From their molecular formulas, classify each of these...

From their molecular formulas, classify each of these straight-chain hydrocarbons as an alkane, an alkene, or an alkyne.
(a) $\mathrm{C}_{21} \mathrm{H}_{44}$
(b) $\mathrm{C}_{4} \mathrm{H}_{6}$
(c) $\mathrm{C}_{8} \mathrm{H}_{16}$

Key Concepts

Alkynes

Alkynes are a type of unsaturated hydrocarbon that contain at least one carbon–carbon triple bond. Their general formula is CnH2n-2, reflecting the removal of four hydrogen atoms relative to alkanes. This multiple bond structure imparts unique chemical reactivity compared to alkanes and alkenes.

Alkenes

Alkenes are unsaturated hydrocarbons characterized by the presence of at least one carbon–carbon double bond. The general formula for acyclic alkenes is often written as CnH2n, representing a loss of two hydrogen atoms compared to alkanes, which accounts for the double bond.

Alkanes

Alkanes are saturated hydrocarbons containing only single bonds between carbon atoms. Their general formula is CnH2n+2. This formula serves as a benchmark; if a hydrocarbon fits this formula, it indicates the absence of double or triple bonds, hence classifying the molecule as an alkane.

Degree of Unsaturation

This concept involves calculating the number of rings and/or multiple bonds present in a compound based on its molecular formula. A common method is using the formula for unsaturation (or double bond equivalents) which helps in determining if the hydrocarbon is saturated (alkanes) or unsaturated (alkenes or alkynes). It forms the basis for classifying the type of hydrocarbon by comparing the hydrogen count with the expected count in a fully saturated molecule.

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