Question

(a) What are hydrocarbons? Explain with examples.
(b) Explain the meaning of saturated and unsaturated hydrocarbons with two examples each.
(c) Give the names and structural formulae of one saturated cyclic hydrocarbon and one unsaturated cyclic hydrocarbon.
(d) Give one example of a hydrocarbon, other than pentane, having more than three isomers.
(e) How many isomers of the following hydrocarbons are possible?
(i) C₃H₈
(ii) C₄H₁₀
(iii) C₅H₁₂
(iv) C₆H₁₄

Answer 1

(a) Hydrocarbon is a compound of hydrogen and carbon. Examples for hydrocarbons are methane (CH₄), ethane (C₂H₆) and ethyne (C₂H₂). All these compounds are hydrocarbons as they are only made of hydrogen and carbon elements which is evident from their respective molecular formulae.

(b) (i) Saturated hydrocarbons are hydrocarbons in which the carbon atoms are connected by single bonds only. They are also called alkanes, and their general formula is C_nH_2n+2, where n is the number of carbon atoms in each of their molecules.
Examples: Methane (CH₄) and butane (C₄H₁₀) are saturated hydrocarbons with 1 and 4 carbon atoms respectively, and their names end with 'ane'. The atoms are only connected by single covalent bonds.
The structures are represented below:



ii) Unsaturated hydrocarbons are hydrocarbons in which two carbon atoms are either connected by a double bond or triple bond. If the two carbon atoms are connected by a double bond, then it is called an alkene, and its general formula is C_nH_2n.
Example: Ethene (C₂H₄) is an alkene in which two carbon atoms are connected by a double bond.
The structure is as follows:


If two carbon atoms are connected by a triple bond, then it is called an alkyne, and its general formula is C_nH_2n-2.
Example: Ethyne (C₂H₂) is an alkyne in which two carbon atoms are connected by a triple bond.
The structure of ethyne is as follows:



(c) Name of one saturated cyclic hydrocarbon is cyclohexane. It's molecular formula is C₆H₁₂.
Structure of cyclohexane is as follows:


Name of one unsaturated cyclic hydrocarbon is benzene. It's molecular formula is C₆H₆.
Structure of benzene is as follows:


(d) Hexane (C₆H₁₄) is a hydrocarbon having 5 structural isomers. They are hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane.

(e)
(i) C₃H₈ doesn't have isomers, as isomerism is only possible if the number of carbon atoms is 4 or more.
(ii) C₄H₁₀ - Two isomers are possible for this compound, butane. They are n - butane and 2-methylpropane.
(iii) C₅H₁₂ - Three isomers are possible for this compound, pentane. They are n - pentane, 2-methylbutane and 2,2-dimethylpropane.
(iv) C₆H₁₄ - Five isomers are possible for this compound, hexane. They are n - hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane.