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Lesson 1 of 6Organic Chemistry
Unit 1 · Lesson 1 of 6

Hydrocarbons: Naming and Isomers

Alkanes, alkenes, alkynes, cyclic hydrocarbons and benzene: how to name them with IUPAC rules, how to draw them, and why molecules with the same formula can be different compounds.

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Key things to remember

  1. 1Alkanes (saturated); alkenes ; alkynes ; cycloalkanes .
  2. 2Name = branch locants and names (alphabetical) + parent prefix + ending, e.g. 3-ethyl-2,2-dimethylpentane.
  3. 3The parent chain is the longest continuous chain, and it must contain any double or triple bond, which gets the lowest number.
  4. 4Structural isomers share a molecular formula but differ in connections; more branching means weaker London forces and a lower boiling point.
  5. 5Cis-trans isomers need restricted rotation (C=C or a ring) and two different groups on each double-bond carbon.
  6. 6Benzene, , has delocalized ring electrons, so it is more stable and less reactive than an alkene.

Organic chemistry is the chemistry of carbon compounds (apart from a few simple ones such as carbonates, carbon dioxide and carbon monoxide, which are treated as inorganic). Carbon is special because each atom forms four covalent bonds, and carbon atoms bond strongly to each other. That lets carbon build long chains, branched chains and rings, with single, double or triple bonds between carbons. The result is millions of known compounds, from the methane in natural gas to the octane in gasoline. The simplest organic compounds are hydrocarbons, which contain only carbon and hydrogen. Because there are so many, chemists use one systematic naming system (IUPAC) so that every name points to exactly one structure.

FamilyBondingGeneral formulaName endingExample
Alkaneonly C–C single bonds (saturated)-anepropane,
Alkeneat least one C=C double bond (unsaturated) (one C=C)-enepropene,
Alkyneat least one C≡C triple bond (unsaturated) (one C≡C)-ynepropyne,
Cycloalkanesingle bonds joined in a ringcyclo- … -anecyclohexane,
Aromaticbenzene ring with shared (delocalized) electronsbenzene is -benzenemethylbenzene,
The hydrocarbon families. Saturated means every carbon holds the most hydrogen atoms it can.

The same molecule can be drawn several ways. A molecular formula only counts atoms: . A structural formula shows every bond. A condensed formula groups each carbon with its hydrogens, e.g. , with branches in brackets: . A line (skeletal) diagram draws only the carbon skeleton as a zigzag: every corner and line end is a carbon atom, and the hydrogens on carbon are left out because you can work them out (each carbon needs four bonds in total). The molecular formula alone is not enough to identify an organic compound, as you will see with isomers.

CarbonsPrefixStraight-chain alkaneBoiling point (about)
1meth-methane, −162 °C
2eth-ethane, −89 °C
3prop-propane, −42 °C
4but-butane, −1 °C
5pent-pentane, 36 °C
6hex-hexane, 69 °C
7hept-heptane, 98 °C
8oct-octane, 126 °C
9non-nonane, 151 °C
10dec-decane, 174 °C
Carbon-count prefixes. Boiling points rise with chain length because longer molecules have stronger London dispersion forces.

Naming Branched Alkanes

  1. Find the parent chain: the longest continuous chain of carbons, even if it bends around corners in the drawing. Its length gives the prefix; the ending is -ane. If two chains tie for longest, choose the one with more branches.
  2. Number the chain from the end that gives the branches the lowest numbers (locants). Compare the two numberings at the first point where they differ.
  3. Name each branch (alkyl group) by dropping -ane and adding -yl: is methyl, is ethyl, is propyl. Halogen branches are fluoro, chloro, bromo, iodo.
  4. Group identical branches with di-, tri-, tetra-, and give every branch its own number: 2,2-dimethyl, not 2-dimethyl.
  5. Write branches in alphabetical order, ignoring di-/tri- when alphabetizing (ethyl comes before dimethyl because e comes before m).
  6. Punctuate: commas between numbers, hyphens between numbers and letters, and the last branch runs straight into the parent name with no space: 3-ethyl-2,2-dimethylpentane.
Look for the hidden longest chain
A chain drawn in a straight line is not always the longest. If a molecule is drawn as a butane chain with an ethyl group on carbon 2, follow the ethyl group into the chain: you get five carbons in a row with a methyl on carbon 3. The correct name is 3-methylpentane, and "2-ethylbutane" is never a correct name.

Alkenes, Alkynes, Rings and Benzene

For an alkene or alkyne, the parent chain must contain the multiple bond, and the multiple bond gets the lowest possible number, even if that gives the branches higher numbers. (This is the convention used in high-school chemistry; the 2013 IUPAC recommendations pick the longest chain first, but follow the convention here.) The number of the first carbon of the double or triple bond goes just before the ending: is but-2-ene (older resources write 2-butene; both point to the same molecule). Ethene, propene, ethyne and propyne need no number because the multiple bond can only be in one place. Cyclic hydrocarbons add cyclo- to the parent name: cyclopentane, cyclohexene (in a cycloalkene the double-bond carbons are numbered 1 and 2). Benzene, , is a flat ring of six carbons whose bonding electrons are shared evenly around the whole ring instead of sitting in three fixed double bonds, which is why it is drawn as a hexagon with a circle inside. That shared electron cloud makes benzene much more stable and far less reactive than an ordinary alkene. Substituted benzenes are named as ___benzene: methylbenzene (also called toluene), chlorobenzene, 1,2-dimethylbenzene.

