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Unit 14B Organic Chemistry
- Institution
- PEARSON (PEARSON)
Distinction graded assignment for unit 14b in btec extended diploma in applied science level 3.
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Structures, reactions, uses and properties of benzeneBenzene, C6H6, is a planar molecule containing a ring of six carbon atoms, each with a hydrogen
atom attached. The six carbon atoms form a perfectly regular hexagon.
Hybridisation
Because each carbon is only joining to three other atoms, when the carbon atoms hybridise their
outer orbitals before forming bonds, they only need to hybridise three of the orbitals rather than all
four. They use the 2s electron and two of the 2p electrons, but leave the other 2p electron
unchanged.
The new orbitals formed are called sp2 hybrids, because they are made by an s orbital and two p
orbitals reorganising themselves. The three sp2 hybrid orbitals arrange themselves as far apart as
possible - which is at 120° to each other in a plane. The remaining p orbital is at right angles to
them.
Each carbon atom is joined to two other similar carbon atoms instead of just one. Each carbon
atom uses the sp2 hybrids to form sigma bonds with two other carbons and one hydrogen atom.
Kekulé structure
Kekulé was the first to suggest a sensible structure for benzene. The
carbons are arranged in a hexagon, and he suggested alternating
double and single bonds between them. Each carbon atom has a
hydrogen attached to it.
1
BTEC Assignment Brief v1.0
BTEC Internal Assessment QDAM January 2015
, Problems with the Kekulé structure
Due to benzene having three double bonds, we would expect benzene to have reactions like ethene;
ethene undergoes addition reactions in which one of the two bonds joining the carbon atoms
breaks, and the electrons are used to bond with additional atoms.
Benzenehowever, rarely does this. Instead, it usually undergoes substitution reactions in which one
of the hydrogen atoms is replaced by something new.
Problems with the shape
Benzene is a planar molecule and that would also be true of the Kekulé structure. The problem is
that C-C single and double bonds are different lengths:
C-C 0.154 nm
C=C 0.134 nm
That would mean that the hexagon would be irregular if it had the Kekulé structure, with
alternating shorter and longer sides. In real benzene all the bonds are exactly the same -
intermediate in length between C-C and C=C at 0.139 nm. Real benzene is a perfectly regular
hexagon.
Problems with the stability of benzene
Real benzene is a lot more stable than the Kekulé structure would give it credit for. Every time you
do a thermochemistry calculation based on the Kekulé structure, you get an answer which is wrong
by about 150 kJ mol-1. This is most easily shown using enthalpy changes of hydrogenation.
Hydrogenation is the addition of hydrogen to something. In order to do a fair comparison with
benzene, we can compare it with cyclohexene. Cyclohexene, C6H10, is a ring of six carbon atoms
containing just one C=C. When hydrogen is added to this, cyclohexane is formed. The "CH" groups
become CH2 and the double bond is replaced by a single one. The enthalpy change for this reaction
is -120 kJ mol-1.
Applying this to the Kekulé structure for benzene, we would expect an enthalpy change of -360 kJ
mol-1, because there are exactly three times as many bonds being broken and made as in the
cyclohexene case. However, what you actually get is -208 kJ mol-1.
Looking at the enthalpy diagram below, notice that in each case heat energy is released, and in each
case the product is the same (cyclohexane). That means that all the reactions "fall down" to the
same endpoint.
Real changes are shown by heavy lines, solid arrows and
bold numbers. Dotted lines and italics show predicted
changes.
The most important point to notice is that real benzene is
much lower down the diagram than the Kekulé form
predicts. The lower down a substance is, the more
energetically stable it is.
2
BTEC Assignment Brief v1.0
BTEC Internal Assessment QDAM January 2015
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