Ch 2: Alkanes

Chapter 2 : Alkanes

Alkanes

Simplest organic molecules with only C and H atoms.
Contain only C-C and C-H σ bonds
Commercially important as fuels and oils.

Nomenclature:

Summary:

Functional group suffix = -ane
Functional group prefix = alkyl-

e.g. CH₃CH₃ = ethane
e.g. CH₃CH₂- = ethyl

More details on alkane nomenclature?

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").

Intermolecular forces in alkanes

Structure:
Consist of only sp³ hybridised C and H atoms connected by Ïƒ bonds.

C-H σ bond is formed by the interaction of Csp³-H1s orbitals.
C-C σ bond is formed by the interaction of Csp³-Csp³ orbitals.

Alkanes have a generic formula of C_nH_2n+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.

methane CH₄ (b.pt. = -160^oC)
ethane C₂H₆ (b.pt. = -89^oC)
propane C₃H₈ (b.pt. = -42^oC)
butane C₄H₁₀ (b.pt. = -0.4^oC)

Stability:

More branched alkanes are more stable that linear alkanes, e.g. 2-methylpropane is more stable than n-butane.

Reactivity:

Since C and H atoms have very similar electronegativities, so all the bonds in alkanes (C-C and C-H) are non-polar.
As a result, alkanes are not particularly reactive as functional groups go.
Infact, it is often convenient to regard the hydrocarbon framework of a molecule as an unreactive support for the more reactive functional groups.

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