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Summary A level OCR chemistry chapter 25 notes
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Chapter 2525.1- Introducing benzene
Give three properties of benzene Colourless, sweet smelling, highly flammable
Where is it found? In crude oil is a component of petrol and also found
in cigarette smoke
What is benzene classified as? Carcinogen
Draw kekule’s structure of benzene
3 ways kekule’s structure was disproven? Lack of reactivity of benzene, the lengths of the
carbon-carbon bonds in benzene, hydrogenation
enthalpies
Lack of reactivity If benzene contained C=C bonds then it would
decolourise bromine HOWEVER benzene does no
undergo electrophilic addition reactions. It does no
decolourise bromine under normal conditions. This
led scientists to suggest that benzene cannot have
any C=C bonds.
The lengths of the carbon-carbon bonds in Using a technique called X-ray diffraction, it is
benzene possible to measure bond lengths in a molecule.
When benzene was examined by Kathleen Lonsdale
it was found that all the bonds in benzene were
0.139nm in length. This bond length was between
the length of a single bond- 0.153nm and a double
bond, 0.134nm
Hydrogenation enthalpies If benzene had kekules structure, then it would be
expected to have an enthalpy change of
hydrogenation that is three times that of
cyclohexene. When cyclohexene is hydrogenated,
one double bond reacts with hydrogen. The
enthalpy change is -120KJmol-1 but as kekules
structure has 3 C=C is should be -360KJmol-1. The
actual value is -208KJmol-1
What is the current model? The delocalise model of Benzene
Benzene is a ____, ___, ____ hydrocarbon Planar, cyclic, hexagonal. 6c and 6H
containing how many carbon and how many
, Chapter 25
hydrogen atoms?
In this model how many electrons does each Each carbon atom uses three of its available four
carbon atom use up and what are they electrons in bonding to two other carbon atoms and
bonded to. to one hydrogen atom
Where is the 4th electron located? In a p-orbital at right angles to the plane of the
bonded carbon and hydrogen atoms
How do the p orbitals overlap? They overlap sideways, in both directions above
and below the plane of the carbon atoms to form a
ring of electron density.
What does this overlapping create? A system of pi bonds which spread over all six of
the carbon atoms in the ring structure
What are the 6 electrons said to be? delocalised
How do you name a benzene that has one They are shown as prefixes to benzene, eg short
substituent group? alkyl chains, halogens, nitro groups
How long does the alkyl chain have to be for 7 or more carbon atoms
benzene to become the substituent Prefix becomes phenyl
What is the prefix?
Benzoic acid
Phenylamine
Benzaldehyde 2-phenyloctane
2-chloromethylbenzene
3-chloromethylbenzene
4-chloromethylbenzene
Naphthalene
right= anthracene
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