Question

Identify alkene and alkyne from the following.
(i) ${{C}_{2}}{{H}_{6}}$
(ii) ${{C}_{3}}{{H}_{4}}$
(iii) ${{C}_{3}}{{H}_{6}}$
(iv) ${{C}_{3}}{{H}_{8}}$

Answer 1

Both alkenes and alkynes are the categories of unsaturated hydrocarbons i.e. they have less hydrogen atoms making them unsaturated and different from the alkane group (saturated hydrocarbons).

Complete step-by-step answer: Let us study the basics of three categories of hydrocarbons;
Alkane-
The saturated hydrocarbons that would contain single bonds and will have general formula as ${{C}_{n}}{{H}_{2n+2}}$ .
Alkene-
The unsaturated hydrocarbons that would at least contain one double bond and will have general formula as ${{C}_{n}}{{H}_{2n}}$ .
Alkyne-
The unsaturated hydrocarbons that would at least contain one triple bond and will have general formula as ${{C}_{n}}{{H}_{2n-2}}$ .
Now, according to the given definitions we can classify the above given molecules as follows;
(i) ${{C}_{2}}{{H}_{6}}$ - This molecule resembles the general formula as ${{C}_{n}}{{H}_{2n+2}}$ and thus it is an alkane.
(ii) ${{C}_{3}}{{H}_{4}}$ - This molecule resembles the general formula as ${{C}_{n}}{{H}_{2n-2}}$ and thus it is an alkyne.
(iii) ${{C}_{3}}{{H}_{6}}$ - This molecule resembles the general formula as ${{C}_{n}}{{H}_{2n}}$ and thus it is an alkene.
(iv) ${{C}_{3}}{{H}_{8}}$ - This molecule resembles the general formula as ${{C}_{n}}{{H}_{2n+2}}$ and thus it is an alkane.
Thus, ${{C}_{3}}{{H}_{6}}$is the alkene and ${{C}_{3}}{{H}_{4}}$is the alkyne from all of the above-mentioned molecules.

Note: Do not that the molecular formula alone can be used to determining and identifying the actual category of a compound.
For example, ${{C}_{6}}{{H}_{12}}$ can be said unsaturated hydrocarbon (alkene) according to the general formula as ${{C}_{n}}{{H}_{2n}}$ and also as the saturated cyclic hydrocarbon.