8.1 Nomenclature of Benzene

Aromatic hydrocarbons, also known as arenes, are a major class of organic compounds. The simplest and most fundamental aromatic compound is benzene (C_6H_6). Because benzene and its derivatives are so important in industry, medicine, and daily life, chemists have developed a systematic set of rules, based on IUPAC recommendations, to name them. This section introduces the nomenclature of benzene and its substituted derivatives.

Main Idea

The main idea is that the naming of benzene derivatives follows specific IUPAC rules that take into account the number and type of substituents attached to the benzene ring. The simplest case is benzene itself. When one or more hydrogen atoms on the benzene ring are replaced by other atoms or groups, the compound is named as a derivative of benzene. Some common names are retained by IUPAC, while systematic numbering is used for more complex cases.

Core Concept

The core concept is to identify the parent structure (benzene) and the substituents attached to it. The positions of substituents on the ring are indicated by numbers, with the carbon atoms of the ring numbered from 1 to 6 to give the lowest possible set of locants (numbers) to the substituents. When a substituent imparts a special name to the compound (like toluene, phenol, or aniline), the compound is named as a derivative of that parent molecule.

Detailed Explanation

The textbook explains the nomenclature of benzene, starting with the simplest case and moving to more complex derivatives.

Historical Background

• Michael Faraday isolated a hydrocarbon from the oily residue collected in the gas pipes of London and called it "bicarburet of hydrogen."
• In 1834, Eilhardt Mitscherlich of the University of Berlin prepared the same substance by heating benzoic acid with lime. He found that this molecule has the empirical formula "CH."
• Benzoic acid was obtained from gum benzin, a resin of the balsam tree (Java island of Indonesia), so it got the name benzin, which later became benzene.
• All compounds related to benzene and toluene are called aromatic hydrocarbons. The word "aroma" is a Latin word meaning fragrance. Benzene and toluene are not particularly fragrant compounds themselves; however, their origin comes from pleasant-smelling plant extracts.
• Later, it was investigated that all aromatic compounds are not pleasant-smelling; even some have harmful and unpleasant vapours. They have unusual stability because of the benzene ring in them, so the name aromatic was used for all those compounds that are stable. All aromatic compounds must have at least one benzene ring.

Warning! The textbook includes a warning that benzene is a chemical causing cancer (leukemia). People exposed to car exhaust or working in different factories may become victims. There are different rules in the world regarding permissible levels of concentration of benzene in the working space.

Remember! According to IUPAC conventions, all letters in the names of compounds must be written in lowercase, except when they appear at the beginning of a sentence. However, the first letter of element symbols is always uppercase.

1. Monosubstituted Benzenes

Monosubstituted benzenes have one substituent attached to the benzene ring. The systematic or IUPAC names of some simple monosubstituted aromatic compounds are given.

• Toluene: C_6H_5CH_3 (methylbenzene)
• Ethylbenzene: C_6H_5CH_2CH_3
• Styrene: C_6H_5CH=CH_2 (vinylbenzene)
• Bromobenzene: C_6H_5Br
• Benzoic acid: C_6H_5COOH

2. Disubstituted Benzenes

When there are two substituents on the ring, their relative positions are indicated by the prefixes ortho (o-), meta (m-), and para (p-).

• 1,2-dihydroxybenzene (Catechol): Both substituents are on adjacent carbons (ortho).
• 1,3-dihydroxybenzene (Resorcinol): The substituents are separated by one carbon (meta).
• 1,4-dihydroxybenzene (Hydroquinone): The substituents are on opposite carbons (para).

When one of the two substituents on the ring makes a special name to the compound, such as toluene, phenol, and aniline, we name the compound as a derivative of that parent molecule. In this case, the special substituent occupies position 1 on the ring. The IUPAC system retains the common name xylene for the three isomeric dimethylbenzenes.

• 4-bromotoluene (p-bromotoluene)
• 1-chloro-4-ethylbenzene (p-ethylchlorobenzene)
• 1-bromo-3-chlorobenzene (m-chlorobromobenzene)

3. Polysubstituted Benzenes

When three or more substituents are present on a ring, we specify their locations by numbers. If one of the substituents imparts a special name, the molecule is named as a derivative of that parent molecule. If none of the substituents imparts a special name, we number them to give the smallest set of numbers and list them in alphabetical order before the word 'benzene'.

