Resonance structures are different forms of the same molecules, where electrons are rearranged. The molecule is at the same time, a hybrid of all of these forms. Resonance topic is very important in organic chemistry, and while it gets introduced in the beginning of the first semester, it is very useful throughout the whole year of ochem.
How to Draw Resonance forms?
Arrows
We use curved arrows to show the movement of electrons and to draw the correct resonance forms. An arrow has a head and a tail. The head of the arrow shows where electrons are going and tail shows where they are coming from.
Therefore, an arrow always originates from something that has electrons, such as a lone electron pair or a double bond. An arrow will point to something that can accept electrons such as a carbon with a positive charge.
It takes a great deal of practice to get accustomed to drawing resonance correctly.
Let's first learn some rules:
1. Do not break a single bond (don't draw an arrow coming from a single bond)
2. Do not give second row elements more than an octet
3. All resonance forms must have the same total charge
Common resonance patterns include:
a double bond next to a positive charge
a double bond next to a negative charge
a lone pair of electrons next to a double bond
Common Resonance Patterns
Let's take a look at some examples:
Lone pair next to a double bond resonance examples:
Carbocation next to a double bond resonance examples:
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