6.1.3 Carboxylic Acids and Esters

How are carboxylic acids formed from primary alcohols?

complete oxidation of primary alcohols: heat under reflux with acidified dichromate

What is the carboxylic acid functional group?

COOH

What colour change acompanies the formation of carboxylic acids from primary acohols?

orange to green

Which carboxylic acids are soluble in water?

the first 4

Describe the polarity of carboxylic acids?

• the carboxyl group is polar

• the hydrocarbon chain is non-polar

Which part of the carboxykic acid attracts water?

carboxyl group (COOH)

Draw a diagram of the carboxyl group with water?

How does solubility of the carboxylic acid change as the length of the hydrocarbpon chain increases?

as it increases, solubility decreases

Why does the solubility of carboxylicc acids decrease as chain length increases?

a greater proportion of the molecule is hydrophobic

Define acid

proton donor

Define strong acid

acid that completely dissociates into its ions

Define weak acid

an acid that partially dissociates into its ions in aqueous solutions

What type of acids are carboxylic acids?

weak acids

Write the equation for the dissociation of ethanoic acid

CH3COOH ⇌ CH3COO- + H+

Describe the equilibrium position of ethanoic acid

far left

Why is the equilibrium position of ethanoic acid dissociation far to the left

the concentration of protons is low compared to the concentration of undissociated acid

How do you calculate acid dissociation constant?

[H+][A-] / [HA]

What does acid dissociation constant indicate?

extent of acid dissociation

When will Ka be a large number?

when [H+] and [A-] are large (strong acid)

What does a Ka vaue greater than 1 suggest?

equilibrium is to the right

When will Ka be a low value?

when [H+] and [A-] are low

What does a Ka value of less than 1 suggest?

equilibrium lies far to the left

How does Ka change as acids get stronger?

it gets larger

Draw the equation for propanoic acid and sodium oxide

Write the equation for propanoic acid and sodium oxide

propanoic acid + sodium oxide → odium propanoate + water

What is the observation when propanoic acid and sodium oxide react?

solid dissolves

Draw the equation for propanoic acid and sodium hydroxide

Write the equation for propanoic acid and sodium hydroxide

propanoic acid + sodium hydroxide → sodium propanoate + water

What is the observation when propanoic acid reacts with sodium hydroxide?

no visible change

Draw the equation for propanoic acid and magnesium carbonate

Write the equation for propanoic acid and magnesium carbonate

propanoic acid + magnesium carbonate → magnesium propanoate + water + carbon dioxide

What is observed when magnesium carbonate reacts with propanoic acid?

solid dissolves and fizzing

What is the chemical test for the presence of a carboxylic acid?

sodium carbonate: positive result shows fizzing and the solid dissolves

Draw the equation for propanoic acid and calcium

Write the equation for propanoic acid and calcium

propanoic acid + calcium → calcium propanoate + hydrogen

What is observed when calcium reacts with propanoic acid

solid dissolves and fizzing

What type of reaction is that between a carboxylic acid and a metal?

redox

Draw calcium propanoate

Draw the equation for propanoic acid and ammonia

Write the equation for propanoic acid and ammonia

propanoic acid + ammonia → ammonium propanoate

What would be observed when ammonia reacts with propanoic acid

no visible change

Define salt

the compound produced when the H+ of an acid is replaced by a positive metal or ammoium ion

Define base

a proton acceptor

Define alkali

base that dissolves in water and releases hydroxide ions in aqueous solutions

What is the ester functional group?

-COO

Draw the ester functional group?

What are the reagents for the preparation of an ester fro a carboxylic acid and an alcohol?

• carboxylic acid

• alcohol

What are the conditions needed for a carboxylic acid and alcohol to react?

