Draw the Skeletal Line Structure of Pentyl Butanoate
Part 6. The Chemistry of Carboxylic Acids and their Derivatives
Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry preparation and uses of esters, balanced equations, apparatus, reagents, use as solvents, in perfumes & fragrances
6.8 Esters - preparation, physical and chemical properties and uses including transesterification (interesterification), solvents and perfume fragrances
Sub-index for this page
6.8.1 Methods for preparing of esters - esterification
6.8.2 Physical properties of esters
6.8.3 Hydrolysis of esters with acid or alkali
6.8.4 Transesterification of esters (swapping the acid or alcohol component)
6.8.5 Uses of esters (see also Part 6.14 too, overlap)
For basic IGCSE/GCSE revision notes see Esters, chemistry and uses including perfumes, solvents and more advanced notes and links are on this page for advanced organic chemistry students.
See also 6.9 Natural esters - triglyceride fats and oils, manufacture of margarine and biodiesel
For polyesters see Part 6.10 The manufacture, molecular structure, properties and uses of polyesters
Three questions based on the formulae and nomenclature of esters
Q1. Name the given esters from their skeletal formulae and draw the abbreviated structural formulae
Q2. Name the given ester compounds from their abbreviated structural formulae and draw their skeletal formula
Q3 From the given name draw the skeletal formula and the abbreviated structural formula
INDEX of all carboxylic acids and derivatives notes
All Advanced A Level Organic Chemistry Notes
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6.8.1 Methods of making esters
Reminder: Here, an ester is a chemical compound derived from an organic acid and an alcohol/phenol. in which at least one �OH hydroxyl group of the organic acid is replaced by an �O� alkyl (alkoxy)/aryl group e.g. as in the esterification substitution reaction of a carboxylic acid and an alcohol.
In principle the reaction equations for the two methods described here are:
(a) alcohol/phenol + carboxylic acid ===> ester + water
This reaction is fully described on this page and elsewhere too!
(b) alcohol/phenol + acid chloride ===> ester + hydrogen chloride
This reaction is also described in section 6.7A
Reactions of acid chlorides including with alcohols/phenols
(c) alcohol/phenol + acyl (acid) anhydride ===> ester + carboxylic acid
This reaction is also described in section 6.7B
Reactions of acid/acyl anhydrides including with alcohols/phenols
Naming esters
The first part of the ester name is derived from the alcohol or phenol e.g. methyl, ethyl, phenyl.
The second part of the name is derived from the carboxylic acid e.g. ...oate or ... carboxylate.
Examples are shown in table below in section 6.8.2.
The reverse is 'usually' true when writing out the abbreviated formula, so take care!
See Structure & naming of carboxylic acids and derivatives
Brief descriptions of the preparation methods with equations
(a) Esters from carboxylic acid plus alcohol using an acid catalyst
This method of esterification is also an example of a condensation reaction - two molecules link together to give a larger molecule with the elimination of a small molecule - water in this case.
Concentrated sulphuric acid acts as a catalyst in this reaction.
General word equation for esterification:
carboxylic acid + alcohol == acid catalyst ==> ester + water
e.g. ethanoic acid + ethanol
ethyl ethanoate + water
+
+ H2O
sometimes more simply written as
CH3COOH + CH3CH2OH
The reaction is reversible and the mixture reaches equilibrium, and only about 2/3rds of the carboxylic acid and alcohol have been converted to the ester.
Without a strong acid catalyst e.g. conc. sulfuric acid, the reaction is very slow and the mixture is heated to further increase the rate of reaction (see details of the method below).
See another page for a full description of making an ester like ethyl ethanoate
Its easy to do a simple laboratory demonstration
Mix a few cm3 (~2:1 ratio) of small quantities of ethanoic acid and butan-1-ol with an even smaller volume of concentrated sulfuric acid (6 drops is plenty).
The mixture is gently warmed in beaker of warm-hot water for 5-10 minutes.
Don't overheat in case the boiling point of the ester is less than 100oC.
ethanoic acid + butan-1-ol
CH3COOH + CH3CH2CH2CH2OH
The mixture is then poured into a beaker of sodium hydrogencarbonate solution.