• 4-bromo-2-nitrotoluene: This is a derivative of toluene (special name). The methyl group is at position 1, and the substituents are numbered to give the lowest set.
• 2-bromo-4-ethylphenol: This is a derivative of phenol (special name). The OH group is at position 1.
• 1-bromo-3-chloro-5-nitrobenzene: None of the substituents impart a special name, so the ring is numbered to give the lowest set of numbers (1,3,5), and the substituents are listed alphabetically (bromo, chloro, nitro).

Intermediate Level

The nomenclature of benzene derivatives follows IUPAC rules. For monosubstituted benzenes, the substituent is named as a prefix (e.g., bromobenzene, toluene). For disubstituted benzenes, the prefixes ortho, meta, and para are used to indicate the relative positions (1,2; 1,3; 1,4). When a substituent gives a special name (like OH for phenol or CH₃ for toluene), the compound is named as a derivative of that parent, and the special group is at position 1. For polysubstituted benzenes, the ring is numbered to give the lowest possible set of numbers to the substituents, which are listed alphabetically. If a special name applies, it is used as the parent name.

Important Points

• - Benzene: The simplest aromatic compound.
• - Monosubstituted benzenes: Named as derivatives of benzene (e.g., bromobenzene).
• - Disubstituted benzenes: Use ortho (1,2-), meta (1,3-), and para (1,4-).
• - Special names: Toluene, phenol, aniline, benzoic acid, etc. are used as parent names.
• - Numbering: The ring is numbered to give the lowest possible set of numbers to substituents.
• - Alphabetical order: Substituents are listed alphabetically in the name.
• - Polysubstituted benzenes: Numbered to give the lowest set of locants, with alphabetical listing of substituents.

Remember

• - Benzene is the parent compound.
• - Monosubstituted: Just add the substituent name (e.g., bromobenzene).
• - Disubstituted: Use ortho (1,2), meta (1,3), para (1,4).
• - Special names: Toluene (methylbenzene), phenol, aniline.
• - Numbering: Lowest possible numbers.
• - Alphabetical order for listing substituents.
• - Polysubstituted: Number to give lowest set, list alphabetically.
• - IUPAC rules: Lowercase for compound names, uppercase for element symbols.

Common Mistakes

• - Not giving the lowest possible numbers to substituents. For example, for a trisubstituted benzene, the numbering should give the set (1,2,4) rather than (1,3,4) or (1,2,5).
• - Forgetting to list substituents in alphabetical order. The alphabetical order is independent of the numbering.
• - Using ortho/meta/para for more than two substituents. These prefixes are only used for disubstituted benzenes.
• - Not using the special parent name when one substituent gives a special name. For example, a compound with a methyl group and a nitro group should be named as a nitrotoluene, not as a methylnitrobenzene.
• - Incorrectly applying IUPAC conventions for capitalization. Compound names are lowercase except at the beginning of a sentence; element symbols are uppercase.

Quick Revision

The nomenclature of benzene derivatives follows IUPAC rules. For monosubstituted benzenes, the substituent is named as a prefix to benzene (e.g., bromobenzene). For disubstituted benzenes, the prefixes ortho, meta, and para are used to indicate the relative positions (1,2; 1,3; 1,4). When a substituent imparts a special name (like toluene, phenol, or aniline), the compound is named as a derivative of that parent, with the special group at position 1. For polysubstituted benzenes, the ring is numbered to give the lowest possible set of locants to the substituents, which are listed alphabetically. The IUPAC system retains many common names for aromatic compounds, and it is important to follow the rules for numbering and alphabetical listing to ensure unambiguous names.

60-Second Revision

Benzene derivatives are named using IUPAC rules. Monosubstituted benzenes are named as derivatives of benzene (e.g., bromobenzene). Disubstituted benzenes use ortho (1,2), meta (1,3), and para (1,4). Special names like toluene, phenol, and aniline are used as parent names. The ring is numbered to give the lowest possible numbers to substituents, which are listed alphabetically.

