Chemical Nomenclature

Introduction to Chemical Nomenclature

General Rules for Naming Aromatic Organic Compounds

Monosubstituted Aromatic Compounds

Disubstituted Aromatic Compounds

Disubstituted Aromatic Compounds - Further Examples

Trisubstituted Aromatic Compounds

Phenyl or Benzyl: The Main Difference

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Introduction to Chemical Nomenclature

The International Union of Applied Chemistry (IUPAC) is responsible for systems used to name chemical compounds in chemistry, including aromatic organic compounds.

For more details on naming for MATSEC A-Level Chemistry, click on this link.

This document is issued by the MATSEC Examinations Board.

General Rules for Naming Aromatic Organic Compounds

Identify the Parent Compound - The parent compound is the longest continuous carbon chain or the most complex structure (e.g., benzene, naphthalene).
Number the Ring - Start numbering at the substituent that will give the lowest set of numbers for all substituents. When multiple numbering options give the same set of numbers, use alphabetical order to decide where to start numbering.
Name the Substituents - List substituents in alphabetical order as prefixes to the name of the parent compound. Use prefixes (di-, tri-, tetra-, etc.) for multiple identical groups.
Use Common Names When Appropriate - For widely known compounds, common names may be used alongside IUPAC names.

Originally named aromatic compounds due to their often strong smell, benzene and its derivatives earned this name. Despite benzene itself not having a particularly pleasant odour, it is considered the fundamental aromatic molecule. When depicting the benzene ring with its three double bonds, it's important to recognize that it symbolizes just one of the contributing resonance forms within a pair. Alternatively, the ring can be illustrated as a regular hexagon with a circle in the middle.

Monosubstituted Aromatic Compounds

Many monosubstituted benzenes are named by adding a substituent prefix to the word “benzene.” However, this is not always the case.

Fluorobenzene (left) and Nitrobenzene (right)

If a functional group is directly bonded to the ring, then the name of the molecule depends on whether the group is referred to with a prefix or suffix. For the MATSEC A-Level, three functional groups are used as a suffix. These are:

Acid groups (RCOOH, RSO3H)
Acid derivative groups (RCOCl, RCONH, RCN, RCOOR' )
Aldehyde group (-CHO)

Two notable names that we must know are:

Phenol - This is when a hydroxy (OH) group is bonded to the ring. This called phenol and not 'hydroxybenzene'

Benzenamine -This is when an amine (NH2) group is bonded to the ring. This is called benzenamine.

For alkyl groups where one or more hydrogens are replaced with halogens, brackets are used to show that the halogen is part of the alkyl group and not bonded to the ring e.g. (chloromethyl)benzene and (bromoethyl) benzene shown below.

(2-bromoethyl)benzene

The halogen (bromine) is not bonded directly to the benzene ring. When writing the chemical name, we write it in brackets to show that the chlorine is not bonded directly to the ring. In this case, the '2' shows the carbon in the ethyl chain that bromine is bonded to.

(chloromethyl)benzene

The halogen (chlorine) is not bonded directly to the benzene ring. When writing the chemical name, we write it in brackets to show that the chlorine is not bonded directly to the ring.

Disubstituted Aromatic Compounds

When two substituent groups are attached to the benzene ring, their positions relative to each other are indicated using prefixes: ortho- (o-), meta- (m-), and para- (p-), or the equivalent locants (1,2- for ortho; 1,3- for meta; 1,4- for para).

Ortho- (o-) indicates that the substituents are adjacent.
Meta- (m-) means the substituents are separated by one carbon.
Para- (p-) signifies that the substituents are opposite each other.

Alternatively, you can number the ring to give the lowest possible numbers to the substituents, and use these numbers as locants before the name.

1,2-dichlorobenzene

o-dichlorobenzene

1,3-dichlorobenzene

m-dichlorobenzene

1,4-dichlorobenzene

p-dichlorobenzene

Disubstituted Aromatic Compounds - Further Examples

If the substituents are different, they are listed in alphabetical order. The numbering is affected by the order of predence (or priority) that each group has. The order of precedence can be found here.

1-bromo-2-nitrobenzene

1-bromo-2-chlorobenzene

3-hydroxybenzoic acid

Trisubstituted Aromatic Compounds

For compounds with three substituents, use the lowest possible numbering to indicate the positions of the substituents. The substituents are listed in alphabetical order before "benzene." There are no special prefixes like ortho-, meta-, or para- for trisubstituted benzene derivatives; instead, you use numbers to indicate the positions.

The benzene ring is numbered to assign the substituents at positions that reflect the lowest possible numbers. This rule aims to minimize the numerical values in the name for all substituent groups. When deciding how to number the ring, the goal is to ensure that the substituents receive the lowest sequence of numbers possible.

The order of functional group precedence is COOH > CHO > OH > NH2 (see order of predence of groups above). For example, this means that in 3-hydroxybenzoic acid, the carbon on the benzene ring bonded to COOH will be carbon number 1 because COOH has a higher precedence than OH.

1,2,4-trinitrobenzene

For 1,2,4-trinitrobenzene, numbering starts at one nitro group and proceeds in such a way that the three nitro groups are placed at the lowest possible numbers, which are 1, 2, and 4. This numbering avoids higher numbers that would result from alternative numbering schemes (e.g., 1, 3, 5 or 2, 4, 6) and is chosen over a 1,3,5-trinitrobenzene configuration to ensure the substituents have the lowest locants.

Trinitro specifies that there are three nitro (NO2) groups substituents attached to the benzene ring. The prefix "tri-" indicates the quantity of nitro groups.

1-bromo-2,3-dimethylbenzene

In 1-bromo-2,3-dimethylbenzene, the numbering starts near the bromo group to ensure that the combined locants of all substituents (bromo and methyl groups) are as low as possible. The bromo group receives the number 1, indicating it is the first point of reference for numbering the ring, followed by the two methyl groups positioned at carbons 2 and 3, respectively.

Phenyl or Benzyl: The Main Difference

Phenyl

The phenyl group is essentially a benzene ring (C 6 H 6) minus one hydrogen atom, leaving a −C 6H 5 group that can attach to another atom or group of atoms. It is a direct attachment of the aromatic ring to the rest of the molecule without any additional atoms between the ring and the molecule.

When a phenyl group is attached to a molecule, it is directly connected through one of the carbons of the benzene ring. For example, in phenyl chloride (C6H5Cl) the chlorine atom is directly attached to the benzene ring. Note that phenyl chloride is normally called chlorobenzene.

The presence of a phenyl group in a molecule is typically denoted by the prefix “phenyl” in the compound's name.

Benzyl

Structure: The benzyl group consists of a phenyl group (−C6H5) attached to a methylene (-CH 2 ) group. It is the structure of a benzene ring plus an additional -CH2 group making it a −C6H5CH2 group in total.

In contrast to the phenyl group, when a benzyl group is attached to a molecule, it is connected through the CH2 group, which acts as a linker between the benzene ring and the rest of the molecule. For example, in benzyl alcohol C6H5CH2 OH, the hydroxyl group (OH) is connected to the benzene ring through the CH2 group.

The presence of a benzyl group is often indicated by the prefix "benzyl" in the name of the compound.

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