Carboxylic acids

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Carboxylic
acids and
their
derivatives

ESTERS, ACYL CHLORIDES, ACID
ANHYDRIDES, AMIDES

Carboxylic acids – you should already know:

-Bonding and structure
-Reactions of acids
-Naming
-Solubility

Open Ended

Name the three carboxylic acids.

​Rules for naming carboxylic acids.

• the carbon in COOH is ALWAYS carbon 1.

• select the longest chain, name it after an alkane and add -oic acid

• add all the substituents in the alphabetical order

• the simplest aromatic carboxylic acid is called benzoic acid

Statements

The functional group in carboxylic acids is the carboxyl group.

Dicarboxylic acids have two carboxyl groups.

All carboxylic acids are weak acids.

Carboxylic acids with up to 4 carbon atoms are soluble in
water.

Draw

1.     Draw a displayed formula of 2-methylpropanoic acid. Add partial charges.

(remember what a displayed formulae is supposed to show)

Multiple Choice

2-methylpropanoic acid is an example of a monoprotic weak acid. Which statement explains best what a weak acid is?

• It dissociates in water.

• It dissociates in water into H⁺ ions.

• It partially dissociates in water into H⁺ ions.

• It partially dissociates into hydroxyl ions.

Dissociate 1 H⁺ ion per molecule.

(see the topic Acids, buffers and pH)

​Monobasic (monoprotic) acids

Dissociate partially in water producing H⁺ ions.

Carboxylic acid can only dissociate the hydrogen from the COOH group.

Weak acids

Weak acids and monobasic acids

Draw

Write an equation for the partial dissociation of propanoic acid.

Draw

Oxalic acid (found in rhubarb) is a dicarboxylic acid with two carboxyl groups and the molecular formula of C₂H₂O₄. Draw a displayed formula of this acid.

Poll

Is oxalic acid a monoprotic acid?

• Yes

• No

Oxalic acid is a diprotic acid

Oxalic acid has got two COOH groups so can dissociate 2 H⁺ ions per molecule.

The strength of carboxylic acids compared to phenols

Carboxylic acids v. phenols

Carboxylic acids are stronger acids than phenols.

The strength of two acids can be compared using their pKa values.
The lower pKa, the stronger the acid is.
pKa=4.87 (2-methylpropanoic acid)                pKa= 9.95 (phenol)
(See topic Acids, buffers and pH)

The first four carboxylic acids are well soluble in water and then the solubility decreases.

​

Solubility of carboxylic acids

Multiple Choice

Why are the first four acids soluble in water?

• London forces form

• There is a permanent dipole in molecules

• Covalent bonds are weak and break easily.

• Hydrogen bonding forms

Open Ended

Suggest why the solubility decreases from pentanoic acid.

Reason for decreasing solubility

The non-polar carbon chain becomes longer and its effect becomes greater on the solubility of the molecule.

Draw

Draw a hydrogen bond between methanoic acid and a water molecule.

Draw

HO(CH₂)₄OH can be oxidised to form HOOC(CH₂)₂COOH. What are the required conditions and
reagents? Write an equation for this oxidation, using [O].

Draw

Draw skeletal formulae of two other compounds that can be oxidised to form the same carboxylic acid. Write equations for these oxidations, using [O].

Acid reactions of carboxylic acids

Carboxylic acids react in a similar way to non-
organic acids (such as HCl or H2SO4).

a)Neutralisation reactions

b)Redox reactions.

Neutralisation reactions of carboxylic acids with bases

a) reactions with metal oxides such as: MgO, Na2O

b) reactions with alkali such as: NaOH, Ca(OH)2

c) reactions with carbonates: e.g. Na2CO3

Naming the products:

The organic product is a carboxylate salt. The name is formed by replacing -ic
acid with -ate; e.g. methanoic acid forms methanoate salts.

Write a balanced equation to show the products of the following reactions. Name the products

Butanoic acid solution and calcium oxide

Write a balanced equation to show the products of the following reactions. Name the products

2-hydroxybenzoic acid + sodium hydroxide

Remember: the product is a
salt made of positive ions
(the metal ions) and
negative ions (from the
acid). The bonding between
the ions is IONIC. You can
show the charges on the
ions but make sure you
have both, negative and
positive signs.

Draw

Write balanced equations for the following reactions.

a)     Propanoic acid and sodium carbonate.

b)     Butanoic acid and copper(II) oxide.

ESQ p.12

Method 1:

-add Tollens reagent to all 3 compounds– silver mirror forms – it’s the aldehyde

-to the other 2 solutions add 2,4-DNP – orange precipitate – the ketone

-the third solution is the carboxylic acid

Method 2:

-add 2,4-DNP to all compounds – the one that doesn’t form an orange precipitate is the carboxylic acid

-to the other 2 compounds add Tollens reagent – if silver mirror forms – it’s the aldehyde, the other is the ketone

Method 3:

-add Tollens reagent to all 3 compounds– silver mirror forms – it’s the aldehyde

-add a solution of sodium carbonate to the other two – fizzing – the carboxylic acid, the third one is a ketone

Redox reactions of carboxylic acids

Important things to remember:

◦A) the products – a salt and hydrogen gas

◦B) balancing

◦C) the formula and the name of the salt – the carboxylate ion is 1- (but check if there are more than 1
COOH group)

◦D) observations - fizzing

Write balanced equations:
a) Butanoic acid + calcium

 

b) Oxalic acid, HOOC-COOH, + sodium

Butanoic acid solution and calcium

oxalic acid, HOOC-COOH + sodium