Transcript 16.6 Reduction of Aldehydes and Ketones

Outline
16.1 The Carbonyl Group
16.2 Naming Aldehydes and Ketones
16.3 Properties of Aldehydes and Ketones
16.4 Some Common Aldehydes and Ketones
16.5 Oxidation of Aldehydes
16.6 Reduction of Aldehydes and Ketones
16.7 Addition of Alcohols: Hemiacetals and Acetals
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Goals
1. What is the carbonyl group?
Be able to recognize the carbonyl group and describe its polarity and shape.
2. How are ketones and aldehydes named?
Be able to name the simple members of these families and write their structures,
given the names.
3. What are the general properties of aldehydes and ketones?
Be able to describe such properties as polarity, hydrogen bonding, and water
solubility.
4. What are some of the significant occurrences and applications of
aldehydes and ketones?
Be able to specify where aldehydes and ketones are found, list their major
applications, and discuss some important members of each family.
5. What are the results of the oxidation and reduction of aldehydes and
ketones?
Be able to describe and predict the products of the oxidation and reduction of
aldehydes and ketones.
6. What are hemiacetals and acetals, how are they formed, and how do they
react?
Be able to recognize hemiacetals and acetals, describe the conditions under
which they are formed, and predict the products of hemiacetal and acetal
formation and acetal hydrolysis.
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16.1 The Carbonyl Group
• Carbonyl compounds are distinguished by the
presence of a carbonyl group C O and are classified
according to what is bonded to the carbonyl carbon.
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16.1 The Carbonyl Group
• Since oxygen is more electronegative than
carbon, carbonyl groups are strongly polarized.
• The polarity of the carbonyl group gives rise to
its reactivity.
• The bond angles between the three substituents
on the carbonyl carbon atom are 120°.
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16.1 The Carbonyl Group
• Aldehydes and ketones have similar properties
because their carbonyl groups are bonded to carbon and
hydrogen atoms that do not attract electrons strongly.
– Aldehyde A compound that has a carbonyl group bonded to at
least one hydrogen, RCHO. Always ends a carbon chain.
– Ketone A compound that has a carbonyl group bonded to two
carbons in organic groups that can be the same or different,
RCOR’. Always within a carbon chain.
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16.2 Naming Aldehydes and Ketones
• The simplest aldehydes are known by their
common names, which end in -aldehyde.
• In the IUPAC system, the final -e of the name of
the alkane with the same number of carbons is
replaced by -al.
• When substituents are present, the chain is
numbered beginning with the carbonyl carbon.
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16.2 Naming Aldehydes and Ketones
• Most simple ketones are best known by common names
that give the names of the two alkyl groups bonded to
the carbonyl carbon followed by the word ketone.
• Ketones are named systematically by replacing the
final -e of the corresponding alkane name with -one
(pronounced own). The numbering of the alkane chain
begins at the end nearest the carbonyl group.
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16.2 Naming Aldehydes and Ketones
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Chemical Warfare Among the Insects
Insects have evolved extraordinarily effective means of
chemical protection.
The millipede Apheloria corrugata protects itself by
discharging benzaldehyde cyanohydrin.
While cyanohydrin is safe, the decomposition reaction
releases deadly hydrogen cyanide gas.
The weapon of the bombardier beetle is benzoquinone.
When threatened, the bombardier beetle initiates the
enzyme-catalyzed oxidation of dihydroxybenzene by
hydrogen peroxide.
A hot cloud (up to 100 °C) of irritating benzoquinone
vapor shoots out of the beetle’s defensive organ with
such force that it sounds like a pistol shot.
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16.3 Properties of Aldehydes and Ketones
• The polarity of the carbonyl group makes
aldehydes and ketones moderately polar.
• They boil at a higher temperature than
alkanes with similar molecular weights.
• Individual molecules do not hydrogen-bond
with each other, which makes aldehydes and
ketones lower boiling than alcohols.
• In a series, the alkane is lowest boiling, the
alcohol is highest boiling, and the aldehyde
and ketone fall in between.
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16.3 Properties of Aldehydes and Ketones
• Aldehydes and ketones are soluble in common
organic solvents.
• Those with fewer than five or six carbon atoms
are soluble in water because they are able to
accept hydrogen bonds.
• Simple ketones are excellent solvents because
they dissolve polar and nonpolar compounds.
