Aromatic compounds By Dr Md Ashraful Alam Assistant

Aromatic compounds By: Dr. Md Ashraful Alam Assistant Professor

Hydrocarbons aliphatic alkanes alkenes aromatic alkynes

Aliphatic compounds: open-chain compounds and ring compounds that are chemically similar to open-chain compounds. Alkanes, alkenes, alkynes, dienes, alicyclics, etc. Aromatic compounds: unsaturated ring compounds that are far more stable than they should be and resist the addition reactions typical of unsaturated aliphatic compounds. Benzene and related compounds.

AROMATIC COMPOUNDS Early History • In the early 19 th century, whale oil was an important commercial product. Pyrolysis of the oil produced a gas used to light theaters and public buildings in London. A volatile aromatic (pleasant smelling) liquid separated when the gas was compressed for distribution in tanks. The famous English scientist Michael Faraday, Director of the Royal Institution, investigated this liquid and found that it contained carbon and hydrogen in equal atomic proportions. • This same liquid was later isolated from the tar distilled from coal and was named benzene. Other aromatic compounds were similarly isolated from plant and animal sources during the first half of the 19 th century. 4

Discovery of Benzene • Isolated in 1825 by Michael Faraday who determined C: H ratio to be 1: 1. • Synthesized in 1834 by Eilhard Mitscherlich who determined molecular formula to be C 6 H 6. • Other related compounds with low C: H ratios had a pleasant smell, so they were classified as aromatic. Chapter 16 Michael Faraday 5

Production • Benzene may result whenever carbon-rich materials undergo incomplete combustion. It is produced naturally in volcanoes and forest fires, and is also a component of cigarette smoke. • Up until World War II, most benzene was produced as a byproduct of coke production in the steel industry. However, in the 1950 s, increased demand for benzene, especially from the growing plastics industry, necessitated the production of benzene from petroleum. Today, most benzene comes from the petrochemical industry, with only a small fraction being produced from coal. • Three chemical processes contribute equally to industrial benzene production: catalytic reforming, toluene hydrodealkylation, and steam cracking.

Uses • In the 19 th and early 20 th centuries, benzene was used as an aftershave because of its pleasant smell. Prior to the 1920 s, benzene was frequently used as an industrial solvent, especially for degreasing metal. As its toxicity became obvious, other solvents replaced benzene in applications that directly exposed the user to benzene. • As a gasoline additive, benzene increases the octane rating and reduces knocking. As a result, gasoline often contained several percent benzene before the 1950 s, when tetraethyl lead replaced it as the most widely used antiknock additive. However, with the global phaseout of leaded gasoline, benzene has made a comeback as a gasoline additive in some nations. By far the largest use of benzene is as an intermediate to make other chemicals. The most widely produced derivatives of benzene are styrene, which is used to make polymers and plastics, phenol for resins and adhesives (via cumene), and cyclohexane, which is used in Nylon manufacture. Smaller amounts of benzene are used to make some types of rubbers, lubricants, dyes, detergents, drugs, explosives and pesticides.

Health Effects • Breathing very high levels of benzene can result in death, while high levels can cause drowsiness, dizziness, rapid heart rate, headaches, tremors, confusion, and unconsciousness. Eating or drinking foods containing high levels of benzene can cause vomiting, irritation of the stomach, dizziness, sleepiness, convulsions, rapid heart rate, and death. • The major effect of benzene from chronic (long term) exposure is to the blood. Benzene damages the bone marrow and can cause a decrease in red blood cells leading to anemia. It can also cause excessive bleeding and depress the immune system, increasing the chance of infection. • The US Occupational Safety and Health Administration (OSHA) has set a permissible exposure limit of 1 part of benzene per million parts of air (1 ppm) in the workplace during an 8 -hour workday, 40 -hour workweek. Exposure – vystaveni

A Structural Puzzle • In 1834 the German chemist Eilhardt Mitscherlich produced benzene from benzoic acid: • His work showed the molecular formula of benzene is C 6 H 6. Aliphatic and Aromatic Hydrocarbons • By the mid-19 th century, the Theory of Valence (Kekule, Couper, Butlerov) showed that most known organic compounds had about twice as many hydrogen atoms as carbon atoms. A pattern of atomic linkages was rapidly emerging, except for the “aromatic” compounds. Two broad classes of organic compounds were defined: aliphatic (“fatlike”) and aromatic. The two types differed in their hydrogen/carbon ratios and their chemical reactivities. The aromatic compounds retained a sixcarbon unit during chemical changes, and benzene (C 6 H 6) was the parent compound to the other aromatics. 9

The Kekule Structure for Benzene • In 1865 August Kekule proposed a structure for benzene consistent with the new Theory of Valence. The tetravalency of carbon was satisfied by a hexagon with alternating single and double bonds. The low hydrogen to carbon ratio in benzene was now understood. 10

Aromatic compounds • Aromatic compounds are compounds which contain a benzene ring in their molecules Sources of Aromatic Hydrocarbons • From high temperature distillation of coal tar • Heating petroleum at high temperature and pressure over a catalyst

Aromatic Compounds At the time of its discovery, many compounds containing benzene had fragrant odors, so the family of benzene compounds became known as aromatic compounds.

