Introduction to General, Organic and Biochemistry

Frederick A. Bettelheim, William H. Brown, Mary K. Campbell

Chapter 16

Aldehydes and Ketones - all with Video Answers

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Chapter Questions

Problem 1

Answer true or false.
(a) The one aldehyde and the one ketone with a molecular formula of $\mathrm{C}_{3} \mathrm{H}_{6} \mathrm{O}$ are constitutional isomers.
(b) Aldehydes and ketones both contain a carbonyl group.
(c) The VSEPR model predicts bond angles of $120^{\circ}$ about the carbonyl carbon of aldehydes and ketones.
(d) The carbonyl carbon of a ketone is a stereocenter.

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Problem 2

What is the difference in structure between an aldehyde and a ketone?

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Problem 3

What is the difference in structure between an aromatic aldehyde and an aliphatic aldehyde?

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Problem 4

Is it possible for the carbon atom of a carbonyl group to be a stereocenter? Explain.

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Problem 5

Which compounds contain carbonyl groups?

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Problem 6

Following are structural formulas for two steroid hormones.
(a) Name the functional groups in each.
(b) Mark all stereocenters in each hormone and state
how many stereoisomers are possible for each.

Allison Krajewski

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Problem 7

Draw structural formulas for the four aldehydes with the molecular formula $\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}$. Which of these aldehydes are chiral?

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Problem 7

Answer true or false.
(a) An aldehyde is named as an alkanal, and a ketone is named as an alkanone.
(b) The names for aldehydes and ketones are derived from the name of the longest carbon chain that contains the carbonyl group.
(c) In an aromatic aldehyde, the carbonyl carbon is bonded to an aromatic ring.

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Problem 9

Draw structural formulas for these aldehydes.
(a) Formaldehyde
(b) Propanal
(c) 3,7 -Dimethyloctanal
(d) Decanal
(e) $4-$ Hydroxybenzaldehyde
(f) 2,3 -Dihydroxypropanal

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Problem 10

Draw structural formulas for these ketones.
(a) Ethyl isopropyl ketone
(b) 2 -Chlorocyclohexanone
(c) 2,4 -Dimethyl- 3 -pentanone
(d) Diisopropyl ketone
(e) Acetone
(f) 2,5 -Dimethylcyclohexanone

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Problem 11

Write the IUPAC names for these compounds.

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Problem 12

Write the IUPAC names for these compounds.

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Problem 13

Explain why each name is incorrect. Write the correct IUPAC name for the intended compound.
(a) $3-$ Butanone
(b) $1-$ Butanone
(c) $4-$ Methylbutanal
(d) 2,2 -Dimethyl-3-butanone

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Problem 14

Explain why each name is incorrect. Write the correct IUPAC name for the intended compound.
(a) $2-$ Pentanal
(b) Cyclopentanal
(c) $3-$ Ethyl- 2 -butanone
(d) 5 -Aminobenzaldehyde

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Problem 15

Answer true or false.
(a) Aldehydes and ketones are polar compounds.
(b) Aldehydes have lower boiling points than alcohols with comparable carbon skeletons.
(c) Low-molecular-weight aldehydes and ketones are very soluble in water.
(d) There is no possibility for hydrogen bonding between molecules of aldehydes and ketones.

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Problem 16

In each pair of compounds, select the one with the higher boiling point.
(a) Acetaldehyde or ethanol
(b) Acetone or 3 -pentanone
(c) Butanal or butane
(d) Butanone or 2-butanol

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Problem 17

Acetone is completely soluble in water, but 4 -heptanone is completely insoluble in water. Explain.

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Problem 18

Account for the fact that acetone has a higher boiling point $\left(56^{\circ} \mathrm{C}\right)$ than ethyl methyl ether $\left(11^{\circ} \mathrm{C}\right)$ even though their molecular weights are almost the same.

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Problem 19

Pentane, 1-butanol, and butanal all have approximately the same molecular weights but different boiling points. Arrange them in order of increasing boiling point. Explain the basis for your ranking.

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Problem 20

Show how acetaldehyde can form hydrogen bonds with water.

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Problem 21

Why can't two molecules of acetone form a hydrogen bond with each other?

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Problem 22

Answer true or false.
(a) The reduction of an aldehyde always gives a primary alcohol.
(b) The reduction of a ketone always gives a secondary alcohol.
(c) The oxidation of an aldehyde gives a carboxylic acid.
(d) The oxidation of a primary alcohol gives a ketone.
(e) Tollens' reagent can be used to distinguish between an aldehyde and a ketone.
(f) Sodium borohydride, $\mathrm{NaBH}_{4}$, reduces an aldehyde to a primary alcohol.
(g) The addition of one molecule of alcohol to the carbonyl group of a ketone gives a hemiacetal.
(h) The reaction of an aldehyde with two molecules of alcohol gives an acetal, plus a molecule of water.
(i) The formation of hemiacetals and acetals is
reversible.
(j) The cyclic hemiacetal formed from 4-hydroxypentanal has two stereocenters and can exist as a mixture of $2^{2}=4$ stereoisomers.