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16.3 Properties of Aldehydes and Ketones
Some naturally-occurring aldehydes and ketones
have distinctive odors:
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16.3 Properties of Aldehydes and Ketones
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Properties of Aldehydes and Ketones
Aldehydes and ketone molecules are polar due to the
presence of the carbonyl group.
Since aldehydes and ketones cannot hydrogen-bond
with one another, they are lower boiling than alcohols
but higher boiling than alkanes because of their
polarity.
Common aldehydes and ketones are typically liquids.
Simple aldehydes and ketones are water-soluble due
to hydrogen bonding with water molecules, and
ketones are good solvents.
Many aldehydes and ketones have distinctive odors.
Simple ketones are less toxic than simple aldehydes.
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16.4 Some Common Aldehydes and Ketones
FORMALDEHYDE (HCHO): TOXIC BUT USEFUL
• At room temperature, formaldehyde is a
colorless gas with a pungent, suffocating
odor.
• Low concentrations in the air (0.1–1.1 ppm)
can cause eye, throat, and bronchial
irritation, and higher concentrations can
trigger asthma attacks.
• Skin contact can produce dermatitis.
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16.4 Some Common Aldehydes and Ketones
FORMALDEHYDE (HCHO): TOXIC BUT USEFUL
• Formaldehyde is formed during incomplete
combustion of hydrocarbon fuels and is
partly responsible for the irritation caused
by smog-laden air.
• Formaldehyde can cause serious kidney
damage, coma, and sometimes death; it is
a breakdown product of methyl alcohol,
and is one of the reasons that drinking
methanol is so toxic.
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16.4 Some Common Aldehydes and Ketones
FORMALDEHYDE (HCHO): TOXIC BUT USEFUL
• Formaldehyde is commonly sold as a 37%
aqueous solution under the name formalin.
• It kills viruses, fungi, and bacteria by
reaction with amino groups in proteins,
allowing for its use in disinfecting and
sterilizing equipment.
• On standing, formaldehyde polymerizes
into a solid known as paraformaldehyde.
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16.4 Some Common Aldehydes and Ketones
FORMALDEHYDE (HCHO): TOXIC BUT USEFUL
• Formaldehyde is found in polymers such as adhesives
for binding plywood, foam insulation for buildings, textile
finishes, and hard and durable manufactured objects.
• The first completely synthetic and commercially
successful plastic was a polymer of phenol and
formaldehyde known as Bakelite.
• Urea–formaldehyde polymers are now more widely used
than Bakelite. Once the final polymerization of such
materials is finished, no further melting and reshaping is
possible because of its cross-linked, three-dimensional
structure.
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16.4 Some Common Aldehydes and Ketones
FORMALDEHYDE (HCHO): TOXIC BUT USEFUL
• Because of concern over the toxicity of formaldehyde
from polymeric materials, their use in most household
applications is now limited.
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16.4 Some Common Aldehydes and Ketones
ACETALDEHYDE (CH3CHO) SWEET SMELLING BUT NARCOTIC
• Acetaldehyde is a sweet-smelling, flammable liquid
formed by the oxidation of ethyl alcohol.
• It is less toxic than formaldehyde, and small amounts are
produced in the normal breakdown of carbohydrates.
• At one time, acetaldehyde was used in the production of
acetic acid and acetic anhydride, but it is a general
narcotic, and large doses can cause respiratory failure.
• It is most commonly used for the preparation of
polymeric resins, and in the silvering of mirrors.
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16.4 Some Common Aldehydes and Ketones
ACETONE (CH3COCH3) A SUPER SOLVENT
• Acetone is one of the most widely used of all organic
solvents. It dissolves most organic compounds and is
also miscible with water.
• Acetone is volatile and is a serious fire and explosion
hazard when allowed to evaporate in a closed space.
• No chronic health risk has been associated with casual
acetone exposure.
• When the breakdown of fats and carbohydrates is out of
balance, acetone is produced in the liver.
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16.5 Oxidation of Aldehydes
• Alcohols can be oxidized to aldehydes or ketones.
• Aldehydes can be further oxidized to carboxylic
acids.
• In aldehyde oxidation, the hydrogen bonded to the
carbonyl carbon is replaced by an —OH group.
