IUPAC Nomenclature (DP IB Chemistry: SL): Revision Note

• When naming compounds with side chains or functional groups, number the carbon atoms in the longest chain, starting from the end that gives the lowest possible numbers to the substituents

• In structural formulas, a side chain is often shown in brackets.

  • E.g. CH₃CH(CH₃)CH₂CH₃

• Side chains are named by changing the -ane ending of the parent alkane to -yl

• These groups are known as alkyl groups

Naming organic compounds with one alkyl side chain

• 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

• 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

Naming alkenes

• Alkenes follow the general formula C_nH_2n

• They are classified as unsaturated because they contain at least one C=C double bond

• Alkenes are named using the format: alk + ene (e.g. ethene, propene)

• When the carbon chain contains four or more atoms, the position of the double bond must be indicated

• To name:

  • Number the carbon chain from the end closest to the double bond

  • The lowest-numbered carbon involved in the double bond is used in the name

  • This number is placed just before the “-ene” suffix (e.g. but-1-ene, but-2-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

• Alkynes 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 (naming substituted benzenes is not required) and is covered in our Benzene revision note

• Naming depends on whether the benzene ring is considered the main structure or a substituent

• Benzene as the main structure:

  • Use the rule: alkyl group + benzene (e.g. methylbenzene)

  • If there is only one alkyl group, no numbering is needed

  • If there are two or more substituents, apply a numbering system:

    • Number the ring so that the substituents get the lowest possible numbers; if there is a choice, carbon-1 is the substituent that comes first alphabetically

    • List substituents in alphabetical order in the name

Examples of alkyl substituted aromatic compounds

• 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

Halogenoalkanes

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

• Halogenoalkanes are named using the prefix fluoro-, 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 aldehydes 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
	HCHO	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 (the lone pairs on the C=O oxygen form hydrogen bonds with water molecules)

• 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