Isomers

Isomers are compounds with the same molecular formula but different arrangements of atoms, so they are different substances with different properties. Structural isomers have atoms joined in a different order. has two: butane (a straight chain) and 2-methylpropane (a branched chain). has three: pentane, 2-methylbutane and 2,2-dimethylpropane. Their boiling points are about 36 °C, 28 °C and 10 °C. The more branched a molecule is, the more compact and ball-like it becomes, so it has less surface in contact with its neighbours, weaker London dispersion forces, and a lower boiling point.

Structural isomers (C₄H₁₀)Cis-trans isomers (C₄H₈)butaneCH₃CH₂CH₂CH₃b.p. about −1 °C2-methylpropaneCH₃CH(CH₃)CH₃b.p. about −12 °Csame formula, different connectivityeach line end or bend = one carbonCCCH₃HCH₃HCCCH₃HHCH₃cis-but-2-eneCH₃ on same sidetrans-but-2-eneCH₃ on opposite sidesC=C cannot rotate, so the two forms differneeds two different groups on each C of C=C
Fig 1Structural isomers of C4H10 and the cis and trans isomers of but-2-ene.

Stereoisomers have the same atoms joined in the same order but arranged differently in space. The type you need here is cis-trans (geometric) isomerism. A C=C double bond cannot twist, so groups attached to the two double-bond carbons are locked on the same side or on opposite sides. In cis-but-2-ene the two groups are on the same side; in trans-but-2-ene they are on opposite sides. The two are separate compounds: the cis isomer is slightly polar, while in the trans isomer the bond dipoles cancel. A C–C single bond, by contrast, rotates freely, so alkanes have no cis-trans isomers (small rings, which also cannot rotate, can).

The two-different-groups test
Cis-trans isomers exist only if each carbon of the double bond carries two different groups. But-2-ene passes (each double-bond carbon has an H and a ). But-1-ene, , fails: carbon 1 carries two identical H atoms, so flipping it gives the same molecule.
AStructural isomers
  • Same molecular formula
  • Atoms connected in a different order (different chain, branch position or multiple-bond position)
  • Different names, e.g. butane and 2-methylpropane
  • Converting one to the other means breaking bonds
BCis-trans isomers (stereoisomers)
  • Same molecular formula
  • Same connections; different arrangement in space
  • Same name with a cis- or trans- prefix, e.g. cis-but-2-ene
  • Need a double bond or ring that stops rotation, and two different groups on each locked carbon

Worked examples

Reveal one step at a time and try to beat it to the next line.

Example 1
Name the alkane
Example 2
Name .
Example 3
Do pent-1-ene and pent-2-ene have cis-trans isomers? If so, describe them.
Example 4
A student names a compound "2-ethylbutane". Draw what they meant and give the correct IUPAC name.

Where marks get lost

  • Using the chain as it is drawn instead of the longest continuous chain, which produces impossible names such as 2-ethylbutane (really 3-methylpentane) or 1-methylbutane (really pentane).
  • Numbering an alkene to give the branch the lowest number instead of the double bond, e.g. calling 2-methylbut-3-ene instead of 3-methylbut-1-ene.
  • Alphabetizing by the di- or tri- prefix, writing 2,2-dimethyl-3-ethylpentane instead of 3-ethyl-2,2-dimethylpentane.
  • Claiming cis-trans isomers for propene or but-1-ene, where one double-bond carbon carries two identical H atoms.
  • Treating benzene as if it had three ordinary double bonds that react like an alkene; its delocalized electrons make it much less reactive.

Flashcards 9

Saturated vs unsaturated hydrocarbon
General formula of an alkane
General formula of an alkene with one double bond (or a cycloalkane)
General formula of an alkyne with one triple bond
+ 5 more

Practice

1
What is the IUPAC name of ?
2
Which compound can exist as cis and trans isomers?
3
How many hydrogen atoms are in one molecule of hept-2-yne?
4
How many structural isomers have the molecular formula ?
5
A structure is drawn as a butane chain with an ethyl group on carbon 2. Give its correct IUPAC name.
6
Give the IUPAC name of , a benzene ring carrying one methyl group.
7
Why does 2,2-dimethylpropane boil at a lower temperature (about 10 °C) than pentane (about 36 °C)?
8
What is the molecular formula of cyclopentane?
9
Order these hydrocarbons from lowest to highest boiling point.
  1. 12-methylbutane
  2. 2hexane
  3. 3pentane
  4. 42,2-dimethylpropane
10
Match each structure to its hydrocarbon family.
a ring of six groups
ring with delocalized electrons

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