B. Organic Chemistry · Organic ChemistryIsomers and the Physical Properties of Hydrocarbons
B3.5 B3.2 B2.1 B2.3 A1.6 A1.8About 70 minutes
Learning goals- I can define isomers and tell structural isomers from stereoisomers.
- I can classify structural isomers as chain, position or functional-group isomers.
- I can draw and name all the structural isomers of a small alkane or alkene.
- I can explain, using intermolecular forces, how branching and cis-trans geometry affect boiling and melting points.
Words to know
isomers
Different compounds with the same molecular formula.
structural (constitutional) isomers
Isomers whose atoms are joined in a different order.
chain isomers
Structural isomers with different carbon skeletons (straight or branched).
position isomers
Structural isomers with the same skeleton but a substituent, multiple bond or functional group in a different position.
functional-group isomers
Structural isomers that belong to different families, such as an alcohol and an ether.
stereoisomers
Isomers with the same order of bonded atoms but a different arrangement in space; includes cis-trans and optical isomers.
London dispersion forces
Weak attractions between all molecules caused by momentary uneven distributions of electrons; they grow with molecular size and contact area.
Same formula, different molecules
Butane is the fuel in disposable lighters. Its isomer, 2-methylpropane (isobutane), is used as a refrigerant and in aerosol cans. Both are C₄H₁₀, but they are different compounds: butane boils at −1 °C and 2-methylpropane at −12 °C.
Compounds with the same molecular formula but different structures are isomers. Isomers have different names, different properties and sometimes very different behaviour. The number of possible isomers grows explosively with size: C₄H₁₀ has 2 alkane isomers, C₅H₁₂ has 3, C₆H₁₄ has 5, C₇H₁₆ has 9, C₈H₁₈ has 18 and C₁₀H₂₂ has 75.
Kinds of isomers
Structural isomers have atoms joined in a different order.
- Chain isomers: different carbon skeletons. Pentane, 2-methylbutane and 2,2-dimethylpropane are all C₅H₁₂.
- Position isomers: same skeleton, but a branch, halogen, multiple bond or functional group sits in a different place. But-1-ene and but-2-ene; 1-chloropropane and 2-chloropropane.
- Functional-group isomers: different families. C₂H₆O can be ethanol (an alcohol, CH₃CH₂OH) or methoxymethane (an ether, CH₃OCH₃). C₃H₆ can be propene (an alkene) or cyclopropane (a cycloalkane).
Stereoisomers have the same connections, but a different arrangement in space.
- Cis-trans isomers (Lesson 3): groups on the same or opposite sides of a C=C bond or a ring.
- Optical isomers: molecules that are mirror images of each other, like your left and right hands. You will study them in the lesson Comparing the Classes: Structure, Properties and Optical Isomers.
Watch out: same molecule, different drawing. Because single bonds rotate, CH₃CH₂CH₂CH₃ can be drawn straight, bent or curled. These are all butane, not isomers. The quickest test is to name each drawing: if the names are the same, the molecules are the same.
Worked exampleExample 1: All the isomers of C₆H₁₄
Draw and name every alkane isomer of C₆H₁₄.
- Work systematically from the longest chain down, so you don't miss or repeat any.
- 6-carbon chain: CH₃CH₂CH₂CH₂CH₂CH₃, hexane.
- 5-carbon chain + one methyl: the methyl can go on C2 (2-methylpentane) or C3 (3-methylpentane). C4 is the same as C2 counted from the other end; C1 just makes hexane again.
- 4-carbon chain + two methyls: both on C2 (2,2-dimethylbutane) or on C2 and C3 (2,3-dimethylbutane). An ethyl on a 4-carbon chain would make a longer chain, so it is not new.
- A 3-carbon chain cannot hold three more carbons without creating a longer chain, so we are done.
Five isomers: hexane (bp 69 °C), 2-methylpentane (60 °C), 3-methylpentane (63 °C), 2,3-dimethylbutane (58 °C) and 2,2-dimethylbutane (50 °C).
Worked exampleExample 2: Same, isomers, or neither?
Classify each pair: (a) CH₃CH₂CH₂CH₂Cl and CH₃CH₂CHClCH₃; (b) CH₃CH(CH₃)CH₂CH₃ and CH₃CH₂CH(CH₃)CH₃; (c) CH₂=CHCH₂CH₃ and cyclobutane; (d) propane and propene.
- (a) Both C₄H₉Cl; Cl on C1 vs C2. Names: 1-chlorobutane and 2-chlorobutane → position isomers.
