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SCAN0042 - /— p Dipole-Dipole Interactions H J C 5-3'0...

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Unformatted text preview: /—\ p: Dipole-Dipole Interactions H § J C 5-3 '0 . K, w-e' ° / l \a The molecules must he polar, have a dipole moment. H H H H /Cl l-l \ ‘3 . Exam les are CH F, co, PHI. so , H2CO.CH Cl C “O / \ H ’ , ‘01 P 3 3, 2 . 2 2 H / __) J \O “F” The positive end of a molecule is attracted to the negative end of the other. These molecules align themselves to maximize these interactions. The greater the attraction between molecules in a liquid, greater is the energy needed to separate them. So, high. boiling point liquids have high intermolecular attractions. Thus, low boiling point liquids have low hitermolecular attractions. 31m; s5“ 3*; $3 3 Wetter“? .wwWW‘N-«c L“ ' N v w ../’/‘r i For example A5113 is polar with a Bl? of ~62°C (MW =_7§_ g/mole) whereas Geri-4 {MW =21 g/rnole) is nonp lar with a BP of —90°C. V \l, i ,i < «i lordshitemfi Mews» saga m» g fietaafit be: tilmfit’ fl “ii “3“? Cli'pel€~dtpcla. shew digil‘ifieiiégrésg; London Forces — Instantaneous Dipole/Induced Dipole +3169 3 \GSS usual?) On the other end, when the molecule is non-polar, there occurs an instantaneous moment when a non-polar molecule might for an instant have an instantaneous dipole. When this occurs, a molecule next to it, will be induced to have a dipole, thus the instantaneous dipole induces a dipole in its neighbor. So, what is so important about this? It is actually the polarizability. . ..the higher the molar mass, the higher the surface area, the larger the electron cloud is on the surface that can be induced and thus the greater polarizability. For example, methane has a boiling point of -164°C and hexane has a boiling point of 69°C. Both compounds are non-polar — thus should only have London Forces as the intermolecular attractions. So why is hexane with a higher boiling point, because it is more polarizable, with more electrons, more surface area. Consider pentane and 2,2—dimethylpropane. They have the same MW but pentane has a boiling point of 36.1 °C and the other has a 95°C boiling point. Here it is the shape of the molecule that influences the boiling point. These two compounds can be mixed in all proportions and interact using London Forces. ...
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