Aldehydes and Ketones
Naming aldehydes
Establish parent name based on longest chain with aldehyde
included.
Drop the suffix –e from the parent chain and add –al to
indicate an aldehyde.
Aldehydes are ALWAYS on the end of a molecule because of
the terminal hydrogen, so you DO NOT need to tell the
position of the aldehyde. It is assumed to be 1.
Other branches are named and numbered just like before.
Practice
1
2
3
Priority of groups for
IUPAC nomenclature
We have pretty much learned how to name
molecules in their order of priority…this means that
each new type of nomenclature is the highest
priority.
You will need to refer to the sheet I am handing out
to name molecules that are lower priority.
For example, if you have a molecule with an alcohol
and aldehyde, the aldehyde takes priority, so the
branch term for the alcohol is hydroxy-
Common aldehydes
4.jpg
5.jpg
Naming Ketones
Identify the longest continuous chain with the ketone
present.
Use the parent name but drop the suffix –e and add –one.
If the chain is longer than 4 carbons, it is numbered so that
the ketone has the smallest number possible.
Other branches/groups are named as we have done before.
Ketones are not branched… they have too high of a priority.
Bonding in Aldehydes
and Ketones
The carbon atom of the carbonyl
(C=O), is sp2 hybridized. This
means there is a sigma (s
overlap) and a pi (p overlap)
bond present.
The carbonyl group itself is polar
due to the high electronegativity
of oxygen with respect to
carbon. This create a dipole
moment, and it is this
characteristic that dictates a lot
of the reactivity and
functionality.
Bonding and Physical
Properties
They CANNOT
hydrogen bond to
themselves! This is due
to the fact that no
hydrogen is directly
attached to an oxygen
atom.
Therefore, they have
lower boiling points
than alcohols of similar
molar masses.
Solubility of aldehydes
and ketones decreases
markedly after
lengthening the carbon
chain to 5 or more.
Low molar mass
aldehydes have a
rancid, permeable odor
and are responsible for
the bad taste and smell
of rotten foods.
Fragrances
It is high molar mass
and aromatic
aldehydes and ketones
that are especially
fragrant.
These molecules are
used widely in the
flavoring and perfume
industries.
Stereoisomers
Stereoisomers are isomers
that have the same element
in relatively the same order,
but they are still different
in their orientation in
space.
Optical isomers are one
form of stereoisomers
because of the differences
in the way they react with
plane polarized light.
Simple substances
composed of two
stereoisomers are called
enantiomers.
The enantiomer that
rotates plane polarized
light in a clockwise manner
is a (+) enantiomer.
The enantiomer that
rotates plane polarized
light in a counter clockwise
manner is a (-).
Enantiomers
Substances that consist
of a mixture in equal
proportions of these
enantiomers are called
racemic mixtures.
Enantiomers have the
same physical
properties, but they
are mirror images of
each other.
Biologically, the R and
S may not both have
the same activity.
It is difficult to
separate enantiomers,
but luckily, the nonactive ones are not
harmful. They are
merely inactive.
Chemical Reactions of
Aldehydes and Ketones
Oxidation:
Aldehydes are easily
oxidized to carboxylic
acids. Reactants like
potassium dichromate in
sulfuric acid (K2Cr2O7 +
H2SO4), and even milder
oxidizing agents like Cu2+ or
Ag+.
Most ketones do not
undergo oxidation, so this
is a way to differentiate
The Tollens test (silver
mirror test) for
aldehydes is based on
the ability of silver ions
to oxidize them.
A silver mirror is
deposited on the inside
of the test tube when
an aldehyde is added
to silver nitrate and
ammonia.
Other Aldehyde tests
Fehling and Benedict testsThese solutions contain Cu2+
ions in a basic solution. They
oxidize the aldehyde to an
acid and the blue ions (Cu2+)
are reduced to Cu2O, a brick
red precipitate.
The Benedict test is
commonly used to detect
carbohydrates (which often
have an aldehyde group).
Many aldehydes and
ketones are used in
industry because they are
easily and cheaply made
from petroleum, natural
gas and oxygen in the air.
Simple hydrocarbons are
oxidized to form
acetaldehyde, which can be
further oxidized to acetic
acid.
Reduction Reactions
Both aldehydes and
ketones are easily reduced
to alcohols.
This reaction is
accomplished by elemental
hydrogen in the presence
of a catalyst or by reducing
agents like, LiAlH4 or
NaBH4).
Aldehydes make primary
alcohols and ketones make
secondary alcohols.
Reduction reactions
Biochemical processes
Addition Reactions
Aldehydes and ketones can be added to alcohols to
make hemiacetals and hemiketals.
Hemiacetal- 1 alkoxy (ether) and 1 hydroxy (alcohol)
group off the same carbon
Aldehydes react with alcohols in a small amount of
acid to make hemiacetals.
Acetals and Ketals
To make acetals or ketals, 2 alkoxy (ether) groups are
on the same carbon. If the carbon has another alkyl
group and a hydrogen, it is an acetal. If it has 2 other
alkyl groups, it is a ketal.
These substances are unstable in acidic solutions, but
stable in basic solutions.
Can you circle the
hemiacetal in each?
Addition of Hydrogen
Cyanide
When you add hydrogen cyanide to aldehydes and
ketones, you form cyanohydrins.
The –CN and –OH are on the same carbon atom. This
is a typical carbonyl addition reaction.
Aldol Condensation
Reactions
In a carbonyl compound, the carbons next to the carbonyl
carbon are labeled with the Greek alphabet, starting with
alpha, then beta…
The Hydrogen atoms attached to the alpha carbon are more
easily removed as protons than other hydrogens within the
molecule.
Molecules that contain this alpha carbon can add
themselves to other molecules that also contain the alpha
carbon. The result is a molecule with a carbonyl and
alcohol group in the molecule. This must be done in diluted
basic medium.
Aldol Condensation
Practice
1) Write the aldol condensation reaction of propanal.
2) Show the mechanism for the aldol condensation of
3-pentanone.
Condensation Polymers
A polymer is a long chain of a molecule with many
repeating units.
Monomers are the individual units that repeat to
form the polymer.
When monomers join together to form polymers, a
dehydration usually occurs to join them together.
Important classes of polymers are: polyesters,
polyamides, phenol-formaldehyde polymers, and
polyurethanes.
Phenol-Formaldehyde
Polymers