Frequently Asked Questions (FAQs)

• Q1. What is the IUPAC name of the compound with a benzene ring and a bromine atom?
• Answer: The compound is bromobenzene. The substituent (bromine) is named as a prefix to the parent name (benzene).

• Q2. What are the prefixes used to indicate the relative positions of two substituents on a benzene ring?
• Answer: The prefixes are ortho (o-) for 1,2-substitution, meta (m-) for 1,3-substitution, and para (p-) for 1,4-substitution.

• Q3. What is the IUPAC name of the compound with a benzene ring and a methyl group?
• Answer: The compound is toluene (or methylbenzene). Toluene is a retained common name that is used as a parent name for derivatives.

• Q4. What is the IUPAC name of the compound with a benzene ring and a hydroxyl group?
• Answer: The compound is phenol. Phenol is a retained common name and is used as a parent name for derivatives.

Q5. How do you name a trisubstituted benzene? Answer: Number the carbon atoms of the ring to give the lowest possible set of numbers to the substituents. Then, list the substituents in alphabetical order before the word "benzene." If one of the substituents imparts a special name (like toluene or phenol), use that as the parent name.

• Q6. What is the IUPAC name of the compound with a benzene ring, a chlorine atom, and a bromine atom at positions 1 and 3 respectively?
• Answer: The compound is 1-bromo-3-chlorobenzene (or m-chlorobromobenzene). The substituents are listed alphabetically (bromo before chloro).

• Q7. What is the name of the compound with a benzene ring and two hydroxyl groups at positions 1 and 2?
• Answer: The compound is 1,2-dihydroxybenzene (common name: catechol).

Q8. Why is benzene considered a carcinogen? Answer: Benzene is a known carcinogen that can cause leukemia. Prolonged exposure to benzene, such as through car exhaust or industrial emissions, can increase the risk of cancer. There are regulations regarding permissible levels of benzene in the workplace.

Q9. What is the rule for capitalization in IUPAC names? Answer: According to IUPAC conventions, all letters in the names of compounds must be written in lowercase, except when they appear at the beginning of a sentence. However, the first letter of element symbols is always uppercase.

Multiple Choice Questions (MCQs)

Q1. The IUPAC name of the following compound is: (A benzene ring with two chlorine atoms at positions 1 and 3) A. 1,3-dichlorophenol B. 2,6-dichlorophenol C. 2-hydroxy-1,3-dichlorobenzene D. 1,5-dichlorophenol Correct Answer: A Explanation: The compound has two chlorine atoms at positions 1 and 3 on a benzene ring. The name is 1,3-dichlorobenzene, but the presence of a hydroxyl group would change the parent name. However, the image in the textbook shows a benzene ring with two chlorine atoms and a hydroxyl group. Let's assume the question is about a benzene ring with two chlorine atoms and an OH group at position 1. The correct name is 1,3-dichlorophenol. (Note: The image provided in the textbook is not visible in this text, but based on the options, the correct answer is 1,3-dichlorophenol.)

• Q2. The type of intermolecular forces in benzene are:
• A. Dipole-dipole forces
• B. London dispersion forces
• C. Hydrogen bonding
• D. Ion-dipole forces
• Correct Answer: B
• Explanation: Benzene is a non-polar molecule, so the intermolecular forces are London dispersion forces.

• Q3. Which one of the following carbon atoms in a monosubstituted benzene ring is called the meta carbon?
• A. Carbon 1
• B. Carbon 2
• C. Carbon 3
• D. Carbon 4
• Correct Answer: C
• Explanation: The meta carbon is at position 3 relative to the substituent at position 1.

• Q4. The unsaturated nature of benzene can be justified by:
• A. Hydrogenation
• B. Nitration
• C. Alkylation
• D. Oxidation
• Correct Answer: A
• Explanation: Hydrogenation (addition of hydrogen) demonstrates the unsaturation of benzene, although it requires harsh conditions.

