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
Part 6.2
The physical properties of carboxylic acids - odours, melting points, boiling points and
solubility
Sub-index for this page
6.2.1
Examples of the
melting/boiling points, solubility and smell of carboxylic acids - general
trends discussed and explained
6.2.2
More
details on the boiling
point trend of linear monocarboxylic acids
6.2.3
More details on the solubility of carboxylic acids
INDEX of all
My carboxylic
acids
and derivatives notes
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Chemistry Notes
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6.2.1
DATA TABLE: Examples of the melting points,
boiling points and solubility of carboxylic acids
Abbreviations used: mpt =
melting point; bpt = boiling point; dec = thermally
decomposes
Physical state at room temperature
Monocarboxylic acids are colourless
liquids or white waxy solids.
All aromatic carboxylic acids are crystalline solids at room
temperature.
The odour of carboxylic acids
Aromatic carboxylic acids have
relatively faint odours, but aliphatic monocarboxylic acids all have
strong odours - think of the dilute aqueous solution of ethanoic acid,
known as vinegar! Butter turns
and smells rancid because butanoic acid is formed by the action of
bacteria on butter fat.
(a) Names of
linear monocarboxylic acids |
(b)
Abbreviated formula |
(c) oC melting
point |
(d) oC boiling
point |
(e) Solubility g/100g water at
20oC |
(f) Electrons in molecule |
(g) Old trivial name |
1. Methanoic acid |
HCOOH |
8 |
104 |
miscible |
24 |
Formic acid |
2. Ethanoic acid |
CH3COOH |
17 |
118 |
miscible |
32 |
Acetic acid |
3. Propanoic acid |
CH3CH2COOH |
2-1 |
141 |
miscible |
40 |
Propionic acid |
4. Butanoic acid |
CH3(CH2)2COOH |
-6 |
164 |
miscible |
48 |
Butyric acid |
5. Pentanoic acid |
CH3(CH2)3COOH |
-35 |
186 |
5.0 |
56 |
Valeric acid |
6. Hexanoic acid |
CH3(CH2)4COOH |
-2 |
205 |
1.1 |
64 |
Caproic acid |
7. Heptanoic acid |
CH3(CH2)5COOH |
-8 |
223 |
0.24 |
72 |
? |
8. Octanoic acid |
CH3(CH2)6COOH |
17 |
240 |
0.068 |
80 |
Caprylic acid |
9. Nonanoic acid |
CH3(CH2)7COOH |
13 |
254 |
0.03 |
88 |
Pelargonic acid |
10. Decanoic acid |
CH3(CH2)8COOH |
32 |
269 |
0.015 |
96 |
Capric acid |
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(a) Names of
dicarboxylic acids |
(b)
Abbreviated formula |
(c) oC melting
point |
(d) oC boiling
point |
(e) Solubility g/100g water |
(f) Electrons in molecule |
(g) Old trivial name |
Ethanedioic
acid |
HOOCCOOH |
190 dec. |
dec. |
9.5 |
46 |
Oxalic acid |
Propanedioic
acid |
HOOCCH2COOH |
136 dec. |
dec. |
7.3 |
54 |
Malonic acid |
Butanedioic acid |
HOOCCH2CH2COOH |
182 |
235 dec. |
5.8 |
62 |
Succinic acid |
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(a) Names of
aromatic acids |
(b)
Abbreviated formula |
(c) oC melting
point |
(d) oC boiling
point |
(e) Solubility g/100g water |
(f) Electrons in molecule |
(g) Old trivial name |
Benzoic acid |
C6H5COOH |
122 |
249 |
0.29 |
64 |
Benzoic acid |
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Notes on the above data table
(a)
Names
The first four monocarboxylic acids
have retained their trivial names, but after that the name uses pent,
hex, hept ... anoic acid for the systematic name.
