GCSE · Chemistry · Edexcel · Spec 1CH0

Alcohols: methanol to butanol

Methane boils at −164 °C. Swap one hydrogen for an –OH group and you get methanol, which boils at 65 °C. One tiny group, over 200 degrees.

Chemistry · Reading a molecule

Ethanol, taken apart

Showing 1 layer: Ethanol

Explore

Tap each part of the molecule

This is ethanol drawn as a displayed formula: every atom and every covalent bond is shown. Tap a part to find out what it does.

Which part makes this molecule an alcohol, and which part gives it its name?

Chemistry · Naming alcohols

From a carbon count to a name and formula

An alcohol has three carbon atoms in its longest chain. Name it, and write its structural and molecular formulae. Then make the chain one carbon longer and do it again.

It has an –OH group, so it's an alcohol and its name ends in -ol.The family comes first. Now the carbon count gives the start of the name.
missing step

Which line is step 2?

Three carbons gives the prefix prop-, so the alcohol is propanol.Three carbons gives the prefix but-, so the alcohol is butanol.Three carbons gives the prefix prop-, so the alcohol is propane.

Chemistry · Three ways to write it

Structural, molecular or displayed?

Which kind of formula is each one? Pick a formula, then pick its category.

Still to sort

Structural formula (0)

Shows how the atoms are arranged.

Where the line is: If you can see the OH written as its own group at the end, the formula is showing arrangement, so it's structural.

Molecular formula (0)

Shows only how many atoms of each element there are.

Where the line is: All the hydrogens are added up into one number and the O stands alone, so you can't see where the –OH is.

Displayed formula (0)

Shows every covalent bond as a line.

Where the line is: The only type that draws the bonds. Every carbon has four lines.

7 of 7 still to sort.

Watch out: CH₃OH and CH₄O aren't two different substances. Both are methanol: one shows how the atoms are arranged, the other just counts them.

Chemistry · Boiling points

Where does each one boil?

Drag each compound to where you think it boils, in °C. The tag tells you the carbons and the family: C2-ane is the alkane with two carbons (ethane), C2-ol is the alcohol with two carbons (ethanol).

C1-ane — methaneC1-ol — methanolC2-ane — ethaneC2-ol — ethanolC3-ol — propanolC5-ol — pentanol

Chemistry · Why the gap?

Why does ethanol boil so much higher than ethane?

Ethanol and ethane both have two carbon atoms. Ethanol also has an –OH group, and it boils at a far higher temperature.

Which idea is closest to what you think right now?

When ethanol boils, its covalent bonds break, and the O–H bond takes extra energy to break.The forces between ethanol molecules are stronger, so it takes more energy to pull the molecules apart.Ethanol has an extra oxygen atom, so its molecules are heavier. That's why it boils higher.

How sure are you?

I'm sureI think soI'm guessing

When ethanol boils, its covalent bonds break, and the O–H bond takes extra energy to break.

This is a common misconception

This is the big one to fix. Boiling doesn't break any covalent bonds. Ethanol vapour is still made of whole ethanol molecules, exactly the same molecules as in the liquid, just spread far apart. The C–C, C–H, C–O and O–H bonds all survive. What boiling has to overcome is the pull BETWEEN one molecule and the next.

The forces between ethanol molecules are stronger, so it takes more energy to pull the molecules apart.

This matches the evidence

Yes. To boil, molecules have to escape from each other, so what matters is how strongly neighbouring molecules attract. These attractions between molecules are called intermolecular forces. They're stronger between alcohol molecules than between alkane molecules, so more energy is needed to overcome them, and that means a higher boiling point.

Ethanol has an extra oxygen atom, so its molecules are heavier. That's why it boils higher.

This is a common misconception

Test that idea on methanol and ethane. Methanol, CH₄O, has a relative formula mass of 12 + 4 + 16 = 32. Ethane, C₂H₆, has 24 + 6 = 30. Almost the same mass, yet methanol boils at a far higher temperature. Mass can't explain a gap that big. The –OH group makes the forces between the molecules stronger, and that's what raises the boiling point.

Boiling pulls whole molecules away from each other; the bonds inside them stay intact. Alcohols boil higher because the forces between their molecules are stronger, so more energy is needed to overcome them.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Count the carbons, add -ol, and every next alcohol is just one CH₂ longer.