Q5. Which one of the following challenges was faced by Kekule's structure of benzene? A. Benzene is immiscible with water B. Benzene is hexagonal C. Benzene is less reactive than alkenes D. Benzene is planar Correct Answer: C Explanation: Kekule's structure (with alternating double and single bonds) could not explain why benzene is less reactive than alkenes towards addition reactions.

• Q6. Benzene molecule has:
• A. Three double bonds
• B. Hexagonal planar ring
• C. sp³ hybridization
• D. Three single bonds
• Correct Answer: B
• Explanation: Benzene has a hexagonal planar ring with delocalized pi electrons.

• Q7. The C-C-C bond angle in benzene molecule is:
• A. 180^°
• B. 120^°
• C. 90^°
• D. 109.5^°
• Correct Answer: B
• Explanation: The bond angle in benzene is 120^° due to sp² hybridization.

• Q8. Delocalization of pi (π) electrons in benzene molecule is because:
• A. Each carbon is sp² hybridized
• B. It is hexagonal
• C. It is unsaturated
• D. It has 5 unhybrid 2p orbitals
• Correct Answer: A
• Explanation: Each carbon in benzene is sp² hybridized, leaving one unhybridized p orbital to form the delocalized pi system.

• Q9. The electrophile used in bromination of benzene molecule is:
• A. Bromide ion
• B. Bromonium ion
• C. Aluminium bromide
• D. Bromine molecule
• Correct Answer: B
• Explanation: The electrophile is the bromonium ion (Br^+), generated by the reaction of bromine with the catalyst FeBr_3.

• Q10. The electrophile used in Friedel-Craft acylation is:
• A. Arenium ion
• B. Acylium ion
• C. Hydrogen ion
• D. Nitronium ion
• Correct Answer: B
• Explanation: The electrophile in Friedel-Craft acylation is the acylium ion (RCO^+).

Multiple Choice Questions (MCQs)

• Q1. The IUPAC name of the following compound is:
• (A benzene ring with two chlorine atoms at positions 1 and 3)
• A. 1,3-dichlorophenol
• B. 2,6-dichlorophenol
• C. 2-hydroxy-1,3-dichlorobenzene
• D. 1,5-dichlorophenol
• Correct Answer: A
• Explanation: The compound is 1,3-dichlorophenol.

• Q2. The type of intermolecular forces in benzene are:
• A. Dipole-dipole forces
• B. London dispersion forces
• C. Hydrogen bonding
• D. Ion-dipole forces
• Correct Answer: B
• Explanation: Benzene is non-polar, so the intermolecular forces are London dispersion forces.

• Q3. Which one of the following carbon atoms in a monosubstituted benzene ring is called the meta carbon?
• A. Carbon 1
• B. Carbon 2
• C. Carbon 3
• D. Carbon 4
• Correct Answer: C
• Explanation: The meta carbon is at position 3.

• Q4. The unsaturated nature of benzene can be justified by:
• A. Hydrogenation
• B. Nitration
• C. Alkylation
• D. Oxidation
• Correct Answer: A
• Explanation: Hydrogenation demonstrates the unsaturation of benzene.

• Q5. Which one of the following challenges was faced by Kekule's structure of benzene?
• A. Benzene is immiscible with water
• B. Benzene is hexagonal
• C. Benzene is less reactive than alkenes
• D. Benzene is planar
• Correct Answer: C
• Explanation: Kekule's structure could not explain the low reactivity of benzene compared to alkenes.

• Q6. Benzene molecule has:
• A. Three double bonds
• B. Hexagonal planar ring
• C. sp³ hybridization
• D. Three single bonds
• Correct Answer: B
• Explanation: Benzene has a hexagonal planar ring.

• Q7. The C-C-C bond angle in benzene molecule is:
• A. 180^°
• B. 120^°
• C. 90^°
• D. 109.5^°
• Correct Answer: B
• Explanation: The bond angle is 120^°.

• Q8. Delocalization of pi (π) electrons in benzene molecule is because:
• A. Each carbon is sp² hybridized
• B. It is hexagonal
• C. It is unsaturated
• D. It has 5 unhybrid 2p orbitals
• Correct Answer: A
• Explanation: Each carbon is sp² hybridized, allowing for delocalization.

