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C H A P T E R 2 3

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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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 ketone-

containing 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 red-

brown 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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SOLUTIONS TO EXERCISES

1. Names of aldehydes.

(a) H2C����O methanal; formaldehyde

(b)

(c)

(d)

(e)

2. Names of aldehydes

(a)

(b)

(c)

(d)

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(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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(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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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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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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(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:

CH3CH2COO�NHþ

4

(b) The ketone is reduced:

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(c) The aldehyde is oxidized in the Benedict test:

(d) The aldehyde is reduced:

CH3CH2CH2CH2CH2CH2CH2OH

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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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

(b) CH3CH2CH2CH2 C

OH

COOH

CH3

(c) CH3 C

CH3

CH3

CH

OH

COOH

25. Aldol condensation

(a) butanal

(b)

26. Aldol condensation

(a)

(b)

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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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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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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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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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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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