What you need to know

An alcohol is any compound with the –OH functional group. A functional group is the atom or group of atoms responsible for the way a compound reacts.
An alcohol's name is a carbon-count prefix plus -ol: meth (1 carbon), eth (2), prop (3), but (4).
Structural, molecular and displayed formulae are three ways of writing the same molecule.
Alcohols boil higher than the alkanes with the same number of carbons, and higher still as the chain gets longer.

The big picture

Alcohols are a homologous series of compounds that all contain the –OH functional group. Their names end in -ol, and the prefix counts the carbons in the longest chain: meth (1), eth (2), prop (3), but (4). So methanol, ethanol, propanol and butanol each differ from the next by one CH₂. One molecule can be written three ways: structural (CH₃CH₂OH), molecular (C₂H₆O) or displayed, with every bond shown. Alcohols boil far higher than alkanes with the same number of carbons, because the forces between their molecules are stronger, and their boiling points rise as the chain gets longer.

Key points

1Alcohols form a homologous series of compounds that all contain the –OH functional group.

2The first four alcohols are methanol, ethanol, propanol and butanol. The prefix counts the carbons in the longest chain: meth 1, eth 2, prop 3, but 4.

3Methanol: CH₃OH, molecular formula CH₄O. Ethanol: CH₃CH₂OH (also written C₂H₅OH), C₂H₆O. Propanol: CH₃CH₂CH₂OH, C₃H₈O. Butanol: CH₃CH₂CH₂CH₂OH, C₄H₁₀O.

4Each alcohol has one more carbon in its chain than the one before, so each molecular formula is one CH₂ more than the last.

5A structural formula shows how the atoms are arranged. A molecular formula shows only the number of atoms of each element. A displayed formula shows every covalent bond, with four bonds on every carbon atom and the OH group drawn out.

6Boiling points: methane −164 °C, methanol 65 °C; ethane −89 °C, ethanol 78 °C; propane −42 °C, propanol 97 °C. Each alcohol boils far higher than its alkane.

7The forces between alcohol molecules are stronger, so more energy is needed to overcome them. Along the alcohol series, boiling point rises as the chain gets longer: pentanol boils at 137 °C.

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Worked example

Problem

An alcohol molecule has just one carbon atom. Name it, write its structural and molecular formulae, and describe what its displayed formula shows.

⚠ Watch out

Treating CH₃CH₂OH and C₂H₆O as two different substances. They're the same molecule, ethanol: the structural formula shows how the atoms are arranged, and the molecular formula only counts them.

🧠

Memory hook

Monkeys Eat Peeled Bananas: meth 1, eth 2, prop 3, but 4. The –OH adds the -ol, and every step along the series adds one CH₂.

✓

Check yourself

Cover the page. Name CH₃CH₂CH₂CH₂OH and give its molecular formula. Which boils higher, propane or propanol, and why? (Butanol, C₄H₁₀O. Propanol: stronger forces between its molecules need more energy to overcome.)

Flashcards

(15)

What is a functional group?

The atom or group of atoms responsible for the way a compound reacts.

Which functional group do all alcohols contain?

The –OH group.

What do the prefixes meth-, eth-, prop- and but- tell you?

The number of carbon atoms in the longest chain: 1, 2, 3 and 4.

What ending does every alcohol's name have, and why?

-ol, because the molecule contains the –OH group.

Methanol: structural and molecular formula?

CH₃OH and CH₄O.

Ethanol: structural and molecular formula?

CH₃CH₂OH (or C₂H₅OH) and C₂H₆O.

Propanol: structural and molecular formula?

CH₃CH₂CH₂OH and C₃H₈O.

Butanol: structural and molecular formula?

CH₃CH₂CH₂CH₂OH and C₄H₁₀O.

How does each alcohol's molecular formula differ from the one before it?

By one CH₂ unit, because the chain has one more carbon.

What does a structural formula show?

How the atoms are arranged in the molecule, e.g. C₂H₅OH.

What does a molecular formula show?

Only the number of atoms of each element, e.g. C₂H₆O.

What does a displayed formula show?

Every covalent bond as a line, with four bonds on each carbon and the O–H drawn out.

Boiling points of methane and methanol?

Methane −164 °C; methanol 65 °C.

Why does an alcohol boil higher than the alkane with the same number of carbons?

The forces between alcohol molecules are stronger, so more energy is needed to overcome them.

How does boiling point change along the alcohol series?

It increases as the carbon chain gets longer (e.g. pentanol, 137 °C, is above propanol, 97 °C).

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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From a carbon count to a name and formula

Structural, molecular or displayed?

Where does each one boil?

Why does ethanol boil so much higher than ethane?

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