General molecular formula

General empirical formulas are mainly used in organic chemistry to indicate the number of atoms in a molecule or the number ratio between the types of atoms of the homologous series of chemical compounds . The sum formulas of the individual compounds can be determined from the general sum formulas .

Example:

A butane molecule from the homologous series of alkanes has four carbon atoms ( n = 4). The general formula for alkanes is C _n H _{2 n +2} . The number of hydrogen atoms then results from

${\displaystyle 2\cdot n+2=2\cdot 4+2=10}$

The molecular formula of butane is therefore C ₄ H ₁₀ .

In the case of compounds whose functional groups have a carbon atom and which are explicitly specified, n in the empirical formula is to be reduced by 1 for each functional group that contains a carbon atom. A molecule of butanoic acid (butyric acid) from the homologous series of alkanoic acids with the general empirical formula C _n H _{2 n +1} COOH with n = 3 has 4 carbon atoms, one of which is in the carboxy group (-COOH). From this follows the empirical formula of butanoic acid C ₃ H ₇ COOH.

General empirical formulas of homologous series

• Alkanes : C _n H _{2 n +2} ( n = 1, 2, 3, 4, ...)
• Alkenes : C _n H _{2 n} ( n = 2, 3, 4, ...)
• Alkynes : C _n H _{2 n −2} ( n = 2, 3, 4, ...)
• monovalent alkanols : C _n H _{2 n +1} OH ( n = 1, 2, 3, 4, ...)
• monovalent alkanals : C _n H _{2 n +1} CHO ( n = 0, 1, 2, 3, 4, ...)
• monovalent alkanoic acids : C _n H _{2 n +1} COOH ( n = 0, 1, 2, 3, 4, ...)

More general sum formulas

• Cycloalkanes : C _n H _{2 n +2 (1− x )} ( x is the number of rings)
• Simple sugars (monosaccharides): C _n H _{2 n} O _n [Exceptions: deoxy sugars (e.g. deoxyribose , C ₅ H ₁₀ O ₄ )]
• Double sugars (disaccharides): C _n H _{2 n −2} O _{n −1}
• Arsane : As _n H _{2 n +2}
• Boranes : B _n H _m ( n , m = integer)
• Teutons : Ge _n H _{2 n +2}
• Silicas (ring-shaped): [Si (OH) ₂ –O–] _n
• Silanes : Si _n H _{2 n +2}

See also

• Chemical structure
• Ratio formula
• Molar ratio

Individual evidence

1. ^ ^{A b} Jürgen Schatz, Robert Tammer: First Aid - Chemistry and Physics for Physicians . Springer-Verlag, 2015, ISBN 978-3-662-44111-4 , pp. 455 ( Google Books ).