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Problem 23

Draw a structural formula for the principal organic product formed when each compound is treated with $\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} / \mathrm{H}_{2} \mathrm{SO}_{4} .$ If there is no reaction, say so.
(a) Butanal
(b) Benzaldehyde
(c) Cyclohexanone
(d) Cyclohexanol

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Problem 24

Draw a structural formula for the principal organic product formed when each compound in Problem 23 is treated with Tollens' reagent. If there is no reaction, explain why.

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Problem 25

What simple chemical test could you use to distinguish between the members of each pair of compounds? Tell what you would do, what you would expect to observe, and how you would interpret your experimental observation.
(a) Pentanal and 2-pentanone
(b) 2 -Pentanone and 2 -pentanol

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Problem 26

Explain why liquid aldehydes are often stored under an atmosphere of nitrogen rather than in air

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Problem 27

Suppose that you take a bottle of benzaldehyde liquid, bp $179^{\circ} \mathrm{C}$ ) from a shelf and find a white solid in the bottom of the bottle. The solid turns litmus red; that is, it is acidic. Yet aldehydes are neutral compounds. How can you explain these observations?

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Problem 28

Explain why the reduction of an aldehyde always gives a primary alcohol and the reduction of a ketone always gives a secondary alcohol.

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Problem 29

Write a structural formula for the principal organic product formed by treating each compound with $\mathrm{H}_{2}$ transition metal catalyst. Which products are chiral?

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Problem 30

Draw a structural formula for the principal organic product formed by treating each compound in Problem 29 with $\mathrm{NaBH}_{4}$ followed by $\mathrm{H}_{2} \mathrm{O}$.

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Problem 31

1,3-Dihydroxy-2-propanone, more commonly known as dihydroxyacetone, is the active ingredient in artificial tanning agents such as Man-Tan and Magic Tan.
(a) Write a structural formula for this compound.
(b) Would you expect it to be soluble or insoluble in water?
(c) Write a structural formula for the product formed by its reduction with $\mathrm{NaBH}_{4}$

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Problem 32

Draw a structural formula for the product formed by treatment of butanal with each set of reagents.
(a) $\mathrm{H}_{2} /$ transition metal catalyst
(b) $\mathrm{NaBH}_{4},$ then $\mathrm{H}_{2} \mathrm{O}$
(c) $\quad \mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}^{+}$ (Tollens' reagent)
(d) $\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{2} / \mathrm{H}_{2} \mathrm{SO}_{4}$

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Problem 33

Draw a structural formula for the product formed by treatment of acetophenone, $C_{6} H_{5} C O C H_{3},$ with each set of reagents given in Problem 32

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Problem 34

Mark each statement true or false.
(a) Keto and enol tautomers are constitutional isomers.
(b) For a pair of keto-enol tautomers, the keto form generally predominates.

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Problem 35

Which of these compounds undergo keto-enol tautomerism?

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Problem 36

Draw all enol forms of each aldehyde and ketone.

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Problem 37

Draw a structural formula for the keto form of each enol.

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Problem 38

Warfarin is widely used in medicine as a blood anticoagulant.
(a) Identify all the functional groups in warfarin.
(b) The $S$ enantiomer is more active than the $R$ enantiomer. Draw a stereorepresentation of the $R$ enantiomer
(c) Warfarin, as drawn in the Chemical Connections box, contains an enol group. Draw the structural formula of the keto form of warfarin.
(d) Draw the product formed by treating warfarin with $\mathrm{NaBH}_{4}$
(e) Warfarin exerts its anticoagulation property in many individuals at a dose of 4 mg/day. How many molecules of warfarin are present in a 4 mg tablet?

Allison Krajewski

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Problem 39

What is the characteristic structural feature of a hemiacetal? Of an acetal?

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Problem 40

Which compounds are hemiacetals, which are acetals, and which are neither?

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Problem 41

Which compounds are hemiacetals, which are acetals, and which are neither?

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Problem 42

Draw the hemiacetal and then the acetal formed in each reaction. In each case, assume an excess of the alcohol.
(a) Propanal + methanol $\rightarrow$
(b) Cyclopentanone + methanol $\rightarrow$

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Problem 43

Draw the structures of the aldehydes or ketones and alcohols formed when these acetals are treated with aqueous acid and hydrolyzed.

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Problem 44

The following compound is a component of the fragrance of jasmine:
From which carbonyl-containing compound and alcohol is this compound derived?

