Organic chemistry has two main divisions. One division deals with aliphatic (fatty) compounds, the first compounds you encountered in Organic Chemistry I. The second division includes the aromatic (fragrant) compounds, of which benzene is a typical example.
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I
Aromatic Compound
Organic chemistry has two main divisions. One division
deals with aliphatic (fatty) compounds, the first compounds you
encountered in Organic Chemistry I. The second division includes the
aromatic (fragrant) compounds, of which benzene is a typical example.
Chemists proposed many structures for benzene. However, the facts
didnโt support any of the possibilities until Kekulรฉ proposed a ring
structure in 1865. Some of the proposed structures, including Kekulรฉโs
one.
I. Nomenclature of Aromatic Cpd.s
1.General rules for naming;
1. Benzene derivatives are named by pre๏ฌxing the name of the
substituent group to the word benzene, e.g. chlorobenzene and
nitrobenzene.
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2. Disubstituted benzene with identical groups; the relative positions
have to be identi๏ฌed.
(1,2 = o; ortho โ 1,3 = m; meta โ 1,4 = para)
o-dimethylbenzene m-dibromobenzene
3.Disubstituted benzene with different groups;
According to the priority, parent name is detected.
Aniline. Phenol, Benzaldehyde and benzoic acid will be considered the
parent name.
p-Chlorostyrene p-methylphenol
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4. When more than two substituent,, use the no. to express the positions.
If the groups are different, then the last-named group is understood to
be in position 1.
5.Long carbon chain has the priority,,,
Unsaturated chain has the priority,,,
(that rule can be neglected)
(2E-2-phenyl-2-butane phenyldecan
*benzene as a substituent;
-Aryl group; Ar-:
Aromatic hydrocarbons are sometimes called arenes.
Ar- is the aromatic group that remains after the removal of a hydrogen
atom from an aromatic ring.
examples of aryl groups:
examples of generic aryl groups:
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-phenyl group; Ph (or โ ):
The phenyl group is used in the name just like the name of an alkyl
group.
examples of phenyl groups:
-benzyl group;
The seven-carbon unit consisting of a benzene ring and a methylene
(-๐ช๐ ๐-) group.
examples of benzyl group:
*practical quiz;
Name the following structures:
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II. Study of Benzene ring
Rx. Of benzene ring!
If benzene contains three double bonds, why, his critics asked, doesnโt
it show the reactions typical of alkenes?
It is easier to break ๐ bond than breaking ๐ bond but it broke ๐ as the
๐ bond gains stability from benzene structure system.
1.Measurement of the benzene ring stability
Using halogenation!!! As a process used to measure the stability of
benzene in a comparison with series of related compounds that have
๐ bond giving a difference in the energy values,,, stabilization energy..
The presence of resonance in the benzene makes it harder to break the
3 double bonds and to break it, it costs high energy.
***Note;
- Def. of,,
*Stabilization energy ;
the difference between the supposed released energy and the actually
released energy.
*Entropy ; freedom of electrons.
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So energetically , benzene cannot behave as an alkene,,
2. Theories of benzene stability explanation
I. Resonance Theory
*(overview)
Resonance;
the extra stabilization provided by delocalization, compared with a
localized structure.
delocalization is gained from migration of e' of ๐ bonds as the weak
overlapping or unshared e'.
Contributing Structures
๐บ๐ท ๐
hybridization
Give reason;
Resonance increase the stability.
Resonance cause dispersion of charges (-ve charges of e')
and it's spread over the atoms so, it decreases the entropy and increase
the stability.
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1. The ๐ bond = ๐ orbital + ๐ orbital ,,, that remain after hybridization
( ๐บ๐ท ๐
).
2. Resonance forms are imaginary, not real.
3. Resonance structures differ only in the positions of their electrons.
4. Different resonance forms do not have to be equivalent.
5. The more resonance structures there are, the more stable the
molecule is.
6. Orbital;The SPACE that provide an opportunity for e' migration.
*Kekule structure & its explanation for the benzene stability;
As the approch of Kekule's structure and
the above theory, it would be a conclusion
that the stability of the benzene structure is
a result of the resonance som it's the aromaticity!
"What we need to catch a reason for it".
