Question

Write condensed structural formulas for all the alkynes, i.e. unsaturated compounds with triple bonds, with a molecular formula of \(\mathrm{C}_{5} \mathrm{H}_{8}\)

Short answer

There are two possible alkynes with a molecular formula of C5H8: 1. 1-Pentyne: \(H-C\equiv C-C-C-C-H\) 2. 2-Pentyne: \(H_{3}C-C\equiv C-C-C-H\)

Step-by-step solution

We have 5 carbon atoms, and we need 1 triple bond (degree of unsaturation is 2) between two of the carbon atoms. #Step 2: Draw the first structural isomer with a triple bond at terminal carbon atom#

Start with the terminal carbon atom (carbon-1) and put a triple bond: \(H- C\equiv C-C-C-C-H \) This is 1-Pentyne (IUPAC name). #Step 3: Draw the second structural isomer with triple bond at non-terminal carbon atom#

Place the triple bond between second and third carbon atoms in the chain: \( H_{3}C-C\equiv C-C-C-H \) This is 2-Pentyne (IUPAC name). #Step 4: List condensed structural formulas and names of alkynes with molecular formula C5H8#

There are two possible alkynes with a molecular formula of C5H8: 1. 1-Pentyne: \(H-C\equiv C-C-C-C-H\) 2. 2-Pentyne: \(H_{3}C-C\equiv C-C-C-H\)

Key concepts

Structural Isomers

When dealing with organic compounds that have the same molecular formula but differ in their connectivity, we refer to these variations as structural isomers. In the context of alkynes, these isomers differ in the placement of the triple bond within the carbon chain. For a molecule like \(\mathrm{C}_{5} \mathrm{H}_{8}\), the presence of a triple bond creates a distinct feature at one point in the carbon chain.

• 1-Pentyne has the triple bond between the first and second carbon atoms, creating a terminal alkyne.
• 2-Pentyne features the triple bond between the second and third carbon atoms, which places the triple bond more centrally in the chain.

These structural isomers illustrate how different arrangements of atoms can lead to unique compounds, even though they share the same molecular formula. This concept is crucial for understanding the diversity of organic chemistry structures.

Degree of Unsaturation

The degree of unsaturation in a compound helps us determine the number of rings and multiple bonds (like double or triple bonds) that are present. For each multiple bond or ring, the degree of unsaturation increases by one. The formula to find the degree of unsaturation for hydrocarbons is given by:\[\text{Degree of unsaturation} = C + 1 - \frac{H}{2}\]where \(C\) is the number of carbon atoms and \(H\) is the number of hydrogen atoms.For example, with \(\mathrm{C}_{5} \mathrm{H}_{8}\), we calculate:\[5 + 1 - \frac{8}{2} = 2\]So, the degree of unsaturation is 2, indicating that there are two pi bonds present in the molecule which might be due to one triple bond, as found in alkynes. Understanding this concept helps in predicting possible structures based on the chemical formula.

Molecular Formula

The molecular formula provides a fundamental overview of the molecule by showing the number of atoms of each element present. For alkynes like those with the molecular formula \(\mathrm{C}_{5} \mathrm{H}_{8}\), it tells us several things:

• There are five carbon atoms.
• There are eight hydrogen atoms.

What the molecular formula doesn't convey directly is the structure or the arrangement of these atoms. It helps chemists start to deduce possible isomers and the degree of unsaturation, but without providing specific connectivity or layout details. Hence, while the molecular formula is crucial, it's only one part of understanding a compound's complete chemical identity.

IUPAC Nomenclature

The naming of organic compounds follows the International Union of Pure and Applied Chemistry (IUPAC) rules, a standardized method that ensures each compound has a unique name that describes its structure. For alkynes, the naming involves identifying the longest carbon chain containing the triple bond and numbering the chain to give the carbon atoms involved in the triple bond the lowest numbers possible. For example: - **1-Pentyne**: The triple bond is between the first and second carbon atoms of the chain. With the prefix 'Pent-' indicating a five-carbon chain, the name '1-Pentyne' precisely describes the structure. - **2-Pentyne**: This name indicates that the triple bond occurs between the second and third carbon atoms. It follows a similar logic but shifts the location of the bond, demonstrating the flexibility in naming based on the position within the carbon framework. IUPAC names help ensure clear communication among chemists globally, preventing confusion about the compounds being discussed.