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chemistry Unit 14 learning aim B
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chemistry Unit 14 learning aim B
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Unit 14BP2: Explain the structure of benzene using sigma and pi bonding providing
evidence for the structure
Benzene has 6 carbon and a hydrogen atom bonded to it in an hexagon shape
with a formula of C6H6. carbon atoms has 6 sigma bonds in between it and 4
electrons on each of the carbon atom and it also has 3pi bonds.
Kekule’ structure of benzene
Kekule’ look for a way on how to create the structure of benzene which made
sense and the carbon is arranged in an hexagonal shape and he taught of an
alternating double and single bonds between the carbon atom which was
adding an hydrogen on each of the carbon atom. Here are the examples of
benzene shown:
Hybridisation- each carbon is joining to three other atoms, when the carbon
atom hybridise their outer orbital. Each of the carbon is SP2 hybridised and
their 3SPZ orbital that forms into 3 double bond and its delocalised. The
carbon atom in benzene is bonded with the other one through a double and
single bond.
Thermo chemical evidence- the calculated enthalpy of formation for one
mole of gaseous benzene is +252kjmol-1. However, the actual enthalpy of
formation had been tested before and lower at +82kjmol-1. This indicates
structure of benzene is stable than kekule’’ s model of benzene. Thermo
chemical evidence against kekule’s model comes from the theoretical
enthalpy change for the hydrogenation of benzene when cyclohexane, with
its one double carbon-carbon bond, undergoes hydrogenation the enthalpy
change is -119.6kjmol-1 The theoretical enthalpy change for the
hydrogenation of benzene can be calculated by multiplying the enthalpy
change for the hydrogenation of cyclohexane by three, resulting in a value
of -358.8kjmol-1.
, X-ray diffraction – uses crystal x-ray diffraction patterns the single
carbon-carbon bond lengths in cyclohexane and having single carbon-
carbon bonds and a double carbon-carbon bond with a cyclic structure
were measured to be 0.154nm.
0.133nm was the value of the bond length of double carbon-carbon
bond in cyclohexane measured by X-ray diffraction. Assuming kekule ’
structure was right it will specify that the alternating double carbon-
carbon and single carbon-carbon bonds having different lengths but if
the bond lengths in a molecule in benzene was calculated using the
same method it will result in having the same lengths of 0.139nm.
Sulfonation of benzene
it is the heating of C6H6 with fuming sulphuric acid (H2SO4 +SO3) to produce
benzene sulphuric acid. The reaction can be reversed to produce benzene
sulfonic acid by adding sulfur trioxide and fuming sulfur acid. The reaction
reverses by adding hot aqueous acid to benzene sulfonic acid to produce
benzene.
Mechanism
fuming sulfuric acid and sulfur trioxide produces benzenesulfonic acid. Fuming
sulfuric acid is a concentrated solution of dissolved sulfur trioxde
(electrophillic) in sulfuric acid. The electrophillic in sulfur trioxide has oxygens
that pull electrons away from it.The benzene attacks the sulfur and subsequent
protons transfers occur to produce benzenesulfonic acid.
Nitration of benzene
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