Draw a six-carbon alkyne that can exist as diastereomers. You do not need to specify wedge or dashed bonds. Select Draw Rings More Erase / MICH 5 3 2 o
The Correct Answer and Explanation is:
Correct Answer:
3-Hexyne with two different substituents on the internal carbons – specifically 3-hexyn-2,4-diol
(Skeletal formula):
HO–CH₂–C≡C–CH(OH)–CH₃
This compound has a triple bond between carbons 2 and 3, and chiral centers at carbons 2 and 4 due to the hydroxyl (–OH) groups.
Explanation (300+ words):
To determine a six-carbon alkyne that can exist as diastereomers, we must understand both structural requirements:
- Alkyne means the molecule must contain at least one carbon–carbon triple bond (C≡C).
- Diastereomers are stereoisomers that are not mirror images (unlike enantiomers), which implies the molecule must have two or more stereocenters (chiral centers).
Let’s explore the molecular design needed:
- A six-carbon alkyne can be linear like hex-1-yne, hex-2-yne, or hex-3-yne. However, none of these basic hydrocarbons have stereocenters, so they cannot form diastereomers on their own.
- To form diastereomers, the molecule must contain at least two chiral centers.
- A triple bond does not allow cis-trans (E/Z) isomerism due to its linear geometry (180° bond angle), so that kind of geometric isomerism is excluded.
The best way to introduce chirality is to substitute groups on carbons near the triple bond in such a way that two or more carbons become chiral.
Let’s take 3-hexyne and add hydroxyl groups at carbons 2 and 4, yielding 3-hexyn-2,4-diol:
Structure:
HO–CH₂–CH(OH)–C≡C–CH(OH)–CH₃
- Carbon 2 and Carbon 4 are both attached to four different groups, making them chiral centers.
- With two chiral centers, this molecule can form up to four stereoisomers: a pair of enantiomers and two diastereomeric pairs.
Because diastereomers are non-mirror image stereoisomers, at least two of these will be diastereomers to each other.
Thus, 3-hexyn-2,4-diol is a valid six-carbon alkyne that can exist as diastereomers.