The sodium hydrogencarbonate neutralises the acid catalyst and any unreacted 'smelly' carboxylic acid. The excess of alcohol should dissolves in the water too.
You should get some drops of ester left on the surface which can be carefully smelled to appreciate the aroma of the ester, which should have a fruity smell.
You can do this is as a nice class experiment with various alcohols and ethanoic acid - far less rancid than butanoic acid, as long as you don't mind a super vinegar smell !!!
Notes what students think the esters smells like (likely to be 'fruity') alongside appreciating its molecular structure too!
If you repeat the experiment with butanoic acid (rancid smell at first) and ethanol you get ethyl butanoate which also smells of pineapples and it is used as a flavour enhancer in processed orange juices.
CH3CH2CH2COOH + CH3CH2OH
The mechanism of acid catalysed esterification (simplified)
e.g. the formation of ethyl ethanoate with conc. sulfuric acid as the catalyst
CH3COOH + CH3CH2OH
(i) The acid protonates the carboxylic acid
CH3COOH + H2SO4 ===> [CH3COOH2]+ + HSO4 -
(ii) The protonated carboxylic acid breaks down to give an acylium cation
[CH3COOH2]+ ===> [CH3CO]+ + H2O
(iii) The alcohol molecule donates a pair of electrons to acylium ion, the ester molecule is formed and a simultaneously a proton is released and picked up by the hydrogensulfate ion to reform the sulfuric acid molecule catalyst.
[CH3CO]+ + CH3CH2OH ===> CH3COOCH2CH3 + H+
H+ + HSO4 - ===> H2SO4
(b) Reaction between an acid/acyl chloride and an alcohol
The acyl chloride and alcohol usually readily react at room temperature, especially if both are aliphatic.
Examples of nucleophilic addition of an alcohol to acid/acyl chlorides, followed by elimination to give the ester and hydrogen chloride.
(i) ethanoyl chloride + ethanol ===> ethyl ethanoate + hydrogen chloride
+ CH3-CH2-OH ===>
+ HCl
(ii) ethanoyl chloride + phenol ===> phenyl ethanoate + hydrogen chloride
===>
+ HCl
(iii) pentanoyl chloride + propan-1-ol ===> propyl pentanoate + hydrogen chloride
+
===>
+ HCl
Unlike the acid catalysed esterification described in method (a) with ~67% yield, this reaction gives a very high yield >90%.
For more details and mechanism see 6.7 Preparation & reactions of acid/acyl chlorides
(c) Reaction between an acid/acyl anhydride and an alcohol
(RCO)2O + 2R'OH ===> RCOOR' + RCOOH (R = alkyl or aryl, R' = alkyl)
With ethanoic anhydride, the reaction is called ethanoylation - a particular case of acylation, adding an CH3-C=O group to a molecule.
e.g. ethanoic anhydride + ethanol ===> ethyl ethanoate + ethanoic acid
+
===>
and
pentanoic anhydride + propan-1-ol ===> propyl pentanoate + pentanoic acid
+
+
This reaction is also described in section 6.7B
Reactions of acid/acyl anhydrides including with alcohols/phenols
6.8.2 Physical properties of esters
The lower members of the aliphatic acid esters are colourless liquids at room temperature with relatively low boiling points.
A few of the lower members are quite soluble in water, but most are ~insoluble or sparingly soluble.
Table of abbreviated structural formula and boiling point in oC, plus comments on water solubility and isomerism.
| Table of names, abbreviated structural formula and boiling point in oC. | |||
| Abbreviated formula | Name | Boiling point | Comments |
| 1. HCOOCH3 | methyl methanoate | 31 | Very soluble in water. |
| 2. CH3COOCH3 | methyl ethanoate | 57 | Moderately soluble in water. |
| 3. HCOOCH2CH3 | ethyl methanoate | 54 | Moderately soluble in water, 2. & 3. are isomers. |
| 4. CH3COOCH2CH3 | ethyl ethanoate | 77 | Slightly soluble in water. |
| 5. HCOOCH2CH2CH3 | propyl methanoate | 81 | Slightly soluble in water, 4. and 5. are isomers. |
| 6. CH3CH2CH2COOCH2CH2CH3 | propyl butanoate | 143 | Very slightly soluble in water. |
| 7. CH3CH2COOCH2CH2CH2CH3 | butyl propanoate | 146 | Very slightly soluble in water, 6. and 7. are isomers |
| 8. C6H5COOC6H5 | phenyl benzoate | 314 | Insoluble in water, melting point 71oC. |
Notes and further comments on the information table above.