• Ketones do not have this hydrogen and do not react
cleanly with oxidizing agents.
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16.5 Oxidation of Aldehydes
• Because ketones cannot be
oxidized, treatment with a mild
oxidizing agent is used as a test to
distinguish between aldehydes
and ketones.
• Tollens’ reagent consists of a
solution containing silver ion in
aqueous ammonia. Treatment of
an aldehyde with this reagent
rapidly yields the carboxylic acid
anion and metallic silver.
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16.5 Oxidation of Aldehydes
• Benedict’s reagent contains blue
copper(II) ion, which is reduced to
give a precipitate of red copper(I)
oxide in the reaction with an
aldehyde.
• Benedict’s reagent does not
unequivocally distinguish between
ketones and aldehydes.
• At one time, Benedict’s reagent
was extensively used as a test for
sugars in the urine.
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16.6 Reduction of Aldehydes and Ketones
• The reduction of a carbonyl group occurs
with the addition of hydrogen across the
double bond to produce an —OH group.
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16.6 Reduction of Aldehydes and Ketones
• Aldehydes are reduced to primary
alcohols, and ketones are reduced to
secondary alcohols.
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16.6 Reduction of Aldehydes and Ketones
• Aldehydes are reduced to primary alcohols, and
ketones are reduced to secondary alcohols.
• Reductions occur by formation of a bond to the
carbonyl carbon atom by a hydride ion H2–
accompanied by bonding of a hydrogen ion H+
to the carbonyl oxygen atom.
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16.6 Reduction of Aldehydes and Ketones
• A hydride ion has a lone pair of valence
electrons. Both electrons are used to form a
covalent bond to the carbonyl carbon.
• This leaves a negative charge on the carbonyl
oxygen. Aqueous acid is then added, H+ bonds
to the oxygen, and a neutral alcohol results.
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16.6 Reduction of Aldehydes and Ketones
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16.6 Reduction of Aldehydes and Ketones
• In biological systems, the reducing agent for a
carbonyl group is often the coenzyme
nicotinamide adenine dinucleotide (NAD+),
which cycles between acting as a reducing
agent (NADH) and an oxidizing agent (NAD+)
by the loss and gain of a hydride ion.
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16.6 Reduction of Aldehydes and Ketones
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How Toxic Is Toxic?
The term dose refers to the amount of substance that enters the
body at one time.
One standard method for reporting toxicity is the LD50 or lethal dose,
50%, which is a measure of the toxicity of a single dose, known as
acute toxicity.
A substance is fed in varying doses to laboratory animals, frequently
rats or mice, and the mortality rate of the animals recorded. The
result of the test is reported as the the dose that kills 50% of the
animals in a uniform testing laboratory population.
By comparing LD50 values, relative toxicities of various laboratory
chemicals can be evaluated.
For therapeutic use, a compound must show a comfortably wide
margin between the dose that produces the desired effect and the
dose that produces an acute toxic effect.
The LD50 test is controversial; it has many drawbacks and many
advantages.
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16.7 Addition of Alcohols: Hemiacetals and Acetals
HEMIACETAL FORMATION:
• Aldehydes and ketones undergo addition
reactions in which an alcohol combines
with the carbonyl carbon and oxygen.
• The initial product of addition reactions
with alcohols are known as hemiacetals.
• Hemiacetals have both an alcohol-like —
OH group and an ether-like —OR group
bonded to what was once the carbonyl
carbon atom.
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16.7 Addition of Alcohols: Hemiacetals and Acetals
HEMIACETAL FORMATION:
• The H from the alcohol bonds to the
carbonyl-group oxygen, and the OR from
the alcohol bonds to the carbonyl-group
carbon.
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16.7 Addition of Alcohols: Hemiacetals and Acetals
HEMIACETAL FORMATION:
• Hemiacetals rapidly revert back to aldehydes or
ketones by loss of alcohol and establish an
equilibrium with the aldehyde or ketone.
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16.7 Addition of Alcohols: Hemiacetals and Acetals
HEMIACETAL FORMATION:
• Hemiacetals are often too unstable to be isolated.
• A major exception occurs when the —OH and CHO
functional groups that react are part of the same
molecule.
• Because of their greater stability, most simple sugars
exist mainly in the cyclic hemiacetal form.