For more details see
Molecular structure and nomenclature of
carboxylic acids and derivatives
(b) Structure
For more details see
Molecular structure and nomenclature of
carboxylic acids and derivatives
(c) Melting points
No obvious pattern with the linear
monocarboxylic acids, you don't get a systematic trend as you do with
boiling points i.e. a steady increase with increase in electrons in the
molecule with increase in carbon chain length.
The melting points of linear aliphatic
monocarboxylic acids
One reason for this lack of obvious trend is the
ability of carboxylic acids to form 'dimers' via hydrogen bonding
between two carboxylic acid group - illustrated in the diagram above.
This is especially so with the
lower members of the aliphatic monocarboxylic acids like methanoic
acid, ethanoic acid. It is
not a true dimer in the sense that the two molecules are covalently
bonded together, but the hydrogen bonding is strong enough to hold
them together, not only in the crystalline state, but when liquid or
dissolved in most solvents other than water.
This effectively doubles the size of the
molecules and roughly doubles the instantaneous dipole - induced
dipole intermolecular forces between the neighbouring dimers.
A greater kinetic energy is needed to melt the
'dimer' compared to the 'monomer', hence the higher than expected
melting points for the lower members of the series.
However, as the hydrocarbon chain length
increases, the hydrogen bonding between he carboxylic acid groups is
increasing hindered.
Decanoic acid is the first member of this homologous
series to be a solid at room temperature.
It is a white solid with a
strong rancid odour - they all have strong odours!
Think of the smell of pure ethanoic acid -
100% concentration of vinegar!
Higher members do follow a more systematic
trend of increasing melting point as the instantaneous dipole -
induced dipole forces dominate much more than the hydrogen
bonding.
The melting points of aromatic
carboxylic acids. Benzoic
acid,
(64 electrons), mpt 122oC, and propylbenzene
(66 electrons), mpt -100oC.
Aromatic acids also form hydrogen bonded dimers,
which raises the melting point e.g. comparing the melting points of two
aromatic molecules with similar numbers of electrons in the molecule.
The benzoic acid 'dimer' has a much higher melting
point than the aromatic hydrocarbon propyl benzene.
Isomer differences
(i) CH3CH2CH2COOH,
butanoic acid melts at -6oC and boils at 164oC.
(ii) Isomeric (CH3)2CHCOOH,
2-methylpropanoic acid melts at -47oC and boils at 154oC.
(ii) is a more compact molecule with less surface
to surface contact with neighbouring molecules, so the instantaneous
dipole - induce dipole forces are reduced - lower kinetic energies
and enthalpies required to effect the change of state.
(d) Boiling points
of linear monocarboxylic acids
The boiling points of aromatic
acids. The boiling points
show a steady trend of increase with increase in length of the carbon
chain of the molecule (and increase in total electrons).
The boiling points of linear monocarboxylic acids are discussed in
detail in 6.2.2
The boiling points of aromatic acids.
Benzoic acid,
(64 electrons), mpt 122oC, and propylbenzene
(66 electrons), mpt -100oC.
Aromatic acids also form hydrogen bonded dimers,
which raises the melting point e.g. comparing the melting points of two
aromatic molecules with similar numbers of electrons in the molecule.
The benzoic acid 'dimer' has a much higher melting
point than propyl benzene.
(e) Solubility in water
For the linear monocarboxylic linear
aliphatic acids you get a significant decrease in solubility as the
hydrophobic carbon chain gets longer.
However, the solubility of lower members is enhanced
by hydrogen bonding with water.
The solubility of carboxylic acids is discussed in
detail in
6.2.3
(f) Electrons in molecule
Equals the sum of the
atomic numbers in the molecular formula.
TOP OF PAGE
and sub-index
6.2.2
The boiling point trend of linear monocarboxylic acids and
intermolecular forces
Lower in this homologous series
are colourless liquids. Higher
members of the series are white coloured waxy solids.