(i) Ester isomers tend to have similar boiling points and solubility and have similar polarity.
With equal numbers of electrons in the molecule, they have similar intermolecular forces - mainly instantaneous dipole - induced dipole plus permanent dipole - permanent dipole attractive forces.
They are polar molecules, but the boiling points of esters (RCOOR) are much lower than isomeric carboxylic acids, which can hydrogen bond between the RCOOH molecules (R = alkyl or aryl).
There is no hydrogen bonding between ester molecules.
(ii) Despite being polar molecules, they generally have a low solubility in water because they cannot strongly hydrogen bond with water.
The first few lower members are quite or very soluble in water, but, with increasing length of the molecule, too many hydrogen bonds between water molecules are disrupted without compensating solute-solvent intermolecular attractions, so the solubility rapidly decreases with increase in carbon chain length.
The solubility of lower members in water is attributed to their polar nature AND they can accept a hydrogen bond donation from water molecules (often not mentioned in text books).
Hydrogen bond between an ether and water molecule: R2O:δ- llll δ+H-OH
(iii) Alternative names
propyl butanoate can also be called 1-propyl butanoate, to distinguish it from structural isomer
CH3CH2CH2COOCH(CH3)2, which can called 2-propyl butanoate (1-methylethyl butanoate)
Odour - smell - fragrance!
Esters tend to have pleasant odours and are used in flavourings and perfumes and they occur naturally in many plants.
6.8.3 Hydrolysis with acid or alkali - the opposite of esterification
Esters can be hydrolysed by refluxing with strong mineral acids or alkali.
Hydrolysis is very slow with pure water, much faster with dilute mineral acid (HCl(aq) or H2SO4(aq)) and even faster with a strong base (NaOH(aq) or KOH(aq/ethanol).
The reaction is the opposite of esterification.
(a) Hydrolysis of an ester with dilute hydrochloric or sulfuric acid
RCOOR' + H2O
This reaction cannot go to completion because it is a reversible reaction, despite the use of a catalyst and heating the mixture under reflux.
You can go to a high % yield to the right using an aqueous solution with lots of excess water.
e.g. butyl propanoate + water
CH3CH2COOCH2CH2CH2CH3 + H2O
(b) Hydrolysis of an ester with aqueous sodium hydroxide
Hydrolysing an ester with strong alkali e.g. aqueous or ethanolic sodium/potassium hydroxide is called saponification, i.e. its a specific name for a particular type of hydrolysis reaction.
This strong base hydrolysis method is much faster than just water and also faster than acid catalysis.
RCOOR' + NaOH ===> RCOONa + R'OH
RCOOR' + OH- ===> RCOO- + R'OH
e.g. using the same ester as in (a) for this non-reversible reaction ...
e.g. butyl propanoate + sodium hydroxide ===> sodium propanoate + butan-1-ol
CH3CH2COOCH2CH2CH2CH3 + NaOH ===> CH3CH2COONa + CH2CH2CH2CH3OH
CH3CH2COOCH2CH2CH2CH3 + OH- ===> CH3CH2COO- + CH2CH2CH2CH3OH
This reaction is not reversible and goes to 100% completion forming the sodium salt of the acid.
The carboxylic acid is freed by adding excess of a strong hydrochloric or sulfuric mineral acid.
e.g. RCOONa + HCl ===> RCOOH + NaCl or RCOO- + H+ ===> RCOOH
CH3CH2COONa + HCl ===> CH3CH2COOH + NaCl
CH3CH2COONa + H+ ===> CH3CH2COOH
Note on hydrolysis and SOAP making
What is it? How is it made? and it is another use of esters!
'Traditional' soap is a product of the hydrolysis of fats from animals and vegetable oils from plants.
