RUŌOD Lab

Level 7 · Perfume Chemistry and Analytical Methods

Reading a Molecular Structure

Learn to read skeletal formulas as a perfumer: implied carbons and hydrogens, rings, double bonds and the formula counts you can check on PubChem.

Advanced 25 min study By the RUŌOD Lab team Updated

In short

A skeletal formula draws a molecule by its carbon framework. Each line end and corner is a carbon, hydrogens on carbon are left out, and any other atom is written in. Once you can read one, a structure tells you the molecule's size, shape and functional groups, and that lets you make sensible predictions about how it might behave in a formula.

What you will learn

  • Read a skeletal formula, counting carbons and hydrogens
  • Identify rings, double bonds and heteroatoms
  • Check your reading against a molecular formula on PubChem

Before this lesson: Keep an Evaluation Sheet: Level 6 Project

Why a perfumer should bother

You will not need to synthesize anything. But structures help you to anticipate things: whether a material is likely to oxidize, whether it might react with another line, how volatile it might be, and why two names may be related. Your encyclopedia pages show a structure for many single molecules, so you can practice with materials you already own.

The rules

  1. Every line end and corner is a carbon, unless another atom is written there.
  2. Carbon makes four bonds. Any bond not drawn is to a hydrogen.
  3. A double line is a double bond, which counts as two of the four.
  4. Letters are other atoms: O is oxygen, N nitrogen, S sulfur. Oxygen usually makes two bonds, and an OH is drawn with its hydrogen.
  5. A ring closes back on itself. A six-membered ring with alternating double bonds, or a circle inside it, is an aromatic ring.
  6. Wedges and dashes show bonds toward or away from you, and they matter for chirality, which comes later in this level.

Worked reading: phenylethyl alcohol

Phenylethyl alcohol is an aromatic ring, then two carbons, then OH.

  • Carbons: 6 in the ring + 2 in the chain = 8.
  • Hydrogens: 5 on the ring (one ring position is occupied by the chain) + 2 + 2 on the chain + 1 on the oxygen = 10.
  • Oxygen: 1.
  • Molecular formula: C8H10O.

Compare that with the entry on PubChem. If your count differs, you have an implied hydrogen wrong, usually at a branch.

A check using unsaturation

A quick test for a molecule containing C, H and O is the degree of unsaturation:

DU = (2C + 2 − H) ÷ 2

Each ring or double bond counts as one, and an aromatic ring counts as four (one ring plus three double bonds). For C8H10O, DU = (16 + 2 − 10) ÷ 2 = 4, which matches a single aromatic ring. For linalool, C10H18O, DU = (20 + 2 − 18) ÷ 2 = 2, which matches its two double bonds and no ring. Oxygen does not affect the sum. This check catches most counting errors.

Reading for clues

What to look for, and what it may suggest. A tendency, not a rule.
FeatureMay suggest
Small size, few oxygensHigher volatility
Large size, or several polar groupsLower volatility
Aldehyde groupReactivity, oxidation
Double bonds beside CH2 or CH groups, as in terpenesSensitivity to air
Phenol groupDiscoloration; often strong
Amine or other nitrogenPossible reactions with aldehydes

These are prompts for checking, not conclusions. Odor in particular cannot be read off a drawing, as the lesson on structure and odor explains.

Practical exercise: Read four perfumery molecules

About 40 minutes · You will have: worked atom counts for four molecules, each checked against PubChem

Look up each molecule's structure on its encyclopedia page or on PubChem. Cover the molecular formula before you start.

  1. For phenylethyl alcohol, vanillin, linalool and limonene, count C, H and O.
  2. Calculate DU for each, and name the rings and double bonds it accounts for.
  3. Compare with PubChem. Vanillin is C8H8O3, so DU = (16 + 2 − 8) ÷ 2 = 5: an aromatic ring (4) plus the aldehyde C=O (1).
  4. Write one prediction for each molecule from the clues table, and check it against what you have observed on your strips.

You have done this well if your counts match PubChem and your predictions are marked right or wrong.

Common mistakes

  • Forgetting implied hydrogens. Count bonds at every carbon.
  • Treating a drawing as a smell. It suggests behavior, and it rarely predicts odor.
  • Trusting a single internet image. Check against a record with a CAS number.

Key terms

Skeletal formula
A drawing of a molecule in which carbon atoms are implied at line ends and corners, and most hydrogens are not shown.
Heteroatom
In organic chemistry, any atom in a molecule other than carbon or hydrogen, commonly oxygen, nitrogen or sulfur in fragrance materials.
Molecular formula
The count of each kind of atom in a molecule, such as C10H18O for linalool.

Knowledge check

1. In a skeletal formula, what does a corner in a line usually represent?
Show the answer

A carbon atom. Carbons are implied at line ends and corners unless another atom is drawn.

2. How many hydrogens does a neutral carbon with two single bonds to other carbons usually carry?
Show the answer

Two. Carbon forms four bonds. Two are used, so two hydrogens are implied.

3. Linalool and geraniol are both C10H18O, yet they smell different. What does this show?
Show the answer

A molecular formula does not tell you the arrangement. They are constitutional isomers: the same atoms, arranged differently.

Sources

Further reading and materials

Written and reviewed by the RUŌOD Lab team. This is general education, not legal, regulatory, medical or safety advice. Example dilutions and exercises are for learning, are not validated products, and have not been safety-assessed. Completing a lesson confers no qualification. How we check these lessons.