Chem 2412: Ch. 18 Aldehydes and Ketones. Classes of Carbonyl Compounds.
1. 2 Structures of Aldehydes Ketones 3 Both aldehydes and ketones contain a carbonyl ( C=O) group.
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Transcript of 1. 2 Structures of Aldehydes Ketones 3 Both aldehydes and ketones contain a carbonyl ( C=O) group.
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Structures of Aldehydes & Ketones
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• Both aldehydes and ketones contain a carbonyl ( C=O) group.
RC
H
O
ArC
H
O
aldehydes
RC
R
O
ArC
R
O
ketones
ArC
Ar
O
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•In a linear expression, the aldehyde group is often written as:
CHO
H3CC
H
O
is equivalent to CH3CHO
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•In the linear expression of a ketone, the carbonyl group is written as:
CO
H3CC
CH3
O
is equivalent to CH3COCH3
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Naming Aldehydes & Ketones
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IUPAC Rules for Naming Aldehydes1. To establish the parent name, select the
longest continuous chain of carbon atoms that contains the aldehyde group.
2. The carbons of the parent chain are numbered starting with the aldehyde group. Since the aldehyde group is at the beginning (or end) of a chain, it is understood to be number 1.
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IUPAC Rules for Naming Aldehydes
3. Form the parent aldehyde name by dropping the –e from the corresponding alkane name and adding the suffix –al.
4. Other groups attached to the parent chain are named and numbered as we have done before.
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H3CC
H
O
ethanal
HC
CH2CH2CHCH2CH3
O
CH34-methyhexanal
2 3 4 5 61
Naming Aldehydes
4-methylhexanal
ethanal
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Common Names for Aldehydes
HC
H
O
formaldehydeH
CCH3
O
acetaldehyde
CH
O
benzaldehyde
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IUPAC Rules for Naming Ketones
1. To establish the parent name, select the longest continuous chain of carbon atoms that contain the ketone group.
2. Form the parent name by dropping the –e from the corresponding alkane name and add the suffix –one.
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IUPAC Rules for Naming Ketones
3. If the chain is longer than four carbons, it is numbered so that the carbonyl group has the smallest number possible; this number is prefixed to the parent name of the ketone.
4. Other groups attached to the parent chain are named and numbered as we have done before.
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Naming Ketones
H3CC
CH3
O
propanone
H3CH2CC
CH2CH2CHCH2CH3
O
CH36-methyl-3-octanone
2 3 4 51H3C
CCH2CH2CH3
O
2-petanone
4 51 23 6 7 8
2-pentanone
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Common Names for Ketones
H3CC
CH3
O
propanoneacetone
H3CC
CH2CH3
O
butanonemethyl ethyl ketone, MEK
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Bonding and Physical Properties
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Bonding• The carbon atom of the carbonyl group
is sp2-hybridized and is joined to three other atoms by sigma bonds.
• The fourth bond is made by overlapping p electrons of carbon and oxygen to form a pi bond between the carbon and oxygen atoms.
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Bonding• Because the oxygen atom is considerably
more electronegative than carbon, the C=O group is polar.
• Many of the chemical reactions of aldehydes and ketones are due to this polarity.
C O + -
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Properties• Unlike alcohols, aldehydes and ketones
cannot hydrogen-bond to themselves, because no hydrogen atom is attached to the oxygen atom of the carbonyl group.
• Aldehydes and ketones, therefore, have lower boiling points than alcohols of comparable molar mass.
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Effect of Hydrogen Bonding on Physical Properties
Alcohols can undergohydrogen bonding resulting in higher boiling points andhigher solubility.
Aldehydes and ketones cannot undergo hydrogen bonding resulting in lower boiling points and lower solubility.
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Mole Weight Boiling point oC
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Chemical Properties of Aldehydes &
Ketones
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Reactions of Aldehydes & Ketones
• Oxidation– aldehydes only
• Reduction– aldehydes and ketones
• Addition– aldehydes and ketones
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Oxidation of Aldehydes• Aldehydes are easily oxidized to carboxylic
acids by a variety of oxidizing agents, including (under some conditions) oxygen of the air.