You reflux the fat or vegetable oil with aqueous sodium hydroxide.
A saponification reaction
'Soapy' soaps (not modern 'soapless' detergents) are the sodium salts of long chain fatty acids formed by heating fatty oils with concentrated alkalis like sodium hydroxide or potassium hydroxide to hydrolyse them.
This is known as a saponification reaction and a typical equation is illustrated above and the general word equation quoted below.
vegetable oil/animal fat + sodium hydroxide ==> soap molecule + glycerol
In the example illustrated above, the saponification (hydrolysis) product is the soap sodium palmitate.
The fatty acid can be freed by adding a strong dilute mineral acid (e.g. hydrochloric acid), which produces a waxy precipitate.
simple equation: RCOONa + HCl ===> RCOOH + NaCl
ionically and more correctly: RCOO-(aq) + H+(aq) ===> RCOOH(s)
A case of a strong mineral acid displacing a weaker organic carboxylic acid.
The original ester is a saturated fat found in olive oil, palm oil, and body lipids.
This reaction breaks the fat molecule down into one glycerol molecule (a triol alcohol) and three sodium salts of the long chain carboxylic fatty acids that formed part of the original oil/fat ester.
Sodium stearate is another soap molecule - it is actually a salt of a long chain fatty acid like stearic acid shown below, which is freed by adding dilute strong mineral acid (e.g. hydrochloric acid).
See also 6.9 Natural esters - triglyceride fats and oils, manufacture of margarine and biodiesel
6.8.4 Transesterification of esters (swapping the acid or alcohol component - interesterification)
Since the acid catalysed esterification reaction is reversible, you can actually swap the alcohol or carboxylic acid derived component in the ester molecule.
You gently reflux the ester with another alcohol or another carboxylic acid to effect the transesterification.
So, there are two possibilities e.g.
(a) Swapping the alcohol derived component of the ester
RCOOR' + R"OH
ester 1 + alcohol 1
So this is effectively exchanging the alcohol reactant in the esterification.
Examples
(i) ethyl ethanoate + propan-1-ol
+ 
+
(ii) butyl pentanoate + ethanol
+ 
+ 
The first ester can be named 1-butyl pentanoate.
(b) Swapping the carboxylic derived component of the ester
RCOOR' + R"COOH
ester 1 + carboxylic acid 1
So this is effectively exchanging the carboxylic acid reactant in the esterification.
Examples
(i) propyl pentanoate + butanoic acid
(ii) methyl ethanoate + benzoic acid
+
+
A transesterification reaction is used in the production of margarine and biodiesel
See 6.9 Natural esters - triglyceride fats and oils, manufacture of margarine and biodiesel
6.8.5 The uses of esters
Esters are used in perfumes and flavourings and most are derived from natural sources especially aromatic plants.
Esters are found in essential oils from plants and in animal pheromones.
Solvents may contain esters e.g. nail varnish remover includes some ethyl ethanoate.
Esters such as ethyl ethanoate and butyl ethanoate are used as high quality solvents for a range of materials including plastics, plasticizers, resins, and lacquers and used as synthetic lubricants and plasticizers too.
Esters are polar, but not highly so, so can dissolve a wide range of organic compounds.
Fragrances and food additives
Perfumes are complex mixtures of esters and ketones - blended to a give a particular volatility and aroma.
Many esters are used as flavour enhancers in fruit drink and cakes etc. etc.!
Much of our sensation of taste is actually due to the smell of the food we are eating e.g. odours from esters. When eating, we smell these ester flavours - think of fruity odors/tastes and these odours make the food more appetising.
The flavour molecules such as esters, are sufficiently volatile to reach the olfactory sensory receptors in the nose - food smells good - but note that if you have a heavy cold and blocked nose, your appetite seems decreased and the food doesn't taste as good as the olfactory receptors are block too!
A selection of esters are shown below as skeletal formulae and notes on them including abbreviated structural formulae, their odour and natural sources, though all are synthetically made now, and some of their uses mentioned in the text below as well as above.
I've devised two questions based on the diagram and text below which serve as the answers to the questions.
