Question

Draw the condensed structural formula for each of the following alkynes: (a) 1-propyne (b) 4 -octyne (c) 2-butyne (d) 3-decyne

Short answer

(a) CH\(\equiv\)C-CH₃, (b) CH₃CH₂C\(\equiv\)CCH₂CH₂CH₂CH₃, (c) CH₃C\(\equiv\)CCH₃, (d) CH₃CH₂C\(\equiv\)CCH₂CH₂CH₂CH₂CH₂CH₃.

Step-by-step solution

Understand Alkynes

Alkynes are hydrocarbons that contain at least one carbon-carbon triple bond. Their general formula is \( C_nH_{2n-2} \). Condensed structural formulas show each atom in a molecule and how they're connected in a minimal way, often omitting the bonds between carbon atoms and using parentheses for repeating groups.

Analyze 1-Propyne

The name 1-propyne indicates a three-carbon alkyne with the triple bond starting at the first carbon. The formula is \( C_3H_4 \). The first carbon has a triple bond with the second carbon, and the third carbon is singly bonded to the second carbon. The condensed structural formula is CH\(\equiv\)C-CH₃.

Analyze 4-Octyne

The name 4-octyne indicates an eight-carbon alkyne with the triple bond starting at the fourth carbon. Count carbons ensuring the triple bond is between the fourth and fifth carbons: CH₃CH₂C\(\equiv\)CCH₂CH₂CH₂CH₃.

Analyze 2-Butyne

The name 2-butyne suggests a four-carbon alkyne with the triple bond starting at the second carbon. The condensed structural formula is CH₃C\(\equiv\)CCH₃, with the triple bond between the second and third carbons.

Analyze 3-Decyne

The name 3-decyne indicates a ten-carbon alkyne with the triple bond starting at the third carbon. Count carbons and insert the triple bond between the third and fourth carbons: CH₃CH₂C\(\equiv\)CCH₂CH₂CH₂CH₂CH₂CH₃.

Key concepts

Condensed Structural Formula

In chemistry, the condensed structural formula is a way of representing a molecule to clearly show the atoms present and their connections, but in a more compact form. This type of formula is particularly useful in organic chemistry where molecules can be quite large. The goal is to provide detailed information about the molecule without drawing out all individual bonds and hydrogen atoms.

Here's how to interpret and write a condensed structural formula:

• Each atom in the molecule is written out sequentially.
• The atoms bonded to carbon are usually grouped together to simplify the structure. For example, CH₃CH₂ represents two carbon atoms, where the first carbon is bonded to three hydrogens, and the second carbon is bonded to two hydrogens.
• Atoms connected directly in a linear fashion can be written consecutively, such as in CH₃C≡CCH₃, where the triple bond is implied.
• Repeating groups can often be shown using parentheses, such as (CH₂)n, to indicate a series of methylene (CH₂) units.

This representation makes it easier to visualize structures and is essential for efficiently communicating molecular structures in alkyne chemistry, where carbon-carbon triple bonds play a significant role.

Carbon-Carbon Triple Bond

An integral feature of alkynes is the carbon-carbon triple bond. Understanding this key structural element is crucial for distinguishing alkynes from other types of hydrocarbons like alkanes and alkenes. Here's why the triple bond is important:

• The triple bond consists of one sigma (σ) bond and two pi (π) bonds. These create a linear structure with 180° angles around the triple-bonded carbons.
• The presence of a triple bond significantly affects the physical properties of a compound, such as boiling and melting points, due to increased bond energy and rigidity.
• Alkynes are unsaturated hydrocarbons, meaning they have fewer hydrogen atoms than alkanes, which influences their reactivity and applications in chemical synthesis.
• The ability to form stable carbon-carbon triple bonds allows for more complex molecular structures and chemical reactions, making alkynes a valuable subset in organic chemistry.

Recognizing and handling the triple bond's unique chemical behavior is essential for anyone studying or working with alkyne compounds.

Naming Alkynes

When it comes to naming alkynes, there is a systematic approach to ensure that the names reflect the structure and timeline of discovery in a standardized way, as per IUPAC guidelines.

Some key tips for naming alkynes include:

• The root name of the alkyne is based on the longest carbon chain that contains the triple bond. For example, an eight-carbon chain with a triple bond is an "octyne."
• The position of the triple bond is indicated by the smallest possible number, counted from the end of the chain nearest to the triple bond. So "4-octyne" means the triple bond is between the fourth and fifth carbons.
• For multiple triple bonds, use prefixes like di-, tri-, etc., to indicate the number of triple bonds (e.g., "diacetylene" for two triple bonds).
• Substituents attached to the main chain are named and numbered based on their position and type, similar to other organic compounds.

These naming conventions help in accurately describing and understanding the structure of alkynes, ensuring effective communication in both educational and professional chemical environments.