Like, charges repel, and junk
Organic ions usually involve two specific groups: amino groups and carboxylic acid groups, though a few more unusual organic ions also exist. Amines are basic (alkaline) and prefer to become positively charged. Carboxylic acids become negatively charged.
Ammonia itself is also strongly basic and readily becomes positively ionized. When this happens, the NH3 molecule acquires a hydrogen nucleus, that is, a proton. Therefore we say it is protonated. The resulting ion is called ammonium and has the formula NH4+. It now has four hydrogen atoms instead of three, and its positive charge is evenly distributed over all the hydrogen atoms, just as if they were conjugated.
Amines also form ions by becoming protonated. Methylamine, with formula CH3NH2, ionizes and becomes CH3NH3+. Trimethylamine ionizes to CH3NH+(CH3)2.
Carboxylic acids form negative ions by giving up a hydrogen atom while keeping its electron. Since the resulting carboxylate ion has given up a proton, we say it is deprotonated. Acetic acid CH3CO2H becomes the acetate ion CH3CO2-. Since positive and negative ions have opposite charges, they attract each other pretty strongly.
Generally, if an atom in an organic molecule has a negative charge, it will be missing a bond, such as the carboxylate ion having an oxygen atom with only one bond. If an atom in an organic molecule has a positive charge, it will have an extra bond, just like the ammonium ion having an extra hydrogen. Ions don't have to be bonded to hydrogen; oddities like tetramethyl ammonium (N+(CH3)4) are known and even occur naturally. Anthocyanins, the purple and blue pigments present in many plants, have an O+ atom with a single bond to one carbon and a double bond to another carbon, forming a total of 3 effective bonds.
The two oxygen atoms of a carboxylate ion are actually conjugated. They continuously switch back and forth which one has the single bond and which one has the double bond. The negative charge is therefore equally distributed between them.
The carboxylic acids, by the way, also called fatty acids because of their oily nature and their presence in fats, have names similar to the alkanes but with a different ending. Methanoic acid has one carbon atom; ethanoic acid has two; propanoic has three, etc. However, several of the fatty acids are also known by common names. These include, but are by no means limited to:
| Common Name | Meaning | IUPAC Name | Carbons | Skeletal formula | Smells like |
|---|---|---|---|---|---|
| Formic acid | ants | Methanoic acid | 1 | vinegar, crushed ants | |
| Acetic acid | vinegar < sour | Ethanoic acid | 2 | vinegar | |
| Propionic acid | "first fat" | Propanoic acid | 3 | vinegar, cheesy | |
| Butyric acid | butter, lit. cow cheese |
Butanoic acid | 4 | vomit, rancid butter | |
| Valeric acid | valerian | Pentanoic acid | 5 | sour, cheesy | |
| Caproic acid | goat | Hexanoic acid | 6 | sweaty, barnyard, fecal | |
| Oenanthic acid | grapes, lit. wine flower |
Heptanoic acid | 7 | parmesan, sour | |
| Caprylic acid | goat | Octanoic acid | 8 | rancid, sour, cheesy | |
| Pelargonic acid | geranium < stork | Nonanoic acid | 9 | cheesy, waxy | |
| Capric acid | goat | Decanoic acid | 10 | rancid, fatty |
There are others too. Lauric acid has 12 carbons, myristic acid has 14, and stearic acid has 18. Lauric and myristic acids smell fatty and soapy, while stearic acid has a weak tallow smell, probably due to impurities rather than the acid itself. The list is admittedly a little confusing... capric, caproic, and caprylic? It seems whoever named them must have been studying goats! Small wonder that IUPAC decided to go with Greek number prefixes for those three. But many of the informal names of fatty acids are in common use and it's good to be at least somewhat familiar with them.
Another common fatty acid is isovaleric acid, with the skeletal formula . It is the cause of sweaty foot stink.
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