Chemistry · Rung 12 of 24

Carbon chemistry

Organic chemistry is the chemistry of carbon, and it does not get its own syllabus by whim: carbon forms four bonds and joins to itself in chains and rings without limit, which is why there are millions of compounds where any other element would give hundreds. What decides how one behaves is not the chain, which is nearly inert, but the functional group: the −OH of an alcohol, the C=O of an aldehyde and a ketone, the −COOH of an acid, the −COO− of an ester, the −NH₂ of an amine. Two compounds with the same formula and a different arrangement are isomers and may have nothing in common: ethanol and dimethyl ether are both C₂H₆O, and one of them is a drink. And an organic reaction is told with curly arrows, which are not decoration: each arrow starts where the electrons are — a lone pair or a bond — and ends where they end up.

Where this sits

Comes after
The shape of molecules, Reactions and balancing
Opens up
Carbohydrates, proteins and fats

Exercises from this rung

Generated by the app, from easiest to hardest

  1. Which functional group does this molecule have?

    • Alcohol
    • Carboxylic acid
    • Aldehyde
    • Ketone

    Why An −OH hanging off a carbon chain is an alcohol, named with the suffix -ol: this is ethanol, the one in drinks. The −OH group is polar and can make hydrogen bonds, so ethanol mixes with water in any proportion, while ethane — the same chain without the −OH — is a gas that will not dissolve. Three atoms change the whole substance.

    easier

  2. Ethanol and dimethyl ether are both C₂H₆O. What are they to each other?

    • Isomers
    • Allotropes
    • Isotopes
    • The same compound

    Why Same formula, different arrangement: that is what isomers are, and they may have nothing in common. Ethanol boils at 78 °C and is drinkable; dimethyl ether boils at −24 °C and is a gas. The difference is where the oxygen sits: in ethanol it hangs off one end with a hydrogen, in the ether it sits between the two carbons. Which is why a formula alone is not enough and you have to draw the structure.

    in between

  3. Benzene is drawn with three double bonds, yet it does not react like an alkene. Why?

    • Because its electrons are delocalised
    • Because it has six carbons
    • Because the double bonds are shorter
    • Because the ring is flat

    Why The six electrons of the double bonds are not in three fixed places: they are spread around the whole ring in a cloud above and below the plane. That is aromaticity, and it stabilises the molecule so much that benzene would rather have a hydrogen substituted than break the system. Which is why all six C—C bonds are exactly the same length, somewhere between single and double, and the three-double-bond drawing is a compromise: the reality is a circle.

    harder