`color{green}(★)` Besides forming bonds with hydrogen, carbon has a tendency to form bonds with other elements like halogens, oxygen, nitrogen and sulphur. In a hydrocarbon chain hydrogen can be raplaced by these elements but the condition is that valency of carbon must be satisfied.

`color{green}(★)` The element replacing hydrogen is referred to as a heteroatom.


`color{green}(★)` The heteroatoms and the group containing these confer specific properties to the compound, regardless of the length and nature of the carbon chain and hence are called functional groups.

`color{green}(★)` The presence of a functional group dictates the properties of the carbon compound, regardless of the length of the carbon chain. For example, the chemical properties of `color{red}(CH_3OH, C_2H_5OH, C_3H_7OH)` and `color{red}(C_4H_9OH)` are all very similar.


`color{green}(★)` A series of compounds in which the same functional group substitutes for hydrogen in a carbon chain is called a homologous series.
For eg:

`color{green}("𝟏 𝐇𝐨𝐦𝐨𝐥𝐨𝐠𝐨𝐮𝐬 𝐬𝐞𝐫𝐢𝐞𝐬 𝐟𝐨𝐫 𝐚𝐥𝐤𝐚𝐧𝐞𝐬:")`

`color{red}(CH_4)` and `color{red}(C_2H_6)` — these differ by a `color{red}(–CH_2)` – unit

`color{red}(C_2H_6)` and `color{red}(C_3H_8)` — these differ by a `color{red}(–CH_2)` - unit


`color{green}("𝟐 𝐇𝐨𝐦𝐨𝐥𝐨𝐠𝐨𝐮𝐬 𝐬𝐞𝐫𝐢𝐞𝐬 𝐨𝐟 𝐚𝐥𝐤𝐞𝐧𝐞𝐬")`

`color{green}("𝟑 𝐇𝐨𝐦𝐨𝐥𝐨𝐠𝐨𝐮𝐬 𝐬𝐞𝐫𝐢𝐞𝐬 𝐨𝐟 𝐚𝐥𝐤𝐲𝐧𝐞𝐬")`

`color{green}(★)` As the molecular mass increases in any homologous series, a gradation in physical properties is seen because the melting and boiling points increase with increasing molecular mass. Other physical properties such as solubility in a particular solvent also show a similar gradation.

`color{green}(★)` But the chemical properties, which are determined solely by the functional group, remain similar in a homologous series.

`color{green}(★)` The names of compounds in a homologous series are based on the name of the basic carbon chain modified by a `color{red}("“prefix” “phrase before” or “suffix” “phrase after”")` indicating the nature of the functional group.

𝐍𝐚𝐦𝐢𝐧𝐠 𝐚 𝐜𝐚𝐫𝐛𝐨𝐧 𝐜𝐨𝐦𝐩𝐨𝐮𝐧𝐝 𝐜𝐚𝐧 𝐛𝐞 𝐝𝐨𝐧𝐞 𝐛𝐲 𝐭𝐡𝐞 𝐟𝐨𝐥𝐥𝐨𝐰𝐢𝐧𝐠 𝐦𝐞𝐭𝐡𝐨𝐝 –

(i) Identify the number of carbon atoms in the compound. A compound having three carbon atoms would have the name propane.

(ii) In case a functional group is present, it is indicated in the name of the compound with either a prefix or a suffix (as given in Table 4.4).

(iii) If the name of the functional group is to be given as a suffix, the name of the carbon chain is modified by deleting the final ‘e’ and adding the appropriate suffix. For example, a three-carbon chain with a ketone group would be named in the following manner – Propane `color{red}("– ‘e’ = propan + ‘one’ = propanone.")`

(iv) If the carbon chain is unsaturated, then the final ‘ane’ in the name of the carbon chain is substituted by ‘ene’ or ‘yne’ as given in Table

4.4. For example, a three-carbon chain with a double bond would be called propene and if it has a triple bond, it would be called propyne.