IUPAC nomenclature Dr Margaret A L Blackie mblackiesun

IUPAC nomenclature Dr Margaret A. L. Blackie mblackie@sun. ac. za

Naming of the Alkanes

Naming of the alkanes 1. Identify the main functional group 2. Identify the longest unbroken carbon chain 7 – carbon chain 8 – carbon chain octane base This step is not necessary for the alkanes – there is no functional group.

NB: If the first substituent (side chain) has the same number, use the second substituent to separate the two etc … 3. Number the chain so that the first substituent has the lowest possible number 1 8 2 7 3 6 4 5 First substituent at carbon no. 2 Correct numbering: 5 4 6 3 7 2 8 1 First substituent at position no. 3 Substituents at 2, 5 and 6

4. Identify the substituents (side chains and the position on the main chain or ring) 1 2 -methyl 5 -ethyl 6 -methyl 2 3 4 5 6 7 8

5. Use the prefixes (di-, tri-, tetra-) depending on the number of identical substituents bonded to the main chain 1 2 -methyl 5 -ethyl 6 -methyl 2 3 4 5 6 7 8 2, 6 -dimethyl 5 -ethyl

6. Write down the side chains in alphabetical order, preceded by the number: (don’t write down di-, tri-, tetra-etc. in alphabetical order) 1 2 3 4 5 6 7 5 -ethyl-2, 6 -dimethyloctane Dimethyl comes after ethyl, because e is before m – the d of di is not considered. 8

The name should not be separated (there are some exceptions which will be addressed later. ) Numbers separated by a comma Dimethyloctane is written as one word 5 -ethyl-2, 6 -dimethyloctane Numbers separated from words by a hyphen (N shows a position and is considered as a number when naming the amines and amides) Note: The name is WRONG if any one of these three rules are disobeyed!

Important: The name is WRONG if any one of these rules are disobeyed! 5 -ethyl-2, 6 -dimethyloctane What is wrong with the names below? 5 -ethyl-2 -6 -dimethyloctane Name is wrong – the hyphen between the numbers are incorrect 5, ethyl, 2, 6, dimethyloctane The name is wrong – the comma between a number and a word is incorrect 5 -ethyl-2, 6 -dimethyl octane Name is wrong – There should be no space between dimethyl and octane

Important points for purposes of explanation

The longest unbroken carbon chain is not necessarily straight Longest straight chain contains only 5 carbons Longest unbroken chain contains 8 carbons Therefore the name is octane and not pentane

If there are two options for an unbroken carbon chain: Choose the chain with the most side chains as the base Two substituents Three 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 the first side chain, look at the following side chain, if there is no difference at the second, look at the third, etc. 1 2 3 4 5 6 6 5 4 3 2 Correct numbering First substituent is at position 2 in both numberings Second substituent at positions 3 and 4 respectively. Correct name : 2, 3, 5 -trimethylhexane 1

When there is no difference in the numbers of the positions of the substituents : choose the numbers according to alphabetical order of the substituents, that is the first substituent alphabetically gets the lowest number 1 2 3 4 5 3 -methyl-4 -ethylhexane 6 6 5 4 3 2 3 -ethyl-4 -methylhexane Correct name 1

When a functional group occurs

At grade 12 level the following step is unnecessary, but is included for educators. Identifying the main functional group is the first step in naming the compound. Functional groups are prioritised according a list (see next slide) The main functional group is the functional group with the highest priority. The base is the longest unbroken carbon chain containing the main functional group.

FUNCTIONAL GROUPS Highest to lowest priority General name Carboxylic acid Ester Aldehyde General structure Name as suffix -oic acid -oate -al Ketone -one Alcohol -ol Alkene -ene Alkyne -yne

The highest priority group must get the lowest possible number 1 2 3 4 Subsituents: 3 -bromo 4 -methyl 5 Number from left to right Base – pentan-2 -one Main functional group – ketone – suffix ‘one’ Longest unbroken carbon chain containing the main functional group – 5 carbons ‘pentane’ Correct name: 3 -bromo-4 -methylpentan-2 -one

1 2 3 4 5 correct name: 3 -bromo-4 -methylpentan-2 -one 3 -bromo-4 -methyl-2 -pentanone is also acceptable It is better to write the number directly in front of the main functional group

1 2 3 4 5 Correct name: 3 -bromo-4 -methylpentan-2 -one Consider: Main functional group – ketone – prefix ‘one’ Longest unbroken carbon chain containing the 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: ketone – prefix ‘one’ correct name: Pentan-2 -one Number necessary because the carbonyl group can be at position 2 or 3 Pentan-1 -al is NOT acceptable Main functional group : aldehyde – prefix ‘al’ No number on the aldehyde because the carbonyl group must be on the first carbon correct name: Pentanal Revise: the main functional group always gets the lowest possible number. The carbonyl group is always at the end or beginning of the chain, therefore it must be in position number 1.

Number or no number? correct name: But-2 -ene But-1 -ene is a possibility, so the number is important. ‘ There is no alternative, propene is prop-1 -ene The locant is redundant in this particular case and should be dropped. correct name: Propene The redundancy is associated with this particular molecule not the nature of the functional group so inclusion of the locant is acceptable

Questions 1. 2. 3. 4. 5. On the previous slide But-2 -ene and But-1 -ene are called… ? Give a brief explanation for the names given in question 1. Write down the molecular formula for the above compounds. Write down the structural formulae for 2 more possible structures. Name the structures completed in question 4. Answers 1. ISOMERS 2. Isomers are compounds with the same molecular formula but different structural formulas. 4. Write down the structural formulae for 2 more possible structures. H CH 2 H 3 C C H CH 2 -Methyl. Propene H H H C C H H H Cyclobutane

Carboxylic acids and derivatives Carboxylic acids Propanoic acid Propanoyl chloride (not for exam) Propanoate ion

Identify carboxylic acid that produces the ester This part is named as with the corresponding ion - propanoate Methylpropanoate Identify the substituent on the carboxylate oxygen This part is named as the alkyl group – methyl Esters