# What is the condensed structural formula for ethyne?

Sep 6, 2017

$H C \equiv C H$.........

#### Explanation:

A very useful idea that is used to rationalize the formulae of organic compounds is the $\text{degree of unsaturation}$.  An alkane is fully $\text{saturated}$, and it contains the MAXIMUM ALLOWABLE number of $C - H$ bonds.

Alkanes are $\text{FULLY saturated}$ and have a general formula of ${C}_{n} {H}_{2 n + 2}$. Try this out for $\text{methane,}$ $\text{ethane,}$ .........$\text{pentane, etc.}$

Each degree of unsaturation, an olefinic bond OR a ring junction, corresponds to 1""^@ of unsaturation. So according to the scheme, $\text{ethane}$ has the saturated formula of ${H}_{3} C - C {H}_{3}$, but $\text{ethylene}$, ${H}_{2} C = C {H}_{2}$ has ${1}^{\circ}$ of unsaturation. And here, acetylene, with 2 formal double bonds, has ${2}^{\circ}$ of unsaturation. Halogen atoms count for one hydrogen; for nitrogen atoms, substract $N H$ from the formula before assessing unsaturation; i.e. for $\text{ethylamine,}$ ${H}_{2} N C {H}_{2} C {H}_{3}$ $\rightarrow {C}_{2} {H}_{6}$, i.e. $\text{no degrees of unsaturation}$.

How many degrees of unsaturation does benzene have? What about pyridine?