Worked example 1
Name the compound CH₃–CH(CH₃)–CH₂–CH₂–CH₃.
Try it first: How many carbons are in the longest continuous chain?
0 of 4 steps revealed.
Organic ChemistryHydrocarbonsContent level: Core 18 min
Hydrocarbons and IUPAC Nomenclature
What you'll be able to do: Recognise alkanes, alkenes and alkynes from a structure or formula and name a branched hydrocarbon using IUPAC rules.
Introduction
Organic chemistry starts with carbon chains. Carbon forms four bonds, so a handful of simple rules generates every alkane, alkene and alkyne, and one naming system covers all of them.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Distinguish alkanes, alkenes and alkynes by bonding and general formula
- Convert between molecular, condensed and skeletal structures
- Name a branched hydrocarbon: parent chain, substituents, lowest locants
- Draw a structure from an IUPAC name
Lesson
Carbon makes four bonds
Carbon has four valence electrons, so it forms four covalent bonds. Those bonds can be four single bonds, two singles and a double, or a single and a triple. Chains, branches and rings all come from that one fact, which is why so few elements give so many compounds.
CH₄, C₂H₆, C₃H₈ — each carbon still has four bonds.
The three hydrocarbon families
Alkanes contain only C–C single bonds and follow CₙH₂ₙ₊₂; they are saturated. Alkenes contain at least one C=C double bond and follow CₙH₂ₙ for one double bond. Alkynes contain a C≡C triple bond and follow CₙH₂ₙ₋₂. Each degree of unsaturation (a double bond or a ring) removes two hydrogens.
Count hydrogens against CₙH₂ₙ₊₂. Every 2 H you are short is one double bond or one ring.
Prefixes for chain length
The first ten roots are meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6), hept- (7), oct- (8), non- (9), dec- (10). The ending tells you the family: -ane, -ene, -yne.
pentane = 5 carbons, all single bonds; pent-2-ene = 5 carbons, C=C starting at carbon 2.
Naming in four steps
1) Find the longest continuous carbon chain that contains the multiple bond — that is the parent. 2) Number the chain from the end that gives the multiple bond, or failing that the first substituent, the lowest number. 3) Name each branch as a substituent: methyl, ethyl, propyl, and halogens as fluoro, chloro, bromo, iodo. 4) List substituents alphabetically with their locants, using di-, tri-, tetra- for repeats.
locant-substituent + parent chain + ending
Skeletal structures
Chemists draw lines instead of letters: each vertex and each line end is a carbon, and hydrogens on carbon are not drawn. A zig-zag of four line segments is pentane. Reading skeletal structures quickly is the single most useful organic skill.
In a skeletal drawing, every carbon silently carries enough hydrogens to reach four bonds.
Key ideas
Definition
Hydrocarbon
A compound of carbon and hydrogen only.
Definition
Saturated
Every C–C bond is single, so the molecule holds the maximum possible hydrogens.
Rule
Parent chain
The longest continuous chain containing the multiple bond sets the name.
Rule
Lowest locants
Number so the multiple bond gets the lowest number; if there is none, the first substituent does.
Key concept
Degrees of unsaturation
Each double bond or ring lowers the hydrogen count by 2 from CₙH₂ₙ₊₂.
Equation
Alkane general formula
CₙH₂ₙ₊₂
- n = number of carbon atoms
Equation
Alkene (one C=C)
CₙH₂ₙ
- n = number of carbon atoms
Equation
Alkyne (one C≡C)
CₙH₂ₙ₋₂
- n = number of carbon atoms
Worked examples
Worked example 2
Give the IUPAC name of CH₃–CH=CH–CH₂–CH₃.
Try it first: Where does the double bond start when you number from each end?
0 of 3 steps revealed.
Common mistakes
Choosing the horizontal chain as the parent because that is how it is drawn.
Why it's wrong: The parent is the longest continuous chain, which often turns a corner in the drawing.
Check instead: Trace every path through the skeleton and count carbons before naming.
Numbering from the left out of habit.
Why it's wrong: The direction is decided by which end gives the lower locants, not by the page.
Check instead: Number both ways, write both locant sets, and keep the lower one.
Treating C₄H₈ as an alkane.
Why it's wrong: An alkane with 4 carbons is C₄H₁₀; two hydrogens short means one double bond or one ring.
Check instead: Compare the hydrogen count to CₙH₂ₙ₊₂ before assigning a family.
Practice this skill
No practice questions are available for this topic yet. You can still practice the whole unit.
What you should now know
Hydrocarbons contain only carbon and hydrogen, and split into alkanes (all single bonds, CₙH₂ₙ₊₂), alkenes (a C=C, CₙH₂ₙ) and alkynes (a C≡C, CₙH₂ₙ₋₂). IUPAC names are built by finding the longest chain containing the multiple bond, numbering it so the multiple bond or first substituent gets the lowest locant, and listing branches alphabetically with their positions. Skeletal drawings hide carbons at vertices and all hydrogens on carbon.
- Carbon always forms four bonds, which is why chains, branches and rings are possible
- Alkane CₙH₂ₙ₊₂, alkene CₙH₂ₙ, alkyne CₙH₂ₙ₋₂
- Each double bond or ring removes 2 hydrogens from the saturated count
- Parent chain = longest continuous chain containing the multiple bond
- Number to give the multiple bond, then the first substituent, the lowest locant
Sources and further reading
This lesson is original Chem Help content. No external sources were adapted.