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Part 6.
The Chemistry of Carboxylic Acids and their Derivatives
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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
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All my revision notes on the
chemistry of carboxylic acids and derivatives
All my advanced A level organic
chemistry revision notes
Index of GCSE level oil and basic organic chemistry notes
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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
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
CH3COOCH2CH3 + H2O
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
butyl ethanoate + water
CH3COOH
+ CH3CH2CH2CH2OH
CH3COOCH2CH2CH2CH3
+ H2O
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
CH3CH2CH2COOCH2CH3
+ H2O
The mechanism of acid catalysed
esterification (simplified)
e.g. the formation of ethyl
ethanoate with conc. sulfuric acid as the catalyst
CH3COOH + CH3CH2OH
CH3COOCH2CH3 + H2O
(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
TOP OF PAGE
and sub-index
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-llllδ+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.
TOP OF PAGE
and sub-index
 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
RCOOH + R'OH
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
propanoic acid + butan-1-ol
CH3CH2COOCH2CH2CH2CH3
+ H2O
CH3CH2COOH + CH2CH2CH2CH3OH
(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
TOP OF PAGE
and sub-index
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
RCOOR" + R'OH
ester 1 + alcohol 1
ester 2 + alcohol 2
So this is effectively exchanging the
alcohol reactant in the esterification.
Examples
(i)
ethyl ethanoate +
propan-1-ol
propyl ethanoate + ethanol
+
+
(ii)
butyl pentanoate +
ethanol
ethyl pentanoate + butan-1-ol
 +
+
The first ester can be named 1-butyl
pentanoate.
(b) Swapping the
carboxylic derived component of the ester
RCOOR' +
R"COOH
R"COOR' + RCOOH
ester 1 + carboxylic
acid 1
ester 2 + carboxylic acid 2
So this is effectively exchanging the
carboxylic acid reactant in the esterification.
Examples
(i)
propyl pentanoate +
butanoic acid
propyl butanoate + pentanoic acid
+
+
(ii)
methyl ethanoate +
benzoic acid
methyl benzoate + ethanoic 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
TOP OF PAGE
and sub-index
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
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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