Examples of fruity fragrances and odours
Most of the molecules illustrated above (skeletal formulae), and listed below, with their abbreviated structural formulae, are used in the food flavouring or perfume industries.
1. CH3COOCH2CH3 ethyl ethanoate, found in pears, used in nail polish remover, paints, glue formulations
2. CH3CH2CH2CH2COOCH2CH3 ethyl pentanoate, found in apples,
3. CH3COOCH2CH2CH(CH3)2 3-methylbutyl ethanoate, found in pears and bananas,
Extra note on the name: On the alkyl group from the original alcohol, the carbon attached to the ester linkage is deemed carbon atom 1, hence the 3-methyl ....
4. CH3CH2CH2COOCH2CH2CH2CH3 butyl butanoate, found in pineapples,
CH3CH2COOCH2CH2CH2CH3 butyl propanoate is used in pear drop sweets.
5. HCOOCH2CH3 ethyl methanoate, found in raspberries,
6. CH3COOCH2C6H5 benzyl ethanoate, found in pears, raspberries and jasmine, its aroma is used perfumes and cosmetics, in food flavouring and giving apple and pear flavours to drinks.
7. CH3CH2CH2COOCH2CH2CH2CH2CH3 pentyl butanoate, found in apricots, pineapples, strawberries and pears
8. C6H5CH=CHCOOCH3 methyl cinnamate, found in strawberries,
9. C6H5COOCH2CH3 ethyl benzoate, found in oil of wintergreen and cherry grapes,
10. CH3CH2CH2COOCH2CH3 ethyl butanoate, found in strawberries, pineapples and bananas,
11. CH3COOCH2CH2CH2CH3 butyl ethanoate, found in apples and honey,
CH3COOCH2CH2CH2CH2CH3 pentyl ethanoate, found in banana odour
CH3COO(CH2)7CH3 octyl ethanoate, is the main contributor to orange odour
12. CH3CH2CH2CH2CH2COOCH2CH3 ethyl hexanoate, found in pineapples and bananas,
13. (CH3)2CHCH2COOCH2CH3 ethyl 3-methylbutanoate, found in apples,
14. CH3COOCH3 methyl ethanoate, used as a solvent e.g. in glue formulations.
Use of esters as plasticisers/plasticizers (note the two spellings e.g. for US/UK)
A plasticizer/plasticiser is a substance that is added to a material like polymers to make a plastic softer and more flexible i.e. to increase its plasticity for the final commercial product or to decrease friction during the processing and manufacture of a plastic materials.
Plasticisers are required because the 'pure' polymer often does not have the desired optimum properties for a particular application e.g. 'pure' PVC is a hard tough polymer that is to inflexible to be used as insulation on electrical cable, mix in a plasticizer and your cable to an appliance is flexible!
Unplasticized PVC is good for gutters, pipes and window frames, but you need plasticized PVC for anything that needs to be flexible ranging from cable insulation to plastic raincoats.
The plasticiser allows polymer chains to move over each other and disrupts close contact making the polymer less crystalline and more softer.
However, over time, the plasticizer make diffuse out and evaporate leaving the polymer more rigid and less flexible, possibly leading to cracking when stressed.
For obvious reasons, the ester molecule must not be too volatile to evaporate from the polymer formulation, so it needs to quite large.
The esters of benzene-1,2-dicarboxylic acid (phthalic acid) shown on the right, have been used extensively in the past, but their use is being restricted due to health concerns.
1. Phthalic acid (benzene-1,2-dicarboxylic acid), reacted with excess alcohol to give the ester 2.
2. A dialkyl ester of phthalic acid, a phthalate ester.
3. Rather than phthalic acid, phthalic anhydride is better to use to make phthalate esters to use as plasticisers.
Examples of phthalate esters used as plasticisers.
4. diethyl phthalate, dibutyl phthalate is also used.
5. di-2-ethylhexyl phthalate,
6. dinonyl phthalate, the alkyl section can also be derived from isomeric alcohols of nonyl alcohol (nonan-1-ol, 1-nonanol) e.g. 7-methyloctan-1-ol
Concerns over the use of phthalate esters as plasticizers.