• concentrated H2SO4

• heat

What catalyst is used to react a carboxylic acid and alcohol?

concentrated sulfuric acid

What is the reaction between a carboxylic acid and alcohol called?

esterification

What type of reaction is esterification?

condensation

Why is esterification a condensation reaction?

a water molecule is lost when the carboxylic acid and alcohol molecules join together to form an ester

What is lost when a carboxylic acid and alcohol react to form an acid?

a water molecule

Is esterification reversible or irreversible?

reversible

Describe the yield of ester when formed from a carboxylic acid and alcohol?

low

Why is the yield of ester when formed from a carboxylic acid and an alcohol low?

the reaction is reversible

How are esters named?

alkyl (from the alcohol) alkanoate (from the carboxylic acid)

What is the name of the ester formed when methanol and propanoic acid react?

methyl propanoate

Draw 2-butyl methanoate

Draw ethyl-2-methyl butanoate

Draw 2-propyl 3-methyl heptanoate

What reagents are needed for the formation of an ester from an acid anhide?

• acid anhydride

• alcohol

What conditions are needed for the reaction between an acid anhydride and alcohol?

• gentle heat

• anhydrous conditions

Draw ethanoic anhydride

Compare the yield of ester from an acid anhydride compared to esterification

higher

Why is the yield of an ester from an acid anhydride greater?

it is irreversible

IS the reaction between an acid anhydride and alcohol reversible?

no

What are the products from the reaction of an acid anhydride and alcohol

• ester

• carboxylic acid

Draw the reaction between propanoic anhydride and methanol

When the acid anhydride is non-symetrical, how many possible products are there in the reaction with an alcohol?

4

Draw the reaction between butanoic anhydride and pental-2-ol

Define hydrolysis

the breaking of a covalent bond by its reaction with water

What happens when an ester is hydrolysed?

the ester group reacts with water

Which bond breaks when an ester is hydrolysed?

C-O sigma bond

Give 2 types of ester hydrolysis

• acid catalysed

• base catalysed

What reagents are needed for acid catalysed hydrolysis?

• aqueous acid

• ester

What are the conditions for acid-catalysed hydrolysis?

heat under reflux

What is formed from acid hydrolysis of an ester?

• carboxylic acid

• alcohol

Is acid hydrolysis of an ester reversible?

yes

What are the reagents for base catalysed hydrolysis?

• aqueous alkali

• ester

What are the conditions for base-catalysed hydrolysis?

heat under reflux

What are the products of base catalysed hydrolysis of an ester?

• carboxylate salt

• alcohol

Is base-catalysed hydrolysis of an ester reversible?

no

Why is base catalysed hydrolysis of an ester irreversible?

formation of the carboxylate salt

What is the acyl chloride functional group?

COCl

Draw the acyl chloride functional group

What is always carbon 1 of an acyl chloride

The carbon that is part of the acyl chloride functional group

How are acyl chlorides named?

-oyl chloride (e.g. ethanoyl chloride)

How are acyl chlorides formed?

from the reaction of a carboxylic acid and SOCl2

What reagents are needed for the formation of acyl chlorides?

• carboxylic acid

• SOCl2

What conditions are needed for the formation of acyl chlorides?

anhydrous conditions

What is the equation for the formation of ethanoyl chloride?

ethanoic acid + SOCl2 → ethanoyl chloride + SO2 + HCl

After the reaction between the carboxylic acid and SOCl2 how is the acyl chloride obtained?

it is warmed gently and separated by distilation

Why must the preparation of acyl chlorides happen in anhydrous condutions?

acyl chlorides react readliy with water

What are anhydrous conditions?

• apparatus completely dry

• ethanoic acid not aqueous

• water not allowed to enter the apparatus during the experiment

How are the openings of apparatus treated to stop water from entering?

they are protected by a drying tube that contains a drying agent (CaCl2)

What will a drying tube contain?

drying agent

What is a drying agent?

an anhydrous inorganic salt that readily takes up water to become hydrated

Give 2 examples of drying agents

• CaSO4

• MgSO4

Why are acyl chlorides useful in synthesis?

they are very reactive and can easily be converted into many different products