Properties, Uses, Preparation and Naming of Benzene Compounds – Benzene (Senior 6 Chemistry) | Ganzaa

Benzene

Lessons

Senior 6 · Chemistry

Benzene

4 lessons

Lesson 2 of 4

Properties, Uses, Preparation and Naming of Benzene Compounds

25 min

A useful but dangerous liquid

Benzene is made from crude oil in very large amounts. Almost none of it reaches a shop as benzene: it is turned into plastics such as polystyrene, into nylon, dyes, medicines and detergents. Yet it is also one of the few everyday chemicals known for certain to cause cancer in humans. You will meet it wherever there is petrol, because petrol contains a little benzene, and wherever things burn.

Physical properties

Table 2.1: Physical properties of benzene

Property	Value or description	Explanation
Appearance	colourless liquid with a sweet smell	simple molecular substance
Melting point	5.5 °C	flat, symmetrical molecules pack closely in the solid (hexane melts at −95 °C)
Boiling point	80.1 °C	only weak London (dispersion) forces between molecules
Density	0.88 g cm⁻³	less dense than water, so it floats
Solubility	immiscible with water; mixes with non-polar solvents	non-polar molecules cannot form hydrogen bonds with water
Burning	very flammable; burns with a smoky, yellow flame	high ratio of carbon to hydrogen: much unburnt carbon (soot)

Complete combustion: $\mathrm{2C_6H_6(l) + 15O_2(g) \rightarrow 12CO_2(g) + 6H_2O(l)}$ . In a limited air supply, much of the carbon is released as soot and carbon monoxide.

Uses

Table 2.2: What benzene is turned into

Benzene is converted to	which is used to make
ethylbenzene, then phenylethene (styrene)	polystyrene: packaging, cups, insulation
(1-methylethyl)benzene (cumene)	phenol and propanone: resins, adhesives, medicines
cyclohexane	nylon for clothing, ropes and fishing nets
nitrobenzene, then phenylamine (aniline)	dyes, polyurethane foams, rubber chemicals, paracetamol
alkylbenzenes, then their sulfonates	detergents

Warning

Benzene causes cancer. The World Health Organization describes exposure to benzene as a major public health concern: long-term exposure damages the bone marrow and causes leukaemia, and the International Agency for Research on Cancer classifies benzene as carcinogenic to humans. People are exposed through petrol vapour and vehicle exhaust, tobacco smoke, and the burning of fuels. Benzene is therefore not used in school laboratories; methylbenzene, which is much less toxic, is used as a solvent instead. Never siphon petrol by mouth, and fill vehicles in the open air.

Preparing benzene

In industry, benzene is obtained from crude oil. In catalytic reforming, alkanes from the naphtha fraction are passed over a platinum catalyst at about 500 °C and lose hydrogen to form rings:

$\mathrm{C_6H_{14}(g) \rightarrow C_6H_6(g) + 4H_2(g)}$

In the laboratory, three reactions are used:

Decarboxylation of sodium benzoate by heating it with soda lime (Figure 2.1): $\mathrm{C_6H_5COONa(s) + NaOH(s) \rightarrow C_6H_6(l) + Na_2CO_3(s)}$ . The $\mathrm{-COONa}$ group is replaced by hydrogen.
Polymerisation of ethyne: passing ethyne through a red-hot iron tube joins three molecules into a ring, $\mathrm{3C_2H_2(g) \rightarrow C_6H_6(l)}$ .
Reduction of phenol by heating it with zinc dust: $\mathrm{C_6H_5OH(l) + Zn(s) \rightarrow C_6H_6(l) + ZnO(s)}$ .

Figure 2.1: Preparing benzene by decarboxylation (a teacher demonstration in a fume cupboard only).

Definition

Phenyl group: the group $\mathrm{C_6H_5-}$ , a benzene ring less one hydrogen atom, sometimes written Ph. Example: phenylethene, $\mathrm{C_6H_5CH=CH_2}$ .

Positional isomers: compounds with the same molecular formula and the same functional groups, differing only in where the groups are attached to the ring. Example: 1,2-, 1,3- and 1,4-dichlorobenzene.

Naming benzene compounds

With one substituent, the name is the group as a prefix plus "benzene", or a special name:

Table 2.3: Names of monosubstituted benzenes

Formula	Name	Older or common name
$\mathrm{C_6H_5CH_3}$	methylbenzene	toluene
$\mathrm{C_6H_5Cl}$	chlorobenzene
$\mathrm{C_6H_5NO_2}$	nitrobenzene
$\mathrm{C_6H_5OH}$	phenol	carbolic acid
$\mathrm{C_6H_5NH_2}$	phenylamine	aniline
$\mathrm{C_6H_5COOH}$	benzoic acid
$\mathrm{C_6H_5CHO}$	benzaldehyde
$\mathrm{C_6H_5COCH_3}$	phenylethanone	acetophenone

With two or more substituents, number the ring carbons to give the lowest possible numbers (locants), and list the substituents in alphabetical order. If the compound has a special name (phenol, benzoic acid, phenylamine), the carbon bearing that group is carbon 1. Older names use ortho (1,2), meta (1,3) and para (1,4).

