Destroyer Q 79 (2009 edition)

[BiomajorPreDent]

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It says that allene's are optically active.

I looked it up in my text and it says Allene's are chiral even though they do not have asymmetric centers.

Ok, I get that, but does this apply to ALL allenes? or only certain ones??

 

[UCB05]

It says that allene's are optically active.

I looked it up in my text and it says Allene's are chiral even though they do not have asymmetric centers.

Ok, I get that, but does this apply to ALL allenes? or only certain ones??

The two substituents on one C must not be identical, and the two substituents on the other C must not be identical. You can, however have two of the same substituents, but on either C.

e.g.

H,Me-C=C=C-Et,Me is optically active
Me,Me-C=C=C-Et,H is not

 

[BiomajorPreDent]

great, thanks alot

 

[Shinpe]

The two substituents on one C must not be identical, and the two substituents on the other C must not be identical. You can, however have two of the same substituents, but on either C.

e.g.

H,Me-C=C=C-Et,Me is optically active
Me,Me-C=C=C-Et,H is not

It also seems to me that if the substituents on 2 sides are the same, but each side has 2. It's still chiral. Can you confirm?

Like
HMe-C=C=C-HMe

 

[UCB05]

It also seems to me that if the substituents on 2 sides are the same, but each side has 2. It's still chiral. Can you confirm?

Like
HMe-C=C=C-HMe

Not sure, but I would guess so. Changing the connectivity of one pair of substituents gives a non-superimposable diastereomer of the original

 

[sixkiller]

It also seems to me that if the substituents on 2 sides are the same, but each side has 2. It's still chiral. Can you confirm?

Like
HMe-C=C=C-HMe

Designate left side as R1 and R2

Designate the right side as R3 and R4

Draw the mirror image....you will see that it is non-super impossable

A non-superimpossable mirror image is an enantiomer....it has optical activity.

For allenes, as long as R1 is not equal to R2 and R3 is not equal to R4 it is optically active; thus has an enantiomer.

 

[Shinpe]

Designate left side as R1 and R2

Designate the right side as R3 and R4

Draw the mirror image....you will see that it is non-super impossable

A non-superimpossable mirror image is an enantiomer....it has optical activity.

For allenes, as long as R1 is not equal to R2 and R3 is not equal to R4 it is optically active; thus has an enantiomer.

Yup that's what I thought, Just checking. Thanks

 

[BiomajorPreDent]

Designate left side as R1 and R2

Designate the right side as R3 and R4

Draw the mirror image....you will see that it is non-super impossable

A non-superimpossable mirror image is an enantiomer....it has optical activity.

For allenes, as long as R1 is not equal to R2 and R3 is not equal to R4 it is optically active; thus has an enantiomer.

Thanks all! 😀