Lesson · Grade 11 · Chemistry
Isomers: Same Formula, Different Structure
- Grade 11
- Chemistry
- 1 min 51 s
- British Columbia
- BC11-SC-15
C4H10 forms two molecules: butane, an unbranched chain, and 2-methylpropane, which is branched. Their 3D ball-and-stick models turn and bend to show that only a different order of bonded atoms makes a new isomer, and that the branched one boils lower (about −12 °C against about −1 °C) because it touches its neighbours less. The same diagram then shows that ethanol (about 78 °C) boils far higher than dimethyl ether (about −24 °C) because only ethanol has a hydrogen on oxygen that can form hydrogen bonds.
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In one sentence
The question this lesson answers
Can two different molecules share one molecular formula?
A molecular formula counts the atoms: C four H ten means four carbon atoms and ten hydrogen atoms.
Transcript
What the lesson says
Can two different molecules share one molecular formula?
A molecular formula counts the atoms: C four H ten means four carbon atoms and ten hydrogen atoms.
Join the four carbons in one chain, and you get butane.
Or join three in a chain and put the fourth on the middle one, as a branch: 2-methylpropane.
These are isomers: same molecular formula, different structure. Their atoms are joined in a different order.
Turning a model in three dimensions does not change the molecule.
The butane chain can bend around a single bond, but its carbons still run one, two, three, four: still butane, not a new isomer.
Butane boils at about minus 1 degree Celsius, but 2-methylpropane boils at about minus 12.
Dispersion forces, weak attractions between molecules, grow with contact. Compact 2-methylpropane touches less, so it boils lower.
Another formula, C two H six O, has two isomers: ethanol and dimethyl ether.
Ethanol boils at about 78 degrees Celsius, but dimethyl ether boils at about minus 24.
In ethanol, a hydrogen on oxygen is attracted to the oxygen of the next molecule: a strong hydrogen bond.
Dimethyl ether has no hydrogen on oxygen, so its molecules form no hydrogen bonds, and it boils far lower.
Isomers have the same molecular formula but a different structure, which gives them different properties.
Curriculum
Where this fits
- British Columbia, Grade 11
Chemistry 11, Organic chemistry: organic compoundsBC11-SC-15
Previous: Functional groups: the part that gives a molecule its behaviourNext: Polymers: Long Chains of Repeating Units
The outcomes and strands follow the British Columbia curriculum.
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