What makes an allene optically active?

[topdent1]

This is driving me mad

---

Members don't see this ad.

 

[drgreen]

Optical activity is a result of chirality. Chirality is when one molecule is NOT superimposable with it's mirror image. If you have an allene with four different substituents, since there is no rotation about the double bonded carbons, the mirror image of itself is not superimposable, it's chiral.

 

[topdent1]

Optical activity is a result of chirality. Chirality is when one molecule is NOT superimposable with it's mirror image. If you have an allene with four different substituents, since there is no rotation about the double bonded carbons, the mirror image of itself is not superimposable, it's chiral.

But in order for a molecule to be chiral, doesn't it require 4 DIFFERENT substituents?

 

[Maygyver]

But in order for a molecule to be chiral, doesn't it require 4 DIFFERENT substituents?

I know there is already a thread about the chirality of allenes. In short, an allene can be cis or trans. Perhaps that will lead to optical activity? Not sure to be honest. Just a shot in the dark.

 

[mddang]

Okay, let's say we have an allene (1,2-propadiene). We'll designate the carbon at one end C1 and we'll designate the carbon at the other and C3.


For this allene to be chiral, the two substituents bonded to C1 cannot be identical to each other, and the two substituents bonded to C3 cannot be identical to each other.

You have to visualize the molecule to understand why it is chiral. If you have one of those little organic chem model kits, build it.


Examples of chiral allenes include..
1,3-dichloro-1,2-propadiene
1-chloro-3-methyl-1,2-propadiene
1,3-dichloro-1,3-dimethyl-1,2-propadiene

 

[Andrew324]

Like Allene on page 81 of destroyer question 52 and 59.

Which C is the chiral carbon? it seems like none of them are bonded to 4 different things

 

[Mstoothlady2012]

Like Allene on page 81 of destroyer question 52 and 59.

Which C is the chiral carbon? it seems like none of them are bonded to 4 different things

52 has to be inactive coz the first carbon has 2 hydrogen bonds, unless I have a different version of destroyer

59 a) is definitely chiral b/c 1st & 3rd carbons both have different substituents, making it chiral.

 

[Andrew324]

Yeah the explaination of 52 is what I was looking at really, the chiral example is the one in 59.

Which carbon has 4 different things bonded to it?

Starting from the left.
3 Hydrogens
1 hydrogen, 1 methyl, 2 to the same carbon
2 to one carbon, 2 to another carbon
2 to the carbon, 1 methyl, 1 ethyl
3 hydrogens, 1 carbon
1 carbon, 1 methyl, 2 hydrogens

if that makes sense

 

[Mstoothlady2012]

Yeah the explaination of 52 is what I was looking at really, the chiral example is the one in 59.

Which carbon has 4 different things bonded to it?

Starting from the left.
3 Hydrogens
1 hydrogen, 1 methyl, 2 to the same carbon
2 to one carbon, 2 to another carbon
2 to the carbon, 1 methyl, 1 ethyl
3 hydrogens, 1 carbon
1 carbon, 1 methyl, 2 hydrogens

if that makes sense

wow I have no idea what you are asking for? Are you talking about #59? You want to know why #59 is chiral or why #52 is not chiral?

 

[Andrew324]

why 59 is chiral

 

[Mstoothlady2012]

why 59 is chiral

Why 59 a) is chiral right? My molecule doesn't look like the way you explained before or may be I just didnt understand your list of description.

Do you see that the first carbon has 1 hydrogen and 1 methyl group and double bond? That's chiral. In order for an allene to be chiral, the substituents should be different. Now if this molecule had 2 hydrogens attached to that first carbon, it wouldn't be chiral. Same thing for the third carbon as well. Both of the substituents connected to the third carbon are different, making it chiral as well.

So 1st and 3rd carbons are chiral, making the molecular chiral. Hope you get it now...

 

[Andrew324]

A double bonded carbon being chiral...? Pretty strange

I found it on wiki

"It is also possible for a molecule to be chiral without having actual point chirality. Common examples include 1,1'-bi-2-naphthol (BINOL) and 1,3-dichloro-allene, which have axial chirality, and (E)-cyclooctene, which has planar chirality."

An axis about which a set of ligands is held so that it results in a spatial arrangement which is not superposable on its mirror image. For example with an allene abC=C=Ccd the chiral axis is defined by the C=C=C bonds; and with an ortho-substituted biphenyl the atoms C-1, C-1', C-4 and C-4' lie on the chiral axis.

 

[Mstoothlady2012]

lol i thought you were aware of this concept that double bonded molecules could be chiral. I thought you were just not understanding why 59 is chiral. Anyways yea double bonded molecules can be chiral as long has the 2 substituents attached to the carbon are DIFFERENT from each other!! mddang did a good job explaining, read his explanation in this thread.

 

[niktacal82]

but i thought double bonds are achiral even they have 3 different( it can't be 4) subs attached to them; but allene(c=c=c) is not just a double bond as dr green said it's mirror image is not superimposable.
Can u even use chirality for all double bonds such as -c=c-?