Part 5. Carbonyl chemistry of
ALDEHYDES & KETONES
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
Part 6.2
The physical properties of aldehydes and ketones - intermolecular forces,
physical state, boiling points, solubility and odour!
Sub-index for this page
5.2.1
The boiling points of aldehydes
and ketones
5.2.2
The solubility of aldehydes and
ketones
5.2.3
The odours of aldehydes and
ketones
INDEX of ALDEHYDES
and KETONES revision notes
All Advanced A Level Organic
Chemistry Notes
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5.2.1
The boiling points of aldehydes and ketones
Structure reminder
Abbreviations: bpt = boiling point 0oC),
na = not applicable, and get used to:
CO representing C=O in
abbreviated carbonyl compound formulae for ketones
and CHO representing H-C=O in abbreviated carbonyl
compound formulae for aldehydes.
and note butanone is a simpler and unambiguous name
for butan-2-one, but you need the latter if there is a substituent atom
or group on one of the non carbonyl carbon atoms in the chain.
A comparison of isomeric aldehydes
and ketones (all
their full structures are
on a separate page)
name of aldehyde |
methanal |
ethanal |
propanal |
butanal |
pentanal |
formula of aldehyde |
HCHO |
CH3CHO |
CH3CH2CHO |
CH3CH2CH2CHO |
CH3CH2CH2CH2CHO |
bpt of aldehyde/oC |
-21 |
21 |
49 |
76 |
103 |
bpt of ketone/oC |
na |
na |
56 |
80 |
102 |
formula of ketone |
na |
na |
CH3COCH3 |
CH3COCH2CH3 |
CH3COCH2CH2CH3 |
name of ketone |
na |
na |
propanone |
butanone |
pentan-2-one |
bpt of alkanes
with similar number of electrons |
-88 |
-42 |
0 |
36 |
69 |
You can see from the table that, for the
same carbon chain number, the boiling points of aldehydes and ketones are
quite similar and both follow a steady trend of increasing boiling point
with increase in carbon length.
Note the boiling point of
methylpropanal
is 62, lower than its structural isomer butanal
because it is a more compact molecule, which reduces the surface to
surface intermolecular contact forces - the instantaneous dipole -
induced dipole forces.
Similarly the more compact methylbutanone
boils at 94oC, a bit lower than pentan-2-one
.
Simple aliphatic
aldehydes and ketones are colourless gases, liquids or white solids for high
members in the series. Pure
methanal is a gas at room temperature, ethanal is a very volatile liquid,
the rest of the lower members are liquids.
All lower members of the ketone homologous series are
liquids. They are all polar
molecules because of the presence of the permanent polar bond, the
δ+C=Oδ-
dipole.

and
 extra
δ+ δ-
attractions
You can see that the polar bond adds a
significant extra permanent dipole - permanent dipole attraction to the
total intermolecular bonding force.
The boiling point trend of
linear aldehydes are now discussed in detail and compared
with other homologous series.
Graph 1
grey purple line = aldehydes
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.
For the
'purple
line of linear aldehydes',
the graph goes from methanal HCHO to
decanal CH3(CH2)8CHO.
It is a similar graph line for methyl
ketones (2-ones, RCOCH3, R = alkyl), but I haven't plotted the
data (yet!).
So, in this discussion we are comparing the red line
(linear alkanes) with the purple line (linear aldehydes) AND
comparing molecules with the same 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
functional groups.
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, weak, but not insignificant forces,
known as instantaneous dipole - induced dipole forces.
From Graph 1 you can see
the effect of the
permanently polar oxygen - hydrogen bond (Cδ+=Oδ-)
increases the intermolecular forces of attraction, and raising the boiling point compared to non-polar molecules
of similar size in terms of numbers of electrons (clouds).
The bond is highly polar because
of the great difference in electronegativity (carbon 2.5, oxygen 3.5).
Even so, for most polar molecules, the
majority of the intermolecular force is still due to the instantaneous
dipole - induced dipole attractions.
Total intermolecular force =
(instantaneous dipole – induced dipole) + (permanent dipole – permanent dipole) +
(permanent dipole – induced dipole)
For propanone (14.2% instantaneous dipole
–
induced dipole force) + (78.4% permanent dipole – permanent dipole) + (7.4%
permanent dipole – induced dipole)
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.
However, because
aldehydes and ketones cannot hydrogen bond with themselves,
their boiling points are still less than hydrogen bonded
alcohols - check out
yellow line for linear primary alcohols on
Graph 1 above..
