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