1. Aldehydes
Lecture Presented by:
Victor R. Oribe
Presented to:
Dr. Leonisa O. Bernardo

2. Aldehydes contain carbonyl functional
group.
A carbonyl group is a carbon atom double
bonded to an oxygen atom.
The structural representation for a
carbonyl group is:

3. Carbon-oxygen and carbon-carbon double bond
differ in a major way.
A carbon-oxygen bond is polar and a carbon-
carbon double bond is nonpolar.
The electronegativity of oxygen (3.5) is much
greater than that of carbon (2.5).
Hence the carbon-oxygen double bond is
polarized, the oxygen atom acquiring a fractional
negative charge and the carbon atom acquiring a
fractional positive charge.

4. All carbonyl groups have a trigonal planar
structure.
The bond angles between the three atoms
attached to the carbonyl carbon atom are 1200 ,
as would be predicted using VSEPR theory.

5. An Aldehyde is a carbonyl-containing organic
compound in which the carbonyl carbon atom
has at least one hydrogen atom directly attached
to it.
The remaining group attached to the carbonyl
carbon atom can be hydrogen, an alkyl group, a
cycloalkyl group, or an aryl group (Ar).

6. In interpreting general condensed functional
group structure such as RCHO, remember that
carbon always has four bonds and hydrogen
always has only one.
In RCHO, one of carbon’s bonds
goes to the R group and one to H;
therefore, two bonds must go to O.

7. Linear notation for an aldehyde functional
group and for an aldehyde itself are –CHO and
RCHO respectively.
Note that the ordering of the symbol H and O in
these notations is HO, not OH (which denotes a
hydroxyl group)
In an aldehyde, the carbonyl group is always
located at the end of a hydrocarbon chain.
O
║
CH3 – CH2 – CH2 – CH2 – C - H

8. An aldehyde functional group can be bonded to
only one carbon atom because three of the four
bonds from an aldehyde carbonyl carbon must
go to oxygen and hydrogen.
Thus, an aldehyde functional group
is always found at the end of the
carbon chain.
Cyclic aldehydes are not possible.
For an aldehyde carbonyl carbon atom to be part
of a ring system it would have to form two bonds
to ring atoms, which would give it five bonds.

9. Aldehydes are related to alcohol in some
manner that alkenes are related to alkanes.
Removal of hydrogen atoms from each of two
adjacent carbon atoms in an alkane produces an
alkene.
In a like
H
manner, removal of a
O O hydrogen atom from the
║ –OH group of an alcohol
C
-2H
C and from the carbon
atom to which the
Aldehyde
H hydroxyl group is
attached produces a
Alcohol carbonyl group.

10. Physical Properties of Aldehydes
The C1 and C2 aldehydes are gases at room
temperature.
The C3 through C11 straight-chain saturated
aldehydes are liquids, and the higher aldehydes
are solid.
The presence of alkyl groups tends to lower both
boiling points and melting points, as does the
presence of unsaturated in the carbon chain.
The boiling point of aldehydes are intermediate
between those of alcohols and alkanes of similar
molecular mass.

11. Aldehydes have higher boiling points than
alkanes because of dipole-dipole attractions
between molecules.
Carbonyl group polarity makes these dipole-
dipole interactions possible.
Unbranched Aldehydes
O C
C1 C3 C5 C7
║ ║
C C2 C4 C6 C8
O
gas liquid
Dipole-dipole
A physical state summary for
attraction
unbranched aldehydes at room
temperature and pressure

12. Aldehydes have lower boiling points than the
corresponding alcohols because no hydrogen
bonding occurs as it does with alcohols.
Water molecule can hydrogen-bond with
aldehyde molecule.
H R
H C H
O H O
O H
Aldehyde-water hydrogen bonding

13. This hydrogen bonding causes low-
molecular-mass aldehydes to be water
soluble.
As the hydrocarbon portion get larger, the
water solubility of aldehydes decreases.
Low-molecular –mass aldehyde have
pungent, penetrating, unpleasant odors, higher-
molecular-mass aldehydes (above C8 ) are more
fragrant, especially benzaldehyde derivatives.

14. Naturally Occurring Aldehydes
Aldehydes occurs widely in nature.
Naturally occurring aldehydes are of higher
molecular masses, usually have pleasant odor
and flavors and are often used for these
properties in consumer products (perfume, air
fresheners, and the like). Cinnamaldehyde
cinnamon flavoring
Vanillin Benzaldehyde
Vanilla flavoring almod flavoring

15. Nomenclature for Aldehyde
The IUPAC rules for naming aldehydes:
1. Select as the parent carbon chain the longest
chain that includes the carbon atom of the
carbonyl group. 1 methane
O 2 ethane
5 4 3 2 1║ 3 propane
CH3 – CH2 - CH2 – CH – C – H
4 butane
Pentane CH2
5 pentane
CH2
Parent chain

16. 2. Name the parent chain by changing the –e
ending of the corresponding alkane to –al.
Pentanel
a
3. Determine the identity and location of any
substituents, and append this information to the
front of the parent chain name. O
5 4 3 2 1║
1 methyl CH3 – CH2 - CH2 – CH – C – H
2 ethyl
3 propyl
1 CH2
4 butyl 2 CH3
5 pentyl
2- ethylpentanal

17. 1 methyl
║
3 1
4 2
H 2 ethane
3 propane
butane butanal 4 butane
5 pentane
3- methyl
3-methylbutanal

18. O
5 4 3 2 1║
CH3 – CH2 – CH – CH2 – C - H
OH
Hydroxylaldehyde

19. Assign IUPAC names to the following
aldehydes
O
║
1) CH3 – CH – C- H 2-Methylpropanal
CH3
O
║
2) CH3 – CH – CH- C - H
Cl Cl Dichlorobutanal

