Why are aromatic compounds called aromatic? | Definition, Properties
Such compounds have been obtained from natural sources such as resins, balsam and bitter almond oil, which contain special type of smell.
These compounds are called aromatic compounds due to their special type of smell. The word aromatic originated from the Greek aroma meaning aroma.
The study of these compounds revealed that the percentage of carbon atoms in them is higher than corresponding aliphatic compounds. It has been found that the simplest of these compounds is the derivative of benzene and most other compounds benzene.
Therefore, initially benzene and its derivatives are called aromatic compounds. Later, by detailed study of the properties and structures of some other compounds, it has been found that their properties and structure are more similar to the properties of benzene and its derivatives and their structure. These compounds are also called aromatic compounds.
Therefore, benzene, derivatives of benzene and other compounds that have properties and structure similar to benzene are called aromatic compounds.
Aromatic Compounds Properties
The special properties of aromatic compounds, because of which they are studied in a different group from aliphatic compounds, are called aromatic properties.
Aromatic properties differ from the properties of aliphatic compounds. The following is a comparative description of the properties of aliphatic and aromatic compounds.
Aromatic compounds contain a special smell called aromatic smell. The aliphatic compounds are odorless or have different smells.
The special properties of aromatic compounds, because of which they are studied in a different group from aliphatic compounds, are called aromatic properties.
Aromatic properties differ from the properties of aliphatic compounds. The following is a comparative description of the
properties of aliphatic and aromatic compounds.
Aromatic compounds contain a special smell called aromatic smell. The aliphatic compounds are odorless or have different smells. The special properties of aromatic compounds, because of which they are studied in a different group from aliphatic compounds, are called aromatic properties.
Aromatic properties differ from the properties of aliphatic compounds. The following is a comparative description of the
properties of aliphatic and aromatic compounds.
Aromatic compounds contain a special smell called aromatic smell. The aliphatic compounds are odorless or have different smells.
The ratio of the percentages of carbon and hydrogen in aromatic compounds is often greater than the ratio of percentages of these elements in aliphatic compounds. For this reason, when most aromatic compounds are ignited, they burn with smoky flame while most aliphatic compounds burn with non-smoky flame.
Molecules of aromatic compounds show that they have a high degree of unsaturation, that is, many carbon-carbon double bonds are present in them. In spite of the presence of carbon-carbon bond, aromatic compound aggregation reactions do not display easily. The aliphatic compound exhibits addition reactions easily.
Aromatic compound substitution reactions such as halogenation, nitration, sulfuration and field craft react easily.
Hydrogenation heat data show that the durability of the aromatic compounds is greater than the durability of the aliphatic compound. Hence, most of the reactions of the aromatic compound remain unaffected by their characteristic structure.
Aromatic hydroxy compound (phenol) is acidic whereas aliphatic hydroxy compound (alcohol) is neutral.
Aromatic amines compounds are weaker as compared to aliphatic amines compounds.
Aromatic hydroxy compound (phenol) is acidic whereas aliphatic hydroxy compound (alcohol) is neutral.
Aromatic amines compounds are weaker as compared to aliphatic amines compounds.
The aromatic compound is cyclic. In aromatic helogen compounds, the helogen atoms present in the cycle cannot be easily displaced by other atoms or groups, whereas the helogen atoms present in aliphatic compounds exhibit many substitution reactions.
The special properties of aromatic compounds are due to their structural characteristics. The structure of benzene is cyclic. Benzene cycle is present in derivatives of benzene. Other aromatic compounds either have benzene cycles present or their cyclic structure is similar to the cyclic structure of benzene.
Sources of aromatic compounds
Bitumen has been a major source of aromatic compounds for nearly four decades from now. It derives as a byproduct from distructive distillation of bituminous coul. Aromatic compounds are also found in flora and patrolium. Petrolium industry has grown rapidly in the last four decades. Along with this, patrolium has now become a source of aromatic compounds.
The following substances are mainly found by heating bituminous coal in the retorts of fire clay in the absence of air at 1000 – 14oo°C.
Coal gas: This gas is used to produce fuel and light.
Ammonical liquor: This fluid is used to make ammonia.
Coal Tar: It is a black thick liquid. From this benzene and benzene are derived.
Coke: This solid substance remains as a residue in ritart which is used as a reducing agent in fuel and metal extraction.
Aromatic Substitution
The aromatic compound exhibits halogenatioin, nitration, sulphonation, and other substitution reactions easily. In these reactions, one or more hydrogen atoms present in the benzene cycle are displaced by other atoms or groups.
There are three types of aromatic substitution reactions depending on the nature of the invagination reagent.
- Electrophilic aromatic Substitution Reactions
- Nucleophilic aromatic Substitution Reactions
- Free Radical aromatic Substitution Reactions
In these reactions, the invasive reagents are electrophile, nucleophile, and free radical, respectively. Most substitution reactions are electrophilic.
