IUPAC Nomenclature (DP IB Chemistry)

IUPAC Nomenclature

• IUPAC or systematic nomenclature can be used to name organic compounds and therefore make it easier to refer to them

Naming hydrocarbons

• Hydrocarbons are compounds containing hydrogen and carbon only

• There are four families of hydrocarbons you should know: 

  1. Alkanes

  2. Alkenes

  3. Alkynes

  4. Arenes

Naming alkanes

• Alkanes have the general molecular formula C_nH_2n+2

  • They contain only single bonds and are said to be saturated

• Alkanes are named using the nomenclature rule alk + ane

  • The alk depends on the number of carbons as outlined in the following table 

• The alkanes provide the basis of the naming system and the stem of each name indicates how many carbon atoms are in the longest chain in one molecule of the compound

IUPAC system of naming alkanes table

• If there are any side-chains or functional groups present, then the position of these groups is indicated by numbering the carbon atoms in the longest chain starting at the end that gives the lowest possible numbers in the name

• The hydrocarbon side chain is shown in brackets in the structural formula

CH₃CH(CH₃)CH₂CH₃

• The side-chain is named by adding ‘-yl’ to the normal alkane stem

• This type of group is called an alkyl group

Naming organic compounds with one alkyl side chain

The longest chain provides the main name and the side chain is shown as a numbered alkyl prefix

• If there are more than one of the same alkyl side chain or functional groups, di- (for two), tri- (for three) or tetra- (for four) is added in front of its name

• The adjacent numbers have a comma between them

• Numbers are separated from words by a hyphen

Naming organic compounds with multiple, identical side chains

The longest chain still provides the main name and the side chains are shown as numbered alkyl prefixes

• If there is more than one type of alkyl side chain, the same numbering system applies but the different side chains are listed in alphabetic order

Naming organic compounds with multiple, different side chains

The longest chain still provides the main name and the side chains are still shown as numbered alkyl prefixes but in alphabetical order

Naming alkenes

• Alkenes have the general molecular formula C_nH_2n

• They are said to be unsaturated

• Alkenes are named using the nomenclature rule alk + ene

• In molecules with a straight chain of 4 or more carbon atoms, the position of the C=C double bond must be specified

  • The carbon atoms on the straight chain must be numbered, starting with the end closest to the double bond

  • The lowest-numbered carbon atom participating in the double bond is indicated just before the -ene:

IUPAC system of naming alkenes table

Number of carbon atoms	Displayed formula of straight-chain alkene	Molecular formula of alkene	IUPAC name of alkene
1	-	-	-
2		C2H4	ethene
3		C3H6	propene
4		C4H8	but-1-ene
5		C5H10	pent-2-ene
6		C6H12	hex-3-ene

• There is a distinction to be made between the name of the functional group and the name of the family

  • The name of the family is alkene

  • The name of the functional group is alkenyl

Naming alkynes

• Alkynes have the general molecular formula C_nH_2n-2

• The triple bond makes them unsaturated molecules

• Alkenes are named using the nomenclature rule alk + yne

• In molecules with a straight chain of 4 or more carbon atoms, the position of the C=C triple bond must be specified

  • The carbon atoms on the straight chain must be numbered, starting with the end closest to the triple bond

  • The lowest-numbered carbon atom participating in the triple bond is indicated just before the -yne:

IUPAC system of naming alkynes table

Number of carbon atoms	Displayed formula of straight-chain alkyne	Molecular formula of alkyne	IUPAC name of alkyne
1	-	-	-
2		C2H2	ethyne
3		C3H4	propyne
4		C4H6	but-1-yne
5		C5H8	pent-2-yne
6		C6H10	hex-3-yne

• There is a distinction to be made between the name of the functional group and the name of the family

  • The name of the family is alkyne

  • The name of the functional group is alkynyl

Naming arenes

• Arenes are aromatic compounds, i.e. compounds with one or more rings with pi electrons that are delocalised throughout the ring(s)

• Benzene, C₆H₆, is the only aromatic hydrocarbon that is covered in IB Chemistry and is covered in our Benzene revision note

• Naming aromatic compounds depends on whether the benzene ring is considered the main structure or a functional group

• Benzene as the main structure:

  • Alkyl groups attached to benzene rings are named using the nomenclature rule alkyl group + benzene

  • If there is only one alkyl group attached, then no numbering system is applied to the benzene ring

  • If more than one alkyl group is attached, then a relative numbering system is applied

    • This is where the longest alkyl chain is considered as being attached to carbon-1 of the ring

