[beheada]

So I need a helpful push in the right direction. I know that a chiral molecule is a molecule whose mirror image when superimposed is not the same, but I'm having problems visualizing molecules like 2,5-dimethylheptane and determining whether they are chiral. I have even built a molecular model of the thing, but still it's hard to determine whether or not the WHOLE thing is chiral just by looking at it, since it has two chirality centers. Any helpful hints?

[AWK]

Just draw its structure. Only C5 is chiral!

[beheada]

I don't get why C2 isn't chiral? I have C2 and C5 being chiral... but I'm attempting to figure out if the ENTIRE molecules are chiral. For example, when I was looking at the molecule I built for 2,5-dimethylheptane, I could find a single plane of symmetry, so I just assumed the whole molecule was chiral. But it seems to me, if C5 in this molecule is chiral, then C2 should be as well.

[Dan]

no, C2 is not chiral.
List the four groups on C2, you will see why.

Draw out/make models of both enantiomes. Can you superimpose them?

[AWK]

I don't get why C2 isn't chiral? I have C2 and C5 being chiral... but I'm attempting to figure out if the ENTIRE molecules are chiral. For example, when I was looking at the molecule I built for 2,5-dimethylheptane, I could find a single plane of symmetry, so I just assumed the whole molecule was chiral. But it seems to me, if C5 in this molecule is chiral, then C2 should be as well.

Chiral moleculec cannot show a plane of symmetry!

[beheada]

AWK - Sorry, that was a typo. I meant I couldn't find a plane of symmetry so I figured the entire molecule was chiral.

Dan - I get what you're saying, but I don't understand where to stop drawing the sub groups when using C2 as a chirality center. I mean, if you have to do a mirror image of it, where do you stop, at just the first carbon attached?

For example, in the 2,5-dimethylheptane example, you've got two methyl groups, a hydrogen, and a CH2 group attached to the rest of the chain. How exactly do you draw that? Do you cut off the C4 and just draw it as the quaternary carbon with only the four directly attached carbons?

rayfe

[Dan]

For example, in the 2,5-dimethylheptane example, you've got two methyl groups, a hydrogen, and a CH2 group attached to the rest of the chain. How exactly do you draw that?

You have two methyl groups on C2, so C2 isn't a chiral centre.

There is only one chiral centre, C5, so draw one stereoisomer of the whole molecule, then draw the mirror image. Are the original drawing and the mirror image superimposable?

I have attatched a pic of the molecule with undefined stereochemistry at the chiral centre, C5

[beheada]

They're not superimposable. But could you elucidate why having two methyl groups means that C2 is achiral?

[Yggdrasil]

There seems to be a problem with terminology here.  Maybe I can clear this up.

Chiral is a term which applies to molecules, not individual atoms.  An molecule is chiral if it cannot be superposed with its mirror image (reflection).  A chiral molecule and its mirror image are known as stereoisomers (note: this is not the most general definition of a stereoisomer).

Stereocenter (aka chiral center) is a term which applies to an atom or group of atoms in a molecule.  A group of atoms can be considered a stereocenter if changing the position of any two groups attached to the stereocenter generates a stereoisomer.  For an sp³ hybridized carbon, this rule simplifies to any carbon bearing four distinct chemical groups.

So, to find out whether a molecule is chiral, you compare the molecule to it's mirror image.  To find out which carbon(s) in the molecule are stereocenters, you look to see if the carbon has four distinct chemical groups attached.  You cannot check to see whether a carbon is a stereocenter by drawing the mirror image; this is a test for chial molecules, not stereocenters.

Now, 2,5-dimethylheptane is chiral.  This means that the molecule must posses one or more stereocenters.  C5 is a stereocenter because it has four distinct chemical groups attached to it (execrise for you: what are they?), but C2 is not a stereocenter because it has only three distinct groups attached (exercise for you: what are they?).

[beheada]

Now, 2,5-dimethylheptane is chiral.  This means that the molecule must posses one or more stereocenters.  C5 is a stereocenter because it has four distinct chemical groups attached to it (execrise for you: what are they?), but C2 is not a stereocenter because it has only three distinct groups attached (exercise for you: what are they?).

From what you have said, 2,5-dimethylheptane is a chiral molecule because C5 is a stereocenter because it has four distinct groups, being a methyl, ethyl, hydrogen, and the continuation of the carbon chain. It is a chirality center (stereocenter, etc) because it has four DIFFERENT substituted groups, as opposed to carbon₂, which has three DIFFERENT distinct groups, being a hydrogen, two methyls (same), and the continuation of the carbon chain. So I'm assuming the differentiation comes from having distinct (meaning DIFFERENT) groups on each of the four substituted spots. Correct?

[Borek]

having distinct (meaning DIFFERENT) groups on each of the four substituted spots. Correct?

Isn't it just a definition of "chiral"?

[movies]

Stereocenter (aka chiral center) is a term which applies to an atom or group of atoms in a molecule.  A group of atoms can be considered a stereocenter if changing the position of any two groups attached to the stereocenter generates a stereoisomer.  For an sp³ hybridized carbon, this rule simplifies to any carbon bearing four distinct chemical groups.

Not all stereocenters are chiral centers!  For a carbon with four different substituents, I prefer the term "asymmetric carbon."  Stereocenters are any atom where switch the position of two groups would give a different stereoisomer (a molecule with the same atom-atom connectivity, but a different arrangement of those atoms in space).  Consider the carbons that make up the alkene in 2-butene: if you were to swap the two substituents at C2 (i.e., the H and the Me) you would convert from the E stereoisomer to the Z isomer or vice versa.  Therefore, C2 is a stereocenter but not an asymmetric carbon.

I hope this clarifies the point and doesn't add more confusion!!

[Yggdrasil]

From what you have said, 2,5-dimethylheptane is a chiral molecule because C5 is a stereocenter because it has four distinct groups, being a methyl, ethyl, hydrogen, and the continuation of the carbon chain. It is a chirality center (stereocenter, etc) because it has four DIFFERENT substituted groups, as opposed to carbon₂, which has three DIFFERENT distinct groups, being a hydrogen, two methyls (same), and the continuation of the carbon chain. So I'm assuming the differentiation comes from having distinct (meaning DIFFERENT) groups on each of the four substituted spots. Correct?

An asymmetric carbon will have four different groups on the four substituted spots.  When I said distinct in my previous post I meant different.

Isn't it just a definition of "chiral"?

Chiral is a term which labels only molecules.  Part of the confusion from the thread came from labeling atoms as chiral (vs. labeling them as chiral centers or asymmetric carbons).

[tbuihuu]

I think, we need distinguish between asymmetric carbon and asymmetric molecular. if that is asymmetric carbon then four substituted spots of C need different, but asymmetric molecular isn't. Chiral centre could be asymmetric carbon or not (ex.asymmetric molecular have chiral centre but haven't asymmetric carbon)

[movies]

As stated above, the presence of an asymmetric carbon is not a guarantee that the molecule is chiral (an example is a meso compound like (2S,3R)-butane-2,3-diol, two asymmetric carbons, but achiral because of a mirror plane).  Also, the absense of an asymmetric carbon is not a guarantee that the molecule is achiral, because you can have other elements like an axis of chirality (an example is a chiral allene).

As others have said, the true test of whether or not a molecule is chiral is to draw its mirror image and superimpose the two.