- (b) Name both: each is 2-methylbutane (the second is written backwards) → the same compound.
- (c) Both C₄H₈; one is an alkene, the other a cycloalkane → functional-group (structural) isomers.
- (d) C₃H₈ and C₃H₆ have different formulas → neither: not isomers.
(a) position isomers, (b) same compound, (c) structural (functional-group) isomers, (d) not isomers.
How structure affects physical properties
Hydrocarbons are non-polar, so the only forces between their molecules are London dispersion forces. Two things control how strong these forces are:
- Number of electrons (molecule size). Bigger molecules have more electrons, which are easier to distort, so the forces are stronger. Boiling points rise along a homologous series.
- Surface contact (shape). Long, thin molecules can lie alongside each other and touch along their whole length. Branched molecules are more compact, closer to a ball, and touch over a smaller area. For isomers, more branching means weaker forces and a lower boiling point.
Melting points are trickier, because they also depend on how well molecules pack into a crystal. Very symmetrical molecules pack neatly: 2,2-dimethylpropane, the most branched C₅H₁₂, has the lowest boiling point (10 °C) but the highest melting point (about −17 °C, compared with −130 °C for pentane).
Cis-trans isomers differ too. In cis-but-2-ene the two C–CH₃ bond dipoles do not cancel, so the molecule is very slightly polar, and it boils a little higher (4 °C) than trans-but-2-ene (1 °C). The trans isomer is more symmetrical and packs better, so it melts higher (−106 °C vs −139 °C).
| Isomer of C₅H₁₂ | Boiling point (°C) | Melting point (°C) |
|---|
| pentane | 36 | −130 |
| 2-methylbutane | 28 | −160 |
| 2,2-dimethylpropane | 10 | −17 |
Worked exampleExample 3: Predicting boiling points
Rank by boiling point, lowest first: heptane, 2,2,3-trimethylbutane, 2-methylhexane. (All are C₇H₁₆.)
- Same formula and all non-polar, so the same number of electrons: compare shape.
- Heptane is unbranched: most contact.
- 2-methylhexane has one branch.
- 2,2,3-trimethylbutane has three branches and is the most compact.
2,2,3-trimethylbutane < 2-methylhexane < heptane (measured: 81, 90 and 98 °C).
Hands-on activityBuild every isomer
You need: 6 "carbon" pieces and 14 "hydrogen" pieces (gumdrops, clay balls or bottle caps in two colours), about 20 toothpicks or straws, paper and pencil. Grown-up note: toothpicks are sharp.
- Build hexane, C₆H₁₄. Then, using the same 6 carbons and 14 hydrogens each time, rebuild the pieces into a different molecule. Every carbon must have 4 bonds.
- Draw a skeletal structure of each model and name it before you take it apart. If a name repeats, that model was not new.
- Stop when you think you have them all. (Hint: there are five.)
- Make a data table of your five names and these boiling points: 69, 63, 60, 58 and 50 °C. Match each value to an isomer using the branching rule, then check your matches against Example 1.
- Graph boiling point against the number of branches. Describe the trend and any exception (A1.6, A1.8).
What to look for: your graph should slope downward. Look closely at the one-branch isomers: 3-methylpentane (63 °C) boils a little higher than 2-methylpentane (60 °C), even though both have one branch. The branching rule predicts the overall trend, but small differences depend on the detailed shape, which is why chemists rely on measured data, not only rules.
Just for funThe alkane C₃₀H₆₂ has a little over 4 billion possible structural isomers. If you built one model per second, day and night, you would need more than 130 years.
Your turn
1What do all isomers have in common?
The same molecular formula.
2Name the two alkane isomers of C₄H₁₀.
butane and 2-methylpropane.
3Classify: propan-1-ol (CH₃CH₂CH₂OH) and propan-2-ol (CH₃CH(OH)CH₃).
Same skeleton, –OH in a different position: position isomers.
4Are CH₃CH₂CH₂CH₃ drawn in a straight line and the same chain drawn bent into a U shape isomers?
No: they are the same compound (butane). Rotation about single bonds lets the chain take any shape.
5Which has the higher boiling point: 2-methylpentane or 2,2-dimethylbutane? Explain.
2-methylpentane (60 °C vs 50 °C). It is less branched, so its molecules have more surface contact and stronger London forces.
6Give a functional-group isomer of propene, C₃H₆.
7Why does decane boil far higher than any isomer of pentane?
Decane (C₁₀H₂₂) has many more electrons than any C₅H₁₂, so its London forces are much stronger. Size matters more than shape when the formulas differ.
