Why is 1,3-dimethylallene Optically Active?
In this video created by Chemistry Guru, Singapore's leading JC Chemistry tuition centre, we want to explain why is 1,3-dimethylallene optically active.
Allene functional group is where we have 3 carbons double bonded to each other, ie C=C=C.
Note that this functional group is not in A Level Chemistry syllabus so we do not need to know the reactions of allene.
In syllabus we learn that an sp3 hybridised carbon bonded to 4 different groups will be chiral.
1,3-dimethylallene seems non-chiral since it does not have an sp3 hybridised carbon and it is only bonded to 2 different groups (hydrogen and methyl groups).
There are 2 main ideas we need to discuss to understand why 1,3-dimethylallene is optically active:
1. 1,3-dimethylallene is not planar
2. Mirror images of 1,3-dimethylallene are non-superimposable
1. 1,3-dimethylallene is not planar
An allene looks similar to alkene so we might think that allene is planar as well.
But let us consider the state of hybridisation of the 3 carbons in allene.
The centre carbon is sp hybridised while the 2 carbons at the sides are sp2 hybridised.
Shape with respect to the centre sp carbon will be linear, and to the side sp2 carbons will be trigonal planar.
The centre carbon uses its 2py orbital to form a first pi bond with the left carbon.
Since the first pi bond is in y-direction, the sp2 orbitals for the left carbon will be on the xz plane (as shown in pink box).
The centre carbon uses its 2pz orbital to form another pi bond with the right carbon.
Since this second pi bond is in z-direction, the sp2 orbitals for the right carbon will be on the xy plane (as shown in blue box).
This means that the groups bonded to the two sp2 carbons are lying along different planes and hence allene functional group cannot be planar.
2. Mirror images of 1,3-dimethylallene are non-superimposable
The best way to show that the mirror images are non-superimposable is to use the molecular models to visualise.
In the video we can show that the mirror images cannot overlap exactly, hence they are non-superimposable.
This means that the mirror images of 1,3-dimethylallene are optical isomers and hence they are optically active.
Topic: Intro to Organic Chem, Organic Chemistry, A Level Chemistry, Singapore
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