2.2 - Homologous Series

Meaning and Characteristics of a Homologous Series

• A homologous series is a group of organic compounds classified together for the study of their physical properties and chemical reactions.
• Members of one homologous series have:
  • the same general formula;
  • the same functional group;
  • the same chemical properties;
  • consecutive members differing by $\mathrm{C}{\mathrm{H}}_2$, so their relative molecular masses differ by 14;
  • physical properties that change gradually from one member to the next.
• A functional group is a group of atoms bonded to an organic molecule that determines its chemical properties; chemical reactions occur at this group.

Homologous series	General formula	Functional group / type
Alkane	${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2n+2}$, $n\ge 1$	C–C; saturated hydrocarbon
Alkene	${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2n}$, $n\ge 2$	C=C; unsaturated hydrocarbon
Alkyne	${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2n-2}$, $n\ge 2$	C≡C; unsaturated hydrocarbon
Alcohol	${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2n+1}\mathrm{OH}$, $n\ge 1$	hydroxyl, –OH; non-hydrocarbon
Carboxylic acid	${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2n+1}\mathrm{COOH}$, $n\ge 0$	carboxyl, –COOH; non-hydrocarbon
Ester	${\mathrm{C}}_{\mathrm{m}}{\mathrm{H}}_{2m+1}\mathrm{COO}{\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2n+1}$, $m\ge 0$, $n\ge 1$	carboxylate, –COO–; non-hydrocarbon

Formulae and IUPAC Names

• A molecular formula gives the types and actual numbers of atoms of each element in one molecule.
• A structural formula shows the types of bonds and how the atoms are connected.
• A straight-chain IUPAC name combines a root, which indicates the number of carbon atoms in the longest chain, and a suffix, which identifies the homologous series.

Number of carbon atoms	Root
1	meth-
2	eth-
3	prop-
4	but-
5	pent-
6	hex-
7	hept-
8	oct-
9	non-
10	dec-

• Suffixes: alkane: -ane; alkene: -ene; alkyne: -yne; alcohol: -ol; carboxylic acid: -oic acid; ester: -oate.
• To construct a molecular formula, substitute the carbon number into the general formula. For propane, $n=3$ gives ${\mathrm{C}}_3{\mathrm{H}}_{2(3)+2}={\mathrm{C}}_3{\mathrm{H}}_8$.
• In a structural formula, each carbon has four covalent bonds; adjacent carbon atoms are joined by the bond type of the series.

Alkanes

• Every carbon atom forms four single covalent bonds. Methane, $\mathrm{C}{\mathrm{H}}_4$, is the first member.
• First ten straight-chain members: methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane and decane.
• Examples: ethane, ${\mathrm{C}}_2{\mathrm{H}}_6$, has condensed structural formula $\mathrm{C}{\mathrm{H}}_3-\mathrm{C}{\mathrm{H}}_3$; propane, ${\mathrm{C}}_3{\mathrm{H}}_8$, has $\mathrm{C}{\mathrm{H}}_3-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_3$.
• Uses shown include the waxy alkane coating on fruit and butane, ${\mathrm{C}}_4{\mathrm{H}}_{10}$, as cooking fuel.

Alkenes and alkynes

• Alkenes contain C=C; ethene, ${\mathrm{C}}_2{\mathrm{H}}_4$, is the first member and promotes fruit ripening.
• Alkynes contain C≡C; ethyne, ${\mathrm{C}}_2{\mathrm{H}}_2$, is the first member and is used for cutting metals.
• Number a chain from the end giving the multiple bond the lowest position: for example, but-1-ene and but-1-yne.
• Alkene examples: ethene, ${\mathrm{C}}_2{\mathrm{H}}_4$, $\mathrm{C}{\mathrm{H}}_2=\mathrm{C}{\mathrm{H}}_2$; propene, ${\mathrm{C}}_3{\mathrm{H}}_6$, $\mathrm{C}{\mathrm{H}}_2=\mathrm{CH}-\mathrm{C}{\mathrm{H}}_3$; but-1-ene, ${\mathrm{C}}_4{\mathrm{H}}_8$, $\mathrm{C}{\mathrm{H}}_2=\mathrm{CH}-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_3$.
• Alkyne examples: ethyne, ${\mathrm{C}}_2{\mathrm{H}}_2$, $\mathrm{HC}\equiv \mathrm{CH}$; propyne, ${\mathrm{C}}_3{\mathrm{H}}_4$, $\mathrm{HC}\equiv \mathrm{C}-\mathrm{C}{\mathrm{H}}_3$; but-1-yne, ${\mathrm{C}}_4{\mathrm{H}}_6$, $\mathrm{HC}\equiv \mathrm{C}-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_3$.

Alcohols

• Alcohols contain carbon, hydrogen and oxygen and have the hydroxyl group, –OH. This covalent group is not the hydroxide ion, $\mathrm{O}{\mathrm{H}}^{-}$.
• Select the longest chain containing –OH, replace the final “e” of the corresponding alkane with “ol”, and state the position of –OH where needed: $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ is propan-1-ol.
• Uses pictured include antiseptics, perfumes, solvents and fuels.

Carboxylic acids

• Carboxylic acids contain the carboxyl group, –COOH. Methanoic acid, $\mathrm{HCOOH}$, occurs in ant bites; ethanoic acid, $\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}$, occurs in vinegar.
• The carboxyl group is at the end of the chain and its carbon is numbered 1. The value $n=0$ in the general formula gives the first member, $\mathrm{HCOOH}$.
• Count the carboxyl carbon as part of the main chain and replace the final “e” of the corresponding alkane with “oic acid”. Thus $\mathrm{HCOOH}$ is methanoic acid and $\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}$ is ethanoic acid.

Physical Properties within a Homologous Series

Alkanes, alkenes and alkynes

• They consist of neutral covalent molecules, dissolve in organic solvents, are insoluble in water, cannot conduct electricity in any state and are less dense than water.
• They have low melting and boiling points. As carbon number and molecular size increase, intermolecular van der Waals forces become stronger; more heat is needed to overcome them, so melting and boiling points generally increase.
• At room temperature, the lower members are gases and the subsequent members become liquids as their boiling points rise.

Alcohols and carboxylic acids

• They are non-hydrocarbon compounds containing carbon, hydrogen and oxygen.
• Their boiling points are relatively higher than those of the corresponding alkenes and increase with carbon number.
• The first eleven alcohols and first nine carboxylic acids are liquids at room temperature.
• Methanol, ethanol and propanol are miscible with water in all proportions. Methanoic, ethanoic and propanoic acids are very soluble in water.
• Water solubility decreases as molecular size increases.