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CHAPTER 23
ALDEHYDES AND KETONES
SOLUTIONS TO REVIEW QUESTIONS
1.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
2.
Each has the molecular formula of C3H6O, so aldehydes and ketones appear to be isomeric with each
other.
Each has the molecular formula C4H8O, so the generalization seems to check out. The general formula
for aldehydes and ketones is CnH2nO.
3. The strength of collagen depends on aldol condensations. After collagen is formed, aldehydes add along
its length. Collagen fibers adjacent to each other undergo an aldol condensation. The cross linking
bonds between collagen strands form a strong network, giving collagen its strength.
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- Chapter 23 4. 1-Butanol has a boiling point of 118 C while the boiling point for butanal is 76 C and for butanone,
80 C. Of these three compounds, only 1-butanol can hydrogen bond to itself. This additional bonding
holds the molecules together more tightly and accounts for the much higher boiling point.
5. The versatile aldol condensation reaction allows Streptomyces to synthesize a wide variety of different
antibiotics.
6. Acetaldehyde carries a reactive aldehyde functional group that can bond to many different
biochemicals: by reacting with amino acids, acetaldehyde slows protein synthesis; by reacting with
antioxidants, acetaldehyde increases oxidative damage to the liver; by reacting with specific proteins,
acetaldehyde hampers the liver’s ability to export needed chemicals to the blood stream. Acetaldehyde
can cause liver cirrhosis.
7. A ketone is a carbonyl functional group that is bonded to two alkyl or aryl groups. Cortisone is a ketonecontaining hormone. The sugar, fructose, contains a ketone. Ketones are also found in Vitamin K1.
8. (a) When an aldehyde reacts with Benedict solution, the blue color of copper ion disappears and a redbrown precipitate (Cu2O) forms.
(b) When an aldehyde reacts with Tollens solution, the silver ion in solution forms a thin silver metal
mirror on the inside of the glass container used in the test.
9. The phenol-formaldehyde polymer is rigid because covalent bonds not only form the polymers but also
cross-link the polymers to each other.
10. MEK is an abbreviation for methyl ethyl ketone. Its main use is as a solvent, especially for lacquers and
paints.
11. A ketone group,
, cannot be located at the end of a carbon-carbon chain. Consequently its only
possible location in both propanone and butanone is on C-2 of these ketones. Therefore its location need
not be numbered.
12. There must be an H atom on the alpha carbon adjacent to a carbonyl group of one of the reacting
compounds in the aldol condensation. This hydrogen transfers to the carbonyl of the other reactant and
breaks the carbonyl pi bond leaving intermediates in which a carbon atom of each molecule has three
bonds. The two intermediates then bond together to give the final product.
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- Chapter 23 -
SOLUTIONS TO EXERCISES
1. Names of aldehydes.
O
(a) H2 C
methanal; formaldehyde
(b)
(c)
(d)
(e)
2. Names of aldehydes
(a)
(b)
(c)
(d)
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- Chapter 23 -
(e)
3. Names of ketones
(a) propanone, acetone, dimethyl ketone
(b) 1-phenyl-l-propanone, ethyl phenyl ketone
(c) cyclopentanone
(d) 4-hydroxy-4-methyl-2-pentanone
4. Names of ketones
(a) butanone, methyl ethyl ketone (MEK)
(b) 3,3-dimethylbutanone, t-butyl methyl ketone
(c) 2,5-hexanedione
(d) 1-phenyl-2-propanone, benzyl methyl ketone
5. Structural formulas
(a)
(b)
(c)
(d)
(e)
6. Structural formulas
(a)
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- Chapter 23 -
(b)
(c)
(d)
(e)
7. (a) Acetaldehyde is a common name. The IUPAC name is ethanal.
(b) 2-Methyl-3-butanone is numbered incorrectly. The correct IUPAC name is 3-methyl-2-butanone.
(c) 1-Hexanal is incorrect because the aldehyde needs no number. The correct IUPAC name is
hexanal.
