×

# What are the general formulas for alkane, alkene, alkyne, alkyl, aldehyde, ketone, cycloalkane?

Dec 24, 2015

A useful idea in this context is $\text{the degree of unsaturation}$, which I will outline with the answer.

#### Explanation:

$\text{Alkanes:}$ ${C}_{n} {H}_{2 n + 2}$;

$\text{Alkene:}$ ${C}_{n} {H}_{2 n}$;

$\text{Alkyne:}$ ${C}_{n} {H}_{2 n - 2}$;

$\text{Alkyl residue:}$ ${C}_{n} {H}_{2 n + 1}$;

$\text{Aldehyde/ketone:}$ ${C}_{n} {H}_{2 n} O$;

$\text{Cycloalkane:}$ ${C}_{n} {H}_{2 n}$

A fully saturated hydrocarbon, an alkane, has general formula ${C}_{n} {H}_{2 n + 2}$: $n = 1$, methane; $n = 2$, ethane; $n = 3$, propane. BY reason of their formula alkanes are said to $\text{HAVE NO DEGREES of UNSATURATION.}$

Where the formula is ${C}_{n} {H}_{2 n}$ or ${C}_{n} {H}_{2 n} {O}_{m}$, each 2 hydrogens LESS than $2 n + 2$ represents a $\text{degree of unsaturation}$. Each degree of unsaturation represents a double bond OR a ring. Compare cyclopropane to propane or hexane to cyclohexane to cyclohexene; does this formulation hold?

Where there is nitrogen in the formula we substract $N H$ from the given formula before assessing its degree of saturation. For $\text{methylamine}$, ${H}_{3} C {H}_{2} C N {H}_{2}$, we assess a formula of ${C}_{2} {H}_{6}$, no degrees of saturation. For pyridine, ${C}_{5} {H}_{5} N$, we assess ${C}_{5} {H}_{4}$, ${4}^{\circ}$ of unsaturation, i.e. 3 olefinic bonds, and one ring..........