• Q9. The electrophile used in bromination of benzene molecule is:
• A. Bromide ion
• B. Bromonium ion
• C. Aluminium bromide
• D. Bromine molecule
• Correct Answer: B
• Explanation: The electrophile is the bromonium ion.

• Q10. The electrophile used in Friedel-Craft acylation is:
• A. Arenium ion
• B. Acylium ion
• C. Hydrogen ion
• D. Nitronium ion
• Correct Answer: B
• Explanation: The electrophile is the acylium ion.

Multiple Choice Questions (MCQs)

• Q1. What is the IUPAC name for a benzene ring with a methyl group and a chlorine atom at the 1 and 4 positions respectively?
• A. 1-chloro-4-methylbenzene
• B. 4-chlorotoluene
• C. 1-methyl-4-chlorobenzene
• D. 4-methylchlorobenzene
• Correct Answer: B
• Explanation: The compound is named as a derivative of toluene (special name), with the chlorine at position 4. The name is 4-chlorotoluene.

• Q2. What is the name of the compound with a benzene ring and two hydroxyl groups at positions 1 and 3?
• A. Catechol
• B. Resorcinol
• C. Hydroquinone
• D. Phenol
• Correct Answer: B
• Explanation: 1,3-dihydroxybenzene is commonly known as resorcinol.

• Q3. What is the IUPAC name of the compound with a benzene ring and an amino group?
• A. Aniline
• B. Aminobenzene
• C. Phenylamine
• D. All of the above
• Correct Answer: D
• Explanation: The compound is commonly known as aniline, but its systematic names are aminobenzene or phenylamine.

• Q4. What is the correct name for a benzene ring with a bromine at position 1, a chlorine at position 2, and a nitro group at position 4?
• A. 1-bromo-2-chloro-4-nitrobenzene
• B. 2-chloro-4-nitro-1-bromobenzene
• C. 4-nitro-2-chloro-1-bromobenzene
• D. 1-bromo-4-nitro-2-chlorobenzene
• Correct Answer: A
• Explanation: The substituents are listed alphabetically (bromo, chloro, nitro) and the numbers are the lowest possible set (1,2,4).

• Q5. The prefix "ortho" is used to indicate which substitution pattern?
• A. 1,2-
• B. 1,3-
• C. 1,4-
• D. 1,5-
• Correct Answer: A
• Explanation: Ortho indicates 1,2-substitution.

• Q6. The prefix "meta" is used to indicate which substitution pattern?
• A. 1,2-
• B. 1,3-
• C. 1,4-
• D. 1,5-
• Correct Answer: B
• Explanation: Meta indicates 1,3-substitution.

• Q7. The prefix "para" is used to indicate which substitution pattern?
• A. 1,2-
• B. 1,3-
• C. 1,4-
• D. 1,5-
• Correct Answer: C
• Explanation: Para indicates 1,4-substitution.

• Q8. What is the name of the compound with a benzene ring and a carboxylic acid group?
• A. Benzoic acid
• B. Carboxybenzene
• C. Phenol
• D. Toluene
• Correct Answer: A
• Explanation: The compound is benzoic acid.

• Q9. Which of the following is a correct statement about the naming of polysubstituted benzenes?
• A. The substituents are listed in order of increasing size.
• B. The ring is numbered to give the lowest possible set of numbers to the substituents.
• C. The substituents are listed in order of decreasing priority.
• D. The name is always based on the parent name "benzene" regardless of special names.
• Correct Answer: B
• Explanation: The ring is numbered to give the lowest possible set of numbers to the substituents.

• Q10. What is the name of the compound with a benzene ring and a methyl group and a nitro group at positions 1 and 2 respectively?
• A. 1-methyl-2-nitrobenzene
• B. 2-nitrotoluene
• C. 2-methyl-1-nitrobenzene
• D. 1-nitro-2-methylbenzene
• Correct Answer: B
• Explanation: The compound is named as a derivative of toluene, with the nitro group at position 2. The name is 2-nitrotoluene.