Old Definition of Organic Chemistry
The word organic derived from Latin
word was first introduced by Swedish chemist, Berzelius in 1806 which
means living organism. The term
organic was misleading because, previously organic chemistry was defined as the
chemistry of those compounds which were produced or obtained from living things
(plants or animals) through the agency of vital force.
Modern
Definition of Organic Chemistry
Now a days, organic compounds are considered
to be essential for life. They are compounds of carbon regardless of their
origin. The study of covalent compounds
of carbon containing H, and often O, N, S, P and halogens (Cl, Br and I) is
termed as Organic Chemistry. OR
The study of hydrocarbons and their derivatives is called Organic Chemistry.
Exceptions
However there are certain compounds of carbon
which are not included in organic compounds. These are carbides (CaC2),
cyanides (NaCN), metal carbonates (Na2CO3), metal
bicarbonates (NaHCO3), cyanates (CNO¯), sulphocyanides (CNS¯) and
oxides of carbon (CO and CO2).
Vital
Force Theory
According to this theory, it was assumed that
all living organism contained an unknown, mysterious and super-natural force
known as vital force which was the basic requirement for their production. This
force was considered to be originated by living cells and is beyond human
control. So nobody can synthesize
organic compounds in the laboratory.
In 1828, a German
chemist Friedrich Wohler accidentally prepared Urea (a typical organic
compound) in the laboratory by controlled heating of ammonium cyanate. This
synthesis proved that there is no necessity of vital force for the preparation
of organic compounds. Thus vital force theory was completely over thrown or
discarded.
NH4CNO → (NH2)2CO
Urea/Carbamide
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Catenation
Carbon shows catenation. The property of
carbon atom to bond or link itself with its own atoms to form long chains or
rings is called Catenation. [Any number of carbon atoms can unite with each
other through single, double or triple covalent bonds to form stable chains and
rings of any size and length]. The tendency of carbon for catenation is due to:
(i) Unique
electronic arrangements.
(ii) Tendency
for forming strong covalent bonds capable of holding greater no. of carbon
atoms.
Alkyl
radicals
The residual hydrocarbon group left after the
removal of a hydrogen atom from a saturated hydrocarbon is called alkyl group
or radical. OR The radical
obtained from alkanes by the removal of one H atom are called alkyl groups or
radicals. All alkyl radicals are expressed by a general formula CnH2n+1.
The name of an alkyl radical is derived from the parent alkane by substituting ane of alkane by yl. They are generally
represented by ‘R–‘. Propyl and
higher alkyl groups exist in more than one structure.
R – H → R –
e.g. CH4 → CH3
–
Homologous
Series
The members of the same class of organic
compounds arranged in order of ascending molecular masses with a common
difference of methylene groups (–CH2–) are said to form a Homologous
Series. OR A homologous series is a set or series of similar organic
compounds differing by an integral number of methylene groups (–CH2–)
in their molecular formulae (or by molecular mass of 14) which have same
elements, same functional groups, same general formula and identical chemical
properties. Each member of the series is known as homologue of the other and
the phenomenon itself is called Homology.
Functional
Group
An atom
or group of atoms whose presence confers on an organic molecule characteristics
properties unique to that group is called Functional Group. OR
An atom or group of atoms which is present within the organic molecule and is
responsible for its chemical behaviour and characteristic properties is called
Functional Group. The
functional group determines the basic chemistry of an organic compound. It gives specific and characteristic
properties to an organic molecule, while the remainder hydrocarbon part (alkyl
group) of the molecule has an effect on its physical properties.
1. Open Chain / Aliphatic / Acyclic Compounds
The organic compounds whose molecules are
composed of open chains of carbon atoms either branched or unbranched are
called Acyclic Compounds. In acyclic compounds, there is no link
between first and the last carbon atoms. They are commonly called Aliphatic
Compounds (Greek word; means oil-fats characteristics) as some of the
important members are found in fats. They burn without soot (smoke) due to
complete oxidation. The aliphatic compounds may be saturated or
unsaturated.
2. Cyclic or Ring Compounds
Compounds whose molecules are composed of
closed chains or rings of carbon atoms (with or without atom of other elements)
are called Cyclic Compounds. Cyclic compounds are further divided into two
types:
(a) Heterocyclic or non-carbocyclic compounds
(b) Homocyclic or carbocyclic compounds.
Heterocyclic Compounds
The cyclic compounds having one or more hetero
atoms (like nitrogen, sulphur or oxygen) along with carbon in the ring are
called Heterocyclic or Non-Carbocyclic Compounds. More or less 3600 heterocylic compounds are known.
The various different prefixes are used to indicate the kind and number of
hetero atoms.
Homocyclic
Compounds
These are the cyclic compounds in which the
main skeleton or ring is made up of carbon atoms only. Homocyclic compounds are
further divided into two groups:
(i) Aromatic or Benzenoids.
