alkenes chiral?

[Questions]

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Can alkenes be chiral?

 

[Lou42]

I was just studying this last night. From what I gather alkenes always start of as achiral. Example from my book...

When 1-Butene which is achiral is reacted with HBR it forms a racemic chiral product of (+/-) 2-Bromobutane.

It would be great if someone could explain this in greater detail?

 

[Moabmad]

The definition of chiral is that the mirror image is not superimposable on the original.

The reason alkenes end up as a racemic mixture is because they are trigonal planar. The key being planar, both faces are open for attack with no prejudice either way. Therefore they are attacked on both faces equally ending up with a 50% R and 50% S mixture.

 

[Geekchick921]

Chiral carbons are always sp3 hybridized, so carbons that are part of a C=C bond cannot be chiral. There could be OTHER chiral carbons in that same molecule, though, which would make the molecule itself chiral. Also, as, as the PP said, the carbons that are part of the double bond can react and become chiral carbons (depending on if all the attached groups are different).

 

[UCB05]

the alkene functional center is achiral. If you look at a simple bromoethene, you should be able to see that it is superimposable over its mirror image. All it takes is a little rotation.

The only exception is an allene, R2C=C=CR2. in this case, one double bond lies 90 degrees from the other, which puts the R group on one end at a plane perpendicular to the R groups at the other end.

 

[TexasOMFS]

the alkene functional center is achiral. If you look at a simple bromoethene, you should be able to see that it is superimposable over its mirror image. All it takes is a little rotation.

The only exception is an allene, R2C=C=CR2. in this case, one double bond lies 90 degrees from the other, which puts the R group on one end at a plane perpendicular to the R groups at the other end.

^This is correct. I'd just like to add that the R groups on each carbon have to be different from each other in order for the molecule to be chiral. If the R groups are the same on either end, there is an internal plane of symmetry.

Also, another chiral molecule that has no chiral centers is biphenyl (with substituents in the two ortho positions). A benzene singly bonded to another benzene can rotate around the bond, but if there are other substituents, you can stop free rotation and obtain a configuration similar to allene (w/the two benzenes perpendicular to each other). Again, the ortho substituents on either side (either benzene) have to be different from each other or else there is an internal plane of symmetry.

 

[Questions]

so how can an allene be chiral? aren't all the Carbons sp2?

 

[sketo43]

so how can an allene be chiral? aren't all the Carbons sp2?

Normally for something to be chiral, it has to be sp3. Allenes are an exception. Since R2C=C=CR2 has a fixed configuration, it cannot rotate. It's hard to explain, but if you build it with a molecular model kit, and then build its mirror image, you will see.

 

[Polarmolardmd]

Alkenes are not chiral due to the fact that they are SP2.
alkanes on the other hands could be chiral if there are four different subs attached to it and of course they are SP3.