Module 6 Chapter 25
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Who was benzene first isolated by
Michael Faraday in 1825
What is benzene used for
It is a major feedstock in many industries: Polymers Pharmaceuticals Dyes Explosives
What is a feature of benzene
It is highly carcinogenic, it is colourless, sweet smelling, highly flammable liquid found in crude oil and cigarette smoke.
What is benzene an example of
it is the simplest arena with the empirical formula of CH, and molecular formula of C6H6
What occurred in 1865 to do with benzene
August kekulé suggested a cyclical structure for benzene with three alternating carbon-carbon double bonds.
What were problems with the kekulé structure of benzene
It does not react in the same way as alkenes, and in general seems unreactive. This model does not explain all of its physical and chemical properties.
What did the kekulé model propose to get over this claim
That benzene’s low reactivity was due to a rapid equilibrium between two isomers
Draw the equilibrium between the two isomers of benzene
DRAW IT
What are problems with the kekulé model for benzene (1)
If benzene did contain these c=c bonds, it should decolourise bromine in an electrophilic addition reaction —> Therefore there are no c=c double bonds within its structure
What are problems with the kekulé model for benzene (2)
isomers: compounds like 1,2 - dichlorobenzene should form isomers but only one form exists
What are problems with the kekulé model for benzene (3)
bond length: c-c and c=c bonds have different bonds lengths so benzene would be an irregular hexagon but imaging shows it’s regular with intermediate bond lengths
What are problems with the kekulé model for benzene (4)
Hydrogenation enthalpy: Benzene can be saturated with hydrogen to form cyclohexane With the kekule model having an expected hydrogenation enthalpy of -360KJ/mol, whereas in reality it has a ^H of -208KJ/mol so therefore it’s bonds are more stable than alkenes
What are delocalised electrons
Bonding electrons that are not fixed between two atoms but shared between 3 or more atoms
What is the actual structure of benzene
• Each carbon atoms form there covalent sigma bonds • The remaining electron is found in the p orbital at a right angle above and below the carbon atoms • The p orbitals overlap sideways, crating a ring of delocalised pi electrons above and below the structure. Forming a ring of e- density
What does the ring in benzene allow
Charge to be evenly spread across the molecule making it stable and allowing equal bond lengths.
What is a feature of benzene
it is a planar molecule
What are the delocalised electrons shown as in benzene
A circle inside the benzene ring
What is the nomenclature of benzene
Some groups are shown as prefixes to benzene. In aromatic compounds benzene ring is often considered the parent chain/main chain But if it is attached to an alkyl chain with a functional group or an alkyl chain with 7 or more c it is considered to be a substitutent
What is it called in nomenclature when benzene is substituent
Prefix = phenyl
What are some naming exceptions for benzene
benzoic acid (Benzenecarboxylic acid) Phenylamine (aminebenzene, small nitro group) Benaldehyde (benzene carbaledhye)
What is a feature of benzene
It is the parent Arlene that others are derived from
What is a feature of naming arenes
The position of the substituted hydrogens are numbered to give the lowest possible numbers. —> with one group always occupying position 1
When naming compounds with benzene what are the different suffixes and prefixes.
The ring can be either given of the suffix ‘benzene’ or as the prefix ‘phenyl’ if with alcohol or amine groups
What reactions can alkenes undergo that benzene can’t and why
Electrophilic addition reactions as it would disrupt the delocalised pi electron ring, resulting in the product being less energetically stable