What is an aromatic hydrocarbon

Organic Chemistry for Students / Aromatic Hydrocarbons

The aromatics are a special class of OC compounds. Their main feature is their special bond structure (conjugated double bonds[1]) in ring shape. As a rule, they differ significantly in their properties from the chain-like hydrocarbon compounds (also called aliphatic hydrocarbons).

History [edit]

The first aromatic to be discovered was benzene (C.6H6). The name Aromat (Greek: aroma = fragrance) comes from its smell.

Soon one also found substances with similar properties, which showed a different structure. It quickly became apparent that these compounds, despite their double bonds, were not easily induced to undergo addition reactions.

Common properties [edit]

  • polyunsaturated (i.e. several double bonds)
  • cyclic, almost always planar (!) molecules
  • all atoms of the ring are sp2-hybridized (only LK)
  • relatively inert to the addition at the double bond
  • instead tend to be substituted on the double bond
  • the bond system of conjugate double bonds is responsible for the high stability of the molecule. A comparatively large amount of energy is required to break ties.
The aromatic state is defined by the requirements: cyclic & planar structure, conjugated double bond system and the fulfillment of the most important criterion - the Hückel rule!

Hückel rule: The number of delocalized π electrons in aromatics must correspond to the formula 4n + 2. (e.g. 2, 6, 10, 14)

The best-known representative of aromatics is benzene. (According to Hückel's rule, benzene has 6π electrons.)

Aromatic reactions

Addition reactions on aromatics are difficult due to the firm bond structure. Aromatics react mainly according to the mechanism of electrophilic aromatic substitution (e.g. sulfonation, nitration). Nucleophilic aromatic substitutions are rare.

Aromatics can be substituted several times. Special rules then apply for this.

Further information

Benzene (C.6H6)[To edit]

Properties and aromatic structure

The first time scientists noticed benzene was in the process of degassing hard coal. This was done in the 19th century to extract gas for street lamps, for example. A by-product was found to be a liquid that was colorless at room temperature and smelled particularly aromatic. Liebig called it benzene.

Benzene is particularly thin and burns in the air with an orange and sooty flame. However, even at room temperature it is highly volatile, so that its vapors are particularly flammable.

It should be avoided because it penetrates the body through the skin and is inhaled through the lungs and acts as a cytotoxin in the body. It damages the bone marrow, liver and kidneys, and red blood cell production. Even small amounts cause dizziness and vomiting when inhaled. Unconsciousness follows.

It is also carcinogenic.

Benzene is apolar and therefore does not mix with water. Due to its non-polar character, it is a good solvent for non-polar substances and therefore an important raw material in the chemical industry.

Occurrence and use [edit]

Benzene is found in coal and petroleum. It is therefore released when petrol and diesel are burned. Even if the share in fuels has steadily decreased over the past few years, 75% of emissions are still released by traffic.

It is also produced when tobacco is burned. (10-100 µg per cigarette). It can also arise and be released through volcanic eruptions or incomplete fires (smoldering fires).

Food that is sold at petrol stations has repeatedly attracted attention in recent years due to its relatively high benzene load. The petrol station markets are now filled with overpressure so that only air gets outside when the door is opened and no gasoline gases inside.

Many substances that contained benzene are now banned in Germany. (Except for fuels, where it e.g. increases the knock resistance of gasoline (admixture 1 to 5%!) This practice has long been banned in the USA.

In the chemical industry it is used as a solvent and as a starting material for the synthesis of many organic compounds (especially of plastics) (e.g. aniline, styrene, nylon, synthetic rubber, plastics, detergent substances, insecticides, dyes, etc.)

Multi-ring aromatics and aromatics with substituents are also obtained from benzene (phenol, nitrobenzene, aniline, chlorobenzene, hydroquinone and picric acid, etc.).

History of the structure elucidation of benzene

The chemist August Kekulé (1829-1896) researched the structural formula of benzene. He had to "nibble" on this tough nut for a long time. From the measurements made by the chemist Mitscherlich in 1845, he knew that all 6 atoms of benzene are equivalent and that it is as inert as an alkane. Additions such as those of the alkenes were not possible with benzene.

With the usual alkane formulas, however, this riddle could not be solved until one night in a dream the connection appeared to him in the form of a snake that grasped its own tail. So Kekulé concluded the ring structure of benzene.

Mesomerism [edit]

Kekulé assumed that the double bonds of benzene do not have a fixed place, but rather that they constantly change place.

Today we know that the 6 pz-Electrons of benzene lie above and below the plane of the ring. A ring-shaped π-electron system is formed, which is delocalized by the constant movement of the electrons.

Through this form of mesomerism, the benzene molecule is particularly energetically stabilized and consequently inert. That is why an important characteristic of aromatics is the absence of an addition reaction with, for example, hydrobromic acid or bromine water.

The bond lengths between the carbon atoms are proof of delocalization and the resulting special type of bond. A C-C single bond is 154pm and a C = C double bond is 134pm[2] long. In the case of benzene, all carbons have a bond length of 139 pm.

Further information:


Mesomeric stabilization of benzene

The perpendicular p orbitals (electron clouds) overlap and thus form a common area above and below the benzene ring. This is called the π orbital.

This is the actual state of the electrons in benzene. Mesomeric boundary structures cannot provide this exact representation - but at least give an idea of ​​it!

