Unit 4, Topic 3:

Aromatic Compounds (1.3)

Lesson Instructions

Review the learning goals and success criteria for Topic 3: Aromatic Compounds.
- Review the minds on, action, and consolidation sections of the lesson, completing any tasks as outlined.
- Complete the fill-in-the-blanks handouts that have been provided to you. They have been linked below.
- Practice and apply what you've learned by completing the practice tasks outlined at the end of the lesson.

Learning Goals

I am learning to ...

- Name and draw compounds containing benzene.

Minds On

Aromatic Compounds and Old Books!

Books, both old and new, give off several hundred volatile compounds or VOCs. These compounds have a variety of origins - some are products of degradation, whilst others originate from the type of paper, binding adhesive, and printing ink used in the book's manufacture.

Here are some of the aromatic compounds and scents associated with old books!

Toluene

(Common Name)

Toluene gives old books a sweet odour. The IUPAC name for this compound is methylbenzene.

ethylbenzene

(IUPAC Name)

The organic compound ethylbenzene is also responsible for the sweet aroma of old books.

Vanillin

(Common Name)

This compound provides old books with a vanilla-like odour. The IUPAC name is 4-hydroxy-3-methoxybenzaldehyde.

benzaldehyde

(IUPAC Name)

This compound gives old books an almond-like scent.

Furfural

(Common Name)

Also giving old books an almond-like scent, the IUPAC name for this aromatic aldehyde is 2-Furaldehyde.

Over time, gradual breakdown of cellulose and lignin in paper produces a large number of organic compounds. Type of paper and age of the book affect the compounds produced and their concentrations; for instance, very old books contain more lignin than modern books.

We will learn how to name and draw some of these compounds in this lesson. Other molecules will be explored when we learn about functional groups.

Action

Aromatic Compounds

Aromatic compounds are unsaturated hydrocarbons that have a ring structure and a bonding structure that causes it to be chemically stable.

Benzene

Benzene (C₆H₆), is the simplest aromatic compound. It can be represented by the following structures.

- Electrons are shared over the entire molecule.
- Considered 6 identical bonds (halfway between a single and double bond).

Figure 1. The first structure (left) shows all of the single and double bonds, carbon and hydrogen, that make up benzene. The two centre diagrams show a simplified drawing of benzene that show the position of the double bonds. These are called resonance structures. The picture on the right shows an even further simplified drawing of benzene. The circle drawn in the center indicates the alternating double bonds.

Naming Aromatic Compounds

1. Benzene is the PARENT CHAIN, when the aromatic compound has non-carbon substituents or an alkyl chain with fewer than 5 carbons.
2. The name will end in -benzene.
3. If two more substituents are bonded to a benzene ring, the lowest position number is assigned alphabetically.

Figure 2. 1-ethyl-4-methylbenzene.

Naming Aromatic Hydrocarbons

Example 1. Name the following compound.

Figure 3. 1,3-dichlorobenzene.

Example 2. Name the following compound.

Figure 4. 1-ethyl-2,4-dimethylbenzene.

When a benzene ring has exactly 2 substituents, the prefixes ortho-, meta-, and para- can be used instead of numbers.

Example 3. Name the following compounds.

Figure 5.Left: ortho-ethylmethylbenzeneCentre: meta-ethylmethylbenzeneRight: para-dimethylbenzene

Benzene as a Substituent

- If the aromatic compound has a large alkyl chain or a alkyl chain with a double or triple bond(s), then benzene will be a substituent.
- When benzene behaves as an alkyl group/stituent, it is named using -phenyl.

Example 4. Name the following compound.

This is an example of benzene as a subtituent because the straight chain of single-bonded carbons is considered to be long (the chain has 5 carbons or more).

Figure 6. 3-phenylhexane

Example 4. Name the following compound.

This is an example of benzene as a substituent because the straight chain of carbon has a double bond in it. The double bond takes priority when when numbering the parent chain.