Homologous series – All about the topic

If you imagine a chain to which links can always be added or subtracted, that’s pretty much the principle of the homolog series. The chain links of the chain are always the same and together the chains of different lengths form a group. In the case of molecules, this group is then called the homologous series.

the homologous series are substances with a general molecular formula, the series of which results from the addition of a single chain link. The chemical properties of the series homologues are almost identical, while the physical properties vary slightly with chain length change.

The term homolog series has existed since 1843 and was coined by Charles Frédéric Gerhardt. For organic carboxylic acids and alcohols, however, the homologue series was first demonstrated by Jean Baptiste Dumas.

The term homolog

The term homolog is derived from the words homo and logos away. Homo means «equal», while logos here can be translated as «sensibly summarized». The term homolog is intended to denote a meaningful summary of something of the same kind.

Chemistry involves not only homologous series, but also homologies per se. Included are then substances that are similar in their chemical structure and also have similar properties.

This term is particularly common for chemical elements in periodic table used that are in the same group. Elements above the corresponding element can be referred to as lighter homologues, while elements below it are referred to as higher homologues.

The homologous series of hydrocarbons

There are many different homologue series in chemistry. The best-known homologue series is the homologue series of alkanes. However, alkenes and alkynes also have a homologue series.

alkanes

Alkanes are saturated, mostly acyclic hydrocarbons, which consist of the elements carbon and hydrogen and only have single bonds. They have the general empirical formula CnH2n+2.

Starting with the fourth alkane, the basic structure can consist of branched or unbranched carbon chains. The unbranched alkanes form the homologous series.

carbon count

Name Molecular Formula1MethaneCH42EthaneC2H63PropaneC3H84n-ButaneC4H105n-PentaneC5H126n-HexaneC6H147n-HeptaneC7H168n-OctaneC8H189n-NonaneC9H2010n-DecaneC10H2211n-UndecaneC11H2412n-DodecaneC12H26

The alkanes are flammable and hardly soluble in water. The more CH2 groups an alkane has, the higher the melting and boiling point. This is due to the increased number of bonds that are present. The more bonds there are in a molecule, the more energy is required to break them.

The combustibility of alkanes, on the other hand, decreases in the course of the homologue series due to the increasing number of carbon atoms.

If you want to learn more about alkanes, take a look at their explanation!

alkenes

alkenes are aliphatic hydrocarbons with double bond. Unlike alkanes, alkenes are unsaturated compounds. The general molecular formula of alkenes is CnH2n and begins with ethene.

Aliphatic means that one or more carbon atoms are open, i.e. in a chain. Thus, the word aliphatic describes the opposite of cyclic.

Carbon NumberName Molecular Formula2EtheneC2H43PropeneC3H641-ButeneC4H851-PenteneC5H1061-HexeneC6H1271-HepteneC7H1481-OcteneC8H1691-NonenC9H18101-DeceneC10H20

As with the alkanes, the melting and boiling points of the alkenes also increase as the number of carbons increases because there are now more bonds that require more energy to break.

The water solubility is also only given to a very small extent, although it is already significantly greater than that of the alkanes. The reason for this is that both groups of substances are hydrophobic.

Have a look at the explanation of alkenes for more information!

alkynes

The alkynes are aliphatic hydrocarbons with triple bonds. Like the alkenes, the alkynes are unsaturated compounds. The general molecular formula of the alkynes is CnH2n-2 and starts with the substance ethyne.

An atom or molecule is unsaturated if not all outer electrons are present in a bond and the octet rule is therefore not fulfilled for this particle.

Carbon NumberName Molecular Formula2EthineC2H43PropyneC3H64ButyneC4H85PeptyneC5H106HexyneC6H127HeptyneC7H148OctyneC8H169NonyneC9H1810DecyneC10H20

The alkynes are also hardly soluble in water and easily combustible. With increasing chain length, there are different properties within the homologous series.

If you compare where the boiling temperature of ethyne is and where that of octyne is, you will notice that just a few added carbon atoms, in this case seven, can have a big impact:

Surname

molecular formula

boiling temperature

ethin

C2H2

-83.8℃

Octin

C8H14

126°C

You can find more information in the explanation of alkynes!

The homologous series of carboxylic acids

carboxylic acids are organic compounds with one or more carboxy groups. The monocarboxylic acids having a carboxy group also have homologous series. Their general molecular formula is CnH2nO2.

Carbon numberNameCommon nameMomental formula1methanoic acidformic acidHCOOH2ethanoic acidacetic acidCH3COOH3propanoic acidpropionic acidC2H5COOH4butyric acidbutyric acidC3H7COOH5pentanoic acidvaleric acidC4H9COOH6hexanoic acidcaproic acidC5H11COOH7heptanoic acidoenanthic acidC6H13COOH

The properties of the first four carboxylic acids are mainly determined by the polar carboxyl groups. They are readily water-soluble in water, which is also polar, while carboxylic acids with more than four carbon atoms in the molecule are less water-soluble. Because the polar part of the carboxyl group is increasingly subordinate to the non-polar carbon chain.

The boiling as well as the melting temperature increase with the size of the alkyl radical, since there are again more bonds that have to be broken. In the case of the carboxylic acids, this goes so far that the carboxylic acids are present in the solid state of aggregation starting with the decanoic acid.

Homologous Series – The Most Important

  • The homologous series are substances with a general molecular formula and increasing chain length.
  • The substances of the homologous series have similar chemical properties.
  • The physical properties change with chain length.
  • The best-known series of homologues is that of the alkanes.

proof

  1. knowledge platform-schueler.de: homologous series (08/27/2022)
  2. chemie.de: Alkanes (09/02/2022)
  3. chemie.de: Alkynes (09/02/2022)
  4. abiturwissen.org: The homologous series of carboxylic acids (September 2nd, 2022)