Characteristics of the ties
  • Planar structure of the entire molecule
  • The bond length between all carbon atoms is 139 pm
  • Each carbon atom has a p orbital (also sp2Hybridization). The result is a delocalization of the electrons above and below the ring.

Various aromatics

a) Five-ring aromatics [edit]

b) Six-ring aromatics [edit]

Free work aromatics: 1. What are aromatics? [Edit]

Hückel rule: Aromatics have 4n + 2 π electrons, which are arranged alternately.
  1. Are aromatics with 3 carbon atoms possible?
  2. Draw aromatics with 6, 9, or 10 carbon atoms
  3. Draw the double bonds in the following structural formulas and then determine whether aromatics are present:
  1. Draw all three mesomeric limit forms for naphthalene. C.10H8
  2. Is the compound [10] -annulen an aromatic or not?
  3. Is Cycloheptatriene an Aromatic?
  4. Can you now define what the aromatic state is?

Further information: Hückel rule naphthalene

Free work aromatics: 2. Nomenclature of aromatics [edit]

Hückel rule: Aromatics have 4n + 2 π electrons, which are arranged alternately.
  1. Draw: a) benzaldehyde, b) benzyl alcohol, c) toluene, d) phenol e) benzoic acid
  2. Draw: a) 1,3,5-trimethylbenzene, b) 2,6-dibromo-4-chlorotoluene, c) phenylcyclohexane
  3. Heteroaromatics are characterized by atoms in the ring system that do not consist of carbon. Are They Really Aromatics?
  • Caution: the Hückel rule only applies to simple ring systems, it does not always apply to polycyclic aromatics and heteroaromatics!
    1. For which of the above mentioned aromatics does it not apply?
    2. Do the following polycyclic aromatics correspond to the Hückel rule?
    1. Choose any two connections on this side and paint two mesomeric boundary structures each

    Further information: Polycyclic aromatic hydrocarbons

    Free work aromatics: 3. The electrophilic aromatic substitution

    1. Explain in your words the process and the reaction steps of the catalytic sulfonation of benzene. Note the three mesomeric boundary structures in brackets. In order for the reaction to take place, freshly concentrated sulfuric acid is used, which always includes some sulfur trioxide (SO3) contains. This combines with protons of sulfuric acid to form (HSO3)+.
    2. Look for the process of halogenation of an aromatic (e.g. the bromination of benzene) from two different chemistry books (or from the Internet). Look carefully and look for similarities and differences. Then create your own drawing for your records.
    3. Draw an energy diagram under your version of the aromatic, E.electrophilic substitution and then assign the appropriate substances to the energy diagram.
    4. What would the energy diagram look like if an addition of bromine on the aromatic took place instead?

    Free work aromatics: 4. Phenol and its peculiarities [edit]

    1. Read your book about phenol and its properties and then create a brief profile (occurrence, properties, use). Then answer whether it's more of an alcohol, alkali, base, or acid.
    2. When phenol releases a proton, it is simply negative phenolation. Draw it and find a total of 5 mesomeric boundary structures. (Tip: the charge is not always on the oxygen!)
    3. How do you interpret the following rule: "The more mesomeric boundary structures there are, the more stabilized an ion is and thus its existence is more likely"
    4. Create the protolysis equation for the reaction of phenol with water and the reaction equations for the reaction with sodium and caustic soda. Then mark the sodium phenolation.
    5. What is phenol used for?
    6. Phenol as a substituent on carbon chains or other aromatic rings is called phenyl. What is the molecular formula of phenyl actually?
    7. To the delight of the chemist's heart. Conjugated π-electron systems are colored from a certain length. Phenolphthalein has such an extensive conjugated π-electron system. a) Why is it discolored when acid is added? For nomenclature masochists: name it correctly!

    Further information:

    Free work aromatics: 5. Aniline - is not a lye, but a base [edit]

    1. Read your book about aniline and its properties and then create a brief profile (occurrence, properties, use). Then answer whether it is more of an alkali, acid, or base.
    2. Write down the equation for the acid-base reaction of aniline with nitric acid.
    3. Repeat the Brönstedt acid and base definitions. Then mark the corresponding acid-base pair of the reaction from the previous exercise.
    4. Around 1 billion kg of aniline are required worldwide. What is it actually used for?
    5. What would be the correct IUPAC name for aniline?
    6. How does nitrobenzene compare?

    Further information: aniline

    Free work aromatics: 6. Toluene and other benzene derivatives [edit]

    1. Read your book about toluene, benzoic acid, xylene and styrene and then create brief profiles (occurrence, properties, use).
    2. Dimethylbenzene (= xylene) exists in a total of three isomers. Draw them. They are called ortho-, meta or para-xylene.
    3. Write down the reaction equation for the core or side chain bromination of toluene.[4]
    4. Draw the three isomers of trimethylbenzenes.
    5. Formulate the reaction of benzoic acid with potassium hydroxide.
    6. Draw 2,4,6-trinitromethylbenzene (= TNT). (Nitro- as a substituent denotes nitrite (without charge), i.e. NO2)
    7. What should the intermediate stages of the oxidative production of benzoic acid from toluene look like? Draw and name them:

    Further information:

    1. conjugated means that there are alternating single and double bonds
    2. 1pm = 1/1000 000 000 000 m = 10-12m
    3. Thio = sulfur
    4. The KKK or SSS rule applies here