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.
http://en.wikipedia.org/wiki/Enantiomer
http://en.wikipedia.org/wiki/Diastereomers