about van der Waal' force

1. Which of the following haloalkanes has the lowest boiling point?
A. 1-fluorohexane
B. 1-chlorohexane
C. 1-bromohexane
D. 1-iodohexane

The answer is: A. 1-fluorohexane

On boiling, the molecules in liquid change to be gaseous.Heat is absorbed to overcome the intermolecular forces. In other words, CH3(CH2)5X(l)is changed to CH3(CH2)5X(g), where X = F, Cl,Be or I.

Note that there is no breaking of covalent bonds. Therefore, it is not necessary to consider the strength of C-X bond. Instead, the strength of intermolecular forces must be considered.

In each of the four given compounds, the molecules are held together by van der Waals' forces (permanent dipole - permanent dipole attractions). The larger the molecular size, the stronger the van der Waals' forces between molecules. Since1-fluorohexane has the smallest molecular size among the four given compounds,1-fluorohexane has thus the lowest boiling point.


2. 同埋我想問permanent-dipolepermanent-dipole attration, instantaneous dipole-induced dipole, hydrogen bond 的大小次序???

For molecules with similar molecular size:
Strength: (hydrogen bonds) > (permanent dipole - permanent dipole attractions) > (instantaneous dipole - induced dipole attractions)

However, if the difference in molecular size is significant, it is hardly to compare the strengths of different types of intermolecular forces. This is because permanent dipole - permanent dipole attractions and instantaneous dipole - induced dipole attractions are affected by molecular size.


2011-07-13 03:34:51 補充：
同樣是 molecular substances, 比較 boiling point 的高低就是比較 intermolecular forces 的強弱。