CHEM10006 Study Guide - Final Guide: Dipole, Sigma Bond, Hydrogen Bond
12 Jun 2018
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Department
Course
Professor
orbitals approach
Hybridization: Mixing atomic orbitals into new hybrid orbitals
(with different energies, shapes, etc., than the component
atomic orbitals) suitable for the pairing of electrons to form
chemical bonds; asking for geometry is the areas of electron
density, asking for shape is the shape not including lone pairs
as an area
SP Hybridization: linear
SP
3
Hybridization: tetrahedral
Radicals: Odd electron
species: contain single,
unpaired electrons, often
very reactive e.g. NO, OH,
NO
2
, contain an odd number
of electrons
Pi bonds: Hold two electrons above & below the plane of the
molecule; P orbital overlap effectively locks the atoms in the
plane so rotation about the C-C bond is not possible:
geometric isomerism in alkenes
Triple bond: One sigma bond & two pi bonds which are
perpendicular to each other
Skeletal structures:
Intermolecular forces:
Dipole-dipole forces: Arise
between polar molecules
due to electrostatic
interactions between
dipoles; approximately
2-3kJ per mole (only 1% the
strength of a covalent bond)
Increase in strength as the
number of electrons in a
molecule increases as there
is greater opportunity for
instantaneous dipoles to
arise, therefore boiling point
and melting point of the
straight chain alkanes
increases with the length of
London dispersion forces:
Weak forces that exist
between all molecules
whether polar or not;
strength of 1-2 kJ per mol;
arise due to random
motions of electrons which
lead to momentary,
non-symmetric distributions
of electron density, creating
instantaneous dipoles which
then induce a dipole in a
neighbour; weak and
short-lived;
Benzene: C
6
H
6
: all C-C bond lengths are equal, each SP2
hybridized, exists as a resonance hybrid, six pi electrons are
completely delocalized around the ring: does not behave like a
typical alkene: does not undergo bromine addition, will undergo
substitution with a catals
Aromatic: Organic compounds containing a planar unsaturated
ring of atoms which is stabilized by an interaction of the bonds
forming the ring, e.g. benzene and its derivatives
Requirements for aromaticity: Cyclic, conjugated (alternating
double & single bonds), planar, satisfies Huckel rule
The Huckel rule: 4n + 2 pi electrons exist in a an aromatic,
where n is an integer (cannot be e.g. 1 ½ )
Water: A random 3D network with a local preference for
tetrahedral geometry but a large number of strained or broken
hydrogen bonds (liquid)
●Each molecule can form hydrogen bonds with four
neighbours (tetrahedral)
●Ice has cavities so a lower density (floats on liquid
water)
●Melting of ice: some hydrogen bonds breaking
allowing H
2
O molecules to pack closer together
●High specific heat capacity & heat of vaporization:
stable medium: when energy is added it causes
disruption of hydrogen bonds rather than an increase
in temperature