Graph 2
grey purple line = aldehydes
A plot of the molecular mass
of the linear aldehyde 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.
For a similar molecular mass,
the aldehydes have significantly higher boiling points
than alkanes,
mainly due to the extra force of the permanent dipole -
permanent dipole attraction.
Graph 3
grey purple line = aldehydes
A plot of the carbon number
of the linear aldehyde molecule versus its boiling point (K) shows a steady rise
with a gradually decreasing gradient.
For the same carbon number,
the aldehydes have significantly higher boiling points
than alkanes,
mainly due to the extra force of the permanent dipole -
permanent dipole attraction
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.
5.2.2
The solubility of aldehydes and ketones
How soluble in water?
Aldehydes and ketones are highly polar molecule and
readily bond with water molecules via permanent dipole - permanent dipole
intermolecular forces.
(aldehyde/ketone)
δ+C=Oδ-
llll δ+H-Oδ-
(water) hydrogen bond (llll)
Note that aldehydes and
ketones do not hydrogen bond with themselves, but they can hydrogen bond with
water.
The hydrogen bonding with water enables the lower
aldehydes/ketones to dissolve in water, but after that, the solubility of
them rapidly decreases.
So 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 'hydrocarbon' chain length makes the
carbonyl compound 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 alcohol, without
compensating aldehyde/ketone - water hydrogen bonds.
You can also argue that the instantaneous dipole -
induced dipole forces between the hydrocarbon chain of neighbouring
aldehyde/ketone molecules is stronger than a hydrogen bond, so the longer chain
molecules will come together.
As the carbon chain gets longer, the solubility
decreases, because the intermolecular bonding described above, becomes less
compensated for as more water - water hydrogen bonds are disrupted. Consequently, methanal,
ethanal and propanone are very soluble.
Propanal and butanone are quite soluble.
Butanal, pentan-2-one, pentan-3-one are slightly soluble.
Use
as solvents themselves
Aldehydes are not used as solvents - unpleasant and can be harmful if
inhaled. The lower ketones are like
propanone and butanone are very useful solvents because they dissolve a wide
variety of organic compounds.
5.2.3 The odours of aldehydes and ketones!
When first studying organic
chemistry at an advanced level, the odour of the compounds is
rarely mentioned apart from the pungent 'hydrocarbon' smell of
alkanes, alkenes and aromatic hydrocarbons.
Aldehydes and ketones also
have strong smelling odours e.g.
Ethanal, like other lower
aldehydes, has very strong
pungent unpleasant odour, it is also harmful to breathe in - a
known carcinogen - and note it is produced in one of the
metabolic pathways after drinking an alcoholic beverage!
Propanone and other lower
ketones have moderately
pleasant 'sweeter' odour than aldehydes (like 'acetone' nail varnish remover)
The aromatic aldehyde or benzaldehyde, C7H6O,
has the smell of almonds and used in flavouring food - I love thick
layer of marzipan between a thin slice of sugar icing and a thick layer
of rich Christmas cake! Many more complex molecules
containing an aldehyde group or ketone group occur in essential oils from plants, often
giving, or contributing to, their characteristic odour e.g. citral and
carvone whose skeletal formulae are shown below.
A Citral
(lemonal), C10H16O,
is an aldehyde with all the usual characteristic reactions of an R-CHO
molecule.
Citral also has two alkene groups (C=C) as well as the
aldehyde group.
It is found in several species of lemon plants (e.g. lemon grass oil) and contributes to the
strong citrus 'lemon-like' odour of the fruit.
Citral can exhibit E/Z stereoisomerism via the top C=C bond (geometric
isomers).
Citral cannot exhibit R/S stereoisomerism - no chiral carbon.
Note 'al' in the name 'citral'.
B
Carvone, C10H14O, is a cyclic ketone found in
spearmint and caraway oil.
It has one ketone group and behaves as a ketone, but it
also two C=C double bond groups, so is 'diene'. Both
citral and carvone behaves as a 'double' alkene, reacting quantitatively
with two molecules of bromine.
They are also both a good
test in reading skeletal formulae - match them up with the molecular
formulae!
Carvone cannot exhibit E/Z stereoisomerism via the top C=C bond
(geometric isomers) - one C=C has two identical end groups and the top
left C=C is part of the ring.
Citral can exhibit R/S stereoisomerism - it has a chiral carbon - the
bottom one of the hexagonal ring - see diagram on right for the
non-superimposable mirror image forms.
Note 'one' in the name 'carvone'.
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INDEX of ALDEHYDE
and KETONE revision notes
All Advanced Organic
Chemistry Notes
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