Why is 1,3-dimethylallene Optically Active?

https://www.youtube.com/watch?v=CTF32s7J7Ec

In this video we want to explain why is 1,3-dimethylallene optically active.

why 13 dimethylallene optically active structure of dimethylallene

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.

why 13 dimethylallene optically active hybridisation of allene carbons

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.

why 13 dimethylallene optically active direction of pi bonds and sp2 orbitals

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

why 13 dimethylallene optically active mirror images 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

Back to other previous Organic Chemistry Video Lessons.

Found this A Level Chemistry video useful?

Please LIKE this video and SHARE it with your friends!

Join my 10,000+ subscribers on my YouTube Channel for new A Level Chemistry video lessons every week.

Check out other A Level Chemistry Video Lessons here!

Need an experienced tutor to make Chemistry simpler for you?

Do consider signing up for my JC Chemistry Tuition classes at Bishan, weekly LIVE webinars or on-demand video lessons!