Characteristics and Classification of Organic Compounds - Notes - Class 11

ORGANIC CHEMISTRY
The hydrides of carbon (hydrocarbons) and their derivatives are called organic compounds. The branch of chemistry which deals with these compounds is called organic chemistry.
Berzelius (1808) defined organic chemistry as the chemistry of substances found in living matter and gave the vital force theory. Synthesis of urea, the first organic compound synthesised in laboratory, by Wohler. gave death blow to the vital force theory.
Acetic acid is the first organic compound synthesised from its elements.

REASONS FOR LARGE NUMBER OF ORGANIC COMPOUNDS
(a) Catenation: It is the tendency of self combination and is maximum in carbon. A carbon atom can combine with other carbon atoms by single, double or triple bonds. Thus, it forms more compounds than the others.
(b) Tetravalency and small size: Carbon being tetravalent, is capable of bonding with four other C atoms or some other monovalent atoms. Carbon can form compound with oxygen, hydrogen, chlorine, sulphur, nitrogen and phosphorus. These compounds have specific properties depending upon the nature of the element or group attached with the carbon.
Furthermore, these compounds are exceptionally stable because of the small size of carbon.

GENERAL CHARACTERISTICS OF ORGANIC COMPOUNDS

• These are the compounds of carbon with H, O, N, S, P, F, CI, Br and I.
• These are generally found in living organisms.
  Example: carbohydrates, proteins etc.
• These may be gases, liquids or solids.
• Being covalent in nature, these have low boiling point and melting point and soluble in organic solvents.
• These are generally volatile and inflammable.
• They do not conduct electricity because of the absence of free ions.
• They possess distinct colour and odour.

Classification of Organic Compounds:

Fig: Classification of Organic Compounds

Alicyclic or Closed Chain or Ring Compounds:
These compounds contain chains or rings of atoms in their molecules.

Alicyclic Compounds: These compounds contain a ring of three or more carbon atoms in them. They resemble aliphatic compounds in many of their properties.
Example:

Aromatic Compounds: These have a cyclic system containing at last one benzene ring. The parent member of the family is called benzene. Benzene has a homocyclic hexagonal ring of six carbon atoms with three double bonds in the alternate position. Some of the example of various types of aromatic compounds are
These compounds are also called benzenoid compounds and they exhibit special property known as aromaticity Bicyclic and tricyclic compounds also belong to this category
In addition to these, there are some compounds which show aromaticity but do not contain any benzene ring. These are called non-benzenoids.
Example:
Heterocyclic Compounds: In these compounds, the ring contains one or more atoms of either nitrogen, oxygen or sulphur in addition to carbon atoms. The atom other than carbon (such as N, O, S) present in the ring is called hetero atoms.
Example:
Functional or Characteristic Groups: We have studied that hydrocarbons are parent organic compounds and those formed by replacing one or mom hydrogen atoms with some other atoms or groups are known as the derivatives of hydrocarbons, such atoms or groups are called functional groups and the properties of the organic compounds particularly the chemical properties, depends upon the functional groups present in the compound. Thus, a functional group may be defined as:
“An atom or group of atoms which largely determines the properties of the organic compounds particularly the chemical properties.”
Example:
The remaining part of the molecule mainly affects the physical properties such as melting point, boiling point, density, solubility etc.
Organic compound can also be classified on the basis of functional groups, into families or homologous series.
Homologous Series: Homologous series may be defined as “a series of similarity constituted compound in which the members possess the same functional group and have similar chemical characteristics”. The two consecutive members differ in their molecular formula by - CH₂ - group
All the members in a particular homologous series have the same general. For example, the general formula for the members of the alcohol family is C_nH_2n-1OH (n = 1, 2, 3, 4...etc)
A few members of am family are:
 

An aromatic (or aryl) compound contains a set of covalently bound atoms with specific characteristics

• A delocalized conjugated π system, most commonly an arrangement of alternating single and double bonds.
• 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 even, but not a multiple of 4. That is, 4n + 2π- electrons, where n = 0,1, 2, 3, and so on. This is known as Huckel's rule.

According to Huckel's rule, if a molecule has 4n + 2 π- electrons, it is aromatic, but if it has 4n π-electrons and has characteristics 1 -3 above, the molecule is said to be antiaromatic. Whereas benzene is aromatic (6 electrons, from 3 double bonds), cyclobutadiene is antiaromatic, since the number of n delocalized electrons is 4, which of course is a multiple of 4. The cyclobutadienyl (2-) ion, however, is aromatic (6 electrons). An atom in an aromatic system can have other electrons that are not part of the system, and are therefore ignored for the 4n + 2 rule. In furan, the oxygen atom is sp² hybridized. One lone pair is in the π system and the other in the plane of the ring (analogous to the C-H bond in the other positions). There are 6 π-electrons, so furan is aromatic.

Some examples of aromatic compounds:


CLASSIFICATION OF CARBON ATOMS 
1. On the Basis of Number of C Attached 

• Primary carbon atom: When carbon atom is attached with one other carbon atom only, it is called primary or 1° carbon atom.
• Secondary carbon atom: When carbon atom is attached with two other carbon atoms, it is called secondary or 2°carbon atom.
• Tertiary carbon atom: When carbon atom is attached with three other carbon atoms, it is called tertiary or 3° carbon atom.
• Quaternary carbon atom: When carbon atom is attached with four other carbon atoms, it is called quaternary or 4º carbon atom.

Reactivity order of carbon atoms is as follows 3° > 2° > 1°.

On the Basis of Position of Functional Group

• α – carbon Carbon which is directly attached to the functional group.
• β- carbon Carbon which is directly attached to the n-carbon.

Classification of Hydrogen Atoms

• 1°-hydrogen (primary) attached to 10-carbon.
• 2°-hydrogen (secondary) attached to 2°-carbon.
• 3°-hydrogen (tertiary) attached to 3°·carbon.
• α- hydrogen (s) Hydrogens which are attached to n-carbon atom.
• β – hydrogen (s) Hydrogens which are attached to ~-carbon atom (-R-).