The Criteria for Aromaticity—Hückel’s Rule Four structural criteria must be satisfied for a compound to be aromatic. [1] A molecule must be cyclic. To be aromatic, each p orbital must overlap with p orbitals on adjacent atoms. 13

[2] A molecule must be planar. All adjacent p orbitals must be aligned so that the electron density can be delocalized. Since cyclooctatetraene is non-planar, it is not aromatic, and it undergoes addition reactions just like those of other alkenes. 14

[3] A molecule must be completely conjugated. Aromatic compounds must have a p orbital on every atom. 15

[4] A molecule must satisfy Hückel’s rule, and contain a particular number of electrons. Hückel's rule: Benzene is aromatic and especially stable because it contains 6 electrons. Cyclobutadiene is antiaromatic and especially unstable because it contains 4 electrons. 16

Note that Hückel’s rule refers to the number of electrons, not the number of atoms in a particular ring. 17

Considering aromaticity, a compound can be classified in one of three ways: 1. Aromatic—A cyclic, planar, completely conjugated compound with 4 n + 2 electrons. 2. Antiaromatic—A cyclic, planar, completely conjugated compound with 4 n electrons. 3. Not aromatic (nonaromatic)—A compound that lacks one (or more) of the following requirements for aromaticity: being cyclic, planar, and completely conjugated. 18

Can we predict which compounds will be aliphatic and which ones will be aromatic like benzene? Yes. In order to be aromatic, the compound must be: 1) cyclic with p-atomic orbitals on all members of the ring. and 2) have 4 i + 2 electrons in the p-orbitals of the ring (where i = 0, 1, 2, 3, …). [ eg. = 2 or 6 or 10 or 14 or 18 or 22 or 26… ]

Annulenes: monocyclic compounds with the formula: -[-CH=CH-]n- Hydrocarbons containing a single ring with alternating double and single bonds are called annulenes. • To name an annulene, indicate the number of atoms in the ring in brackets and add the word annulene.

• [10]-Annulene has 10 electrons, which satisfies Hückel's rule, but a planar molecule would place the two H atoms inside the ring too close to each other. Thus, the ring puckers to relieve this strain. • Since [10]-annulene is not planar, the 10 electrons can’t delocalize over the entire ring and it is not aromatic.

• Two or more six-membered rings with alternating double and single bonds can be fused together to form polycyclic aromatic hydrocarbons (PAHs). • There are two different ways to join three rings together, forming anthracene and phenanthrene. • As the number of fused rings increases, the number of resonance structures increases. Naphthalene is a hybrid of three resonance structures whereas benzene is a hybrid of two.

Characteristics of aromatic compounds 1. 2. 3. 4. A delocalized conjugated π system, most commonly an arrangement of alternating single and double bonds : Conjugated Coplanar structure, with all the contributing atoms in the same plane Contributing atoms arranged in one or more rings A number of π delocalized electrons that is, 4 n + 2 number of π electrons, where n=0, 1, 2, 3, and so on. This is known as Hückel's Rule.

Aromatic compounds Non Aromatic compounds

Which of these is aromatic? A) Is aromatic. Count the number of pi bonds in the outer ring. A has 5 which means 10 pi electrons, 4(2)+2=10. B) While B has 6 pi bonds and 12 pi electrons, 4(3)=12. Doesn’t meet the Huckel rule requirements for aromaticity. 26

Is this compound aromatic or antiaromatic? Antiaromatic – cyclic, planar, conjugated , but does not meet Huckel’s rule. 4 doulbe bonds and 2 triple bonds so 4(2) + 2(4)=16 pi electons. 4 n+2 or 4 n? 4(4)=16 27

Indicate which of the following are aromatic and antiaromatic? C is aromatic 4(3)+2=14 A is antiaromatic 4(2)=8 28

Which of these is antiaromatic? B 8 pi electrons 4(2)=8 C and D as well, 8 and 4 respectively 29

Some Heterocyclic Aromatic Compounds 30

Aromatic Compounds in Biochemistry • Two aromatic amino acids that are “essential” (i. e. , must be a part of the human diet) are these: • They are essential because humans, in contrast to many other organisms, cannot synthesize benzene rings. Two biological very important heterocyclic aromatic systems are these: • • In both DNA and RNA, there are two nucleotide monomer types that contain purine moieties and two that contain pyrimidine moieties. 31

Nomenclature

Nomenclature of Aromatic Compounds 1. Monosubstituted Benzenes a. IUPAC name b. Common name

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The Phenyl Group • When a benzene ring is a substituent, the term phenyl is used (for C 6 H 5) – You may also see “Ph” or “f” in place of “C 6 H 5” • “Benzyl” refers to “C 6 H 5 CH 2” 35

Disubstituted Benzenes • Relative positions on a benzene ring – ortho- (o) on adjacent carbons (1, 2) – meta- (m) separated by one carbon (1, 3) – para- (p) separated by two carbons (1, 4) • Describes reaction patterns (“occurs at the para position”) 36

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Naming Benzenes With More Than Two Substituents • Choose numbers to get lowest possible values • List substituents alphabetically with hyphenated numbers • Common names, such as “toluene” can serve as root name 38