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Charge, polarity and resonance together locate a molecule's electron-rich and electron-poor sites; the neighbours of a carbonyl carbon set how electrophilic it is, and resonance can reveal a site one structure hides.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
You will sort sites into nucleophilic and electrophilic, find the most electrophilic carbon, rank carbonyl compounds by electrophilicity, and use resonance to find a hidden reactive site.
Formal charge, resonance and polarity each tell you where electrons are. This lesson uses all three at once to say where a molecule will react.
A nucleophile (nucleus-loving) is electron-rich and gives a pair of electrons. An electrophile (electron-loving) is electron-poor and accepts a pair. A reactive site is the particular atom or bond where that happens.
Ask of each part of a structure:
In ethanoyl chloride, CH3COCl, the carbonyl carbon is bonded to oxygen and to chlorine: it is the most electron-poor atom, the electrophilic site. In the methoxide ion the oxygen's negative charge and lone pairs make it the nucleophilic site. A reaction pairs the two.
Another way: table
Common sites.
| Electron-rich (nucleophilic) | Electron-poor (electrophilic) |
|---|---|
| negative ions: OH−, CH3O−, CN− | cations: CH3+, H3O+ |
| lone pairs: N of NH3, O of H2O | δ+ carbons: C=O, C–Br |
| pi bonds: C=C | the H of an acid |
Not every carbonyl carbon is equally electron-poor. A neighbour that only pulls electrons away makes it more so; one that can push a lone pair back into the C=O by resonance makes it less. So chlorine (an acid chloride) gives the most electrophilic carbonyl, then an aldehyde, then a ketone, whose extra alkyl group pushes a little electron density in; an ester's oxygen and, more strongly, an amide's nitrogen donate a lone pair, making those carbons the least electrophilic. This order predicts how readily each reacts with water or an alcohol.
Resonance can reveal a site one structure hides: in propenal, CH2=CH–CHO, a contributor puts a positive charge on the far CH2 carbon, which some nucleophiles attack.
The largest atom is always the reactive site. Electron supply and demand decide, not size.
The most electronegative atom is where a nucleophile attacks. It is δ−; the atom it pulls from is attacked.
A molecule has one reactive site. Many have a nucleophilic and an electrophilic site.
Only a charged atom can react. Lone pairs, pi bonds and δ+ atoms react too.
Water's oxygen has two lone pairs: electron-rich.
The nucleophilic site.
Bromomethane's carbon is bonded to bromine: δ+.
The electrophilic site.
The oxygen's lone pair bonds to that carbon.
Rich meets poor.
Which is electron-rich?
Cyanide, with its negative charge and lone pair on carbon.
Which atom of ethanal is electron-poor?
The carbonyl carbon, δ+ from its oxygen.
Match each site to whether it is electron-rich (nucleophilic) or electron-poor (electrophilic).
| electron-rich: nucleophilic | electron-poor: electrophilic | |
|---|---|---|
| the carbonyl carbon of propanone | ||
| the oxygen of the methoxide ion, CH3O− | ||
| the carbon of the methyl cation, CH3+ | ||
| the nitrogen of ammonia, NH3 | ||
| the C=C pi bond of ethene |
Ethanoyl chloride is CH3COCl: a CH3 joined to a carbon that carries both a C=O and a C–Cl. Which carbon will an electron-rich reagent attack?
An amino alcohol, H2NCH2CH2OH, is described part by part below. Mark every part that is an electron-rich (nucleophilic) site.
This task has no paper form; do it on a device.
Rank these carbonyl compounds from the most electrophilic carbonyl carbon to the least: ethanoyl chloride CH3COCl, ethanal CH3CHO, propanone CH3COCH3, methyl ethanoate CH3COOCH3, ethanamide CH3CONH2.
Number the steps in order (write the number in the box):
Propenal is CH2=CH–CHO. Besides the carbonyl carbon, which carbon does resonance show to be electron-poor?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
For each species, say whether it reacts as a nucleophile (electron-rich) or an electrophile (electron-poor).
| reacts as | |
|---|---|
| the hydroxide ion, OH− | |
| ammonia, NH3 | |
| the hydronium ion, H3O+ | |
| bromomethane, CH3Br |
You can point to where a molecule will react. Tell someone why an amide is less electrophilic than an acid chloride. Next unit: naming the molecules you have been reasoning about.
8. Your turn: the cyanide ion, CN−, and ethanal, CH3CHO, step 3