Aromatic Compounds

Defining Aromatic Compounds

• Historically:– organic compounds had an aroma or

odour

– unsaturated cyclic hydrocarbons – contain single rings or groups of rings– simplest aromatic compound is benzene– aromatic compounds include benzene or

are benzene based and have benzene-like structures and properties.

Aromatic Compounds

♫♪Sweet Dreams are Made of Benzene ♫♪

• Kekulé – The idea of the

benzene structure came to Kekulé in a dream

Benzene, C6H6

• 6 carbon ring with a hydrogen bonded to each carbon

• one electron from each carbon is free to participate in a double bond

Structure of Benzene

• we can draw two different structures with double bonds for benzene

H

H

H

H

H

H

H

H

H

H

H

H

or

Resonance

• occurs when two or more equally valid structures can be drawn for a molecule

• benzene exhibits resonance

• explains unique properties of benzene and other aromatics– benzene is perfectly flat (electrons are

delocalized, in other words, they are not fixed to one carbon but can move around)

– bending or twisting of the aromatic molecule would disrupt the electron sharing and the stability of the molecule

Resonance Structure for Benzene

• The following structures represent the resonance that exists in the aromatic molecule

More on Resonance

• The structure with the inserted circle illustrates how the electrons can move to form resonance structures, but does not illustrate how the electrons are involved in bonding.

• molecules that have resonance are more stable than molecules without resonance– ie benzene is not as reactive as a

corresponding six-carbon alkene

Properties of Benzene

• Molecular formula of benzene, C6H6, is based on its percent composition and molar mass

• MPbenzene = 5.5°C, BPbenzene=80.1°C• Non-polar molecule• Very unreactive with halogens• Does not undergo addition reactions.• All carbon-to-carbon bonds in benzene are the

same length (determined by x-ray diffraction)• Empirical evidence shows that all carbons are

identical and that each carbon is bonded to one hydrogen

Naming Aromatics

Substituted Benzenes• Compounds containing substituents

(ie alkyl groups, other atoms) in place of one hydrogen atom are named as derivatives of benzene.

Step 1 Identify the branch (alkyl or atom) and write it as the first part of the name.

Step 2 Complete the name with benzene.

Some examples

methylbenzene

CH3

toluene

ethylbenzene

CH2CH3

CH2CH2CH3

propylbenzene

Disubstituted Benzenes

• Some derivatives of benzene have two substituents

• Known as disubstituted benzenes.

Isomers

• Three isomers of disubstituted benzenes exist

• Named according to the location of the substituents on the benzene ring (IUPAC) or with the corresponding prefix (Classical)

Naming Rules

Step 1 Number the carbons, starting at one of the substituents and numbering in the direction which will give the substituents the lowest number.

Step 2 Write the locations of the branches (IUPAC) or use the appropriate classical prefix.

Prefix (Classical)

Positions(IUPAC)

ortho- (o) 1,2

meta- (m) 1,3

para- (p) 1,4

Step 2 Write the location of the branch, followed by the name of the branch.

Step 3 Complete the name with benzene.

Some Examples

CH3

CH3

1,2-dimethylbenzene

o-dimethylbenzene

ortho-dimethylbenzene

CH3

CH3

1,3-dimethylbenzene

meta-dimethylbenzenem-dimethylbenzene

CH3

CH3

1,4-dimethylbenzene

para-dimethylbenzene

p-dimethylbenzene

CH3

CH2CH3

1-methyl-3-ethylbenzene

m-methylethylbenzene

CH2CH3

CH2CH2CH3

1-ethyl-2-propylbenzeneo-ethylpropylbenzene

• For larger organic molecules, the benzene is considered a branch or an alkyl group

• Called phenyl (C6H5-)

Here are some examplesH3C CH CH3 H3C CH CH2 CH2 CH3 H3C CH2 CH CH2 CH3

2-phenylpropane

2-phenylpentane

3-phenylpentane

Can you name these?

CH3CHCHCH2CH2CH3

CH3

H3C C CH CH CH2 CH2 CH2 CH3

CH2

CH3

3-methyl-2-phenylhexane

3-methyl-5-phenyl-3-nonene