Homologous Series (SL IB Chemistry)

• Organic chemistry is the chemistry of carbon compounds
• Carbon forms a vast number of compounds because it can form strong covalent bonds with itself
• This enables it to form long chains of carbon atoms, and hence an almost infinite variety of carbon compounds are known
• The tendency of identical atoms to form covalent bonds with each other and hence form chains is known as catenation
  
  

Examples of catenation using carbon

Catenation in carbon allows an almost infinite variety of chains, branches and rings

• Carbon always forms four covalent bonds which can be single, double or triple bonds
• A functional group is a specific atom or group of atoms which confer certain physical and chemical properties onto the molecule
• Organic molecules are classified by the dominant functional group on the molecule

What is a homologous series?

• Organic compounds with the same functional group, but a different number of carbon atoms, are said to belong to the same homologous series
• One definition of a homologous series is:

A  family of similar compounds, having the same functional group, and so similar chemical properties

• Every time a carbon atom is added to the chain, two hydrogen atoms are also added
  
  

Homologous Series of Alkanes Table

Name of alkane	Number of carbons	Chemical formula	Melting point / oC	State at room temperature	Boiling point / oC
Methane	1	CH4	–182	gas	–162
Ethane	2	C2H6	–183	gas	–89
Propane	3	C3H8	–188	gas	–42
Butane	4	C4H10	–138	gas	–1
Pentane	5	C5H12	–130	liquid	36

The characteristics of a homologous series

• The features of a homologous series are:
  • Each member has the same functional group
  • Each member has the same general formula
  • Each member has similar chemical properties
  • Each member differs by  -CH₂-
  • Members have gradually changing physical properties, for example, boiling point, melting point and density
• As a homologous series is ascended, the size of the molecule increases
  • This has an effect on the physical properties, such as boiling point and density

General formulae of different homologous series

• Each homologous series can be described by a general formula
  
  

General formulae of homologous series table

Homologous series	General formula	Example
alkanes	CnH2n+2	Propane C3H8
alkenes**	CnH2n	Propene C3H6
alkynes	CnH2n–2	Propyne C3H4
halogenoalkanes	CnH2n+1X	Chloropropane C3H7Cl
alcohols	CnH2n+1OH	Propanol C3H7OH
aldehydes	CnH2nO (usually written as R–CHO)	Propanal C3H6O
ketones	CnH2nO (usually written as R–(C=O)–R)	Propanal C3H6O
carboxylic acids	CnH2n+1COOH (usually written as R–COOH)	Propanoic acid C2H5COOH
ethers	CnH2n+2O (usually written as R–O–R)	Methoxymethane CH3OCH3
amines	CnH2n+1NH2	Propylamine C3H7NH2
amides	CnH2n+1NO (usually written as R–CONH–R')	N-methylethanamide CH3CONHCH3
esters	CnH2nO2 (usually written as R–COO–R')	Methyl methanoate HCOOCH3

^** Ethene is the smallest possible alkene as a minimum of two carbons are required to form the carbon-carbon double bond

• Since successive members of a homologous series differ by a single –CH₂– group, they show a trend in physical properties
  
  

Boiling point graph for four different homologous series

As the number of carbons in the straight-chain molecule increases, the boiling point increases

• The broad trend is that boiling point increases with increased molecular size
• Each additional -CH₂- (called the homologous increment) adds 8 more electrons to the molecule
• This increases the strength of the London dispersion forces, which leads to a higher boiling point
• Similar trends are seen with other physical properties such as melting point, density and viscosity
• These trends are followed in other homologous series