Note: Look for the boxes like the one below. If clicked on the gray text, it will expand to show the content of the explanation or exercise. To collapse it back, click on the gray text again.
| ⊕ Interactive explanation/exercise (click to show) |
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Organic compounds consist of molecules, more or less loosely held together by intermolecular forces. Molecules consist of atoms, strongly held together by chemical bonds (atoms, in turn, are made together from subatomic particles held together by even stronger forces).
Brief summary of valence bond theory/molecular orbital theory for organic chemistry:
A common depiction of structure is Lewis structure: electrons are shown as dots (•), and valence orbitals are implied rather than shown, though they can be shown explicitly shown as hollow squares (□) when empty.
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Note that same atom can have different arrangement of electrons on orbitals. These would be different, but possible arrangements (though some may be preferred over others)
| ⊕ Examples of Lewis structures of atoms (click to show) |
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To assemble a molecule from atoms, bring atoms together and make bonds between them using orbitals and electrons, until no more can be made.
Sample Lewis structure (hover/touch on the parts to see the details)
| ⊕ An example how it works (click to show) |
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As a shortcut that works most of the time, an atom can be expected to make as many bonds as it has unpaired electrons (e.g. 1 bond for hydrogen, 4 bonds for carbon, 2 bonds for oxygen). In some case this may be more complicated, since electrons can be moved around.
| ⊕ A more elaborate example (click to show) |
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In some cases, several structures can be put together for the same combination of atoms, even if they stay in the same place. For instance, for the one above:
| ⊕ Getting another structure for the example above (click to show) |
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| ⊕ An exercise on assembly of Lewis structures (click to show) |
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To draw Lewis structures is a lot of work. Chemist, like all people, do not like extra work. To avoid it, parts of the structure can be abbreviated instead of being drawn out - as long as it possible to figure out how to draw it, if needed. Lone pairs (:) and empty (□) can also be omitted, since they can be deduced knowing what atom it is. Example below (hover mouse pointer over it/touch it to see what's abbreviated).
| ⊕ An example of condensing a structure (click to show) |
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When un-condensing condensed structures, at times some careful deliberations may be necessary. The basic approach is: go left to right atom by atoms, trying to make all bonds possible to the atoms on the left before moving right.
| ⊕ An example of un-condensing a structure (click to show) |
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Parentheses: ) and ( can be used in condensed structures to isolate or repeat part of the structure.
| ⊕ An example of of parentheses in condensed structures (click to show) |
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| ⊕ An exercise on un-condensing structures (click to show) |
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Another common way to reduce effort when drawing structures is to use of skeletal formulas (also known as line-angle, or shorthand formulas. In these, things are omitted rather than abbreviated - again, in a way so that it's possible to figure out what's omitted. Organic compounds tend to contain a lot of carbon and hydrogen - skeletal structures focus of trimming these two up.
That cuts down on drawing quite dramatically. Example below (hover the mouse pointer/touch to see what's hidden)
| ⊕ An example of converting a Lewis structure to line-angle (click to show) |
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| ⊕ An exercise on line-angle formulas (click to show) |
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