8. (a) 2-Methyl-3-propanal is numbered incorrectly and the aldehyde needs no number. The correct
IUPAC name is 2-methylpropanal.
(b) Formaldehyde is a common name. The IUPAC name is methanal.
(c) 4-Hexanone is numbered incorrectly. The correct IUPAC name is 3-hexanone.
9. Boiling points depend on molecular size (larger molecules have higher boiling points) and
intermolecular bonding (molecules that hydrogen bond have higher boiling points than molecules that
don’t hydrogen bond; polar molecules have higher boiling points than non-polar molecules).
(a) Although 3-pentanone and 2-pentanol are about the same size, 2-pentanol can hydrogen bond to
itself. 2-Pentanol has the higher boiling point.
(b) Ethanal is both larger and more polar than ethane. Ethanal has the higher boiling point.
(c) 1,3-Propanediol is larger and hydrogen bonds to itself while propanone is smaller and can’t
hydrogen bond to itself. 1,3-Propanediol has the higher boiling point.
(d) Pentanal is both larger and more polar than propane. Pentanal has the higher boiling point.
10. Boiling points depend on molecular size (larger molecules have a higher boiling points) and
intermolecular bonding (molecules that hydrogen bond have higher boiling points than molecules
that don’t hydrogen bond; polar molecules have higher boiling points than non-polar molecules).
(a) Butanone is both larger and more polar than butane. Butanone has the higher boiling point.
(b) Although 2-butanol and butanone are about the same size, 2-butanol can hydrogen bond to itself
while butanone can’t. 2-Butanol has the higher boiling point.
(c) 2-Pentanone is both larger and more polar than propane. 2-Pentanone has the higher boiling point.
(d) 1,2-Propanediol is larger and can hydrogen bond to itself while propanal is smaller and can’t
hydrogen bond to itself. 1,2-Propanediol has the higher boiling point.
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- Chapter 23 11. Aqueous solubility depends on the size of the alkyl chain (the smaller alkyl chain has more aqueous
solubility) and bonding between the solute and water (hydrogen bonding solutes have higher aqueous
solubilities; more polar solutes have higher aqueous solubilities).
(a) Both 2-heptanone and propanone are ketones but propanone has a much smaller alkyl chain.
Propanone has a higher aqueous solubility.
(b) Pentane and pentanal have the same sized alkyl chains but pentanal can hydrogen bond to water.
Pentanal has the higher water solubility.
(c) 3-Pentanone and 2,4-pentanedione have the same sized alkyl chains but 2,4-pentanedione has two
ketone functional groups while 3-pentanone has only one. 2,4-Pentanedione can hydrogen bond
more strongly to water and has the higher aqueous solubility.
12. Aqueous solubility depends on the size of the alkyl chain (the smaller alkyl chain has more aqueous
solubility) and bonding between the solute and water (hydrogen bonding solutes have higher aqueous
solubilities; more polar solutes have higher aqueous solubilities).
(a) 3-Hydroxypentanal has a smaller alkyl chain and two functional groups that can hydrogen bond to
water while 3-hexanone has a larger alkyl chain and only one functional group that can hydrogen
bond to water. 3-Hydroxypentanal has the higher aqueous solubility.
(b) Cyclohexanone and cyclohexane have the same sized alkyl chains but cyclohexanone can
hydrogen bond to water. Cyclohexanone has the higher aqueous solubility.
(c) Propanone and 2-pentanone are both ketones but propanone has the smaller alkyl chain. Propanone
has the higher aqueous solubility.
13. Equations for the oxidation of:
(a) 3-pentanol
This is the same product as for the oxidation by Cu2þ or Agþ.
(b) 3-methyl-l-hexanol
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- Chapter 23 14. Equations for the oxidation of:
(a) 1-propanol
(b)
15. (a)
(b)
(c)
16. (a)
(b)
(c)
17. (a) propanal,
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- Chapter 23 (b) butanone,
(c) butanal,
18. (a)
(b)
(c)
19. (a) An aldehyde group,
, must be present to give a positive Tollens test.
