This document provides an overview of aromatic compounds, ions, and radicals. It begins with an introduction to aromatic compounds, using benzene as an example. It discusses the preparation and reactions of benzene. Next, it covers ions, including the history of ions, examples of cations and anions, and their natural occurrence. Finally, it defines radicals, how they are formed, and factors that contribute to their stability, such as electronegativity and delocalization.
4. INTRODUCTION:
The term aromatic was derived from the Greek word ‘aroma’ meaning “fragrant” and was used in Organic Chemistry for
a special class of compounds.
Further, when aromatic compounds of higher molecular mass were subjected to various methods of degradation, they
often produced benzene or derivatives of benzene
Hence, benzene was recognized as the simplest and the parent member of this class of compounds. So aromatic
hydrocarbons include benzene and all those compounds that are structurally related to benzene.
6. BENZENE:
Benzene was discovered by Michael Faraday in 1825 in the gas produced by the destructive distillation of vegetable oil
and twenty years later it was also found in coal-tar by Hoffmann.
The structure of benzene continued to be a serious problem for chemists for about 40 years.
A German chemist, Kekule at last solved the problem in 1865.
Kekule proposed a cyclic regular hexagonal structure for benzene, which contains three double bonds alternating with
three single bonds.
7. PREPARATION OF BENZENE:
Some of the methods generally used for the preparation of benzene are as follows.
1. Dehydrogenation of Cyclohexane When cyclohexane or its derivative is dehydrogenated we get benzene or a
substituted benzene. The reaction is carried out by the use of a catalyst at elevated temperature.
8. In a laboratory, Benzene can also be obtained by the decarboxylation of aromatic acids. Sodium benzoate on heating with
soda-lime loses a molecule of carbon dioxide and forms benzene
9. REACTION OF BENZENE:
The highly stable, delocalized electrons of benzene ring are not readily available for the nucleophillic attack like the
electrons of alkenes. Therefore, the electrons of benzene ring do not assist in the attack of weak electrophiles. It means
that more powerful electrophiles are required to penetrate and break the continuous sheath of electron cloud in benzene,
e.g., substitution of halogen in benzene requires iron or corresponding ferric halide as a catalyst. Infact iron too is first
converted into FeX3 which further reacts with halogen molecule to produce a powerful electrophile.
Halogenation
The introduction of halogen group in benzene ring is called “Halogenation” Benzene reacts with halogen in the presence of
a catalyst like FeBr3, AlCl3, etc. Chlorination and bromination are normal reactions but fluorination is too vigorous to
control. Iodination gives poor yield.
10.
11. Sulphonation;
The introduction of sulphonic acid group in benzene ring is called Sulphonation. When benzene is heated with fuming
H2SO4 or conc. H2SO4 it yields benzene sulphonic acid.
14. INTRODUCTION:
An ion (/ˈaɪɒn, -ən/)[1] is an atom or molecule with a net electrical charge
The net charge of an ion is not zero because its total number of electrons is unequal to its total number of protons.
A cation is a positively charged ion with fewer electrons than protons[2] while an anion is a negatively charged ion with
more electrons than protons.[3] Opposite electric charges are pulled towards one another by electrostatic force, so cations
and anions attract each other and readily form ionic compounds..
15. HISTORY OF DISCOVERY:
The word ion was coined from Greek ion, neuter present participle of ienai (Greek ἰέναι) "to go" from PIE root *ei- "to
go.", cf. a cation is something that moves down (Greek kato κάτω kat-ion) and an anion is something that moves up
(Greek ano ἄνω, an-ion). So called because ions move toward the electrode of opposite charge.
This term was introduced (after a suggestion by the English polymath William Whewell)[5] by English physicist and
chemist Michael Faraday in 1834 for the then-unknown species that goes from one electrode to the other through an
aqueous medium.
16. CATIONS AND ANIONS:
An anion (−) (/ˈænˌaɪ.ən/ ANN-eye-ən,
from the Greek word ἄνω (ánō),
meaning "up"[12]) is an ion with more
electrons than protons, giving it a net
negative charge (since electrons are
negatively charged and protons are
positively charged).
A cation (+) (/ˈkætˌaɪ.ən/ KAT-eye-ən,
from the Greek word κάτω (káto),
meaning "down"[14]) is an ion with
fewer electrons than protons, giving it a
positive charge
17. NATURAL OCCURRENCE:
Ions are ubiquitous in nature and are responsible for diverse phenomena from the luminescence of the Sun to the
existence of the Earth's ionosphere.
Atoms in their ionic state may have a different color from neutral atoms, and thus light absorption by metal ions gives the
color of gemstones.
20. INTRODUCTION:
In chemistry, a free radical is an atom, molecule, or ion that has at least one unpaired valence electron.[1][2] With some
exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spontaneously dimerize.
Most organic radicals have short lifetimes.
A notable example of a radical is the hydroxyl radical (HO·), a molecule that has one unpaired electron on the oxygen
atom. Two other examples are triplet oxygen and triplet carbene (꞉CH
2) which have two unpaired electrons.
22. Radical formation from spin-paired molecules
Homolysis
Homolysis makes two new radicals from a spin-paired molecule by breaking a covalent bond, leaving each of the
fragments with one of the electrons in the bond
23. Radical formation from other radicals.
Abstraction:
Hydrogen abstraction describes when a hydrogen atom is removed from a hydrogen donor molecule (e.g. tin or silicon
hydride) with its one electron.] Abstraction produces a new radical and a new spin-paired molecule. This is different from
homolysis,
Addition:
Radical addition describes when a radical is added to a spin-paired molecule to form a new radical.[7] The figure on the
right shows the addition of a bromine radical to an alkene. Radical addition follows the Anti -Markovnikov rule, where
the substituent is added to the less substituted carbon atom
24. STABILITY:
Stability of organic radicals
Although organic radicals are generally stable intrinsically, practically speaking their existence
is only transient because they tend to dimerize. Some are quite long-lived. Generally organic
radicals are stabilized by any or all of these factors: presence of electronegativity,
delocalization, and steric hindrance.[9] The compound 2,2,6,6-
tetramethylpiperidinyloxyl illustrates the combination of all three factors. It is a commercially
available solid that, aside from being magnetic, behaves like a normal organic compound.
25. ELECTRONEGATIVITY:
Organic radicals are inherently electron deficient thus the greater the electronegativity of the atom on which the unpaired
electron resides the less stable the radical.]
Between carbon, nitrogen, and oxygen, for example, carbon is the most stable and oxygen the least stable.
