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Classes of Alkanes
The alkanes are hydrocarbons (molecules
that contain only hydrogen
and carbon), those containing only single bonds, such as methane, ethane,
and propane,
Alcanes are classified into several
types according to structure: straight-chain alkanes, branched alkanes
and cycloalkanes.
Nomenclature of Alkanes
The number of carbon
atoms in the chain is specified by a Latin or Greek prefix (meth, eth, prop, but,
pent, hex, hept, oct, non, dec) preceding the suffix -ane,
which identifies the compound as a member of the alkane family
(meth-ane,
eth-ane, prop-ane, but-ane,
pent-ane, hex-ane,
hept-ane, oct-ane).
Physical Properties of Alkanes
The boiling points of
unbranched alkanes increase with the number of carbon atoms.. Molecules
with relatively large surface areas are subject to greater London attractions.
Branched alkanes have
lower boiling points than their unbranched isomers. Branched alkanes have
smaller surface areas than do their unbranched isomers. As a result, they
are generally subject to smaller London attractions and are unable to pack as
well in the crystalline state
Conformational
Analysis
Conformations: the
temporary molecular shapes that result from rotations of groups about single
bonds.
Conformational analysis: the analysis of the energy changes
that a molecule undergoes as groups rotate about single bonds.
Newman Projections
In a Newman projection
, we sight down the C - C bond, and represent
the front carbon by a point and the back carbon by a circle. Each carbon has
three substituents that are placed symmetrically around it..
Potential Energy Diagram
The differences in
potential energy between conformations can be pictured by plotting the energy
changes against the degree of rotation.
Of the two
conformations of ethane, the staggered is more stable than the eclipsed. The
measured difference in potential energy between them is 12 kJ/mol (2.9 kcal/mol).
Combustion of
Alkanes
The alkanes combination with oxygen is known
as combustion and is quite exothermic. All hydrocarbons yield carbon dioxide and
water as the products of their combustion.
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