Aromatic Comp. Lec.1

Dr.M_T The 3rd Vision Academy 01156281369

ASU 1𝑠𝑡 year 2016

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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

didn’t support any of the possibilities until Kekulé proposed a ring

structure in 1865. Some of the proposed structures, including Kekulé’s

one.

I. Nomenclature of Aromatic Cpd.s

1.General rules for naming;

1. Benzene derivatives are named by prefixing 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 identified.

(1,2 = o; ortho – 1,3 = m; meta – 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 ∅ ):

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

(-𝑪𝒉𝟐-) 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, doesn’t

it show the reactions typical of alkenes?

It is easier to break 𝝅 bond than breaking 𝝈 bond but it broke 𝝈 as the

𝝅 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

𝝅bond 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 𝝅 bonds as the weak

overlapping or unshared e'.

Contributing Structures

𝑺𝑷𝟐 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 𝝅 bond = 𝝆 orbital + 𝝆 orbital ,,, that remain after hybridization

( 𝑺𝑷𝟐).

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 following…

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 — Hückel’s 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 Hückel number of π-electrons

*No of 𝝅 electrons = 4n + 2 ,,, " N is a positive integer".

e.g. Benzene no. of e' = 6 ,,, SO, 4n + 2 = 6

4n = 4

∴ 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 Hückel number of π-electrons.

*No. of 𝝅 electrons = 4n + 2 ,,, " n is not a positive integer". but obey …

or, 𝝅 electrons = 4n " n is a positive integer "

e.g. cyclopentadienyl cation no. of e' = 4 ,,, SO, 4n = 4

n = 1 "+ve integer"

∴ 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, nonbonding) 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 𝝅

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 𝑯𝟐 Molecule

that diagram deliver the meaning of the constructive and deconstructive

wave which are formed according to the type of interaction,

*head – head interaction …

*side-by-side interaction …

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"…



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Molecular Orbital Energy-Level Diagram for 𝑯𝟐;

Now, let yourself to draw the same diagram for benzene;

Energy diagram of the molecular orbitals of benzene.

Benzene’s 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 “closed bonding shell”), 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

not…



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*Frost Circle;

A simple method used to estimate the relative π orbital energies of both

planar and cyclic compounds with an uninterrupted π 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.

𝝆𝟏 𝝆𝟐 𝝆𝟑 𝝆𝟒 𝝆𝟓 𝝆𝟔

*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 Hückel’s 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;

Science

Aromatic Comp. Lec.1