Hydrocarbons

1.

Give the general formula of "alkynes". Identify the alkynes from the following :

CH₄, C₂H₆, C₂H₂, C₃H₄, C₂H₄ ?

Sol:

The general formula of alkynes is C_nH_{2n – 2} where n is the number of carbon atoms in one molecule of the alkyne.

Out of the above given hydrocarbons C₂H₂ and C₃H₄ are alkynes (because they correspond to the general formula C_nH_{2n – 2} with n = 2 and n = 3, respectively).

Please note that in an alkyne molecule, "the number of hydrogen atoms" is "2 less than double the number of carbon atoms".

2.

What is the general formula of alkenes ? Identify the alkenes from the following :

C₂H₆, C₂H₄, C₃H₄, C₂H₂, C ₃H₆ ?

Sol:

The general formula of alkenes is C_nH_2n where n is the number of carbon atoms in one molecule of the alkene.

Out of the above given hydrocarbons C₂H₄ and C₃H₆ are alkenes (because they correspond to the general formula C_nH_2n with n = 2 and n = 3, respectively).

Please note that in an alkene molecule, the "number of hydrogen atoms" is exactly equal to "double the number of carbon atoms".

3.

What is the general formula of alkanes ? Identify the alkanes from the following :

CH₄, C₂H₂, C₂H₆, C₃H₆, C₃H₈ ?

Sol:

The general formula of alkanes is C_nH_{2n + 2} where n is the number of carbon atoms in one molecule of the alkane.

Out of the above given hydrocarbons CH₄, C₂H₆ and C₃H₈ are alkanes (because they correspond to the general formula C_nH_{2n + 2} having n = 1, n = 2 and n = 3, respectively).

Please note that in an alkane molecule, "the number of hydrogen atoms" is "2 more than double the number of carbon atoms".

4.

Which of the following organic compounds is unsaturated ?

CH₄, C₂H₄?

Sol:

We know that alkenes and alkynes are unsaturated compounds.

So, all that we have to do here is to find out which of the above compounds is an alkene or alkyne.

That will be the unsaturated compound. Now, out of the above compounds, C₂H₄ is unsaturated (because it is an alkene corresponding to the general formula of alkenes C_nH_2n with n = 2).

5.

Which of the following compounds can have a double bond ?

C ₃H₈, C₃H₆ ?

Sol:

The compound having a double bond is called an alkene.

So, all that we have to do here is to find out which of the above given compounds is an alkene.

Here, C₃H₆ is an alkene (because it corresponds to the general formula for alkenes C_nH_2n with n = 3).

Thus, the compound C₃H₆ will have a double bond in it.

6.

A hydrocarbon molecule has 3 carbon atoms. Write down its molecular formula if it is an :

(i) alkane

(ii) alkene

(iii) alkyne

Sol:

The number of carbon atoms in the molecule of this hydrocarbon is 3,that is, n = 3.

(i) The general formula of an alkane is C_nH_{2n + 2} where n is the number of carbon atoms in one molecule of the alkane.

Here, n = 3

So, putting n = 3 in this general formula, we get C ₃H_{2 * 3 + 2} or C₃H₈

Thus, the molecular formula of the given hydrocarbon, if it is an alkane, is C₃H₈.

(ii) The general formula of an alkene is C_nH_2n where n is the number of carbon atoms in one molecule of the alkene.

Here, n = 3

So, putting n = 3 in this general formula, we get C ₃H_{2 * 3} or C₃H₆

Thus, the molecular formula of the given hydrocarbon, if it is an alkene, is C₃H₆.

(iii) The general formula of an alkyne is C_nH_{2n –2} where n is the number of carbon atoms in one molecule of the alkyne.

Here, n = 3

So, putting n = 3 in this general formula, we get C₃H_{(2 * 3) –2} or C₃H₄

Thus, the molecular formula of the given hydrocarbon, if it is an alkyne, is C₃H₄.

7.

A hydrocarbon molecule contains 4 hydrogen atoms. Give its molecular formula, if it is an :

(i) alkane

(ii) alkene

(iii) alkyne

Sol:

(i) An alkane containing 4 hydrogen atoms in its molecule is methane, CH ₄.

(ii) An alkene containing 4 hydrogen atoms in its molecule is ethene, C₂H₄.

(iii) An alkyne containing 4 hydrogen atoms in its molecule is propyne, C₃H₄.