Here the monocarboxylic acid series discussed is equivalent
to:
CnH2n+1COOH
where n = 0, or a more structurally correct general formula for linear
aliphatic monocarboxylic acids is HCOOH, then CH3(CH2)nCOOH,
where n = 0, 1, 2 etc. which I refer to as '1-ols' primary alkanols
The boiling point trend of
linear monocarboxylic acids are now discussed in detail and compared
with other homologous series.
Graph 1
purple line = RCOOH
The red line graph shows the boiling point of
alkanes from methane CH4 (boiling point -164oC/109 K)
to tetradecane C14H30 (boiling point 254oC/527 K).
[Remember K = oC + 273]
Note:
The red line represents linear alkanes in all
the graphs 1-3 and is a useful baseline to compare the intermolecular
bonding present in other homologous series of non-cyclic aliphatic compounds.
So, in this discussion we are comparing the red line
(linear alkanes) with the linear
aliphatic monocarboxylic molecule AND
comparing molecules with a similar number of electrons.
A plot of number of electrons in
any molecule of a homologous series versus its boiling point (K) shows a steady rise
with a gradually decreasing gradient.
I consider this the best for
comparison of the effects of intermolecular bonding between different
homologous series i.e. carbon chains with a different end functional group.
REMINDER: Intermolecular forces are all about
partially positive (δ+)
sites and partially negative
(δ–)
sites on molecules causing the attraction between neighbouring molecules -
though their origin can differ.
I think Graph 1 is the best graph to look at
the relative effects on intermolecular forces (intermolecular bonding) on
boiling point because it is the distortion of the electron clouds (e.g. in
non-polar alkanes), that gives rise to these, usually weak compared to covalent
bonds, but not insignificant forces,
known as instantaneous dipole - induced dipole forces.
BUT, from Graph 1 you can see
the effect of the
permanently polar oxygen - hydrogen bond (Hδ+-Oδ-)
increases the intermolecular forces of attraction between carboxylic acid
molecules, and raising the boiling point compared to non-polar molecules
of similar size in terms of numbers of electrons (clouds).
As already mentioned in 6.2.1, hydrogen
bonding between the carboxylic acid groups cause the formation of dimers.
However, unlike the lack of a clear melting
point trend, the boiling points do follow the expected rising trend as the
carbon chain gets longer.
The reason being, on melting, and with
increasing rise in temperature, more and more of the hydrogen bonds of the
'dimer' molecules are broken. So,
by the time the liquid boils, most of the hydrogen bonding is randomised
between the molecules, without dimer formation - a similar situation to the
hydrogen bonding in liquid alcohols.
Don't forget, the hydrogen bonding is in addition to
the intermolecular attractive force compared to non-polar molecules.
Even so, for most polar molecules, the
majority of the intermolecular force is still due to the instantaneous
dipole - induced dipole attractions.
The hydrogen bond is directional i.e. the proton
lines up with the lone pair on the oxygen which is effectively the delta
minus and this should come out in a full diagram showing the
hydrogen bonding between molecules.
Total intermolecular force =
(instantaneous dipole – induced dipole) + (permanent dipole – permanent dipole
including hydrogen bonding) +
(permanent dipole – induced dipole)
The increase in
intermolecular attractive forces, means the molecules need a
higher kinetic energy to overcome the intermolecular forces
and escape from the liquid surface, so they
have a higher boiling point and increased enthalpy of
vapourisation compared to alkanes.
For a broader discussion see
on boiling points and intermolecular forces see:
Introduction to Intermolecular Forces
Detailed comparative discussion of boiling points of 8 organic molecules
Boiling point plots for six
organic
homologous series
and for wider reading on
intermolecular bonding forces
Other case studies of
boiling points related to intermolecular forces
Evidence and theory
for hydrogen bonding in simple covalent hydrides
Graph 2 purple line = RCOOH
A plot of the molecular mass
of the linear aliphatic monocarboxylic molecules versus its boiling point (K) shows a steady rise
with a gradually decreasing gradient.
