Draw a six-carbon alkyne that can exist as diastereomers. You do not need to specify wedge or dashed bonds.
The correct answer and explanation is :
A six-carbon alkyne that can exist as diastereomers is 3-hexyne with two different substituents on its internal carbon atoms. The correct structural formula is:
CH₃-CH(Br)-C≡C-CH(Cl)-CH₃
(3-Bromo-4-chloro-3-hexyne)
Explanation (300 words):
Alkynes contain at least one carbon-carbon triple bond (C≡C). Unlike alkenes, which can show cis-trans (E/Z) isomerism due to restricted rotation around the double bond, simple alkynes do not exhibit this type of stereoisomerism because a triple bond is linear (sp-hybridized carbons form a 180° bond angle). However, alkynes can still form diastereomers if additional chiral centers exist in the molecule.
To create a six-carbon alkyne that has diastereomers, we need:
- A triple bond at an internal position (not terminal).
- Two different chiral centers elsewhere in the molecule.
Structure of 3-Bromo-4-chloro-3-hexyne:
- The molecule contains a C≡C bond between C3 and C4.
- Carbon C3 is attached to a Br (bromine) and a methyl (-CH₃) group.
- Carbon C4 is attached to a Cl (chlorine) and an ethyl (-CH₂CH₃) group.
- Since C3 and C4 are both chiral centers (each bonded to four different groups), they can have (R) or (S) configurations, leading to diastereomers.
Why Are They Diastereomers?
- Diastereomers are stereoisomers that are not mirror images of each other.
- Unlike enantiomers, which come in non-superimposable pairs, diastereomers have different physical and chemical properties.
- In our molecule, changing the configuration at one chiral center but not both creates a diastereomeric relationship.
Thus, 3-Bromo-4-chloro-3-hexyne meets all criteria for being a six-carbon alkyne that can exist as diastereomers.