4.5.4: The Importance of Shape

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While we are on the subject of carbon and oxygen containing compounds, let us take a look at one of the most common compounds of carbon and oxygen, carbon dioxide. You can draw the structure of CO₂ with the carbon atom in the middle, double bonded to each of the oxygen atoms. That is, CO₂ has two quite polar bonds in it, and so we might reasonably predict that its boiling point might lie somewhere between dimethyl ether and ethanol. But, as you probably already know, this is not the case. CO₂ exists as a gas at room temperature. In fact CO₂ does not have a liquid phase at standard atmospheric pressure; it changes directly from a solid to a gas, a process called sublimation, at -78 °C. How is such behavior to be understood, particularly given that CO₂ has about the same molar mass as ethanol (CH₃CH₂OH), which is a liquid at room temperature? Once again we have to make sure we have considered all the factors that affect molecular polarity including bond polarity and shape. If you reflect back to the ideas about bond polarity and structure you will see that we have another case here of a molecule with polar bonds, but no overall polarity. CO₂has a linear structure so the bond polarities cancel each other out (they are at 180º from each other). CO₂ has no overall molecular polarity, even though it has polar bonds. Therefore the molecules do not stick together very well and it is a gas at room temperature.

Questions to Answer

1. What is the direction of the molecular dipole moment in ammonia? Draw out a picture showing how you came up with the answer. Does it matter which way you draw the molecule? What if you draw it upside down? Will that affect the direction of the dipole (in the real world)?

2. Why are the interactions between H₂O molecules stronger than those between HF molecules even though the polarity of the HF bond is larger than the polarity of the OH bond?

3. Why don’t more than four water molecules interact with a central water molecule?

4. What would you predict would be the relative boiling points of methanol (CH₃OH) and ethane (CH₃CH₃), which have similar molecular weights?

5. What would you predict would be the relative boiling points of methanol (CH₃OH) and ethanol (CH₃CH₂OH)?

6. What kind of compound (or what structural feature) would you expect might be attracted to the δ+ located on the carbon atom in methanol?

Questions to Ponder

1. What would be the consequences (for life, the universe, and everything) if water molecules were linear?