IUPAC nomenclature
Dr Margaret A.L. Blackie
mblackie@sun.ac.za
Naming of the Alkanes
Naming of the alkanes
2. Identify the longest unbroken carbonchain
8 – carbon chain
7 – carbon chain
octane
base
1. Identify the main functional group
This step isnot necessaryfor thealkanes –there is nofunctionalgroup.
2
1
3
4
5
6
7
8
First substituent at carbon no. 2
3. Number the chain so that the first substituenthas the lowest possible number
3
7
8
6
5
4
2
1
First substituent at position no. 3
Correct numbering:
Substituents at 2, 5 and 6
NB: If the first substituent (side chain) has the samenumber, use the second substituent to separate thetwo etc …
2
1
3
4
5
6
7
8
4. Identify the substituents (side chains andthe position on the main chain or ring)
2-methyl
5-ethyl
6-methyl
5. Use the prefixes (di-, tri-, tetra-) depending onthe number of identical substituents bonded to themain chain
2
1
3
4
5
6
7
8
2-methyl
5-ethyl
6-methyl
2,6-dimethyl
5-ethyl
2
1
3
4
5
6
7
8
6. Write down the side chains in alphabetical order,preceded by the number: (don’t write down di-, tri-,tetra-etc. in alphabetical order)
5-ethyl-2,6-dimethyloctane
Dimethyl comes after ethyl, because e isbefore m – the d of di is not considered.
5-ethyl-2,6-dimethyloctane
The name should not be separated (there are someexceptions which will be addressed later.)
Dimethyloctane iswritten as one word
Numbers separated from words by a hyphen (N shows aposition and is considered as a number when naming theamines and amides)
Numbers separated by a comma
Note: The name is WRONG if any one of these threerules are disobeyed!
5-ethyl-2-6-dimethyloctane
5,ethyl,2,6,dimethyloctane
5-ethyl-2,6-dimethyl octane
Important: The name is WRONG if any one of these rulesare disobeyed!
5-ethyl-2,6-dimethyloctane
Name is wrong – the hyphenbetween the numbers are incorrect
The name is wrong – the commabetween a number and a word isincorrect
Name is wrong – There should be nospace between dimethyl and octane
What is wrong with the names below?
Important points for purposes ofexplanation
The longest unbroken carbon chain is not necessarilystraight
Longest straight chaincontains only 5 carbons
Longest unbroken chaincontains 8 carbons
Therefore the name isoctane and not pentane
If there are two options for an unbroken carbon chain:
Choose the chain with the most side chains asthe base
Three substituents
Two substituents
Correct name: 2,6-dimethyl-4-propyloctane
Incorrect: 3-methyl-5-isobutyloctane
If there is no difference in the number of the position of thefirst side chain, look at the following side chain, if there is nodifference at the second, look at the third, etc.
1
2
3
4
5
6
6
5
4
3
2
1
First substituent is at position 2 in both numberings
Second substituent at positions 3 and 4 respectively.
Correct numbering
Correct name : 2,3,5-trimethylhexane
When there is no difference in the numbers of the positions ofthe substituents : choose the numbers according to alphabeticalorder of the substituents, that is the first substituentalphabetically gets the lowest number
1
2
3
4
5
6
3-methyl-4-ethylhexane
3-ethyl-4-methylhexane
6
5
4
3
2
1
Correct name
When a functional group occurs
Identifying the main functional group is the first step innaming the compound.
At grade 12 level the following step is unnecessary,but is included for educators.
Functional groups are prioritised according a list (seenext slide)
The main functional group is the functional group withthe highest priority.
The base is the longest unbroken carbon chaincontaining the main functional group.
FUNCTIONAL GROUPS
Highest to lowest priority
General name
Generalstructure
Name assuffix
Carboxylic acid
-oic acid
Ester
-oate
Aldehyde
-al
Ketone
-one
Alcohol
-ol
Alkene
-ene
Alkyne
-yne
The highest priority group must getthe lowest possible number
1
2
3
4
5
Number from left to right
Main functional group – ketone – suffix ‘one’
Longest unbroken carbonchain containing the mainfunctional group – 5 carbons -‘pentane’
Base – pentan-2-one
Subsituents:
3-bromo
4-methyl
Correct name:
3-bromo-4-methylpentan-2-one
correct name:
3-bromo-4-methylpentan-2-one
1
2
3
4
5
3-bromo-4-methyl-2-pentanone is also acceptable
It is better to write the number directly in frontof the main functional group
Correct name:
3-bromo-4-methylpentan-2-one
1
2
3
4
5
Consider:
Main functional group – ketone – prefix ‘one’
Longest unbroken carbon chain containingthe main functional group – 5 carbons –unsaturated chain - ‘pentene’-
Base – pent-4-ene-2-one
correct name:
3-bromo-4-methylpent-4-ene-2-one
Number or no number?
Main functional group : aldehyde – prefix ‘al’
correct name: Pentanal
Main functional group: ketone – prefix ‘one’
correct name: Pentan-2-one
No number on the aldehydebecause the carbonyl groupmust be on the first carbon
Number necessary becausethe carbonyl group can be atposition 2 or 3
Revise: the main functional group always gets the lowest possible number. Thecarbonyl group is always at the end or beginning of the chain, therefore it mustbe in position number 1.
Pentan-1-al is NOTacceptable
Number or no number?
correct name: But-2-ene
correct name: Propene
But-1-ene is a possibility, so thenumber is important.‘
There is no alternative,propene is prop-1-ene
The locant is redundant in thisparticular case and should be dropped.
The redundancy is associated with this particularmolecule not the nature of the functional groupso inclusion of the locant is acceptable
Questions
1.On the previous slide But-2-ene and But-1-ene are called… ?
2.Give a brief explanation for the names given in question 1.
3.Write down the molecular formula for the above compounds.
4.Write down the structural formulae for 2 more possible structures.
5.Name the structures completed in question 4.
Answers
1. ISOMERS
2.Isomers are compounds with the same molecular formula butdifferent structural formulas .
4. Write down the structural formulae for 2 more possible structures.
2-MethylPropene
C
C
C
H
H
H
H
H
H
H
H
C
H
2
C
H
3
C
C
H
C
Cyclobutane
Carboxylic acids and derivatives
Carboxylicacids
Propanoic acid
Propanoate ion
Propanoyl chloride (not for exam)
Esters
Methylpropanoate
Identify carboxylic acidthat produces the ester
This part is named as with thecorresponding ion - propanoate
Identify the substituent on thecarboxylate oxygen
This part is named as the alkyl group – methyl