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Summary Unit 14 - Applications of Organic Chemistry BTEC Unit 14 Assignment 2 all criteria
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Unit 14 - Applications of Organic Chemistry BTEC Unit 14 Assignment 2 all criteria /distinction received
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Unit 14 Assignment 2 : B / Understand the reactions and properties of aromatic compounds.Moreen Mero
The structure of Benzene:
o P2 / Explain the structure of benzene using sigma and pi bonding, providing evidence
for the structure:
- What is benzene: this is a chemical organic compound,
and the chemical formula for it is C6H6 which shows that
benzene has 6 carbons that are been joined together in a
ring structure, and there is one hydrogen been attached to
each carbon. And because of this, benzene is classified as
(hydrocarbon) because it is composed by only of carbon
and hydrogen atoms. A three pairs of electrons
(delocalised electrons) are present in benzene, and this is
demonstrated as a line of circle inside the ring of benzene.
- The structure : Kekulé structure:
The structure for benzene was suggested in 1865 by
Kekulé, as his proposal involved a reasonable and sensible
structure depending on the way the carbons are been
arranged in this compound: the proposal was that these
carbons, are been arranged in a shape of hexagon, showing
that the carbons have bonds of single as well as double bonds in between them and each carbon
atom has only one hydrogen been attached to it. Moreover, it was not been suggested by Kekulé
that the double bonds of benzene does move continuously, and neither been proposed that it
has delocalised electron pairs.
- The hybridisation of benzene :
This is when two or more that two atoms form bonds.
Orbitals do mix and combine to firm new orbitals in
which these new orbitals would have the same number
of electrons such as the old orbitals have. The
unhybridized orbitals’ energy properties would be the
same as the new hybridised orbitals’ properties and
energy as well. The process goes of three orbitals, which
are one S and two P’s orbitals, that hybridise to make
new three orbitals, and they are the same or equivalent
to the old ones. Therefore, the outstanding 2P orbitals
would be unaffected, or unchanged:
,Unit 14 Assignment 2 : B / Understand the reactions and properties of aromatic compounds.
Moreen Mero
The hydrogen atoms in benzene : 1s1
The carbon atoms in benzene : 1s2 2s2 2p2
To be able to form the required amount number of bonds,
the atom must have sufficient unpaired electrons. So
therefore it does promote one of 2s2 pair to an empty 2p
orbital to get the desired bonds number.
When there is a gap (energy gap) in between the 2p and 2s
orbitals, there will be electron that is promoted to empty 2p
orbital from 2s orbital, this will give 4 electrons that are
unpaired. This is when the carbon atom will be in an
unsettled state, or (excited state).
Moreover, as each carbon is able to join to only three other carbon atoms; They only hybridise
3 orbitals and not all the 4 of them when the carbon does hybridise the outer orbital before the
bonds form. The two of 2p electrons and the 2s electron are used. However, the other electron
in 2p is been left and unchanged. Thus, there are new orbitals been made using 2 p orbitals and
1 s orbital. There is space of 120 degrees apart from each sp2 hybrid, which therefore forms or
produces the shape of hexagon.
- Delocalization in benzene:
In the benzene ring, in each carbon atom, a p orbital is found, that overlap with another p orbital
from another carbon atom. This overlapping results a producing of pi pond system, that spread
along all the carbon ring. And therefore, there will not be any holding of the electrons between
just two carbon atoms. This will cause the benzene to have a ring of delocalised electrons.
Thus, the pi bonds will delocalise all over the ring forming a constant cloud of electron, which
these electron clouds will be all over the ring causing the benzene molecule to be stable
relatively, because of the delocalised electron in the ring.
The final stable structure of benzene does resists and cannot break down with electrophilic
normal addition. And the delocalisation will not be affected by any substitution of hydrogen
atoms.
- The stability of benzene ring:
➢ The Thermodynamic (thermochemical) evidence: the study of the association between
the heat energy and the chemical reactions is called thermochemistry. And the data of
the thermochemical it is used to demonstrate the molecule’s stability: when heat is
released less, meaning that molecules are been relatively stable. On the other hand,
when the heat is released more, meaning that the molecule is less relatively stable.
➢ The Hydrogenation of benzene : as benzene reacts with hydrogen, by far the enthalpy
change will be exothermic ( -208 kJ ) instead of being ( -360 kJ ). And the amount if
the heat been released will be less that it estimated as benzene is breaking up.
Therefore, this shows that to maintain the benzene structure, a less energy will be
required and that benzene is stable relatively due to its delocalised electrons.
➢ The hydrogenation of cyclohexene : in cyclohexene, each molecule would have one
carbon double bond, and the reaction does have enthalpy change of ( -120 kJ ).
, Unit 14 Assignment 2 : B / Understand the reactions and properties of aromatic compounds.
Moreen Mero
➢ X-ray diffraction evidence : this is been
used as to measure the bonds’ length in the
molecule, because the X-ray does interact
with the electrons being in the molecule,
which cause them to scatter. The scattered
pattern rays in which are produced by the
scattering, it does determine the
distribution of the electron charge in the
molecule. Therefore, this reveals the
bonds’ length in the molecules. Based on
Kekulé’s believes, the benzene bonds been
different in length: the double carbon bonds are very shorter which comes at 0.133nm,
while the single carbon bonds are a little longer at 0.154nm. Nevertheless, the benzene
crystalises when it is been cooled down. However, this propose of Kekulé is invalid,
this is because when using X-ray diffraction to measure the lengths of the bonds, it
found that all the bonds are in the length of 0.139nm.
➢ The infrared data (IR) evidence : when it is needed to confirm what bonds or groups in
molecule are, this method is been used for this. The IR rays will be projected on a
sample that has molecules, then only a specific bond/bonds of that molecule will
absorb a specific wavenumber/wavenumbers of the IR. Then, it can be shown as a
graph that displays the wavenumbers that been absorbed, thus this demonstrates the
kind of bonds that are in the molecule. The wavenumbers are between 1,650 cm-1 and
2,000 cm-1 for the benzene ring absorption. Therefore, there are several peaks been
produced in between these wavenumbers, and this is because of the stretching of the
carbon-hydrogen vibrations, when the carbon atoms will bond, to the delocalised
electrons. Overtones is what these peaks are called, or even are called the benzene
fingers, because they do produce some form of pattern that is similar to the human hand
fingers. (Ghelichkhah, n.d.)
o P3 : Explain the chemical properties of industrially important benzene and
monosubstituted benzene compounds.
The physical properties of benzene :
- Benzene happens to be a liquid which is colourless, it is soluble (in the organic solvents ) but it
is also immiscible (un-mixable ) in water.
- It does have aromatic odour (scent / pleasant scent ) due to being an aromatic compound.
- The density of benzene is 0.87g cm-3, which makes is lighter that the water.
- The carbon-carbon bonds in the benzene have a length of 140pm which is an equal length
because all of the are the same length in the benzene. All the carbons that have Sp2
hybridisation will have bond angle of 120 degrees.
- The boiling point of benzene is on average, which is 80.5 C, and the melting point is high,
coming is 5.5 C. As because attraction in the magnitude of the Vander Waal’s forces increases,
it cause increasing for the homologous series, and along with the molecular mass.
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