For this reason, if any expression is given in relation to aromatic substitution reactions that do not mention the nature of the invagination reagent, it is assumed that the statement is made in relation to the electrophilic substitution reaction.
Following is the mechanism of electrophilic aromatic substitution reactions
This procedure is written for benzene. And E represents electrophilic.
Halogenation
In this reaction one or more hydrogen atoms are displaced by halogen atoms. The halogenation of the aromatic compound is usually carried out by the reaction of halogen in the presence of halogen carrier such as Fe or FeCl3 in the dark.
C6H6 + Cl6 → C6H5Cl + HCl
Halogen carriers act to generate cations of halogen. The cation electrons of halogen act as lubricating electrophile.
FeCl3 + Cl2 ⇌ FeCl4– + Cl+
Nitration
In these reactions one or more hydrogen atoms are displaced by the nitro group(-NO2). There are several methods of nitration of aromatic compounds. The main method of these is the reaction of aromatic compounds by mixing of concentrated HNO3 and concentrated H2SO4 which is also called mixed acid.
C6H6 + HNO3(Conc.) → C6H5NO2 + H2O
The reaction of HNO3 and H2SO4 results in the formation of nitronium ion(NO2+), which acts as an electrophile.
HNO3 + 2H2SO4 ⇌ H3O+ + NO2+ + 2HSO4–
Sulphonation
In this reaction one or more hydrogen atoms are displaced by sulphonic acid group(-SO3H). They are often heated with fuming sulphuric acid to perform sulphonation of aromatic compounds.
Example:
C6H6 + H2SO4 (fuming) → C6H5SO3H + H2O
SO3 or SO3H + act as an electrophile in this reaction.
Friedel – Craft alkylation
In this reaction, one or more hydrogen atoms present in the benzene cycle are displaced by the alkyl group. This reaction is performed by the reaction of aromatic compounds with an alkyl chloride in the presence of a lewis acid such as FeCl3 or AlCl3.
example:
C6H6 + CH3Cl → C6H5CH3 + HCl
In this reaction, CH3+ is obtained by the action of AlCl3 and CH3Cl which act as electrophile.
AlCl3 + CH3Cl ⇌ AlCl4– + CH3+
Fridel craft Acylation
In this reaction, benzene is displaced by one or more hydrogen acyl group(RCO-) such as acetyl group(CH3CO-) present in the cycle. This reaction is performed by the reaction of the aromatic compound with an acyl halide(RCOCl) such as acityl chloride(CH3COCl) in the presence of a luwis acid such as FeCl3 and AlCl3.
C6H6 + CH3COCl → C6H5COH3 + HCl
In this reaction, CH3CO+ is obtained by the reaction of AlCl3 and CH3COCl. Which performs the function of electrophile.
AlCl3 + CH3COCl ⇌ AlCl4– + CH3CO+
Directive Influence of Groups
When a mono substituted benzene is converted to di substituted benzene, the new group entering the benzene ring can take three positions.
Which place the new group (Y) occupies determines the already existing group (X). This effect of an already present group is called directive influence. It is found that there are two types of group (X) –
- Those groups which direct the second entry group to be replaced at ortho and para positions. They are called ortho and para directing groups. -Cl, -Br, -CH3, -OH and –NH2 Groups is ortho and para directing groups. Hence, a mixture of ortho and para nitro toluene is obtained by nitration of toluene.
- Groups that direct the second entering group to be replaced at the meta location. These are called meta directing groups. -NO2, -SO3H, -COOH and -CHO is meta directing groups. Hence meta di nitro benzene is obtained after nitration of nitro benzene.
Several rules have been formulated from time to time to find Directive Influence of Groups, of which the following are the main rules.
- Crum brown – gibson rule
- Vorlander’s rule
- humic and enlingvarth rules
These rules are empirical rules on experiences. They have no theoretical basis.
They also have many exceptions. They are of no use in modern times, only of historical importance. According to modern ideas, Directive Influence of Groups depends on their electron displacement properties. On this basis, Directive Influence of Groups can be known.
Groups that migrate electrons towards benzene ring. There are ortho and para directive groups. These groups increase the electron density of the ortho and para positions of the benzene ring. For this reason the invagination reagent which is positively charged or electrons is lubricated, invades ortho and para positions.
Example: The + M effect of chlorine atom in chlorobenzene increases the electon density of ortho and para conditions.
The groups that displace electrons from benzene ring are meta directive groups. These groups reduce the electron density of the ortho and para positions of the benzene ring. For this reason the invagination reagent which is dense or electron-afflicted invades the meta position compared to the ortho and para positions.
Example: The -M effect of the -COOH group in benzoic acid reduces the electron density of the ortho and para positions.
