Structure and Properties of Alkanoic Acids Chemistry Tutorial

Key Concepts

• Alkanoic acids belong to a class of organic (carbon) compounds known as carboxylic acids.
• Carboxylic acids contain the carboxyl (-COOH) functional group:

  	O ||
  -	C	-OH

• Very long chain carboxylic acids are found in biological systems and referred to as fatty acids.
• General formula of alkanoic acids⁽¹⁾ :

  ⚛ Condensed Structural Formula: C_nH_2n+1COOH or R-COOH

  ⚛ Molecular Formula: C_nH_2nO₂

• Naming alkanoic acids⁽²⁾:

1. Number the longest carbon chain starting with the carbon atom of the COOH functional group.
2. Name the carbon chain as the alkane.
3. Drop the "e" from the name of the alkane.
4. Add the suffix -oic acid

• Physical properties of alkanoic acids:

  ⚛ Alkanoic acids are polar molecules so are expected to be more soluble in polar solvents like water, and to have higher melting and boiling points than the corresponding alkane.

  ⚛ Alkanoic acids are weak acids so are expected to display similar physical and chemical properties to other acids.

• Chemical reactions of alkanoic acids: alkanoic acids react with

  ⚛ bases in neutralisation reactions⁽³⁾

  ⚛ carbonates to produce salt, carbon dioxide gas and water

  ⚛ active metals to produce hydrogen gas

  ⚛ alkanols (alcohols) in esterification reactions

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Naming Alkanoic Acids

Follow the steps below to name a straight-chain alkanoic acid
(also see tutorial Naming Straight-chain Alkanoic Acids):

1. Number the longest carbon chain starting with the carbon atom of the COOH functional group.

   	H |		H |		H |		O ||
   H-	C4	-	C3	-	C2	-	C1
   	| H		| H		| H		|   OH

2. Name the carbon chain as the alkane.

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

   alkane with 4 carbon atoms in the chain is named butane

3. Drop the "e" from the name of the alkane.

   butane becomes butan

4. Add the suffix -oic acid

   butan becomes butanoic acid

The preferred IUPAC names, formulae, and occurrence, of some alkanoic acids is given in the table below:

Physical Properties of Alkanoic Acids

Alkanoic acids are weak acids, and as such are expected to have the properties of acids:

• taste sour
• conduct electricity in solution
• turn blue litmus red

Because alkanoic acids are capable of some dissociation in water, we expect them to display some solubility in water.

Alkanoic acids exist as covalent molecular substances, so their melting points and boiling points are expected to be quite low.

Melting Point and Boiling Point of Alkanoic Acids

Alkanoic acids are polar molecules.

2 hydrogen bonds (shown in dotted red lines) can form between them.

For this reason, alkanoic acids have higher melting and boiling points than other compounds with the same number of carbon atoms, as shown in the table below:

If two organic molecules have the same number of carbon atoms, then the molecule capabale of forming the greatest number of hydrogen-bonds will have the highest melting point and boiling point.

The melting point and the boiling points of alkanoic acids increase with increasing number of carbon atoms in the carbon chain.
The boiling points of alkanoic acids increase by about 20°C for each carbon that is added to the chain, as shown below:

The trend is even more obvious if we graph boiling points of this series of alkanoic acids as shown below:

As the carbon chain becomes longer, the intermolecular interactions between these carbon chains becomes greater, so the weaker Van der Waals forces (dispersion forces or london forces) ⁽⁵⁾ become more significant, and the amount of energy required to separate the molecules so that the liquid will boil becomes greater.

Solubility of Alkanoic Acids in Water

The table below gives the solubility in water of a number of alkanoic acids. Can you see a pattern, or trend, in the data?

Short chain alkanoic acids, that is formic acid (methanoic acid) to butanoic acid (butyric acid), are soluble (miscible) in water as a result of hydrogen bonding between the OH group of the carboxyl group and water molecules.

However, after butanoic acid, as the number of carbon atoms in the carbon chain increases, the solubility of the alkanoic acid in water decreases.
The weak intermolecular forces of attraction between the hydrocarbon chains (Dispersion forces or London forces) become more significant as the chains become longer, so the alkanoic acids are more attracted to each other than they are to the polar water molecules.

Chemical Properties of Alkanoic Acids

Alkanoic acids are weak acids, the acid dissociation constant, K_a, is small.

Soluble alkanoic acids dissociate slightly in water :

The table below gives that value of the acid dissociation constant, K_a, for acetic acid and chlorinated acetic acid derivatives. Can you see a pattern, or a trend, in the data?

As the number of chlorine atoms substituting for hydrogen in the acetic acid molecule increase, the value of the acid dissociation constant also increases.

The acidity of carboxylic acids increases with the substitution of highly electronegative atoms, such as chlorine, in the molecule.

Alkanoic acids react with:

• bases in neutralisation reactions
• carbonates to produce salt, carbon dioxide gas and water
• active metals to produce hydrogen gas
• alkanols (alcohols) in esterification reactions

Alkanoic Acids and Neutralisation Reactions

Alkanoic acids are acids, they react with bases to produce a salt and water in a neutralisation reaction.

The salt produced (metal alkanoate) is the salt of a strong base and weak acid, so the aqueous salt solution produced in the neutralisation reaction will be basic (pH > 7 at 25°C)

Soluble salts of long-chain carboxylic acids (fatty acids) are soaps

Alkanoic Acids React with Carbonates

Alkanoic acids are acids, they react with carbonates (including hydrogen carbonates) to produce a salt, carbon dioxide gas and water.

The standard test for carbon dioxide gas is to bubble the gas through limewater to produce a "milky" precipitate.

Alkanoic Acids React with Active Metals

Alkanoic acids are acids, they react with active metals such as Group 1 (IA or alkali metal) elements to produce a salt and hydrogen gas.

The standard test for hrogen gas is the "pop test".

Esterification Reaction

Esters are produced in a condensation reaction between an alkanoic acid (carboxylic acid) and an alkanol (alcohol).
A condensation reaction is a special type of elimination reaction in which water is the small molecule being eliminated during the reaction.

The reaction that produces an ester is known as an esterification reaction.

Esters commonly have fragrent, fruity odours. You can use your sense of smell to detect the synthesis of an ester.