More practice: warm-up
1What does "structural isomer" mean?
Same formula, atoms joined in a different order.
2Name one pair of cis-trans isomers.
cis-but-2-ene and trans-but-2-ene (other answers possible).
3How many alkane isomers does C₅H₁₂ have?
4What force acts between non-polar hydrocarbon molecules?
London dispersion forces.
5Does branching raise or lower the boiling point of an alkane isomer?
More practice: on your own
1Draw (condensed) and name all the alkene isomers of C₄H₈, including stereoisomers.
but-1-ene CH₂=CHCH₂CH₃; cis-but-2-ene and trans-but-2-ene CH₃CH=CHCH₃; 2-methylpropene CH₂=C(CH₃)₂. That is four alkenes. (Cyclobutane and methylcyclopropane are two more C₄H₈ isomers, but they are cycloalkanes.)
2Name the structural isomers of C₃H₇Br.
1-bromopropane (CH₃CH₂CH₂Br) and 2-bromopropane (CH₃CHBrCH₃): position isomers.
3Classify the pair: CH₃CH₂CH₂CH₂CH₃ and CH₃CH₂CH(CH₃)₂.
Pentane and 2-methylbutane: chain isomers (both C₅H₁₂).
4Use the data: trans-but-2-ene mp −106 °C, cis-but-2-ene mp −139 °C. Explain the difference.
The trans isomer is more symmetrical (straighter), so its molecules pack more tightly in the solid, and more energy is needed to break the crystal apart. Its melting point is higher.
5A student says, "More branching always raises the melting point." Use the C₅H₁₂ data to evaluate the claim.
Not always. 2-methylbutane (one branch) melts at −160 °C, lower than pentane (−130 °C). Only the very symmetrical 2,2-dimethylpropane (−17 °C) melts higher. Melting depends on packing, which depends on symmetry, not just branching.
Challenge
1Find and name all structural isomers of C₄H₉Cl.
Four: 1-chlorobutane, 2-chlorobutane (from the butane skeleton), 1-chloro-2-methylpropane and 2-chloro-2-methylpropane (from the 2-methylpropane skeleton). (2-chlorobutane also exists as two optical isomers; see Comparing the Classes: Structure, Properties and Optical Isomers.)
2How many structural isomers (open-chain and ring) have the formula C₃H₆Cl₂? Name them.
Open chain only (C₃H₆Cl₂ = C₃H₈ with 2 H replaced, no unsaturation left): 1,1-dichloropropane, 1,2-dichloropropane, 1,3-dichloropropane, 2,2-dichloropropane. Four isomers; no ring is possible because the formula is saturated.
3Natural gas companies add a smelly compound to methane so leaks can be detected. Methane boils at −162 °C and is a gas in the pipes even in a Canadian winter. Explain, using intermolecular forces, why methane has such a low boiling point and why propane (−42 °C) can be stored as a liquid in a tank under pressure but methane cannot be liquefied that way at room temperature.
Methane is a tiny, non-polar molecule with only 10 electrons, so its London forces are extremely weak and very little energy separates the molecules. Propane is bigger (26 electrons), so its London forces are stronger; at room temperature, moderate pressure pushes the molecules close enough to condense. Methane's attractions are too weak to liquefy at room temperature by pressure alone: it must be cooled (its critical temperature is about −82 °C), which is why it is shipped as liquefied natural gas only at very low temperatures.
Quiz
2. Which C₅H₁₂ isomer has the lowest boiling point?
3. Ethanol and methoxymethane are both C₂H₆O. What kind of isomers are they?
4. What makes cis-trans isomers possible in alkenes?
5. Which is NOT a pair of isomers?
6. Why does branching lower the boiling point of alkane isomers?
Big ideas (B3.5, B3.2): isomers share a formula but differ in structure; structural isomers differ in connections, stereoisomers in 3-D arrangement. For non-polar hydrocarbons, size and shape control London forces, so branching lowers boiling points.
How to help: Give your teen a formula (C₆H₁₄) and ask them to find all the isomers systematically, naming each one to catch duplicates.
Common difficulties:
- Counting the same molecule twice because it is drawn differently.
- Calling molecules with different formulas "isomers".
- Assuming branching affects melting points the same way as boiling points.
- Forgetting cis-trans isomers when listing alkene isomers.
Skills (A1.6, A1.8): the activity asks your teen to graph data and explain an exception. Ask, "What would you need to measure to test your explanation?"