More atoms, more electron
clouds, more chance of instantaneous dipole - induced dipole
forces, so the overall intermolecular force steadily increases
with carbon number, the hydrogen bonding is a fairly constant
contribution.
Graph 3 purple line = RCOOH
A plot of the carbon number
of the linear aliphatic monocarboxylic molecule versus its boiling point (K) shows a steady rise
with a gradually decreasing gradient.
For the same carbon number,
the primary alcohols have significantly higher boiling points
than alkanes,
mainly due to the hydrogen bonding.
The increase in
intermolecular attractive forces, means the molecules need a
higher kinetic energy to escape from the liquid surface i.e.
have a higher boiling point.
TOP OF PAGE
and sub-index
6.2.3 The solubility of
carboxylic acids
Reminder: Alcohols
are permanently polarised molecule due to the highly polar bond
δ–O–Hδ+ caused by the difference in electronegativities
between oxygen and hydrogen i.e. O (3.5) > H (2.1). This causes the extra
permanent dipole – permanent dipole interaction between neighbouring polar
molecules via
hydrogen bonding
The intermolecular hydrogen bonding in water
Before looking at the solubility of carboxylic acids, a reminder
of the hydrogen bonding in water via the above diagram.
Important note, especially when drawing
hydrogen bonding
diagrams for any molecule! You must clearly
show the directional linearity of the
Oδ--Hδ+ǁǁǁ:Oδ-
arrangement of the hydrogen bond including the single O-H covalent bond
and the lone pair on the other oxygen too!
You must do this accurately in exams
when drawing intermolecular hydrogen bonding diagrams of water, alcohols
and carboxylic acids, because it is the only specifically
spatially directed intermolecular force, all the rest of the
other types of intermolecular bonding forces are randomised.
The lower monocarboxylic acids exist as
dimers in the vapour and pure liquid phases and when dissolved in non-polar
solvents like benzene of hexane.
However, in aqueous solution, carboxylic acids can also hydrogen bond
directly with water - which accounts for why the lower members are so
soluble in water.
Although water - water hydrogen bonds
are disrupted (Oδ--Hδ+ǁǁǁ:Oδ-),
new carboxylic acid - water hydrogen
bonds are formed e.g. (C-Oδ--Hδ+ǁǁǁ:Oδ--Hδ+)
OR (H-Oδ--Hδ+ǁǁǁ:Oδ-=Cδ+) partly compensate for this.
(ǁǁǁ
hydrogen bond)
BUT, there are limits to this effect,
looking at the diagram below, only the first four linear carboxylic
acids are
completely soluble (miscible) in water.
The hydrogen bonding with water enables the first
four carboxylic acids to be miscible with water (completely soluble in each other,
irrespective of proportions), but after that, the solubility rapidly decreases.
BUT, we need to consider solvent - solvent,
solute - solute and solute - solute interactions in terms of intermolecular
bonding attractive forces to explain this trend.
An increase in the length of the
hydrophobic 'hydrocarbon' chain makes the carboxylic acid less
and less able to disrupt hydrogen bonding.
The longer the hydrocarbon
chain, the more water - water hydrogen bonds must be disrupted to dissolve
the carboxylic acid, without
compensating alcohol - water hydrogen bonds.
You can also argue that the instantaneous dipole -
induced dipole forces between the hydrocarbon chain of neighbouring
carboxylic acid
molecules is stronger than the hydrogen bond, so the longer chain alcohol
molecules will come together.
Benzoic acid, an aromatic carboxylic acid, is much
more soluble in hot water than cold water and can be purified and
recrystallised in this way using water as the solvent.
TOP OF PAGE
and sub-index
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INDEX of all
My carboxylic
acids
and derivatives notes
All My Advanced A Level Organic
Chemistry Notes
Index of My GCSE/IGCSE Oil - useful products
and basic organic chemistry notes
TOP OF PAGE
and sub-index
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