Question

Alkynes are hydrocarbons that contain a carbon-carbon triple bond. (a) Write structural formulas for all the isomeric alkynes of molecular formula \(\mathrm{C}_{5} \mathrm{H}_{8}\). (b) Are any of the alkynes chiral? (c) Are any of the alkynes stereoisomeric?

Short answer

(a) 1-pentyne, 2-pentyne. (b) No, there are no chiral alkynes. (c) No, there are no stereoisomeric alkynes.

Step-by-step solution

Identify the Triple Bond Location

The molecular formula for the alkynes is \(\mathrm{C}_5 \mathrm{H}_8\). First, note that a carbon-carbon triple bond must be present in the molecular structure. Positions of the triple bond can vary, leading to different isomers.

Draw the Straight Chain Alkynes

Start with the linear five-carbon structure and place the triple bond at varying positions. 1. Terminal alkyne: HC≡C-CH2-CH2-CH3 (1-pentyne) 2. Internal alkyne: CH3-C≡C-CH2-CH3 (2-pentyne) These represent the different locations for the triple bond along the chain.

Examine for Chiral Centers

A molecule is chiral if it has a carbon atom bonded to four different substituents. In these alkynes, 1-pentyne and 2-pentyne both lack any carbon atom bonded to four distinct groups due to the triple bond type and geometric arrangement. Therefore, no chiral centers are present.

Check for Stereo Isomerism

Stereoisomerism involves the 3D arrangement of atoms. Since alkynes are linear around the triple bond, they do not exhibit cis/trans (geometric) isomerism. Additionally, there are no other features like rings or double bonds that could support stereo isomers.

Key concepts

Structural Isomers

Structural isomers are molecules with the same molecular formula but different arrangements of atoms. This is like playing with building blocks to create unique shapes using the same pieces. For alkynes with the molecular formula \( \mathrm{C}_5 \mathrm{H}_8 \), we focus on rearranging the carbon and hydrogen atoms while keeping one triple bond intact. There are two main types of structural isomers for this alkyne: terminal and internal.

• Terminal Alkyne: The triple bond is at the end of the carbon chain. An example is 1-pentyne, represented as \( \mathrm{HC}\equiv \mathrm{C}-\mathrm{CH}_2-\mathrm{CH}_2-\mathrm{CH}_3 \).
• Internal Alkyne: The triple bond is within the carbon chain. An internal isomer is 2-pentyne, represented as \( \mathrm{CH}_3-\mathrm{C}\equiv\mathrm{C}-\mathrm{CH}_2-\mathrm{CH}_3 \).

Understanding structural isomers helps us predict the properties and reactions of these compounds, giving a window into their chemical behavior.

Chirality

Chirality is a property where molecules are non-superimposable on their mirror images, much like how your left hand is a mirror image but not identical to your right hand. For a molecule to be chiral, it must have at least one carbon atom bonded to four different substituents. Alkynes like 1-pentyne and 2-pentyne don't have such carbon atoms because the triple bond causes two carbon atoms to be bonded linearly to fewer groups—typically only one hydrogen and one group each. As a result, these alkynes lack the complexity required for a chiral center. Thus, they cannot show chirality under normal conditions.

Stereoisomerism

Stereoisomerism occurs when molecules have the same structural formula but differ in the spatial arrangement of atoms, leading to different physical and chemical properties. A familiar type of stereoisomerism is geometric isomerism, such as cis/trans isomers, often found in alkenes. However, stereoisomerism is limited in alkynes due to the linearity imposed by the carbon-carbon triple bond.

• This linear nature means that all atoms attached to the triple bond are in a straight line, eliminating the possibility of having different spatial arrangements.
• Moreover, in the simple structures of 1-pentyne and 2-pentyne, there are no double bonds or rings that could create steric differences among atoms that aren't directly involved in the triple bond.

As a result, alkynes such as these do not exhibit stereoisomerism associated with cis/trans variations.

Triple Bond

The triple bond is a defining feature of alkynes, a type of hydrocarbon characterized by having a carbon-carbon triple bond. This bond is highly unique due to its shape and strength.Triple bonds consist of one sigma (\( \sigma \)) bond and two pi (\( \pi \)) bonds, making the bond very strong and preventing rotation.

• Its high strength is due to the overlap of orbitals, leading to a significant amount of energy required to break it.
• The bond's linear geometry results in a 180° bond angle, giving the associated molecules a distinct line-like shape.

When comparing alkynes with alkenes or alkanes, the presence of the triple bond offers distinctive reactivity and stability, essential features in synthetic and organic chemistry.