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Chapter 2: Alkanes



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Alkanes

Chapter 2: Alkanes

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").
 
 




Intermolecular forces in alkanes

 






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 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
 

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.= -139o
b.pt. = -0.4o
DHf = -125.6 kJ/mol

          (-30.0 kcal/mol)
DHc = -2877 kJ/mol

          (-687 kcal/mol)
isobutane
m.pt.= -161o
b.pt. = -10.2o
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:

Reactivity:


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