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16.7 Addition of Alcohols: Hemiacetals and Acetals
ACETAL FORMATION:
• If a small amount of acid catalyst is added to the reaction
of an alcohol with an aldehyde or ketone, the hemiacetal
initially formed is converted into an acetal.
• An acetal is a compound that has two ether-like groups
bonded to what was the carbonyl carbon atom.
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16.7 Addition of Alcohols: Hemiacetals and Acetals
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16.7 Addition of Alcohols: Hemiacetals and Acetals
ACETAL HYDROLYSIS:
• Reversal requires an acid catalyst and a large quantity of
water to drive the reaction back toward the aldehyde or
ketone.
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16.7 Addition of Alcohols: Hemiacetals and Acetals
CARBONYL ADDITIONS
• A carbonyl is a polarized double bond such
that the carbon carries a partial positive charge
and the oxygen carries a partial negative
charge.
• The electron poor end of the reagent will attach
to the O of the carbonyl and the electron rich
end to the C.
• While only a trace of acid is needed for
formation of a hemiacetal, the presence of H+
is absolutely necessary for further conversion
to the acetal.
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Chapter Summary
1. What is the carbonyl group?
• The carbonyl group is a carbon atom
connected by a double bond to an oxygen
atom, C O.
• Because of the electronegativity of oxygen, the
C O group is polar, with a partial negative
charge on oxygen and a partial positive charge
on carbon.
• The oxygen and the two substituents on the
carbonyl-group carbon atom form a planar
triangle.
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Chapter Summary, Continued
2. How are ketones and aldehydes named?
• The simplest aldehydes and ketones are known by
common names (formaldehyde, acetaldehyde,
benzaldehyde, acetone).
• Aldehydes are named systematically by replacing the
final -e in an alkane name with -al and when necessary
numbering the chain starting with 1 at the CHO group.
• Ketones are named systematically by replacing the
final -e in an alkane name with -one and numbering
starting with 1 at the end nearer the C O group. The
location of the carbonyl group is indicated by placing
the number of its carbon before the name. Some
common names of ketones identify each alkyl group
separately.
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Chapter Summary, Continued
3. What are the general properties of aldehydes
and ketones?
• Aldehyde and ketone molecules are moderately
polar, do not hydrogen-bond with each other, but
can hydrogen-bond with water molecules.
• The smaller ones are water-soluble, and the
ketones are excellent solvents.
• In comparable series of compounds, aldehydes
and ketones are higher boiling than alkanes but
lower boiling than alcohols.
• Many aldehydes and ketones have distinctive,
pleasant odors.
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Chapter Summary, Continued
4. What are some of the significant occurrences
and applications of aldehydes and ketones?
• Aldehydes and ketones are present in many plants,
where they contribute to their aromas. Such natural
aldehydes and ketones are widely used in perfumes
and flavorings.
• Formaldehyde (an irritating and toxic substance) is
used in polymers, is present in smog-laden air, and
is produced biochemically from ingested methanol.
• Acetone is a widely used solvent and is a by-product
of food breakdown during diabetes and starvation.
• Many sugars (carbohydrates) are aldehydes or
ketones.
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Chapter Summary, Continued
5. What are the results of the oxidation and
reduction of aldehydes and ketones?
• Mild oxidizing agents convert aldehydes to
carboxylic acids but have no effect on simple
ketones.
• Tollens’ reagent is used to indicate the presence
of an aldehyde while Benedict’s reagent will give a
positive test result for both aldehydes and alpha
hydroxy ketones.
• With reducing agents, hydride ion (H—) adds to the
C of the C O group in an aldehyde or ketone and
hydrogen ion (H+) adds to the O to produce
primary or secondary alcohols, respectively.
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Chapter Summary, Continued
6. What are hemiacetals and acetals, how are they
formed, and how do they react?
• Aldehydes and ketones establish equilibria with
alcohols to form hemiacetals or acetals.
• The relatively unstable hemiacetals, which have an —
OH and an —OR on what was the carbonyl carbon,
result from addition of alcohol to the C O bond.
• The more stable acetals, which have two —OR groups
on what was the carbonyl carbon, form by addition of a
second alcohol molecule to a hemiacetal.
• The aldehyde or ketone can be regenerated from an
acetal by treatment with an acid catalyst and a large
quantity of water, which is an example of a hydrolysis
reaction.
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