At the beginning of the 18th century, all naturally occurring substances were divided into two main classes: organic and inorganic based on their source. The compounds obtained from living organisms are called organic compounds. eg. sugar obtained from sugarcane. The compounds obtained from non-living sources like minerals and rocks were called inorganic compounds.
Vital force theory
By the middle of the 19th century, there was a common belief that organic compounds were produced only in living organisms under the influence of a natural force called vital force. This hypothesis is known as the vital force which was given by Swedish chemist, Berzelius. This theory means organic compounds could not be prepared artificially.
Rejection to vital force theory
In 1828, a German chemist, Friedrich Wholer prepared the organic compound urea from the urine of animals by heating ammonium cyanate, an inorganic compound.
This result indicates that organic compounds could be prepared in the lab. Urea is the first organic compound prepared in the lab and it is the first step in the rejection of vital force theory. After this break, many organic compounds are prepared. Kolbe synthesized acetic acid, Hennel synthesized ethyl alcohol, and Berthelot synthesized methane. Nowadays more than 95% of organic compounds are obtained by synthesis.
Modern definition of organic compounds
By detailed investigations, organic compounds indicate that all organic compounds contain covalently bonded carbon which almost always contains hydrogen. Organic compounds also contain elements like oxygen, nitrogen, sulphur, halogens, phosphorous, and some metals.
Hence, organic compounds are hydrocarbons ( compounds having carbon and hydrogen) and their derivatives. The study of organic compounds is called organic chemistry.
Reasons for separate study of organic compounds
| Organic Compounds | Inorganic Compounds |
| 1. They are generally made of C, H, O, N, S, P, and halogens. | 1. They are made of all elements including carbon. |
| 2. Their number is more. | 2. Their number is less. |
| 3. They are mostly covalent. | 3. They are mostly ionic. |
| 4. They are mostly insoluble in water. | 4. They are mostly soluble in water. |
| 5. They are generally combustible. | 5. They are generally non-combustible. |
| 6. They have low melting and boiling points. | 6. They have high melting and boiling points. |
| 7. They are molecular and slow. | 7. They are ionic and fast. |
| 8. They generally show isomerism. | 8. They generally do not show isomerism. |
Organic compounds are combustible because they are reacted with oxygen to form CO2.
Properties of Carbon
Tetra-covalency
Carbon forms four covalent bonds by sharing valence electrons with other atoms.
Catenation
Carbon forms a covalent bond with other carbon giving a long chain and rings.
Classification of organic compounds
1. Open Chain or aliphatic compounds
They are the compounds in which one carbon is linked with another carbon in an open chain structure. They may be straight or branched.
i. Alkane: Carbon to carbon single bond.
ii. Alkene: Carbon to carbon at least a double bond.
iii. Alkyne: Carbon to carbon at least a triple bond.
iv. Derivatives of hydrocarbon: They are mainly derived from an alkane. eg. haloalkane, alcohol, ether, carboxylic acid, etc.
CH_{3}-H \xrightarrow[+X]{-H}CH_{3}X2. Closed chain or cyclic compounds
The compounds that contain one or more rings of carbon (or other atoms).
i. Homocyclic: Ring-forming atoms are only carbon.
a. Alicyclic compounds: Those compounds which contain rings of carbon having single, double or triple bonds.
b. Aromatic compounds: The compounds having carbon-to-carbon alternate single and double bonds.
ii. Heterocyclic: Ring containing at least N, O or S.
Heterocyclic compounds are also considered aromatic compounds.
Alkyl groups
They are obtained by the removal of one hydrogen atom from an alkane. They are represented by (R-). Their general formula is CnH2n+1-.
Functional groups
Generally, an organic compound contains two part-one hydrocarbon part and another functional group part. The functional group is active and the hydrocarbon part is inactive. Organic compounds are mostly covalent. Hence only a certain portion actively participates in the chemical reaction.
The functional group is defined as the atom or group of atoms that determine the characteristic properties of an organic compound.
