Summary notes for AQA A-Level Chemistry Unit 3.3.10 - Aromatic chemistry (A-level only) by an Imperial College London MSci Chemistry graduate. Notes divided into the following sections: The Structure and Nomenclature of Benzene, Electrophilic substitution
Aromatic Chemistry
The Structure and Nomenclature of Benzene
The Kekulé Structure of Benzene
In 1865 Kekulé proposed that the structure of benzene (C6H6) was a
hexagonal ring structure of 6 C atoms, each bonded to a single H atom + w/
alternating double + single bonds between the C atoms. This theoretical
model of benzene was known as cyclohexa-1,3,5-triene.
However, there are problems w/ this theoretical model of benzene
(cyclohexa-1,3,5-triene) that show it to be inaccurate…
Kekulé’s/Theoretical structure of benzene Experimental observations
Bond lengths in the carbon ring should alternate All bond lengths in the carbon ring are the
between those of single (C-C) + double (C=C) same + intermediate in length between single
covalent bonds. + double bonds.
It should readily undergo addition reactions in the Benzene can undergo substitution reactions
way that alkenes + bromine rapidly react to but does not undergo addition reactions easily.
decolourise bromine water.
The enthalpy of hydrogenation (adding H atoms to The actual enthalpy of hydrogenation is
all unsaturated bonds) would be -360kJmol-1. -208kJmol-1.
The Delocalised Structure of Benzene
Today’s accepted structure for benzene is of a delocalised model which has the following features:
• It is a planar hexagonal molecule of 6 C atoms.
• All C-C bond lengths are intermediate in length between that of a single C-C + double C=C.
• Each C uses 3 of its outer electrons to form 3 σ bonds to 2 other C atoms, + 1 H atom. This leaves each
C atom w/ one electron in a p orbital.
• The lobes of the p orbitals overlap sideways w/ the neighbouring p orbitals to form a π bond.
• The overall result is a ring of -ve charge (“electron cloud”) above + below the plane of the molecule.
• The 6 p electrons in the π system are delocalised (i.e. they are free to move throughout the π
system).
- This delocalised cloud of π electrons is present in substances called aromatic compounds; as such,
benzene + its derivatives (the arenas) are aromatic.
• A circle is used to represent the ring of delocalised electrons.
Delocalisation of p electrons
makes benzene more stable than
the theoretical molecule
cyclohexa-1,3,5-triene.
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