aromatics 2
TRANSCRIPT
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Aromatic Compounds
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Defining Aromatic Compounds
• Historically:– organic compounds had an aroma or
odour
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– 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
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♫♪Sweet Dreams are Made of Benzene ♫♪
• Kekulé – The idea of the
benzene structure came to Kekulé in a dream
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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
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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
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Resonance
• occurs when two or more equally valid structures can be drawn for a molecule
• benzene exhibits resonance
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• 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
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Resonance Structure for Benzene
• The following structures represent the resonance that exists in the aromatic molecule
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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
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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
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Naming Aromatics
Substituted Benzenes• Compounds containing substituents
(ie alkyl groups, other atoms) in place of one hydrogen atom are named as derivatives of benzene.
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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.
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Some examples
methylbenzene
CH3
toluene
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ethylbenzene
CH2CH3
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CH2CH2CH3
propylbenzene
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Disubstituted Benzenes
• Some derivatives of benzene have two substituents
• Known as disubstituted benzenes.
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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)
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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.
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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
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Step 2 Write the location of the branch, followed by the name of the branch.
Step 3 Complete the name with benzene.
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Some Examples
CH3
CH3
1,2-dimethylbenzene
o-dimethylbenzene
ortho-dimethylbenzene
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CH3
CH3
1,3-dimethylbenzene
meta-dimethylbenzenem-dimethylbenzene
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CH3
CH3
1,4-dimethylbenzene
para-dimethylbenzene
p-dimethylbenzene
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CH3
CH2CH3
1-methyl-3-ethylbenzene
m-methylethylbenzene
CH2CH3
CH2CH2CH3
1-ethyl-2-propylbenzeneo-ethylpropylbenzene
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• For larger organic molecules, the benzene is considered a branch or an alkyl group
• Called phenyl (C6H5-)
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Here are some examplesH3C CH CH3 H3C CH CH2 CH2 CH3 H3C CH2 CH CH2 CH3
2-phenylpropane
2-phenylpentane
3-phenylpentane
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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