(ii) Alicyclic or Non-Benzenoids.
Alicyclic Compounds
They are carbocyclic or
homocyclic compounds which do not contain any benzene ring. They resemble
aliphatic compounds in many ways. They are homocyclic organic compounds which
contain a ring of three or more carbon atoms (other than benzene ring)
resembling aliphatic compounds. They may be saturated or unsaturated. They have
general formula CnH2n or CnH2n–2
Saturated alicyclic compounds
have general formula of CnH2n (n=3, 4, 5, 6) and have two
hydrogen atoms less than their corresponding open chain saturated hydrocarbon
alkanes.
Unsaturated alicyclic compounds
have general formula of CnH2n–2 (n=3, 4, 5, 6) and have four
hydrogen atoms less than their corresponding open chain saturated hydrocarbon
alkanes.
Aromatic Compounds or Arenes
Aromatic compounds are benzenoid
homocyclic (or carbocyclic) compounds containing one or more benzene rings
(which is a hexagonal ring of six carbon atoms with three alternating double
and single bonds) in their molecules. Aromatic compounds and their derivatives
have characteristic smell. The pungency (fragrant odour or aroma) of benzene
gives rise to the name aromatic (Greek, arome = smell) for all the members of
the class (even though many are odourless).
Aromatic
compounds also include the polycyclic (many-ringed) analogues of benzene having
two or more benzene rings fused together in ortho positions. Two of the most
important are naphthalene (bicyclic, C10H8; CnHn–2)
and anthracene (tricyclic, C14H10; CnHn–4).
The formula of polycyclic homologues of benzene is derived by inserting –C4H2–
group into the formula of benzene. Anthracene and phenanthrene are positional
isomers]. Now the term ‘aromatic’ is associated with ‘chemical stability’
rather than any aroma. The modern name of all benzene derivatives is Arenes (ar
from aromatic and –ene, the letter denoting double bonds). All the other
aromatic compounds are considered to be the derivative of benzene.
Aromatic compounds burn with soot or smoke
due to incomplete oxidation owing to high percentage of carbon.
Aromatic compounds are always unsaturated.
But their unsaturated character is masked by the presence of highly stable
delocalized pi bonds, making them highly inert towards addition reactions.
Isomerism
The word isomer is a combination of
two words, iso means same and mers means unit and this term was
invented by Berzelius.
Isomerism
is the existence of different compounds exhibiting different physical or
chemical properties or both having same molecular formula. Isomers refer to
those compounds which have the same molecular formula but differ in physical or
chemical properties or both i.e. Isomers have entirely different physical
properties and in many cases also have distantly different chemical properties
(except chain isomers, metamers).
Isomerism is due to the different
arrangement of atoms or groups in a molecule (structural isomerism) or due to
different spatial configuration of the atoms or groups (stereoisomerism).
Structural
or Constitutional Isomerism
When isomerism is caused by the difference in
the arrangement of atoms within molecule without any reference to space is
called Structural or Constitutional Isomerism. Structural or Constitutional
Isomers are compounds that have same molecular formula but different structural
formulae due to different arrangement of atoms or groups. Structural Isomers have entirely different
physical and in most cases also have distantly different chemical properties. Structural
or Constitutional Isomerism is of following five types:
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1.
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Chain or skeletal
or nuclear isomerism
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2.
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Position
Isomerism
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3.
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Functional
Group Isomerism
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4.
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Metamerism
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5.
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Tautomerism or keto-enol isomerism
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Stereoisomerism
The prefix stereo- is
derived from the Greek word stereos meaning solid. When isomerism is caused by
the different spatial configuration (i.e. three-dimensional arrangement) of
atoms or groups in space is called Stereoisomerism. Stereoisomers have same
molecular formula and also the same structural formula but differ in
arrangement of the bonds (atoms) in space. Stereochemistry is the term applied
to the three-dimensional aspects of molecular structure and reactivity.
Stereoisomerism is of three types:
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1.
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Geometrical/cis-trans
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2.
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Optical
Isomerism
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3.
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Conformational
Isomerism
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Chain Isomerism
1. Isomerism resulting
from varying configuration of main carbon skeleton or chain is called skeletal
or nuclear isomerism. Different compounds which have same molecular formula but
they differ in the configuration of their main carbon skeleton or chains having
different carbon chains are called chain or skeletal isomers.
2. Skeletal isomers are
chemically similar because they posses the same functional group belonging to
the same homologous series but they differ in physical properties as the van
der Waal’s forces between molecules of the straight chain isomer are much
stronger than those between molecules of the other two branched isomers.
3. Skeletal isomerism is
found in all aliphatic homologous series except monosubstituted benzenes.
4. Skeletal isomerism
starts with C4.
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