    • The other alkyl groups are then numbered accordingly 

    • The alkyl groups are still named in alphabetical order 

Examples of alkyl substituted aromatic compounds

Methylbenzene is often called by the common name toluene. 1-ethyl-2-methylbenzene has the longest (ethyl) side chain set as carbon-1, so the methyl side chain is attached to carbon-2

• Benzene as a functional group:

  • The functional group in benzene is known as a phenyl group when attached to other molecules

Benzene as the functional group

The benzene ring is considered a functional group in amine structures, which means that this aromatic compound is called phenylamine

Halogenoalkanes

• Halogenoalkanes have the general molecular formula, C_nH_2n+1X, where X represents a halogen

• Halogenoalkanes are named using the prefix chloro-, bromo- or iodo-, with the ending -ane

• In molecules with a straight chain of three or more carbon atoms, the position of the halogen atom must also be specified

  • The carbon atoms on the straight chain must be numbered, starting with the end closest to the halogen atom

  • The number of the carbon atom attached to the halogen is indicated before the prefix:

Halogenoalkanes Examples Table

Displayed formula of halogenoalkane	Molecular formula of halogenoalkane	IUPAC name of halogenoalkane
	C2H5Cl	chloroethane
	C3H7Br	2-bromopropane
	C5H11I	1-iodopentane
	C5H11Cl	3-chloropentane

• For halogenoalkanes with multiple halogen functional groups, the position and type of functional group must be given

  • For example, an ethane chain with 2 chlorine functional groups on carbon-1 and one chlorine functional group on carbon 2 will be named 1,1,2-trichloroethane

Alcohols

• Alcohols are a family of molecules that contain the hydroxyl functional group, -OH

• Their general formula is  C_nH_2n+1OH

• The nomenclature of alcohols follows the pattern alkan + ol

  • If there are two -OH groups present the molecule is called a diol

Primary alcohols examples table

Displayed formula of primary alcohol	Structural formula of primary alcohol	IUPAC name of primary alcohol
	CH3OH	methanol
	CH3CH2OH	ethanol
	CH3CH2CH2OH	propan-1-ol
	CH3CH2CH2CH2OH	butan-1-ol

• Further information about the classification of alcohols as primary, secondary or tertiary can be found in our Structural Isomers revision note

Carbonyls

• Carbonyl is the collective name for compounds containing the functional group C=O

• The general formula of a carbonyl is C_nH_2nO

• The two sub-families of carbonyls are aldehydes and ketones (known in some countries as alkanals and alkanones)

Aldehydes

• If the carbonyl group is on the end of a chain then it is an aldehyde and has the functional group formula, RCHO

  • The H is written before the O so as not to confuse it with an alcohol

• The nomenclature of carbonyls follows the pattern alkan + al

• There is no need to use numbers in the name as aldehyde will always be on the number 1 carbon atom

Ketones

• Ketones have a minimum of three carbons and have the general functional group formula, RCOR

• The nomenclature of ketones follows the pattern alkan + one

• After butanone, the carbonyl group can have positional isomers, so numbering must be used

  • For example pentan-2-one and pentan-3-one

Carbonyls examples table

Displayed formula of carbonyl	Structural formula of carbonyl	IUPAC name of carbonyl
	CH2O	methanal (also known as formaldehyde)
	CH3CHO	ethanal
	CH3COCH3	propanone (also known as acetone)
	CH3CH2COCH3	butanone

• As they have a very similar functional group arrangement, aldehydes and ketones show similar chemical reactions

• Differences in their chemistry are due to the reactions that involve the H on the aldehyde or the nature of the R group

• The difference in electronegativity between oxygen and carbon means the C=O is polar, leading to dipole-dipole attractions between the molecules which results in:

  • Higher than expected boiling points for small molecules

  • Solubility in water for the lower members of the families

• Aldehydes and ketones with the same number of carbons are functional group isomers

Carboxylic acids

• Carboxylic acid is the name given to compounds containing the functional group carboxyl, -COOH

• The general formula of a carboxylic acid is C_nH_2n+1COOH which can be shortened to just RCOOH

  • (In some countries the family is  called alkanoic acid)

• The nomenclature of carboxylic acids follows the pattern alkan + oic acid

• There is no need to use numbers in the name as the carboxyl group will always be on the number 1 carbon atom

Carboxylic Acids Examples Table

Displayed formula of carboxylic acid	Structural formula of carboxylic acid	IUPAC name of carboxylic acid
	HCO2H	methanoic acid (also known as formic acid)
	CH3CO2H	ethanoic acid (also known as acetic acid)
	CH3CH2CO2H	propanoic acid