Advices For Studying Organic Chemistry

alkanes are gases; the C 5 -C 17 unbranched alkanes are liquids; the unbranched

alkanes with 18 or more carbon atoms are solids.

4.7A BOILING POINTS

1. The boiling points of the unbranched alkanes show a regular increase with
   increasing molecular weight.

Figure 4.3 Boiling points of unbranched alkanes (in red) and cycloalkanes (in
white).

2. Branching of the alkane chain lowers the boiling point (Table 4.2).
   
   
   1. Boiling points of C 6 H 14 : hexane (68.7 °C); 2-methylpentane (60.3 °C);
      3-methylpentane (63.3 °C); 2,3-dimethylbutane (58 °C); 2,2-dimethylbutane
      (49.7 °C).
   
   
   
   


3. As the molecular weight of unbranched alkanes increases, so too does the
   molecular size, and even more importantly molecular surface areas.
   
   
   1. Increasing surface area ⇒ increasing the van der Waals forces between
      molecules ⇒ more energy (a higher temperature) is required to separate
      molecules from one another and produce boiling.
   
   
   
   


4. Chain branching makes a molecule more compact, reducing the surface area