sigma\)-bond is the result of the head-on overlapping of orbitals, while the \(\pi\)-bonds form from the lateral overlap of orbitals.
• These pi bonds are located on opposite sides of the sigma bond, creating a linear geometry around the carbon atoms involved in the triple bond.

This linear shape is what gives alkynes their unique physical properties, such as higher melting and boiling points compared to corresponding alkanes and alkenes. Furthermore, the reactivity of alkynes is largely influenced by the electrons in the pi bonds, which are more exposed compared to those in single or double bonds.

Molecular Structure

Understanding molecular structure is crucial when analyzing different isomers. In organic chemistry, structural isomers are compounds that have the same molecular formula but different structural formulas, meaning the atoms are arranged in different ways.
For pentyne, the molecular formula is \( \mathrm{C}_{5}\mathrm{H}_{8} \), and it can form structural isomers because the location of its triple bond can vary.

• In 1-pentyne, the triple bond is located between the first and second carbon atoms in the chain, giving the structure \( \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{C}\equiv\mathrm{C}-\mathrm{CH}_{3} \).
• In 2-pentyne, the triple bond is between the second and third carbon atoms, resulting in the structure \( \mathrm{CH}_{3}-\mathrm{C}\equiv\mathrm{C}-\mathrm{CH}_{2}-\mathrm{CH}_{3} \).

These structural differences lead to unique properties and reactivities for each isomer. Understanding these distinctions is essential for mastering organic chemistry concepts and predicting the behavior of compounds under various conditions.