Homologous series
A homologous series is a series of similarly arranged compounds containing the same functional group in which the adjacent member differs by a methylene (-CH2-) unit. Each member of the homologous series is called homologue and this process is called homology.
eg. 1. Homologous series of alkane
| Formula | IUPAC Name |
| CH4 | Methane |
| CH3CH3 | Ethane |
| CH3CH2CH3 | Propane |
| CH3CH2CH2CH3 | Butane |
2. Homologous series of alcohol
| Formula | IUPAC Name |
| CH3OH | Methanol |
| CH3CH2OH | Ethanol |
| CH3CH2CH2OH | Propanol |
| CH3CH2CH2CH2OH | Butanol |
3. Homologous series of carboxylic acid
| Formula | IUPAC Name |
| HCOOH | Methanoic acid |
| CH3COOH | Ethanoic acid |
| CH3CH2COOH | Propanoic acid |
| CH3CH2CH2COOH | Butanoic acid |
4. Homologous series of aldehyde
| Formula | IUPAC Name |
| HCHO | Methanal |
| CH3CHO | Ethanal |
| CH3CH2CHO | Propanal |
| CH3CH2CH2CHO | Butanal |
Characteristics of homologous series
- Members in a homologous series can be represented by a common formula called a general formula. For example, the general formula of alcohol is CnH2n+1OH.
- Each member in a homologous series differs from the adjacent member by methylene (-CH2-) unit.
- The physical properties of each member of the homologous series change due to a change in molecular weight.
- The chemical properties of each member of the homologous series remain the same due to a similar functional group.
- Each member in a homologous series can be prepared by a common method of preparation.
- The first member in a homologous series may differ in properties from other members.
Formula of organic compounds
i. Empirical formula: The simplest formula that represents the simplest whole-number ratio of the number of atoms of all elements present in one molecule of the compound. eg. The empirical formula of glucose is CH2O whereas its actual formula is C6H12O6.
ii. Molecular formula: It is the true formula that represents the actual number of atoms of all the elements present in one molecule of the compound. Molecular formula = Empirical formula x n where n=1,2,3…
n = Molecular formula weight/Empirical formula weight
iii. Structural formula: It represents how the atoms are bonded in a molecule of the compound.
iv. Condensed or contracted formula: It is the structural formula in condensed form.
CH3CH3 – Ethane
CH3CH2OH – Ethyl alcohol
CH3COOH – Acetic acid
CH3CH2NH2 – Ethyl amine
v. Bond line structural formula or skeletal structure: A representation of molecular structure in which each covalent bond is represented with a line.
Pyrolysis or cracking
It is the decomposition of alkane by the action of heat.
Aromatization or reforming
When straight-chain alkanes having six or more carbon is heated at 400°C in the presence of Ni catalyst gives aromatic compounds.
Quality of gasoline
Octane number
The number which is used for rating the quality of petrol is called the octane number. It is the percentage of the mixture by volume of n-heptane and iso-octane.
n-Heptane has a low octane number and Isooctane has a high octane number.
If petrol has octane number 80, it means it is a mixture of 80 % isooctane and 20 % n-heptane.
Cetane number
The number which is used for rating the quality of diesel is called the cetane number. It is the percentage of the mixture by volume of n-hexadecane and α – methyl naphthalene.
The diesel having cetane number 75 means it has a mixture of 75% cetane and 25% α – methyl naphthalene.
Anti-knocking agent (gasoline additive)
The external agent added to gasoline to increase the quality of fuel is called the gasoline additive. eg. TEL (tetraethyl lead): (CH3CH2)4Pb.
Some Important Questions
- What is vital force theory? Why is it rejected?
- What are organic compounds? Write the examples of any three organic compounds that contain nitrogen as heteroelement.
- What is the catenation property of carbon?
- Write the functional group of a. ester b. amide c. acid anhydride with one example of each.
- What is a homologous series? Write the homologous series of the first four members of carboxylic acid and aldehyde with the IUPAC name.
- What is the octane number and cetane number?