RC
H
O
+ Cr2O72- + 8 H+
3
RC
OH
O3
+ 3 Cr3+ + 4H2O
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Tollens’ Silver Mirror Test
• Tollens’ reagent,which contains Ag+, oxidizes aldehydes, but not ketones.
• Ag+ is reduced to metallic Ag, which appears as a “mirror” in the test tube.
Ag+ + e– → Ag(s)
RC
H
O
+ 2 Ag+
RC
O-NH4+
O
NH3H2O
+ 2 Ag (s)
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Fehling and Benedict Tests• Benedict’s reagent,
which contains Cu2+ ions in an alkaline medium,
reacts with aldehydes that have an adjacent OH group.
• an aldehyde is oxidized to a carboxylic acid, while Cu2+ is reduced to give brick red Cu2O(s).
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Increasing amounts of reducing sugar
green orange red brown
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Tollens, Fehling & Benedict Tests• Because most ketones do not give a
positive with Tollens, Fehling, or Benedict solutions, these tests are used to distinguish between aldehydes and ketones.
RC
R
O
+ 2 Cu+2 NaOHH2O no reaction
RC
R
O
+ 2 Ag+ NH3H2O no reaction
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Biochemical Oxidation of Aldehydes
• When our cells ‘burn’ carbohydrates, they take advantage of the aldehyde reactivity.
• The aldehyde is oxidized to a carboxylic acid and is eventually converted to carbon dioxide, which is then exhaled.
• This stepwise oxidation provides some of the energy necessary to sustain life.
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Reduction of Aldehydes & KetonesAldehydes and ketones are easily reduced to alcohols using LiAlH4,
NaBH4 , or H2/Ni .
Aldehydes yield primary alcohols (1) while ketones yield secondary alcohols ( 2) .
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Addition Reactions of Aldehydes & Ketones
• Common addition reactions:– Addition of alcohols
• hemiacetal, hemiketal, acetal, ketal– Grignard preparations of alcohols– 2,4-dinitrophenylhydrazine (2,4-DNP)
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Addition of Alcohols
Aldehydes react with alcohols and a trace of acid to yield hemiacetalsas shown here.
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Addition of AlcoholsIn the presence of excess alcohol and a strong acid such as dry HCl,aldehydes or hemiacetals react with a second molecule of the alcohol to yield an acetal.
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Intramolecular Addition of Alcohols
Cyclic hemiacetals or hemiketals can form when the alcohol and the carbonyl group exist within the same molecule .
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Addition of Alcohols to Aldehydes and Ketones
OH
OR'C
RH
OH
OR'C
RR
OR'
OR'C
RH
OR'
OR'C
RR
hemiacetal hemiketal acetal ketal
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Grignard preparations of alcohols• A Grignard reagent is an
organic magnesium halide. It can be either an alkyl or an aryl compound (RMgX or ArMgX). Grignard (pronounced green yard) reagents were first prepared in France around 1900 by Victor Grignard (1871-1935).
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• Grignard reagents are usually made by reacting an organic halide and magnesium metal in an ether solvent:
RX + Mg RMgXether
ArX + Mg ArMgXether
X = Cl, Br, or I
X = Br
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• In the Grignard reagent, the bonding electrons between carbon and magnesium are shifted away from the electropositive Mg to form a strongly polar covalent bond. As a result the charge distribution in the Grignard reagent is such that the organic group (R) is partially negative and the –MgX group is partially positive. This charge distribution directs the manner in which Grignard reacts with other compounds.
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• The Grignard reagent is one of the most versatile and widely used reagents in organic chemistry. We will consider only its reactions with aldehydes and ketones at this time. Grignards react with aldehydes and ketones to give intermediate products that form alcohols when hydrolyzed. With formaldehyde, primary alcohols are formed; with other aldehydes, secondary alcohols are formed; with ketones, tertiary alcohols are formed.
