Simplest organic molecules with only C and H atoms. Commercially important as fuels and oils.

Nomenclature:
Functional group suffix = -ane (review)
Functional group prefix = alkyl-

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, which are easily overcome.  As a result, compared to 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:
Alkanes are the simplest organic compounds, comprised of only sp³ hybridized C and H atoms connected by s bonds.
They 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. = -160oC)
Ethane C2H6 (b.pt. = -89oC)
Propane C3H8 (b.pt. = -42oC)
Butane C4H10 (b.pt. = -0.4oC)

 

Isomeric Alkanes:
The molecular formula for the C1 to C3 alkanes lead to single, unique structures.
However for C₄H₁₀, there are two possible constitutional isomers. It is important to be able to recognize isomers because there can have different chemical, physical properties and biological properties. The constitutional isomers of C₄H₁₀ are shown below along with some properties:
 

n-butane		m.pt.= -139oC  b.pt. = -0.4oC  DHf = -125.6 kJ/mol           (-30.0 kcal/mol) DHc = -2877 kJ/mol           (-687 kcal/mol)
isobutane		m.pt.= -161oC  b.pt. = -10.2oC  DHf = -135.6 kJ/mol           (-32.4 kcal/mol) DHc =-2868 kJ/mol           (-685 kcal/mol)

C₅H₁₂ has three possible constitutional isomers:
 

n-pentane		b.pt. =   36.1 oC DHf = -147 kJ/mol           (-35.1 kcal/mol) DHc = - 3509 kJ/mol           (- 839 kcal/mol)
isopentane		b.pt. =    30 oC DHf = -154.1 kJ/mol           (-36.8 kcal/mol)  DHc = -3502 kJ/mol           (-837 kcal/mol)
neopentane		b.pt. =   9.5 oC DHf = -168.0 kJ/mol           (-40.1 kcal/mol)  DHc = -3493 kJ/mol           (- 835 kcal/mol)

Stability:

• Branched alkanes are more stable than linear alkanes, e.g. 2-methylpropane is more stable than n-butane.

• 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.
• In fact, 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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