Organic Chemistry

Lesson 4/Midterms
Aldehydes and Ketones • Indicate the presence of the carbonyl group by
changing the -ane of the parent alkane -one.
Structure • Number the parent chain from the direction that
gives the carbonyl carbon the smaller number.
• IUPAC retains the common name acetone for 2-
• The functional group of an aldehyde is a carbonyl propanone.
group bonded to a hydrogen atom.
o In methanal, the simplest aldehyde
(formaldehyde), the carbonyl group is bonded
to 2 hydrogens. • To name an aldehyde or ketone that also contains an
o In other aldehydes, it is bonded to 1 hydrogen -OH (hydroxyl) or -NH2 (amino) group:
and 1 carbon group. o Number the parent chain to give the carbonyl
• The functional group of a ketone is a carbonyl group carbon the lower number.
bonded to 2 carbon groups. o Indicate an -OH substituent by hydroxy-, and an
-NH2 substituent by amino-.
o Hydroxy and amino substituents are numbered and
alphabetized along with other substituents.
Nomenclature: Beyond Ordinary Cases
• A carbon atom adjacent to a carbonyl group is called
an a-carbon, and a hydrogen atom bonded to it is
called an a-hydrogen.
Nomenclature
• IUPAC names for aldehydes:
o To name an aldehyde, change the suffix -e
of the parent alkane to -al.
o Because the carbonyl group of an aldehyde
can only be at the end of a parent chain and • On a molecule with a higher priority functional
numbering must start with it as carbon-1, group, C=O is oxo- and -CHO is formyl.
hence, there is no need to use a number to • Aldehyde priority is higher than ketone.
locate the aldehyde group.

• For unsaturated aldehydes, indicate the presence of Common Names

a carbon-carbon double bond by changing the • The common name for an aldehyde is derived from
ending of the parent alkane from -ane to –enal. the common name of the corresponding carboxylic
• Numbering the carbon chain begins with the acid; drop the word "acid" and change the suffix -ic
aldehyde carbonyl carbon. or -oic to -aldehyde
• Name each alkyl or aryl group bonded to the
• Show the location of the carbon-carbon double bond
carbonyl carbon as a separate word, followed by the
by the number of its first carbon. word "ketone”; the alkyl or aryl groups are
generally listed in order of increasing molecular
weight

• The IUPAC system retains common names for some

aldehydes, including these three. Physical Properties
IUPAC names for Ketones: • A C=O bond is polar, with oxygen bearing a partial
negative charge and carbon bearing a partial positive
charge
o Therefore, aldehydes and ketones are polar
• Parent alkane 🡪 longest chain that contains the molecules
carbonyl group.
 Organic Chemistry
Lesson 4/Midterms

• In liquid aldehydes and ketones, the weak

intermolecular attractions are between the partial
Physical Property: Solubility
positive charge on the carbonyl carbon of one
molecule and the partial negative charge on the
carbonyl oxygen of another molecule.
• No hydrogen bonding is possible between aldehyde
or ketone molecules.
• Aldehydes and ketones have lower boiling points
than alcohols and carboxylic acids, compounds in
which there is hydrogen bonding between
molecules.
• Formaldehyde, acetaldehyde, and acetone are
infinitely soluble in water.
• Aldehydes and ketones become less soluble in water
• The polar carbonyl group provides dipole-dipole as the hydrocarbon portion of the molecule increases
interactions. in size.
• Without an H on the oxygen atom, aldehydes and
ketones cannot form hydrogen bonds.
• Aldehydes and ketones can form hydrogen bonds
with the water molecules.
Comparison of Boiling Points Types of isomers
• Aldehydes and ketones have higher boiling points
than alkanes and ethers of similar mass, but lower
boiling points than alcohols.

Chiral objects
For compounds of comparable molecular weight… • Chiral compounds have the same number of atoms
arranged differently in space.
• A chiral carbon atom is bonded to four different
groups.
• Your hands are chiral. Try to superimpose your
Water Solubility thumbs, palms, back of hands, and little fingers.
• Ketones and Aldehydes, like ethers, can function as
hydrogen bond acceptors and smaller compounds Mirror images
have significant water solubility. • The mirror images of chiral compounds cannot be
• Solubility in Water: the electronegative O atom of superimposed.
the carbonyl group of aldehydes and ketones forms • When the H and I atoms are aligned, the Cl and Br
hydrogen bonds with water. atoms are on opposite sites.
 Organic Chemistry
Lesson 4/Midterms
Some everyday chiral achiral objects Reduction

• The carbonyl group of an aldehyde or ketone is

reduced to an -CHOH or -CH 2OH group by
hydrogen in the presence of a transition-metal
catalyst:
o Reduction of an aldehyde gives a primary
Characteristic Reaction: Oxidation alcohol.
• Aldehydes are oxidized to carboxylic acids by a o Reduction a ketone gives a secondary alcohol.
variety of oxidizing agents, including potassium • The most common laboratory reagent for the
dichromate. reduction of an aldehyde or ketone is Sodium
borohydride, NaBH4.
o This reagent contains hydrogen in the form of
hydride ion, H: -.
• Liquid aldehydes are so sensitive to oxidation by O 2 o In a hydride ion, hydrogen has two valence
of the air that they must be protected from contact
electrons and bears a negative charge.
with air during storage.
o In a reduction by Sodium borohydride, hydride
ion adds to the partially positive carbonyl carbon
which leaves a negative charge on the carbonyl
oxygen.
o Reaction of this intermediate with aqueous acid
gives the alcohol.
Oxidation
• Ketones resist oxidation by most oxidizing agents,
including potassium dichromate and molecular O.
o Tollens’ reagent is specific for the oxidation of
aldehydes; if done properly, silver deposits on the
walls of the container as a silver mirror. • Reduction by NaBH4 does not affect a carbon-
carbon double bond or an aromatic ring.

• In biological systems, the agent for the reduction of

aldehydes and ketones is the reduced form of
nicotinamide adenine dinucleotide, abbreviated
NADH (Section 26.3)
o This reducing agent, like NaBH4, delivers a
hydride ion to the carbonyl carbon of the
aldehyde or ketone.
o Reduction of pyruvate, the end product of
glycolysis, by NADH gives lactate.

A reduction of Tollen's reagent (an ammoniacal silver

nitrate complex) by glucose solution to form silver
mirror. Addition of Alcohols
Glucose is oxidized to gluconic acid. This reaction is • Addition of a molecule of alcohol to the carbonyl
used as a test for aldehyde groups in organic chemicals group of an aldehyde or ketone forms a hemiacetal
(as well as for making silver mirror), which includes (a half-acetal).
formaldehyde, acetaldehyde and glucose. o The functional group of a hemiacetal is a carbon
bonded to one -OH group and one -OR group.
 Organic Chemistry
Lesson 4/Midterms
o In forming a hemiacetal, H of the alcohol adds to
the carbonyl oxygen and OR adds to the carbonyl
carbon.

• Hemiacetals are generally unstable and are only

minor components of an equilibrium mixture except
in one very important type of molecule:
• When a hydroxyl group is part of the same molecule
that contains the carbonyl group and a five- or six-
membered ring can form, the compound exists
almost entirely in a cyclic hemiacetal form.

• A hemiacetal can react further with an alcohol to

form an acetal plus water.
o This reaction is acid catalyzed.
o The functional group of an acetal is a carbon
bonded to two -OR groups.

• All steps in hemiacetal and acetal formation are

reversible.
• As with any other equilibrium, we can drive this one
in either direction by using Le Chatelier's principle.
• To drive it to the right, we either use a large excess
of alcohol or remove water from the equilibrium
mixture
• To drive it to the left, we use a large excess of water.