Homologous Series

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

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

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

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

Homologous series	General formula	Example
alkanes	C_nH_2n+2	Propane C₃H₈
alkenes^**	C_nH_2n	Propene C₃H₆
alkynes	C_nH_2n–2	Propyne C₃H₄
halogenoalkanes	C_nH_2n+1X	Chloropropane C₃H₇Cl
alcohols	C_nH_2n+1OH	Propanol C₃H₇OH
aldehydes	C_nH_2nO (usually written as R–CHO)	Propanal C₃H₆O
ketones	C_nH_2nO (usually written as R–(C=O)–R)	Propanal C₃H₆O
carboxylic acids	C_nH_2n+1COOH (usually written as R–COOH)	Propanoic acid C₂H₅COOH
ethers	C_nH_2n+2O (usually written as R–O–R)	Methoxymethane CH₃OCH₃
amines	C_nH_2n+1NH₂	Propylamine C₃H₇NH₂
amides	C_nH_2n+1NO (usually written as R–CONH–R')	N-methylethanamide CH₃CONHCH₃
esters	C_nH_2nO₂ (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

Physical Trends in a Homologous Series

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

Graph showing the boiling points of the first 10 members of 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