Unfortionately, this words fail to explain the followingโฆ
Cyclobutadiene Cyclooctatraiene
Both show resonance but they are NOT show the aromaticity at all
conditions. Cyclobutadiene and cyclooctatetraene have alternating
single and double bonds similar to those of benzene. These compounds
were mistakenly expected to be aromatic.
*Note: all annulenes show resonance but not aromatic.
- Annulene: these cyclic hydrocarbons with alternating single and
double bonds.
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II. Organic math โ Hรผckelโs Rule of aromaticity
Following are the requirements for a species to be aromatic:
I. Aromatic compounds
1. The compound must contain a ring system ,, (cyclic).
2. The system must be planar (or nearly planar), p orbitals are parallel.
3. The system is unsaturated conjugated.
"each atom in the ring must have an unhybridized p-orbital
perpendicular to the plane of the ring,,," (all p orbitals are parallel).
This means that an sp3 atom cannot be part of an aromatic system.
4. The ring system must have a Hรผckel number of ฯ-electrons
*No of ๐ electrons = 4n + 2 ,,, " N is a positive integer".
e.g. Benzene
no. of e' = 6 ,,, SO, 4n + 2 = 6
4n = 4
โด Benzene is AROMATIC n = 1 "+ve integer"
II. Anti-aromatic compounds
1. The compound must contain a ring system ,, (cyclic).
2. The system must be planar.
3. The system is conjugated.
4. Do NOT obey Hรผckel number of ฯ-electrons.
*No. of ๐ electrons = 4n + 2 ,,, " n is not a positive integer".
but obey โฆ
or, ๐ electrons = 4n " n is a positive integer "
e.g. cyclopentadienyl cation
no. of e' = 4 ,,, SO, 4n = 4
n = 1 "+ve integer"
โด cyclopentadienyl cation is ANTI-AROMATIC
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III. Non-Aromatic compounds
*the compound has not one point of the mentioned above*
- Non cyclic conjugated.
Or,
- has non planer configuration.
Or,
- no unhybridized p- orbital for an atom with non-conjugated system.
III. Molecular Orbital Theory & Frost Circle
*MOT (overview)
The key difference is that in molecular orbitals, the electrons are
allowed to interact with more than one atomic nucleus at a time. Just as
with atomic orbitals.
MOT in points;
1. MO = AO + AO So, no. of molecular orbitals = no. of atomic orbitals.
2. There are 3 types (bonding, anti-bonding, non- bonding) that differ
in:
*type of overlap.
*energy.
3. M.O. is formed according to respectable view of:
-Hund's rule.
-Aufbau principle. Just a good knowledge of them is fair.
-Pauli exclusion principle.
*Hund's rule; orbital in a subshell is singly occupied with one electron
before any one orbital is doubly occupied, and all electrons in singly
occupied orbitals have the same spin.
(No pairing even it's being a single in the required position)
*Aufbau principle; electrons orbiting one or more atoms fill the lowest
available energy levels before filling higher levels.
*Pauli exclusion principle; No two electrons in an atom
can have identical quantum numbers.
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principles in developing a molecular orbital representation for
benzene;
1. There are six atomic p orbitals that overlap to form the benzene ๐
system. Therefore, there must be six molecular orbitals.
2. The lowest-energy molecular orbital is entirely bonding, with
constructive overlap between all pairs of adjacent p orbitals. There are
no vertical nodes in this lowest-lying MO.
3. The number of nodes increases as the MOs increase in energy.
4. The MOs should be evenly divided between bonding and antibonding
MOs, with the possibility of nonbonding MOs in some cases.
5. We expect that a stable system will have filled bonding MOs and
empty antibonding MOs.
Are you confused ?! let me simplify that by a simple example at first.
*Molecular Orbitals for the ๐ฏ ๐ Molecule
that diagram deliver the meaning of the constructive and deconstructive
wave which are formed according to the type of interaction,
*head โ head interaction โฆ
*side-by-side interaction โฆ
If you still can't collect the fragments of that point, do not worry at all.
but you must be able to apply the following by yourself, "your hand
writing I mean"โฆ
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Molecular Orbital Energy-Level Diagram for ๐ฏ ๐;
Now, let yourself to draw the same diagram for benzene;
Energy diagram of the molecular orbitals of benzene.