(b) The visible evidence for a positive Tollens test is the formation of a silver mirror on the inner walls
of a test tube.
(c)
20. (a) An aldehyde group,
, must be present to give a positive Fehling test.
(b) The visible evidence for a positive Fehling test is the formation of brick red Cu2O, which
precipitates during the reaction.
(c)
21. (a) The aldehyde is oxidized in the Tollens test:
CH3 CH2 COO NHþ
4
(b) The ketone is reduced:
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- Chapter 23 (c) The aldehyde is oxidized in the Benedict test:
(d) The aldehyde is reduced:
CH3 CH2 CH2 CH2 CH2 CH2 CH2 OH
22. (a) The aldehyde is reduced:
(b) The aldehyde is oxidized in the Benedict test:
(c) The ketone is reduced:
(d) The aldehyde is oxidized in the Tollens test:
23. HCN adds to a carbonyl functional group to form a cyanohydrin. In turn, the cyanohydrin can be reacted
with water to make an a-hydroxy acid.
(a)
(b)
(c)
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- Chapter 23 24. HCN adds to a carbonyl functional group to form a cyanohydrin. In turn, the cyanohydrin can be reacted
with water to make an a-hydroxy acid.
(a) HOCH2COOH
OH
(b) CH3 CH2 CH2 CH2
C
COOH
CH3
CH3 OH
(c) CH3
C
CH
COOH
CH3
25. Aldol condensation
(a) butanal
(b)
26. Aldol condensation
(a)
(b)
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- Chapter 23 27. The completed equations are:
(a)
(b)
(c)
28. The completed equations are:
(a)
(b)
(c)
29. Sequence of reactions:
(a)
(b)
(c)
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- Chapter 23 30. Sequence of reactions:
(a)
(b)
(c)
31.
32.
33. Four aldol condensation products from a mixture of ethanal (E) and propanal (P) are possible.
34. A hemiacetal forms when an alcohol adds to a carbonyl group.
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- Chapter 23 35. (a)
The Tollens test (silver mirror) or Fehling test (red Cu2O) will give positive results with propanal but
not with acetone.
(b)
Bromine will decolorize immediately with the second compound (propenal) but not with propanal.
(c)
Oxidize both compounds: 2-phenylethanol will give 2-phenylethanal and 1-phenylethanol will give
methyl phenyl ketone. 1-phenylethanal will give a positive Tollens or Benedict test and methyl
phenyl ketone will not give a positive test.
36.
37.
38. Pyruvic acid is changed to lactic acid by a reduction reaction.
39. The alcohols which should be oxidized to give these ketones.
(a)
(b)
(c)
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- Chapter 23 40. The following are alll the isomeric aldehydes and ketones with the formula, C5H10O:
41. Ketones will not react in the Fehling test. The following are all the ketones with the formula, C6H12O:
42. (a) 1. This step is an aldol condensation that occurs in dilute NaOH.
2. This step oxidizes the aldehyde to a carboxylic acid. Common oxidizing conditions are set
by the Tollens test (Agþ, NH3, H2O) or the Fehling/Benedict test (Cu2þ, NaOH, H2O).
(b) 1. This step forms a cyanohydrin and the conditions require hydroxide (OH ).
2. This step reacts the cyanohydrin with water to form an a-hydroxy acid. Conditions require
acid (Hþ ) as well as water.
(c) 1. This step is an aldol condensation that occurs in dilute NaOH.
2. This step reduces the aldehyde to form a primary alcohol. Common reducing conditions involve
heat (D), hydrogen gas, and a nickel catalyst.
43.
44. In phenol, three positions, ortho, ortho, and para to the OH group are used in the reaction to form a
thermosetting polymer. However, in p-cresol the para position is occupied by a methyl group and cannot
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- Chapter 23 react with formaldehyde. This leaves the p-cresol molecule as a bifunctional monomer, resulting in a
linear, thermoplastic polymer.
45. (a)
(b)
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