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The Chemistry of Aroma: Pandan Smell in Tap Water

Not all fragrant chemicals are pleasant

Fragrance attracts attention. And even more so when it emanates from our tap water. On 22nd July numerous keen-nosed and concerned Singaporeans alerted PUB to the pandan smell they picked up when they boiled tap water. PUB tested the tap water and removed the affected water rapidly. The reason? Nope, it’s not because the water was drawn from Ulu Pandan Reservoir. The culprit was a pesky little covalent compound called tetrahydrofuran.

PUB officers flushed the pipes to expunge the contaminated water (© PUB, 2020).

What is Tetrahydrofuran?

How chemists draw THF

What a mouthful! While its name is long, tetrahydrofuran (THF) is a simple molecule containing three elements: hydrogen, carbon, and oxygen. Interestingly, its carbon and oxygen atoms are connected in a circle! So it is usually drawn like a ring topped with a diamond by not so romantic chemists.

While we learned that most simple molecules are insoluble in water, THF is an exception. It is in fact highly soluble in water, much like ethanol. This is how THF sneaked into our water supply, despite our stringent water purification.

Why Does THF Give a Pandan Smell?

For us to smell something, we must breathe in the molecules. This means that only gaseous molecules can be detected by our nose as either a fragrance or a stench. We are not unfamiliar to this. In the laboratory, we describe ammonia as pungent precisely because it is gaseous and can hence be detected.

This explains why some Singaporeans only smelled the “pandan” water when they boiled it. THF has a boiling point of 66 °C.

While THF smells like pandan, it does not mean that real pandan leaves contain THF. In our delicious pandan cakes made from pandan leaves, the fragrance is attributed to another covalent compound, called 2-acetyl-1-pyrroline (Wakte et al, 2009).

The Detective Work to Identify THF

PUB ran 400 tests to identify THF as the fragrant culprit.

One of the tests was GC/MS. It involved chromatography. But unlike paper chromatography that works on aqueous mixture, this was a gas chromatography (GC) to separate gaseous mixture. This means that the contaminated water was first vaporised.

With all the solutes separated, it became easier to identify each of the separated solute by a technique called mass spectrometry (MS).

The whole process of collecting water samples and testing them rigorously within a day or two was no small feat. It involved many PUB officers and their Water Quality Specialists. Kudos to them!

Is the Water Harmful?

When PUB and the press reported on the contamination, there were numerous Facebook comments that expressed concerns about the health effects of the pandan-smelling water. Its long name sounds almost diabolic. It is also not very reassuring to know that THF is used as a laboratory and industrial solvent.

However, even industrial chemicals can be friendly. The contrary is true as well, even friendly chemicals like water and oxygen can be deadly. It all depends on the concentration.

A chemical only becomes a poison when its concentration is too high.

According to Professor Shane Snyder from Nanyang Technological University, THF becomes harmful only when its concentration exceeds 0.0001 g/dm3. Thankfully, the concentration of THF in the contaminated water was at least 10 times lower than that. Simply put, even the water with a pandan smell is safe to drink.

Be that as it may, PUB has isolated the source of the THF-water blend. Back to our bland water.

Data-Based Questions à la Paper 2 Section B

QUESTION 1: Separation Techniques
THF has a melting point of -108 °C and a boiling point of 66 °C.

State the physical state of pure THF at room temperature and pressure? [1 mark]

Liquid state.

The room temperature of 25 °C is above the melting point of -108 °C, but below the boiling point of 66 °C. Therefore, it would have melted into a liquid, but not yet boiled.

QUESTION 2: Chemical Bonding
THF is a compound consisting three elements: hydrogen, carbon, and oxygen.

By considering the structure and bonding, explain why THF has a lower boiling point than water. [2 marks]

Both THF and water have a simple molecular structure. As THF has weaker intermolecular forces of attraction that water, less energy is required to overcome the former. Therefore, THF has a lower boiling point.

Marker’s note: As THF is made up of non-metals, it is likely to be a covalent compound with a simple molecular structure.

QUESTION 3: Separation Techniques
When the water contaminated by THF was heated, it emitted a pandan smell.

By considering the boiling points of THF and water, deduce if the smell could be detected before or after the water began to boil vigorously.

The smell could be detected before the water begins to boil vigorously. As the boiling point of THF is lower than that of water, THF would boil first.

QUESTION 4: Separation Techniques
In the laboratory, 25 cm3 of water was mixed with 25 cm3 of THF.

Given that THF is miscible in water, suggest a suitable separation method. Describe the method in detail. [3 marks]

Fractional distillation.

The mixture is heated in a round-bottom flask attached to a fractionating column. As THF has a lower boiling point, it will vaporise and reach the top of the column first, distilling over into a condenser. The gaseous THF will condense back into a liquid to be collected first. This happens at 66 °C.

QUESTION 5: Stoichiometry
THF contains 66.7% carbon, 11.1% hydrogen, and 22.2% oxygen.

(a) Find the empirical formula. [2 marks]
(b) Given that its molar mass is 72 g/mol, hence deduce its molecular formula. [1 mark]

CarbonHydrogen Oxygen
Mass in 100 g sample/ g66.711.122.2
No. of moles/ mol66.7 ÷ 12 = 5.5611.1 ÷ 1 = 11.122.2 ÷ 16 = 1.39
Divide by smallest value5.56 ÷ 1.39 = 411.1 ÷ 1.39 = 81.39 ÷ 1.39 = 1
Simplest ratio481

The empirical formula is C4H8O.

Finding Molecular Formula

Let the molecular formula be C4nH8nOn
n(4×12 + 8×1 + 16) = 72
n(72) = 72
n = 1

As n = 1, the molecular formula is also C4H8O.

QUESTION 6: Stoichiometry
PUB conducted tests on the contaminated water and found out that the concentration of THF was 0.00001 g/dm3.

(a) Find the concentration of THF in mol/dm3. [1 mark]
(b) Find the number of moles of THF in 500 cm3 of contaminated water. [1 mark]

Concentration in mol/dm3 = concentration in g/dm3 ÷ molar mass
Concentration of THF = 0.00001 ÷ 72 = 1.38 × 10-7 mol/dm3

No of moles = concentration × volume in dm3
No of moles of THF = 1.38 × 10-7 × (500/1000) = 6.94 × 10-8 mol

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