Resonance Contributor

Question

How can I draw Resonance contributors ?

Answer 1

You can draw a set (two or more) of resonance contributors using Lewis Structures for molecules and charged particles (ionic grouping of covalently bonded atoms) when there are pi electrons (or lone pair electrons such as those on a carbanion, C^-) and there is a "sink" for them. A sink might be a carbonyl group (C=O), an electron deficient carbon atom (a carbonium ion, C⁺), and other examples. The pi electrons or lone pair electrons must be on an atom adjacent to the electron sink. There cannot be any sp³-hybridized atoms preventing the loosely held electrons  from reaching the sink. The spatial position of all atoms are held constant. The charge on the molecule is spread over the atoms of the contributors, not localized or stuck on one atom. Remember that not all resonance contributors are significant; the "best" resonance contributors are those that have the same number of single and double bonds, no separation of charges are created (no new + or - charges on atoms), and if a formal charge must be included, the most important contributor has negative charges on the most electronegative atoms, and positive charges on the most electropositive atoms.

Above all-all significant contributors MUST obey the octet rule-with the exception of 3rd row elements which may have an "expanded octet" and accept 10, or even 12 electrons (e.g. Si, P, S).

 

Resonance contributors are a concept (thanks again, Linus Pauling!) that helps to understand the stability or instability of a molecule. They help predict the region where a chemical species is most likely to be attacked by a nucleophile, or why they are resistant to attack. In short, they help us when attempting to make sense of the mechanism, or pathway a reaction might take.

 

Resonance is not merely a concept; the resonance energy of a molecule can actually be calculated and verified by experiment. It is essential to remember that the actual structure of a molecule is a hybrid of all of the resonance contributors combined. They are not interconverting or oscillating from one to another like a chemical reaction. Resonance increases stability by increasing the bonding between adjacent (more covalent bonds) atoms and by distributing charge over a greater number of atoms (delocalization). 

 

 

ANSWER TO QUESTION: The easiest way to understand resonance is to get a piece of paper and a pencil and start drawing them. Read and re-read the portion of your textbook that explains resonance. But reading alone will not "sink" the concept into your consciousness; taking pencil in hand and putting pencil to paper signals to your brain that there is something important going on.

 

There are rules that must be adhered to in order to make use of the concept successfully. I won't repeat the rules here; they are in every organic chemistry book. A handbook that my students have found useful for understanding resonance theory is called "Pushing Electrons: A Guide for Students of Organic Chemistry" by Daniel Weeks (ISBN # 978-0030206931)-you can buy an older edition online for under $1. For free, you can download ACD Freeware, which is a program for drawing molecules (and so much more) at acdlabs.com

Answer 2

The following link should prove of principal interest. I would attempt to write out an explanation, but it is easier with diagrams and this is a fairly good explanation itself, if this proves confusing I will try to rephrase it for you. 

 

http://www.chem.ucla.edu/harding/tutorials/resonance/imp_res_str.html

 

Answer 3

Resonance structures are not something you guess at — there is a short procedure, and only three legal moves. Once you know the three moves it becomes mechanical.

The one rule that governs everything: only electrons move. Never atoms. If your second structure has a hydrogen or a carbon in a different place, you have drawn a different molecule (an isomer), not a resonance contributor. Every contributor must have identical atom connectivity and the same total charge.

The only three moves

1. Lone pair → adjacent bond. A lone pair on an atom next to a positive center or next to a pi bond folds down to become a pi bond.

2. Pi bond → adjacent bond. A pi bond shifts one position over — this is what walks charge around a conjugated system or a benzene ring.

3. Pi bond → lone pair. A pi bond collapses onto one of its two atoms, giving that atom a lone pair and a negative charge, and leaving the other atom positive.

That is the whole toolkit. Arrows start at electrons (a lone pair or the middle of a pi bond) and point to where those electrons end up.

The procedure

Draw the Lewis structure with all lone pairs shown. Find a p orbital or lone pair sitting adjacent to a pi bond, a positive charge, or a radical — adjacency is the requirement, because the orbitals must overlap. Push one arrow. Redraw the whole structure. Recount formal charges on every atom you touched. Repeat until no legal move remains.

Ranking them — this is what exam questions actually ask

Best to worst: (1) every atom has a full octet; (2) the fewest formal charges; (3) negative charge on the most electronegative atom, positive charge on the least; (4) no like charges adjacent to each other.

Rule 1 usually beats rule 3 when they conflict. A structure with a complete octet and a charge on carbon typically contributes more than one with an incomplete octet and a charge on oxygen.

What the arrow between structures means. The double-headed arrow (↔) is not an equilibrium. The molecule is not flickering back and forth. The real electron distribution is a single unchanging hybrid, and the contributors are just the best drawings our notation can manage — weighted by that ranking list. The lowest-energy contributor looks most like reality.

The mistakes that cost the most points: breaking a sigma bond (only pi electrons and lone pairs move); exceeding an octet on a second-row atom — C, N, O, and F can never hold more than 8 electrons; forgetting to update formal charge after pushing an arrow; and using ↔ when you meant ⇌.

Practice on the acetate ion, the allyl cation, and nitrobenzene, in that order. Those three cover all three moves.