4.1.1 (a) Organic Nomenclature

Syllabus

(a) application of IUPAC rules of nomenclature for systematically naming organic compounds

{Nomenclature will be limited to the functional groups within this specification. E.g. CH₃CH₂CH(CH₃)CH₂OH has the systematic name: 2-methylbutan-1-ol.}

{Learners will be expected to know the names of the first ten members of the alkanes homologous series and their corresponding alkyl groups.}

What does this mean?

You don't need to know this but IUPAC is the International Union of Pure and Applied Chemistry.

You do need to know that the naming of all Organic compounds is based on the naming of Alkanes - the simplest family of Organic substances.

You may recall Alkanes start with (1 Carbon) Methane, (2 C) Ethane, (3 C) Propane, (4 C) Butane,

(5 C) Pentane, (6 C) Hexane, (7 C) Heptane, (8 C) Octane, (9 C) Nonane, (10 C) Decane.

So, all Organic substances with only 1 Carbon atom begin with the prefix Meth (eg Methanol), if it had two Carbon atoms it would begin with Eth (eg Ethanol) etc.

From 5 onwards the system is the same as for shapes (Pentagon, Hexagon, Heptagon etc).

So you only really need to learn the first 4.

I always remembered My Evil Plane Bombed Paris, Ha Ha.

Meth- Eth- Prop- But- Pent-, Hex- Hept-.

Alkanes and haloalkanes

Have no functional group to name so we would simply name the longest chain of atoms and then look for side-chains or groups.

No side-chains/groups.

6 Carbon atoms long.

No functional groups

Hexane.

7 Carbon atoms but the longest continuous chain is 6 atoms long.

Still a Hexane.

Side group has only 1 Carbon = Methyl

This Methyl group is either on Carbon 2 or Carbon 5, depending on where you count from. Go for the smaller number.

2-Methyl Hexane

Longest Carbon chain is 6. Still a Hexane.

Two Methyl groups - making it a dimethyl hexane

Counting the left 2,3-dimethyl hexane.

Counting from the right 4,5-dimethyl hexane.

So, 2,3-dimethyl hexane is correct

Note - numbers are separated from each other by commas, numbers and letters are separated by dashes.

Even if the methyl groups are on the same part of the Carbon chain they still have to be given individual numbers.

So this is 2,2-dimethylhexane

Another 6 Carbon Hexane.

This time trimethyl-

From left 2,4,4 -trimethyl hexane

From right 3,3,5 - trimethyl hexane

So 2,4,4- trimethyl hexane is correct

Not all side chains are Methyl groups.

This side chain is two carbons long - so it's an Ethyl group.

Making this 4-ethyl octane (counting from the right)

Examiners are fond of drawing molecules weirdly to try to make you name them incorrectly.

At first sight this looks like a 4 Carbon chain with an ethyl group on the second carbon atoms and a methyl group on the 4th.

In other words 2-ethyl-4-methyl butane

Firstly, you can't have a side-chain on the last carbon in a chain!

So maybe it should be 2-ethyl- pentane except that if we count the carbon atoms as in the diagram we can make a chain of six.

Making this molecule 3-methyl hexane

Not 2-ethyl-3-methyl hexane

But 3,4-dimtheyl heptane.

6 Carbon chain makes this a Hexane

Side-groups = Bromo and Chloro,

Lower numbers counting from left

2-bromo-4-chloro hexane

Maximum Carbon chain 4 carbons = butane

Side chains Chloro, Bromo & methyl

These should be written alphabetically.

Counting from left will give us lower numbers so...

3-bromo-2-chloro-2-methyl butane

Notice we don't count to give the bromo the lowest number - we put it first but have the lowest numbers across all the side-groups.

Video

Alcohols.

Functional groups are where reactions tend to happen.

They tend to go at the end of the name - as suffixes.

Simple -OH groups are alcohols.

They end a molecule with -ol

This is a butanol - 4 Carbon atoms, saturated and with an OH

Specifically, it is butan-1-ol because the OH group is on the end carbon atom.

Still a butanol but this time butan-2-ol.

Note that we wouldn't call it butan-3-ol because we would count from the right.

Propane because it has 3 saturated carbons.

1,3 di-ol because there are OH groups at both ends

Notice we put the "e" back in propane because it is followed by a consonant.

In Ethanol, Methanol or Butan-2-ol or Butan-1-ol we drop the e because it is followed by a vowel (even if there's a number between) - examiners don't usually make a fuss if you forget this rule.

It is possible to name an OH group as a prefix but we would only do this if there was another functional group that needed to be at the end of the name.

The -COOH group makes this a carboxyllic acid

The -OH group makes it an alcohol.

Alcohols generally end with -ol

Acids generally end with -oic acid.

So in this case we would call this 2-hydroxy ethanoic acid or just hydroxy ethanoic acid because there's nowhere else it can be but C2

There are lots of other functional groups to learn but for the time being only alcohols can be named as side-groups.

Videos

Exam-style Questions

1. 1. The Alkanes form an homologous series of hydrocarbons. The first four straight-chain Alkanes are shown below.

Methane CH4

Ethane CH3CH3

Propane CH3CH2CH3

Butane CH3CH2CH2CH3

(a) (i) State what is meant by the term hydrocarbon.

..................................................................................................................................................................................................................................

(ii) Give the general formula for the Alkanes.

..................................................................................................................................................................................................................................

(iii) Give the molecular formula for Hexane, the sixth member of the series.

...............................................................................................................................................................................................................................(3)

(b) Each homologous series has its own general formula. State two other characteristics of a homologous series.

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...............................................................................................................................................................................................................................(2)

(c) Branched-chain structural isomers are possible for Alkanes which have more than three Carbon atoms.

(i) State what is meant by the term structural isomers.

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(ii) Name the two isomers of hexane shown below.

Name .........................................................................................................

(iii) Give the structures of two other branched-chain isomers of hexane.

Isomer 3 Isomer 4

(6)

2. The table below gives some of the names and structures of isomers having the molecular formula C4H9Br

Complete the table (2)

Answers

1. (a) (i) Molecule/compound/consists/composed/made up of Hydrogen and Carbon only (1)

(ii) CnH2n+2 (1)

(iii) C6H14 only (1)

(b) Chemically similar / react in same way / same chemistry

Differ by CH2

gradation in physical properties OR specified trend e.g. b.p.

same functional group

Not same molecular formula 2

(c) (i) Same molecular formula (1)

NOT same Mr

different structural formula / structures (1) (or atoms arranged in different way)

NOT different spatial arrangement

Only credit M2 if M1 correct

(ii) 2-methylpentane (1)

2,2-dimethylbutane (1)

(iii)

OR correct condensed / structural formula

Penalise “sticks” once

Penalise absence of vertical bonds once

penalise badly drawn bonds once

2. 1(-)bromobutane 1

correct structure for 1-bromo-2-methylpropane 1

(C–C bonds must be clear where drawn)

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