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Examples
Grignard reagent + formaldehyde → 1º ROHGrignard reagent + other aldehydes → 2º ROHGrignard reagent + ketones → 3º ROH
H2C O + CH3MgBr H2C OMgBr CH3CH2OH + Mg(OH)Br
CH3ether H2O
Formaldehyde
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Examples
C O + CH3MgBr C OMgBr
H H
CH3
ether
CHOH + Mg(OH)Br
CH3
H2OBenzaldehyde
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Examples
CH3CCH3 + CH3CH2MgBr CH3CCH3 CH3CCH3 + Mg(OH)Br
O
CH2CH3
OMgBr
ether H2OCH2CH3
OH
Acetone
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Explanation
• The Grignard reaction with acetone may be explained in this way. In the first step of the addition of ethyl magnesium bromide, the partially positive –MgBr of the Grignard bonds to the oxygen atom, and the partially negative CH3CH2– bonds to the carbon atom of the carbonyl group of acetone.
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CH3CCH3 + CH3CH2MgBr CH3CCH3
O
CH2CH3
O_
+MgBr
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Explanation• In the hydrolysis step, a proton [H+]
from water bonds to the oxygen atom, leaving the hydroxyl group [–OH] to combine with the +MgBr. So, the alcohol is formed.
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CH3CCH3
CH2CH3
O_
+MgBr
+ H OH CH3CCH3 + Mg(OH)Br
OH
CH2CH3
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2,4-dinitrophenylhydrazine (2,4-DNP)
NNO2
NO2
N
H H
H
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2,4-dinitrophenylhydrazine (2,4-DNP)
• The carbonyl carbon in both aldehydes and ketones reacts with 2,4-DNP to form heavy yellow to orange crystalline solids.
• These solids were used extensively for identification purposes before the use of spectrometers.
• The solid is purified by crystallization and its melting point compared to those of known structure.
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Common Aldehydes & Ketones
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Formaldehyde (Methanal)• Formaldehyde is made from methanol by
reaction with oxygen (air) in the presence of a silver or copper catalyst.
• 2 CH3OH + O2 2H2C=O + 2H2O
• Formaldehyde is widely used in the synthesis of polymers.
Ag
heat
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Acetaldehyde (Ethanal)
• Its principal use is as an intermediate in the manufacture of other chemicals, such as acetic acid and 1-butanol.
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Acetone and Methyl Ethyl Ketone
• Acetone is used as a solvent in the manufacture of drugs, chemicals, and explosives. It is also used as a solvent.
• Methyl ethyl ketone (MEK) is also widely used as a solvent, especially for lacquers.
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Aldehydes & Ketones in Nature
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C C C
C
C CO
Violet (Irone)
COH
salicylaldehyde(meadowsweet)
OHC
OOPiperonal(Heliotrope)
OH
C
CH3(CH2)4C H
O
(Eucalyptus) CH3(CH2)10C H
O
(Citrus Fruits)CH3 C C CH3
O O
Raspberries
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C HO
Benzaldehyde(Oil of Almonds)
CHO
CH3O
HO
Vanillin
CH CH C H
O
Oil of Cinnamon
H3CCH3
OCamphor (Mothballs)
CH3
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CH3 CCH3
CHCH2CH2C CH
CH3
C HO
Citral (Lemon Grass Oil)
C C C C C C CO
CH3
C C (C)n CO
n = 4 or 6
Alarm Pheromones in ants
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C C
CH
C HO
Boll Weevil Sex Attractant
C
Citral(Honey Bee Recruiting Pheromone)
HO
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C C
C
(C)12
C O
Musk Ox Sex Attractant
C
C
(C)n
(C)m
C O
Civet Cat Sex Attractant
m = 4, n = 10m = 7, n = 7m = 7, n = 9
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Condensation Polymers
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Leo Baekeland (1863-1944)
6464
A phenolic is a condensation polymer made from phenol as shown here.
Phenol-Formaldehyde Polymers (Bakelite)
This is a section of a phenolic( i.e. Bakelite) which is an example of a thermosetting polymer. These polymers are used in electrical equipment because of their insulating and fire-resistant properties.
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Bakelite products
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Bakelite products
GE Locomotive
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