Chapter 2
Practise reading naphtha's character with two axes — PNA and carbon number. You do not need to memorise structural formulas; "chain, ring, or benzene ring?" and "how many carbons?" are enough.
Entry point: high-school chemistry equations and mole ratios
A structure where the carbons are connected mainly by single bonds.
A structure where the molecule forms a ring.
Molecules containing a benzene ring.
Short for Paraffins / Naphthenes / Aromatics.
The number of carbon atoms in a molecule.
How readily a liquid turns into vapour.
When you first try to read what is inside naphtha, set the individual molecular names aside and group everything into PNA — three bins. It is much easier to organise from there.
| Class | Rough shape | Typical examples | First thing to notice |
|---|---|---|---|
| Paraffins | Predominantly chain-shaped saturated hydrocarbons | Pentane, hexane | Easy to read as lighter / lower density |
| Naphthenes | Saturated hydrocarbons that are cyclic but not aromatic | Cyclohexane | Easy to see the connection to reforming |
| Aromatics | Molecules containing a benzene ring | Benzene, toluene, xylene | Density and octane tendency run higher |
Note: "naphthene" here is a different word from the course title "naphtha." The relationship is simply that naphtha can contain naphthenes among its molecular families.
As the carbon number grows, molecules generally get heavier, boiling points rise, and volatility drops.
| Example | Rough reading |
|---|---|
| Pentane | Light, low boiling point, evaporates readily |
| Hexane | Still light, but a bit heavier than pentane |
| Toluene | Heavier, with visible aromatic character |
| Xylene | Heavier still, density tends to rise further |
Rather than memorising exact numbers, lock in the direction first: "more carbons → heavier."
Even at a comparable cut range, more aromatics tends to pull density and octane tendency upward. More paraffins tends to read as lighter and lower-density.
Lighter and lower-density, fits the steam-cracking storyline.
Cyclic, with a clear link to reforming.
Density and octane tendency both run higher.
"Octane number" here just means how resistant a gasoline is to knocking. Knocking is the abnormal combustion that happens when fuel self-ignites earlier than the spark timing inside an engine — it can cause power loss and engine damage. The higher the octane number, the harder it is for knocking to occur. That is enough for this chapter.
For safety, read both "light vs. heavy" and "which family dominates."
| Axis | What tends to rise |
|---|---|
| Light side | Vapour pressure rises; fire risk and vapour handling need more attention. |
| Aromatic-heavy side | Exposure and regulatory topics tend to come up more. |
This course only tracks trends, not numbers. For real-world safety judgements, always defer to the SDS.
Six questions to firm up how you read PNA and carbon number.
Which molecular family primarily refers to saturated cyclic hydrocarbons?
Answer: B
Naphthenes refer to "cyclic but not aromatic" saturated hydrocarbons, such as cyclohexane.
Which family represents the benzene-ring-containing molecules?
Answer: C
Benzene, toluene, and xylene are all aromatics.
In general, what happens to the boiling point as the carbon number grows?
Answer: A
More carbons generally means heavier, which pushes the boiling point upward.
What does the A in PNA stand for?
Answer: B
PNA stands for Paraffins / Naphthenes / Aromatics.
At the same cut, which side tends to run denser?
Answer: A
More aromatics usually reads heavier and raises density.
In general, which tends to be higher on the light side of the cut?
Answer: A
Lighter components evaporate more readily, so vapour pressure rises while flash point drops.