Benzeneโs six electrons fill the three bonding orbitals,
leaving the antibonding orbitals vacant.
but how we can imagine that more and more
simple..
"picture is better than thousands of books"
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The first MO is all-bonding and
is particularly stable. The second and third (degenerate) MOs
are still strongly bonding, and all three of these bonding
MOs delocalize the electrons over several nuclei. This configuration,
with all the bonding MOs filled (a โclosed bonding shellโ), is
energetically very favorable.
*The most important point is the conclusion that state "if the pairing
occurred in the bonding M.O. so, it's AROMATIC but the must be
completely filled with e' as bonding M.O. are at lower level; therefore,
lower energy and higher stability that means the high stable compounds
are aromatic compounds.
-Using the same concept, we can study if cyclobutadiene is aromatic or
notโฆ
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*Frost Circle;
A simple method used to estimate the relative ฯ orbital energies of both
planar and cyclic compounds with an uninterrupted ฯ electron system.
Steps to draw Frost circle;
1. Draw a circle.
2. Draw the compound into the circle by touching each boundary point
by the circle circumference and starting from below.
3. Put small line at every point of intersection;
-Upper half: Antibonding M.O.
-Diameter: Nonbonding M.O.
-Lower half: Bonding M.O.
๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐
*No e' in non-bonding M.O. so, it is AROMATIC.
e.g: cycloheptatrienyl anion
*The end*
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*Exercise Sheet*
1. Draw the structure of each compound:
(a) o-nitroanisole
(b) 2,4-dimethoxyphenol
(c) p-aminobenzoic acid
(d) 4-nitroaniline
(e) m-chlorotoluene
(f) p-divinylbenzene
(g) p-bromostyrene
(h) 3,5-dimethoxybenzaldehyde
(i) tropylium chloride
(j) sodium cyclopentadienide
(k) 2-phenylpropan-1-ol
(l) benzyl methyl ether
(m) p-toluenesulfonic acid
(n) o-xylene
(o) 3-benzylpyridine
2. Name the following compounds:
3. Give Reason;
Cyclopentadiene is acidic.
(Final 2009, 2013, 2014 midterm, 2015)
4.How can you predict the aromatic, antiaromatic cycles of the
following;
*The answer must be powered by Hรผckelโs criteria, MOT & the aid of
Frost circle*
I. Cyclopropenyl cation.
II. Cylopentadiene anion.
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III. The forced planner form of (8) annulene.
(Final 2005, 2013, 2014)
5.Write the resonance structure for the cyclopentadienyl anion.
6.Give Reason;
Cyclobytadiene is antiaromatic.
(Midterm 2010, 2013, 2014)
7.Discuss the aromaticity of Cyclopentadiene.
8.Study the aromaticity of Cyclooctatraien, Cyclopropene,
Cyclopentadiene, Cycloheptatriene.
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Helpful Sheet
Classes of Aromatic Compounds
Alkylbenzene & Alkenylbenzene;
Toluene o-xylene m-xylene p-xylene
styrene allyl benzene 2-phenyl-2-butene
vinyl benzene 3-phenylpropene
phenyl ethylene
** The simplest alkenyl benzenes are called styrenes.
Aryl Halides;
flurobenzene chlorobenzene bromobenzene Iodobenzene
Aralkyl Halides;
benzyl Chloride 1-phenyl Chloride 2-phenylethyl chloride
1-chloroethylbenzene
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Nitro-compounds;
Nitrobenzene T.N.B T.N.T
Aniline p-Toluidine m-Toluidine
Aromatic Diazonium salts;
Benzene Diazonium Chloride
Phenols;
Phenol O-cresol m-cresol p-cresol
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catechol Resorcinol Quinol
Aromatic Alcohols;
Benzyl alcohol 2-phenylethanol 1-phenylethanol
Aromatic ethers;
Methoxybenzene Ethoxybenzene o-Anisidine o-Phenitidine
Aromatic Aldehyde & Ketones;
Benzaldehyde o-Anisaldehyde Acetylbenzene Buterylbenzene
(Acetaphenone) (Buterophenone)
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Aromatic Acids;
Benzoic acid Salicylic acid Phthalic acid
If you can't explain it simply, you don't
understand it well enough.
- Albert Einstein
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*Table of Functional Group Priorities For Nomenclature;