then how do we deduce if any given molecule is planar anot ?
unless we can somehow deduce, or have the chemical knowledge ,
by just looking at the non 3d structure of various molecules such
as allenes and annulenes and cycloalkanes , they have to conform to
some other conformations due to steric strain/angle strain/ring
strain/vdw repulsions/torsional strain
By looking at the entire structure case-by-case and applying your
knowledge of different aspects of chemistry in combination.
You seem to be whining for a super simple easy way to tell if a
molecule is planar or not. Such simple easy ways are for Ordinary O
Level Chemistry, not Advanced A Level Chemistry.
Of course, Cambridge will use an appropriate range of difficulty
in their questions. They may ask on super easy structures, or
average difficulty intermediate structures, or slightly beyond A
levels difficult structures (for A grade questions, only a few of
such questions will appear in every paper, you shouldn't worry
about such questions, it's a bell-curve, just do your best, and see
if you're better than the rest).
