Unit 14B applied science complete
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Najah Bendriss BTEC Applied Science Chemistry Unit 14B
UNIT 14B
P2:
Benzene is made up of six carbons which are arranged in a cyclic structure, they each have a
hydrogen atom bonded to it.
Between carbon atoms there are six sigma bonds and three pi bonds that alternate. Each
carbon atom has four electrons in bonding electron pairs, three of these electrons are in
sigma bonds and the other is in a pi bond. The P-orbitals from these pi bonds can overlap
above and below the plane of the carbon ring to form a delocalised system above and below
the plane which now contains free moving electrons from the P orbitals.
Evidence:
The length of the bonds in the benzene ring is same for all C-C bonds and has value between
standard length of single and double bonds (C=C bonds are shorter than C-C). If it was to be
as Kekule model (cyclohex-1,3,5-triene) suggests there should be alternating short and
longer bonds.
If benzene didn't have delocalised system but would be just a cyclic alkene (Kekule model),
its enthalpy change of hydrogenation would be three times the enthalpy change of
hydrogenation of cyclohexene (-120 kJ/mol). However, enthalpy change of hydrogenation of
benzene is -208 kJ/mol rather than -360 kJ/mol. The delocalised system gives benzene more
stability.
1
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Najah Bendriss BTEC Applied Science Chemistry Unit 14B
Benzene's resistance to react with compounds that normally alkenes have a reaction with.
One example of this is when benzene is added to bromine water, a regular alkene would
decolourise the bromine water, but benzene does not.
• it did not readily undergo electrophilic addition - no true C=C bond
• only one 1,2 disubstituted product existed
• the ring was thermodynamically more stable than expected
Benzene compounds:
a) Bromobenzene
Br
b) 1,4-dibromobenzene
Br
2
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assignment
written by
najbo123
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Downloaded by: najbo123 | Want to earn £756
Distribution of this document is illegal extra per year?
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Najah Bendriss BTEC Applied Science Chemistry Unit 14B
UNIT 14B
P2:
Benzene is made up of six carbons which are arranged in a cyclic structure, they each have a
hydrogen atom bonded to it.
Between carbon atoms there are six sigma bonds and three pi bonds that alternate. Each
carbon atom has four electrons in bonding electron pairs, three of these electrons are in
sigma bonds and the other is in a pi bond. The P-orbitals from these pi bonds can overlap
above and below the plane of the carbon ring to form a delocalised system above and below
the plane which now contains free moving electrons from the P orbitals.
Evidence:
The length of the bonds in the benzene ring is same for all C-C bonds and has value between
standard length of single and double bonds (C=C bonds are shorter than C-C). If it was to be
as Kekule model (cyclohex-1,3,5-triene) suggests there should be alternating short and
longer bonds.
If benzene didn't have delocalised system but would be just a cyclic alkene (Kekule model),
its enthalpy change of hydrogenation would be three times the enthalpy change of
hydrogenation of cyclohexene (-120 kJ/mol). However, enthalpy change of hydrogenation of
benzene is -208 kJ/mol rather than -360 kJ/mol. The delocalised system gives benzene more
stability.
1
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Najah Bendriss BTEC Applied Science Chemistry Unit 14B
Benzene's resistance to react with compounds that normally alkenes have a reaction with.
One example of this is when benzene is added to bromine water, a regular alkene would
decolourise the bromine water, but benzene does not.
• it did not readily undergo electrophilic addition - no true C=C bond
• only one 1,2 disubstituted product existed
• the ring was thermodynamically more stable than expected
Benzene compounds:
a) Bromobenzene
Br
b) 1,4-dibromobenzene
Br
2
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