Chains, Branches, Rings & Homologous Series

Chains, Branches and Rings

The hydrocarbons may have branched, closed chains or ring or cyclic structures.

Hydrocarbons

Branched Structure

The alkanes containing three or less carbon atoms do not form branches.
CH₄                      Methane
CH₃-CH₃             Ethane
CH₃-CH₂-CH₃    Propane

• The alkane containing four carbon atoms (C₄H₁₀) has two types of arrangement of carbon atoms.
• 

Closed Chains or Cyclic Hydrocarbon

These hydrocarbons contain closed chain or rings of atoms in their molecules. These are of two types:

(A) Alicyclic hydrocarbon: 

• These hydrocarbons contain a ring chain of three or more carbon atoms.
• These cyclic compounds are named by prefixing cyclo before the name of corresponding straight chain hydrocarbon.
  

(B) Aromatic hydrocarbon: 

• These have at least one benzene ring in their molecules.
• It is a special type of ring of six carbon atoms with three double bonds in alternate positions.
• These compounds are called aromatic because they have some identific smell.
  

Will you be my friend? (functional group)

• Carbon forms many compounds with hydrogen. But carbon also forms bonds with other atoms such as halogen, oxygen, nitrogen and sulphur. Therefore, carbon is said to be a very friendly element.
• These compounds are obtained by replacing one or more hydrogen atoms by other atoms such that the valency of carbon remains satisfied. The atom replacing the hydrogen atom is called heteroatom or Functional group.
• Different organic compounds having same functional group have almost same properties. These are called families.

Example:

Properties of CH₃-OH and CH₃-CH₂OH are similar and it is due to the presence of -OH (hydroxyl) group.
This group is known as alcoholic group.
Family of compounds having -OH group is called alcohols.

Some Functional Groups in Carbon compounds 

Homologous Series

"A series of organic compounds having similar structures similar chemical properties and same general formula in which the successive members differ in their molecular formula by -CH₂ group".

The different members of the series are called homologous.

Characteristics of Homologous Series

• All the members of a homologous series can be described by a common general formula. 
  Example: All alkanes can be described by the general formula C_nH_2n+2.
• Each member of a homologous series differ from its higher and lower neighbouring members by a common difference of -CH₂ group.
• The molecular masses of two adjacent homologues differ by 14 mass units because molecular mass of -CH₂ group is 12 + 2 = 14.
• All members of a homologous series display similar chemical properties.
• All members of the series can be prepared using similar methods, known as the general method of preparation.
• They show changes in physical properties as the number of carbon atoms increases.

Table: Some Members of Alkane, Alkene and Alkyne (Homologous Series)

Q. What is a homologous series? Explain with an example.
Solution. 

Conclusion:
(i) Yes, all these compounds are the members of a homologous series for alcohols.
(ii) CH₃OH, C₂H₅OH, C₃H₇OH and C₄H₉OH -increasing carbon atoms. These four compounds form a homologous series.

Homologous Series containing Functional Groups

Aldehydes: 
HCHO, CH₃CHO, CH₃CH₂CHO, CH₃CH₂CH₂CHO
Carboxylic acids: HCOOH, CH₃COOH, CH₃CH₂COOH, CH₃CH₂CH₂COOH
Amines: CH₃NH₂, CH₃CH₂NH₂,CH₃ CH₂CH₂NH₂.
Ketones: CH₃COCH₃, CH₃COCH₂CH₃, CH₃COCH₂CH₂CH ₃
Haloalkanes: CH₃X, CH₃CH₂X, CH₃CH₂CH₂X, CH₃CH₂CH₂-CH₂X
Q. How do physical properties change in a homologous series of hydrocarbons?
The physical properties of the various members of a homologous series change regularly with an increase in the molecular mass.
(i) Melting and Boiling Points : Melting point and boiling point of hydrocarbon in a homologous series increases with an increase in molecular mass.
(ii) Physical State: 
Hydrocarbons containing lesser number of carbon atoms are gases. 
Hydrocarbons containing large number of carbon atoms are solids.
Hydrocarbon containing intermediate number of carbon atoms are liquids.
Example: Hydrocarbon containing 1-4 carbon atoms are gases, these containing 5-16 carbon atoms are liquid and those containing more than 16 carbon atoms are solids.

Nomenclature of Carbon Compounds

Carbon compounds can be called by their common names, but, then remembering millions of compounds by their individual names may be very difficult. Due to this reason, the International Union of Pure and Applied chemistry (IUPAC) has devised a very systematic method of naming these compounds.

Naming a Carbon Compound

The number of carbon atoms in the molecule of a hydrocarbon is indicated by the following stems.

Example: Saturated hydrocarbon.
Alkane → Meth + ane = Methane
Unsaturated hydrocarbon
Alkene → Eth + ene = Ethene
Alkyne → Eth + yne = Ethyne

• In case any functional group is present, it is indicated in the name of compound with either a prefix or a suffix.
• Identify the longest continuous chain of carbon atoms. This gives the name of parent hydrocarbon.
• In the case any substituent appropriate prefix is added before the name of parent hydrocarbon.
• In the case of a functional group, the ending 'e' in the name of the parent hydrocarbon is replaced by the appropriate suffix.

Functional Group

"Functional group may be defined as an atom or a group of atoms which is responsible for most of the characteristic chemical properties of an organic compound".

Prefixes and Suffixes of some Substituents/Functional Group