Figure 2.2: The three positional isomers of $\mathrm{C_6H_4Cl_2}$ . Red numbers show how the ring carbons are numbered.

Worked example

Problem. Name (a) a benzene ring carrying $\mathrm{-CH_3}$ and $\mathrm{-NO_2}$ on neighbouring carbons; (b) phenol with a methyl group on the carbon opposite the $\mathrm{-OH}$ ; (c) benzoic acid with a nitro group on carbon 3.

Solution. (a) No special name, so number to give 1,2 and list alphabetically: methyl before nitro. 1-methyl-2-nitrobenzene (2-nitrotoluene).

(b) The OH carbon is carbon 1 in phenol; the opposite carbon is 4. 4-methylphenol.

Worked example

Problem. How many aromatic isomers have the formula $\mathrm{C_6H_3Cl_3}$ ? Name them.

Solution. Place the first chlorine on carbon 1 and try every arrangement of the other two, keeping the lowest locants: 1,2,3 (all three together); 1,2,4 (two together, one apart); 1,3,5 (alternate). Any other arrangement, such as 1,2,5 or 1,3,4, is the same molecule turned round (1,2,5 renumbers as 1,2,4).

Answer: three isomers: 1,2,3-, 1,2,4- and 1,3,5-trichlorobenzene.

Tip

To count positional isomers, draw each arrangement, then renumber it from every carbon in both directions. If two drawings give the same lowest set of numbers, they are the same compound.

chemistNATE: Ortho, Meta, Para Naming on Benzene (4 min). Watch how the positions 1,2, 1,3 and 1,4 match the older ortho, meta and para names. The video uses the common names toluene and aniline for methylbenzene and phenylamine.

✦

Practice

Explain why benzene ( $M_{r}$ = 78) has a much higher melting point than hexane ( $M_{r}$ = 86).
Explain why benzene burns with a smoky flame, and write the equation for its complete combustion.
Give two ways people are exposed to benzene, and one precaution that reduces the risk.
Write equations for two laboratory preparations of benzene.
Name: $\mathrm{C_6H_5CH_2CH_3}$ ; a benzene ring with Br on carbons 1 and 3; phenol with nitro groups on carbons 2, 4 and 6.
Draw and name the three positional isomers of dimethylbenzene, $\mathrm{C_6H_4(CH_3)_2}$ .
How many aromatic isomers have the formula $\mathrm{C_8H_{10}}$ ? Name them.
A student names a compound "1,5-dichlorobenzene". Explain why the name is wrong and give the correct name.

Show answersHide answers

Benzene molecules are flat and symmetrical, so they pack closely and regularly in the solid; more energy is needed to break up the lattice. Hexane molecules are flexible chains that pack poorly.
It has a high carbon-to-hydrogen ratio (1 : 1), so there is not enough oxygen near the flame to burn all the carbon, which glows and escapes as soot. $\mathrm{2C_6H_6 + 15O_2 \rightarrow 12CO_2 + 6H_2O}$ .
Any two: petrol vapour at filling stations; vehicle exhaust; tobacco smoke; burning fuels or rubbish. Precautions: do not siphon petrol by mouth; fill up in the open air; avoid smoking and second-hand smoke; use a fume cupboard; use methylbenzene instead of benzene as a solvent.
Any two: $\mathrm{C_6H_5COONa + NaOH \rightarrow C_6H_6 + Na_2CO_3}$ ; $\mathrm{3C_2H_2 \rightarrow C_6H_6}$ ; $\mathrm{C_6H_5OH + Zn \rightarrow C_6H_6 + ZnO}$ .
Ethylbenzene; 1,3-dibromobenzene; 2,4,6-trinitrophenol.
1,2-dimethylbenzene, 1,3-dimethylbenzene and 1,4-dimethylbenzene (the methyl groups on neighbouring, alternate and opposite carbons).
Four: ethylbenzene, 1,2-dimethylbenzene, 1,3-dimethylbenzene and 1,4-dimethylbenzene.
Numbering the other way round the ring gives 1,3, which is lower. The correct name is 1,3-dichlorobenzene.

Sources and credits

Health information: Exposure to benzene: a major public health concern, World Health Organization, 2019.
Image: Polistirolo.JPG by Phyrexian, via Wikimedia Commons, licensed under CC BY-SA 3.0.
Video: Ortho, Meta, Para Naming (on Benzene), chemistNATE, embedded from YouTube.
The text, apparatus and isomer diagrams, tables, worked examples and Practice items were written and drawn by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1A useful but dangerous liquid

2Physical properties

3Uses

4Preparing benzene

5Naming benzene compounds

A useful but dangerous liquid

Physical properties

Uses

Preparing benzene

Naming benzene compounds