ALKANOATES (ESTERS) Lesson Note
This SS3 Chemistry lesson note covers ALKANOATES (ESTERS) for First Term, Week 2.
Learning Objectives
- By the end of the lesson, students should be able to:
- Define alkanoates
- Write the general formula and structure of alkanoates
- Name simple alkanoates correctly
- Explain preparation of alkanoates by esterification
- State physical and chemical properties of alkanoates
- State uses of alkanoates
Lesson Note
DEFINITION
Alkanoates are a class of organic compounds derived from alkanoic acids. They are commonly called ESTERS. They are formed when the hydrogen of the -OH group in alkanoic acid is replaced by an alkyl group. They forrm a homologous series with general molecular formula of CnH2n+1COOH where n<0. They are sometimes called fatty acids because some of them are found in natural fats and oils.
The IUPAC name of each homologue is obtain by changing the -e ending of the corresponding alkane to -oic
General formula: RCOOR' where R = H or alkyl from acid, R' = alkyl from alcohol.
Functional group: -COO- (alkanoate group)
Example:
O
||
CH3 - C - O - CH2CH3
Ethyl ethanoate
NOMENCLATURE
Alkanoates are named as Alkyl Alkanoate
First name comes from alkyl group from alcohol (R')
Second name comes from alkanoic acid (RCOO)
Rule: Change -oic acid to -oate.
Examples:
- HCOOCH3 - Methyl methanoate
- CH3COOCH3 - Methyl ethanoate
- CH3COOC2H5 - Ethyl ethanoate
- C2H5COOC2H5 - Ethyl propanoate
- CH3CH2CH2COOCH3 - Methyl butanoate
ESTERIFICATION
The formation of an ester by the reaction between an alkanol and an acid is known as Esterification. This reaction is extremely slow and reversible at room temperature and is catalyzed by a high concentrationof hyrogen ion.
Equation:
Alkanoic acid + Alkanol --conc.H2SO4--> Alkanoate + Water
Example:
CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O
Ethanoic acid + Ethanol ⇌ Ethyl ethanoate + Water
Conc. H2SO4 acts as dehydrating agent and catalyst.
PHYSICAL PROPERTIES
a. Lower alkanoates are colourless liquids with sweet, fruity, pleasant smell.
b. Slightly soluble in water, solubility decreases as molecular mass increases.
c. Soluble in organic solvents.
d. Lower boiling point than corresponding alkanoic acids (no H-bonding between molecules).
e. Lower alkanoates are neutral to litmus.
CHEMICAL PROPERTIES
a. Hydrolysis:
i. Acid Hydrolysis: Reversible, gives acid + alcohol
RCOOR' + H2O --H+--> RCOOH + R'OH
ii. Alkaline Hydrolysis (SAPONIFICATION): Irreversible, gives salt of acid + alcohol
RCOOR' + NaOH → RCOONa + R'OH
This is used for soap production. Example: Hydrolysis of fat (glyceryl alkanoate).
b. Reduction: Ethyl ethanoate is reduced to ethanol by hydrogen from strong reducing agents like Lithium tetrahydrido aluminate (iii)
USES OF ALKANOATES
1. As flavouring in confectioneries and soft drinks (fruity smell)
2. As perfumes and cosmetics
3. As solvents for paints, varnishes, lacquers
4. In production of artificial fibres e.g. Terylene
5. For making nail varnishes
6. As plasticizers
7. In soap making (saponification of fats and oils which are esters)
Evaluation
OBJECTIVE QUESTIONS: Choose the correct option A-D
Alkanoates are also called
A. Ethers
B. Esters
C. Aldehydes
D. Ketones
The general formula for alkanoates is
RCOOH
B. RCOOR’
C. ROH
D. ROR’
The functional group of alkanoates is
-OH
B. -COOH
C. -COO
D. -CHO
Ethyl ethanoate is prepared from
Ethanol + Ethanoic acid
B. Ethanol + Methanoic acid
C. Ethanol + Propanoic acid
D. Methanol + Ethanoic acid
The catalyst used in esterification is
Conc. HCl
B. Conc. HNO3
C. Conc. H2SO4
D. NaOH
Alkaline hydrolysis of alkanoate is known as
Esterification
B. Saponification
C. Polymrization
C. Fermentation
Which of the following alkanaotes is methyl methanoate?
HCOOCH3
B. CH3COOCH3
C. HCOOC2H5
D. C2H5COOCH3
Alkanoates have _______ smell
Pungent
B. Fruity/Sweet
Rotten egg
No smell
An example of a natural alkanoates is
Table salt
C. Soap
C. Fat and Oil
D. Sugar
When ethyl propanoates is hydrolyze with NaOH, the products are
Propanoic acid + Ethanol
B. Sodium propanoate + Ethanol
C. Sodium ethanoate + Propanol
D. Propanol + Ethanol
Theory Questions
1a. Define alkanoate an give its general formula
Write structural formula and IUPAC name of an alkanoate with molecular formula C3H6O2
Describe with equation how ethyl ethanoate can be prepared in the laboratory
2a. Give equation of acid hydrolysis of methyl propanoate
State three physical properties of Alkanoates and its uses