Research on animals has shown plasticisers, and in particular phthalate esters, are harmful and may cause birth defects and liver damage. Traces of plasticizers can leach out from the sides of fluid container and ingested when drinking the fluid and think about young children sucking/chewing plastic toys! Despite these fears, it is deemed safe to use phthalate plasticizers in products for medical applications e.g. flexible plastic tubing and bags.
Safer plasticiser esters can be made from other dicarboxylic acids e.g. butanedioic acid (1,4-butanedioic acid, butane-1,4-dioic acid, butane-1,4-dicarboxylic acid, old name succinic acid)
7. succinic acid, HOOCCH2CH2COOH,
8. diethyl succinate, CH3CH2OOCCH2CH2COOCH2CH3,
9. dibutyl succinate, CH3CH2CH2CH2OOCCH2CH2COOCH2CH2CH2CH3,
Biofuels are made from the production of methyl esters of fatty acids using transesterification.
Medical uses
The ester 2-methyl hydroxybenzoate is found in 'oil of wintergreen' and is used as a liniment.
It is more soluble in fat than water and is absorbed through the skin, and, like aspirin, has an anti-inflammatory effect - reducing pain and swelling.
Esters crop up in all sorts of pharmaceutical products!
Aspirin (right) is an ester of ethanoic acid - but the derived 'alcohol' is actually a phenol called 2-hydroxybenzoic acid.
Some anaesthetics like benzocaine and procaine are esters.
Benzocaine (left) is the ethyl ester of 4-aminobenzoic acid.
Many anaesthetics ('painkillers') are derived from natural products. Synthetic codeine is made from natural morphine, but neither is an ester. However, the related molecule, natural heroin, is a double ester (two CH3COOR groupings - I think you spot the two ester linkages (and a cyclic ether link and tertiary amine group too!!!).
I've already written a lot on the uses of esters on another webpage for basic GCSE/IGCSE chemistry
and Part 6.14 More on the uses of carboxylic acids and derivatives and occurrence in nature
See also aspirin - another ester!
Q1 Name the following esters from their skeletal formulae and draw the abbreviated structural formulae
(ignore the name of 8 at pre-university level)
ANSWERS
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INDEX of all carboxylic acids and derivatives notes
All Advanced Organic Chemistry Notes
For basic IGCSE/GCSE revision notes see Esters, chemistry and uses including perfumes, solvents and more advanced notes and links are on this page for advanced organic chemistry students.
Q2. Name the following ester compounds from their abbreviated structural formulae and draw their skeletal formula
1. CH3COOCH2CH3
2. CH3CH2CH2CH2COOCH2CH3
3. CH3COOCH2CH2CH(CH3)2
4. CH3CH2CH2COOCH2CH2CH2CH3
5. HCOOCH2CH3
6. CH3COOCH2C6H5
7. CH3CH2CH2COOCH2CH2CH2CH2CH3
8. C6H5CH=CHCOOCH3 (ignore name for 8. at pre-university level)
9. C6H5COOCH2CH3
10. CH3CH2CH2COOCH2CH3
11. CH3COOCH2CH2CH2CH3
12. CH3CH2CH2CH2CH2COOCH2CH3
13. ( CH3)2CHCH2COOCH2CH3
14. CH3COOCH3
ANSWERS
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Q3 From the given name draw the skeletal formula and the abbreviated structural formula
1. ethyl ethanoate
2. ethyl pentanoate
3. 3-methylbutyl ethanoate
4. butyl butanoate
5. ethyl methanoate
6. benzyl ethanoate
7. pentyl butanoate
8. C6H5CH=CHCOOCH3 (just do skeletal formula)
9. ethyl benzoate
10. ethyl butanoate
11. butyl ethanoate
12. ethyl hexanoate
13. ethyl 3-methylbutanoate
14. methyl ethanoate
ANSWERS
For basic IGCSE/GCSE revision notes see Esters, chemistry and uses including perfumes, solvents and more advanced notes and links are on this page for advanced organic chemistry students.
For polyesters see Part 6.10 The manufacture, molecular structure, properties and uses of polyesters
Draw the Skeletal Line Structure of Pentyl Butanoate
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