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Chemistry

Memorise the Important Dicarboxylic Acids

IUPAC nomenclature is supposed to save you from memorising names. And mostly it does. But a handful of compounds are so common that problems just call them by their old common names — and leave you to recall the structure yourself.

Dicarboxylic acids are the classic trap. You'll see "succinic acid" in a question, and your only lifeline is memory. So let's fix memory. Today: five important dicarboxylic acids, plus two more, and a trick so you never miscount their carbons again.

What is a dicarboxylic acid?

Simple. A dicarboxylic acid carries two –COOH groups. It can neutralise two OH⁻ ions — two units of a base.

The smallest one is oxalic acid. Just two carboxyl groups stuck together, nothing in between.

Oxalic acid — the smallest dicarboxylic acid

Two acidic H atoms, two units of base neutralised. That's the whole definition in one picture.

Build the rest with a homologous series

Here's the good news: you don't have to memorise five separate structures. They're one family — a homologous series. Same functional group, same general formula, each member one –CH₂– longer than the last.

So you only need the first member. Then keep slipping a –CH₂– group between the two –COOH ends.

Start with oxalic. Add one –CH₂–:

Malonic acid — oxalic + one CH₂

Add another:

Succinic acid — one more CH₂

And keep going:

Glutaric acid
Adipic acid

Five acids, each with one more –CH₂– than the one before. Adipic acid earns extra attention: it's used to make nylon-6,6.

The trick — so you never miscount carbons

Memorising the names is easy. The real trouble is the order, and the number of carbons hiding in each. Get those backwards and you'll draw the wrong structure under exam pressure.

So fix the order first. Read down the list:

Oxalic · Malonic · Succinic · Glutaric · Adipic

Take the first letters — O M S G A — and hang them on a sentence you can't forget:

Oh My Such Good Apple pie.

Or the old coaching-class favourite, "O Mere Saajan Ghar Aaja." Romantic, sure, but it sticks. Make your own if you like — the point is to lock the sequence.

Now the carbons. The position in the list is the carbon count:

  • Oxalic = 2 carbons (1st, but it's just the two –COOH)
  • Malonic = 3
  • Succinic = 4
  • Glutaric = 5
  • Adipic = 6

After oxalic, each step up adds one carbon. Apple pie, two through six. Done.

Your turn. Without looking up, draw succinic acid. How many carbons, and how many –CH₂– groups in the middle?

Check: Succinic is 4th letter region — 4 carbons. Two are the –COOH carbons, leaving two –CH₂– groups between them: HOOC–CH₂–CH₂–COOH.

Two aromatic ones to finish

Just two more, and they matter.

The first is phthalic acid — notice the Ph, the giveaway for a benzene ring. It's a benzene with two –COOH groups ortho to each other (next-door neighbours).

Phthalic acid — two COOH groups ortho on benzene

Move those two –COOH groups to the para position (opposite corners) and you get terephthalic acid — the one that builds PET plastic.

Terephthalic acid — the two COOH groups are para

The whole thing in one breath

  • A dicarboxylic acid has two –COOH groups.
  • The five aliphatic ones form a homologous series: Oh My Such Good Apple pie — Oxalic, Malonic, Succinic, Glutaric, Adipic.
  • Their carbon counts run 2, 3, 4, 5, 6, in that order.
  • Adipic → nylon-6,6.
  • Aromatic two: phthalic (ortho) and terephthalic (para).

Lock the mnemonic now, while it's fresh. Next time a problem throws a common name at you, the structure will already be waiting.