Chapter 2 : Alkanes

The molecular formulae 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 recognise 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)

For 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)

© Dr. Ian Hunt, Department of Chemistry, University of Calgary