I correctly identified the carbon with the red square above it as a chiral centre.
My question is why the one with the GREEN square is NOT a chiral centre ?
Originally posted by Prozen1:I correctly identified the carbon with the red square above it as a chiral centre.
My question is why the one with the GREEN square is NOT a chiral centre ?
Because that carbon is sp2 (trigonal planar) hybridized. Chiral carbons must be sp3 (tetrahedral) hybridized.
Because that carbon has only 3 bonds
Originally posted by SBS2601D:Because that carbon has only 3 bonds
How ? There are four. One to the oxygen, one to the carbon below it, one to the carbon above it and the last to a hydrogen.
Its a benzene ring.
Look it up.
Its not the same as cyclohexane.
Originally posted by SBS2601D:Because that carbon has only 3 bonds
Actually that C atom does indeed have 4 bonds (whether you're considering the resonance contributor or hybrid). In the resonance contributor, it is doubly bonded to 1 C atom, singly bonded to another C atom, and singly bonded to an O atom. 2 + 1 + 1 = 4 bonds. In the resonance hybrid, it has 1.5 bonds to one C atom, 1.5 bonds to the other C atom, and 1 bond with the O atom. 1.5 + 1.5 + 1 = 4 bonds.
What you mean to say, is that the C atom is only bonded to 3 different groups, and thus cannot be a chiral atom / chiral carbon / chiral center / chirality center / asymmetric center / stereogenic center / stereocenter.
Although atoms of other elements may also be chiral (ie. serve as chiral centers for the molecule), but for the H2 syllabus, only chiral carbons will be tested.
And bottomline is, for a carbon to be chiral, it *must* have tetradehdral electron geometry and therefore be sp3 hybridized, *and* be bonded to 4 different groups.
Meso compounds and diastereomers (extension of H2 syllabus)
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Note however, just because a carbon atom may be chiral, doesn't necessarily mean the entire molecule is chiral (ie. optically active). If there is an internal plane of symmetry in the molecule (that contains one or more chiral carbons), then the molecule's mirror image is indeed superimposable upon itself, and therefore the molecule is achiral (ir. optically inactive), despite containing one or more chiral carbons.
Such a molecule (being optically inactive due to the existence of an internal plane of symmetry despite having one or more chiral carbons) is called a meso compound.
Here's a good example which will help students understand meso compounds better :
http://en.wikipedia.org/wiki/Lactide
Note : the R,S classification system (as seen in the image of the lactide optical isomers in the wikipedia page above) for optical isomers is distinct from the (+),(-) system. Note that for some compounds, the (+) isomer might be the R isomer, but for other compounds, the (+) isomer might be the S isomer. For more info, see :
http://en.wikipedia.org/wiki/Chirality_(chemistry)
Stereoisomers are divided into geometric isomers (cis/trans and E/Z) and optical isomers (enantiomers, diastereomers, meso compounds) and conformers (chair, boat, etc).
Although the H2 syllabus only tests on cis/trans (for geometric isomerism) and enantiomers (for optical isomerism), but H2 students who intend to score a distinction grade are encouraged to self-learn about E/Z geometric isomerism, diastereomers and meso compounds, to have a more complete understanding about stereosiomerism.
Diastereomers are optical isomers which are not enantiomers of each other. This exists when there are more than one chiral carbon present in the molecule. For instance, if a molecule has two chiral carbons, and the configuration about both chiral carbons are R, then it's enantiomer (ie. non-superimposable mirror image) would have both chiral carbons with S configuration. Another optical isomer of this compound, might have one chiral carbon with R configuration, and the other chiral carbon with S configuration. Such an optical isomer, would be a diastereomer of our original R,R or S,S optical isomer.