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Chapter 2 : Alkanes |
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Alkanes
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Simplest organic molecules with only C and H atoms.
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Contain only C-C and C-H s bonds
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Commercially important as fuels and oils.
Nomenclature:
Functional group suffix = -ane (review)
Functional group prefix = alkyl- |
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e.g. CH3CH3 = ethane
e.g. CH3CH2- = ethyl |
Physical Properties:
The low polarity of all the bonds in alkanes means that the only intermolecular
forces between molecules of alkanes are the very weak induced dipole
- induced dipole forces. These forces are easily overcome.
As a result, in comparison with other functional groups, alkanes
tend to have low melting and boiling points and very low solubility
in polar solvents such as water (remember "oil and water don't mix" and
the adage "like dissolves like").
Structure:
Consist of only sp3 hybridised C and H atoms connected by
s
bonds.
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C-H s bond is formed by the interaction
of Csp3-H1s orbitals.
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C-C s bond is formed by the interaction
of Csp3-Csp3 orbitals.
Alkanes have a generic formula of CnH2n+2 (a relationship
that also defines the maximum number of hydrogen atoms that can
be present for a given number of C atoms).
Structures of the simple C1 to C4 alkanes are shown below in a variety
of representations.
As the number of C atoms increases then other isomeric
structures are possible.
Stability:
- More branched alkanes are more stable than linear alkanes, e.g. 2-methylpropane
is more stable than n-butane.
Reactivity:
- Since C and H atoms have very similar electronegativities, all the bonds
in alkanes (C-C and C-H) are non-polar.
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As a result, alkanes are not particularly reactive as functional
groups go.
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Infact, it is often convenient to regard the hydrocarbon framework of a
molecule as an unreactive support for the more reactive functional groups.
Want to try a few questions about alkanes ?