20. O
║
3) CH3 – CH2 – CH- C - H
2-Ethylpentanal
CH3 – CH2 – CH2
O
║
4) H
2-Methylbutanal

21. The common name for simple aldehyde
illustrate a second method for counting from
one to four: form - , acet - , propion - , and
butyr -
The common names for aldehydes are one word
rather than two or more.
O O O
║ ║
║
H–C-H CH3 – C - H CH3 – CH2 – C - H
Formaldehyde Acetaldehyde Propionaldehyde

22. IUPAC system for Naming Aromatic
Aldehydes
Aromatic aldehydes are names as derivatives of
benzaldehyde, the parent compound.
O O O
║ ║ ║
C-H Cl C-H C-H
Benzaldehyde
CH3
3-chloro-5-methylbenzaldehyde OH
4-hydroxybenzaldehyde

23. Preparation of Aldehydes
Aldehydes can be produced by the oxidation of
primary alcohol, using mild oxidizing agents such as
KMnO4 or K2 Cr2 O7 .
OH O
║
oxidation
R–C–H R–C–H
Aldehyde
H
Primary When this reaction is used for aldehyde
alcohol preparation, reaction conditions must be
sufficiently mild to avoid further
oxidation of the aldehyde to a carboxylic
acid.

24. The term Aldehyde stems from
alcohol dehydrogenation,
indicating that aldehydes are
related to alcohols by the loss of
hydrogen.

25. Predicting Products in Alcohol Oxidation Reaction
Draw the aldehyde formed from the oxidation of each of the
following alcohols. Assume that reaction conditions are
sufficiently mild that any aldehydes produced are not oxidized
further.
O
║
1) CH3 – CH2 – CH2 - OH CH3 – CH2 – C – H
O
2) CH3 – CH – CH2 - OH ║
CH3 – CH – C - H
CH3 CH3
CH3 CH3 O
3) CH3 – C – CH2 - OH ║
CH3 – C – C - H
CH3
CH3

26. 3) CH3 - CH2 – CH2 – CH2 – CH2 - OH O
║
CH3 - CH2 – CH2 – CH2 – C - H
CH3 CH3 O
║
4) CH3 - C – CH2 – CH2 – OH CH3 - C – CH2 – C – H
CH3 CH3
O
5) CH3 – CH2 – CH – CH2 - OH ║
CH3 – CH2 – CH – C – H
CH3 CH3

27. Oxidation and Reduction of Aldehydes
Aldehyde readily undergo oxidation to carboxylic
acids.
Aldehyde readily undergo oxidation to carboxylic
acids.
O O
║ [O] ║
R–C–H R – C – OH
Aldehyde Carboxylic Acid
Among the mild oxidizing agents that convert
aldehydes into carboxylic acid is oxygen in air.
Thus, aldehydes must be protected from air.

28. Reduction of Aldehydes
Aldehydes are easily reduced by hydrogen has (H2 )
in the presence of a catalysts (Ni, Pt, orm Cu), to
form alcohols.
The reduction of aldehydes produces primary
alcohols.
O OH
║
Ni
CH3 – C - H + H2 CH3 – C - H
H
Ethanal Ethanol

29. Oxidation
Primary
Alcohol
Reduction
Aldehyde reduction to produce alcohols are
opposite of the oxidation of alcohols to produce
aldehydes.

30. Reaction of Aldehydes with Alcohols
Aldehydes react with alcohols to form hemiacetal and
acetals.
Reaction with one molecule of alcohol produces a
hemiacetal, which is then converted to an acetal by
reaction with a second alcohol molecule.
acid
Aldehyde + alcohol
catalyst
hemiacetal
acid
hemiacetal + alcohol
catalyst
acetal

31. Hemiacetal and acetal formation are
very important biochemical reactions,
they are crucial to understanding the
chemistry of carbohydrates.
The Greek prefix hemi- means “half.”
When one alcohol molecule has reacted
with the aldehyde, the compound is
halfway to the final acetal.

32. Hemiacetal Formation
Hemiacetal formation is an addition reaction in which
a molecule of alcohol adds to the carbonyl group of an
aldehyde.
The H portion of the alcohol adds to the carbonyl
oxygen atom, and R – O portion of the alcohol adds
to the carbonyl carbon atom.
O H
O H
R1 C O R2
║
C + C R2
R1 H H
Aldehyde Hemiacetal

33. Formally defined, a hemiacetal is an
organic compound in which carbon atom
is bonded to both a hydroxyl group (-OH)
and an alkoxy (-OR).
The functional group for a hemiacetal is:
OH
The carbon atom of the
C OR hemiacetal functional group is
often referred to as the
4) hemiacetal hemiacetal carbon atom: it
was the carbonyl carbon atom
of the aldehyde that reacted.

34. Indicate whether each of the following
compounds is a hemiacetal
OH
1) CH3 – CH – O – CH3
CH2
4) O CH3
OH CH3
2) CH3 – CH – CH – O – CH3
OH
OH
5) CH3 – C – CH3
O OH
3) O CH3

35. 1. We have an –OH group and an – OR group attached to the
same carbon atom. The compound is a hemiacetal.
2) The –OH and –OR groups present in this molecule are
attached to different carbon atoms. Therefore, the molecule
is not a hemiacetal.
3)We have a ring carbon atom bonded to two oxygen atoms:
one oxygen atom in an –OH substituent and the other
oxygen atom bonded to the rest of the ring (the same as an
R group). This is hemiacetal
4) hemiacetal
5) We have an –OH group and an –OR group attached to the
same carbon atom. The compound is a hemiacetal.