Download Hydrocarbons

Hydrocarbons
Structure and Properties
Hydrocarbons
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Alkanes
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
single C-C bonds
if all C’s have H’s attached molecules are called
saturated hydrocarbons

Alkenes- have one or more C=C double bonds

Alkynes- have one or more CΞC triple bonds

alkenes & alkynes are called unsaturated
hydrocarbons because they have fewer than
the maximum possible number of H atoms
Hydrocarbons


can form either a straight chain or a cyclic (ring)
structure
hydrocarbons which are attached to the main
structure are called alkyl groups and are named
according to the number of carbons
Hydrocarbons


a 4th group are the
aromatic
hydrocarbons which
have a unique ring
structure
simplest is benzene
and all others are
derivatives of
benzene
Alkanes
Physical Properties
of Alkanes
Solubility
Alkanes are insoluble in water
This is because water molecules are polar,
whereas alkanes are non-polar (all the C - C
and C - H bonds are nearly purely covalent)
The O - H bond in a water molecule is highly
polar due to the high electronegativity of oxygen
δ-
H
δ+
O
H
Solubility
The hydrogen atoms
in one water
molecule are
strongly attracted to
the oxygen atoms in
other water
molecules, and the
small size of the H
atoms allows the
molecules to
approach each
other very closely.
Solubility
The mutual insolubility of alkanes and water is
used to advantage by many plants.
Alkanes often make up part of the protective
coating on leaves and fruits (C27H56 and
C29H60).
Similar hydrocarbons are found in beeswax
The major function of plant waxes is to prevent
water loss from the leaves and fruit.
Boiling Points
Alkanes have lower boiling points than most
other organic molecules with a similar
weight.
This is due again to alkanes being non-polar.
Alkane molecules are only held together by weak
intermolecular forces called van der Waals
attractions (London forces).
Electrons are always moving and sometimes for a
instant they can be unevenly distributed in a
molecule and cause temporary dipole (positive
and negative end).
Boiling Points
These attractions are
weak, so the process
of separating the
molecules (boiling)
from one another
requires little energy.
Boiling Points
Since these attractive forces can only
operate over short distances
between the surface of molecules:
The boiling points of alkanes rise
as the chain length increases and
fall as the chains become
branched.
Boiling Points
Note: In a homologous series there is a
gradual increase in boiling point as the
number of carbon atoms increases, for the
very same reason.
Organic compounds in the a
homologous series vary by -CH2