In this lesson, we will learn:
- To understand how a conjugated system affects the reactivity of molecules.
- To understand the nature of a resonance hybrid and contributing resonance structures.
- To predict important resonance forms of organic molecules by the presence of electron-donating and withdrawing groups.
- Organic molecules are held together by shared electrons between atoms. However, many bonds in organic molecules don’t evenly share the electrons that make them.
Because it is electrons that make (and by moving, break) bonds, molecules become reactive when electrons are concentrated or sparse in one specific region of a molecule (more detail in Nucleophiles and electrophiles).
Knowing where electrons are in a molecule, then, is very important in understanding any reactivity.
- Molecules with pi bonds can become stable by delocalizing electrons throughout the structure.
A conjugated system is made when p orbitals combine and delocalize electrons among them. In a conjugated system the electrons are not localized to specific atoms or their orbitals, but instead delocalized, shared amongst multiple atoms. Instead of a painting with only polarizing black (electron-rich) or white (electron-poor) regions, a conjugated system combines to make a single shade of grey.
- This is what benzene does; the alternating single/double (pi) bonds form a conjugated system that delocalizes pi electrons across the p orbitals on carbon.
It is not accurate to draw conjugated systems with just one molecular structure, so chemists represent it by drawing multiple resonance forms. Each resonance form ‘contributes’ to the true structure which is a mix or resonance hybrid of them.
In benzene, both forms contribute equally because they are chemically equivalent.
Depending on the compound though, some resonance forms contribute more to the true structure than others.
In the example of an imine (think C=O carbonyl but with C=N instead) this isn’t true:
This imine has three resonance forms but the red form is extremely unstable and the blue form is somewhat less stable than the form in black. We would say this black resonance form contributes most to the structure and the blue form contributes a small amount; the resonance hybrid is closer to the black structure than the other two, but most importantly its real structure is somewhere in between and can’t be described by one single structure.
- When drawing resonance forms DO NOT use equilibrium arrows. There is no chemical reaction happening!
In this sense, the word ‘resonance’ is misleading; the electrons are not ‘resonating’ or constantly shuffling between the different resonance forms (constantly going between black and white), they are in a structure settled somewhere between these extremes (a solid shade of grey).
- Like the imine, most compounds with resonance forms are not contributed to equally (like the two benzene resonance forms). There are some issues to consider when finding important resonance forms:
- Resonance forms with localized charges are less stable.
Molecules are reactive when they have areas of high charge density, be it positive or negative. Resonance forms stabilize a molecule because charge is dispersed (think back to black and white becoming a shade of grey). Charged atoms are the opposite of this delocalizing effect!
- Incomplete octets make resonance forms less stable. This is especially true of electronegative atoms like nitrogen and oxygen. These atoms ‘demand’ electrons more than other atoms, so a form which leaves them electron-poor will be extremely unstable.
- Stable cations/anions make resonance forms more important. If all resonance forms have localized charge of some sort, consider how stable the ions are.
For a positive charge:
For a negative charge:
- An ion with a filled octet is more stable.
- A more substituted carbon is more stable (primary < secondary < tertiary) due to the positive inductive effect of alkyl groups.
- Adjacent electron donating groups (EDGs) stabilize positive ions; more on this below.
- More electronegative atoms hold negative charge more easily.
- Adjacent electron withdrawing groups (EWGs) stabilize negative ions; more on this below.
- Electron-withdrawing groups and electron-donating groups make some resonance forms very important in organic molecules. This is because by donating or accepting pi electrons they change the overall look of the conjugated system – the ‘shade of grey’ the conjugated molecule reaches is made a lot darker or lighter by taking out (EWGs) or putting in (EDGs) electrons!
Identifying EDGs and EWGs in molecules and drawing resonance forms of these molecules is therefore extremely helpful when finding good nucleophiles and electrophiles.
- Electron-donating groups adjacent to double bonds make an important resonance form where the further carbon in the double bond is negatively charged. This is because the EDG is donating pi electrons; the molecule is nucleophilic at this carbon atom and can attack electrophilic sites from here. See the diagram below:
- Electron-withdrawing groups make the ?-carbon (two carbons away from a functional group) electron-poor because of an important resonance form where the EWG is accepting pi electrons. This leaves a positive charge on the ?-carbon, and it can be attacked by nucleophiles here. This is the resonance structure described in red below: