13 aldehydes-and-ketones

Chapter 13

Aldehydes and Ketones

1

1. Structure
The carbonyl group is a double bond between oxygen and
carbon.
Carbonyl compounds include:
Aldehydes: at least one hydrogen bonded to the carbonyl carbon

Ketones: no hydrogens bonded to the carbonyl carbon.

carboxylic acids and amides. [Tune in NEXT chapter…]

2

1. Properties

The carbonyl group is polar.

Intermolecular forces in carbonyl compounds are
stronger than in alkanes (nonpolar) or ethers (only slightly polar).
weaker than in alcohols, which are polar and can hydrogen bond.

3

1. Boiling points

Structure Name Molar mass Boiling point
CH3CH2CH2CH3 butane 58 -0.5 oC
CH3-O-CH2CH3 methoxyethane 60 7.0 oC
CH3CH2CH2-OH 1-propanol 60 97.2 oC
O
|| propanal 58 49 oC
CH3CH2-C-H
O
|| propanone 58 56 oC
CH3-C-CH3

4

1. Solubility

There is no hydrogen bonding between two carbonyl
groups.

5

1. Solubility

Aldehydes and ketones can hydrogen bond with water.

Aldehydes and ketones with five or fewer carbon atoms are
fairly soluble in water.
Large aldehydes and ketones dissolve only in nonpolar solvents.

6

1. Solubility in water—general guidelines

Molecules with polar groups tend to be more soluble in
water.
Molecules that can hydrogen bond with water are more
soluble than those than cannot.
Increasing the number of polar groups on a molecule
increases its water solubility.
Increasing the number of possibilities for hydrogen
bonding increases solubility in water.
Increasing the molar mass of a molecule decreases its
water solubility.
8

2. Nomenclature

Naming aldehydes
The parent compound is named for the longest continuous
chain containing the carbonyl group.

The final –e of the parent alkane is replaced with –al.

The chain is numbered beginning at the carbonyl carbon.

Substituents are named and numbered as usual.

The aldehyde is always carbon 1, so no number is used for the
carbonyl group.

9

2. Nomenclature

The simplest aldehydes

methanal ethanal

Draw structures for
pencast

7,8-dibromooctanal

trans-2-hexenal (component of olive oil)

10

2. Nomenclature

Name the following compounds.

pencast

11

2. Nomenclature

Ketones
The rules are analogous to the rules for aldehydes.
The –e ending of the parent alkane is changed to –one.
The location of the carbonyl is indicated with a number. [This
number is never 1- because that would be an aldehyde!]
The longest carbon chain is numbered to give the carbonyl the
lowest possible number.

12

2. Nomenclature

The simplest ketones need no number for the carbonyl.
(Why?)

propanone butanone

Draw structures for the following compounds.
2-fluorocyclohexanone
pencast

3-chloro-2-pentanone

13

2. Nomenclature

Name the following compounds.

pencast

14

3. Important aldehydes and ketones

What is the relationship between embalming fluid and
methanol poisoning?

What is the I.U.P.A.C. name of the following component
of lemongrass?

pencast

15


What is the I.U.P.A.C. name for cinnamaldehyde?

pencast

Chanel No. 5 became famous for containing synthetic
aldehydes “in copious quantities”.*

*Perfume Shrine Follow the link for a fun description of
properties of aldehydes and ketones.

16


Aldehydes in perfumery
heptanal: naturally occuring in clary sage and possessing a herbal
green odour
octanal: orange-like
nonanal: smelling of roses
decanal: powerfully evocative of orange rind
citral (a more complicated 10-carbon aldehyde): odor of lemons
undecanal: naturally present in coriander leaf oil
unsaturated undecen-1-al
lauryl aldehyde (12 carbons): evocative of lilacs or violets

17

4. Reactions--oxidation

Preparation is usually by oxidation of an alcohol.
Aldehydes are produced from primary alcohols.

18


Ketones are produced from secondary alcohols.

Tertiary alcohols don’t undergo oxidation (no –H to lose).

19


Oxidation of aldehydes and ketones
Aldehydes can be oxidized to carboxylic acids.

loss of a bond to H, gain of a bond to O

20


Oxidation of aldehydes and ketones
Ketones can’t be oxidized further.

N.R. (no –H to lose)

21


Tollen’s test is used to distinguish between aldehydes
and ketones based on their ability to be oxidized.
When one substance is oxidized, another must be reduced.

+ Ag(NH3)2+  + Ago
Tollen’s reagent elemental silver

22


+ Ag(NH3)2+  + Ago
elemental silver

If an aldehyde is present, a “silver mirror”
forms on the inside of the glass
container.

If a ketone is present, there is no
reaction because it won’t undergo
oxidation.

23


The same process used in Tollen’s test used to be used to
produce silvered mirrors.

This is an article published in 1911 describing the process, if
you are interested!

Go here to see a 2-liter flask turned into a mirror.

For EXTRA CREDIT, explain what precautions need to be taken
when the Tollen’s test is carried out. You’ll have to watch the
video. Post your answer in the Module 4 Journal Answers
journal with the tag Tollen.

24


Draw the structure of the following compounds and their
oxidation products.

2-methyl-2-propanol

2-nonanol
pencast

1-decanol

25


What reaction will pentanal undergo, if any, with Tollen’s
reagent?

pencast

26


Benedict’s test is used to distinguish between reducing
and non-reducing sugars.
A reducing sugar can be oxidized.
The substance reduced is Cu+2.

reducing sugar + Cu+2  oxidized sugar + Cu2O

27


Benedict’s test

+ Cu+2  + Cu2O

28

4. Reactions—blast from the past

Can you remember Chapter 12?
Preparation of alcohols by hydrogenation of aldehydes and
ketones (slide 19)

29

4. Reactions--reduction

Hydrogenation is a reduction reaction.
More bonds to hydrogen, fewer bonds to oxygen

Reduction of an aldehyde or ketone requires a Ni, Pt, or
Pd catalyst. [sound familiar?]

Reduction of an aldehyde produces a primary alcohol.
Reduction of a ketone produces a secondary alcohol.

30

4. Reactions--reduction

What is the product of reduction of

1-chloropropanone?

pencast

2-methylpropanal?

31

4. Reactions--addition

Take the time machine back to Chapter 11 now.
Alkenes undergo addition reactions:
Hydrogenation (addition of H2)
Halogenation (addition of X2)
Hydration (addition of H2O)
Hydrohalogenation (addition of HX)
General addition reaction

32


Addition of an alcohol to an aldehyde:

H+

33


The product is called a hemiacetal (-OH and –OR
attached to the same carbon).
Hemiacetals are very reactive.
They react with an additional alcohol molecule, losing –OH and
adding another –OR.

34


The final product is an acetal (2 –OR groups attached to
one carbon).

hemiacetal acetal

35


Ketones undergo analogous addition reactions with
alcohols.
The initial product is a reactive hemiketal (two –R groups, one –
OH, and one –OR).
An additional –OR group is added to the hemiketal to produce a
ketal.

hemiketal ketal

36


37

4. Reactions

Hemiacetal, acetal, hemiketal, or ketal?

38

4. Reactions

Monosaccharide addition reactions

alcohol
6

5 ]
4

2
1
] aldehyde

3

D-glucose

39

4. Reactions

Monosaccharide addition reactions

6 Hemiacetal:
one –H
5
one –OH
one –OR
one -R
4 1

3 2

The cyclic form is more stable than the linear form and no
further oxidation takes place in this case.
40

4. Reactions: keto-enol tautomers

Tautomers differ from each other in the placement of
one hydrogen and one double bond.

Keto form Enol form
(ketone) (alcohol + alkene)

The two forms exist in an equilibrium mixture, mostly in
the keto form, which is more stable.
41

4. Reactions: keto-enol tautomers

Draw the keto and enol forms of

propanone.

pencast

propanal.

42

4. Reactions: aldol condensation

In an aldol condensation, aldehydes or ketones react to
make a larger molecule by forming a new carbon-carbon
bond between two molecules.

+
OH- or
enzyme

43

4. Reactions: aldol condensation

Write an equation for the aldol condensation of two
molecules of propanal.

pencast

44

13 aldehydes-and-ketones

13 aldehydes-and-ketones

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