Ummatul Choudary

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Problem 45

Consider the formation of an acetal. If the carbonyl oxygen of the starting aldehyde or ketone were labeled with oxygen-18 (Chapter 9) as a chemical tag, would you predict that the oxygen-18 tag would be found as one of the two oxygen atoms of the acetal, or as the oxygen atom of a water molecule? (Hint:
think of the steps by which the reaction occurs; that is, think about the mechanism of acid-catalyzed acetal formation.)

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Problem 46

Following is the structure of immunosuppressant FK$506,$ a molecule shown to disrupt calcineurin-mediated signal transduction in T lymphocytes and first seen in Problem 14.40.
(a) There are three carbon-carbon double bonds present in this molecule. Which of the three has the potential for cis/trans isomerism? Assign a cis or trans configuration to each carbon-carbon double
bond that has this possibility.
(b) How many stereocenters are present in this molecule? How many stereoisomers are possible for it?
(c) Are there any aromatic components in this molecule?
(d) Consider the two carbon atoms marked with asterisks. Assign an $R$ or $S$ configuration of each stereocenter.
(e) Because of the presence of a 21 -member ring, this molecule is described as a macrocycle. This ring is fashioned by several carbon-carbon bonds, one ester, one hemiacetal, and one amide. Locate the ester and the hemiacetal.
(f) Draw the structural formula of the long chain compound that would result if the hemiacetal were to be cleaved to an alcohol and a carbonyl group.

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Problem 47

What is the difference in meaning between the terms "hydration" and "hydrolysis"? Give an example of each.

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Problem 48

What is the difference in meaning between the terms "hydration" and "dehydration"? Give an example of each.

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Problem 49

List the reagents and experimental conditions that might be used to convert cyclohexanone to each of the following compounds.

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Problem 50

Draw a structural formula for an aldehyde or a ketone that can be reduced to produce each alcohol. If none exists, say so.

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Problem 51

Draw a structural formula for an aldehyde or a ketone that can be reduced to produce each alcohol. If none exists, say so.

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Problem 52

1-Propanol can be prepared by the reduction of an aldehyde, but it cannot be prepared by the acidcatalyzed hydration of an alkene. Explain why it cannot be prepared from an alkene.

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Problem 53

Show how to bring about these conversions. In addition to the given starting material, use any other organic or inorganic reagents as necessary.

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Problem 54

Show how to bring about these conversions. In addition to the given starting material, use any other organic or inorganic reagents as necessary.
(a) $1-$ Pentene to 2 -pentanone
(b) Cyclohexene to cyclohexanone

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Problem 55

Describe a simple chemical test by which you could distinguish between the members of each pair of compounds.
(a) Cyclohexanone and aniline
(b) Cyclohexene and cyclohexanol
(c) Benzaldehyde and cinnamaldehyde

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Problem 56

Mark the aldehyde or ketone group in these compounds.

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Problem 57

Draw a structural formula for the product formed by treating each compound in Problem 56 with sodium borohydride, $\mathrm{NaBH}_{4}$

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Problem 58

Draw structural formulas for the (a) one ketone and
(b) two aldehydes with the molecular formula $\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}$

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Problem 59

Draw structural formulas for these compounds.
(a) $1-$ Chloro- 2 -propanone
(b) 3 -Hydroxybutanal
(c) $4-$ Hydroxy- 4 -methyl-2-pentanone
(d) $3-$ Methyl-3-phenylbutanal
(e) $1,3-$ Cyclohexanedione
(f) $\quad 5$ -Hydroxyhexanal

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Problem 60

Why does acetone have a lower boiling point $\left(56^{\circ} \mathrm{C}\right)$ than 2 -propanol $\left(82^{\circ} \mathrm{C}\right)$ even though their molecular weights are almost the same?

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Problem 61

Propanal (bp $\left.49^{\circ} \mathrm{C} \text { ) and } 1 \text { -propanol (bp } 97^{\circ} \mathrm{C}\right)$ have about the same molecular weight, yet their boiling points differ by almost $50^{\circ} \mathrm{C}$. Explain this fact.

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Problem 62

What simple chemical test could you use to distinguish between the members of each pair of compounds? Tell what you would do, what you would expect to observe, and how you would interpret your experimental observation.
(a) Benzaldehyde and cyclohexanone
(b) Acetaldehyde and acetone

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Problem 63

5-Hydroxyhexanal forms a six-membered cyclic hemiacetal, which predominates at equilibrium in aqueous solution.
(a) Draw a structural formula for this cyclic hemiacetal.
(b) How many stereoisomers are possible for 5-hydroxyhexanal? (Chapter 15)
(c) How many stereoisomers are possible for its cyclic hemiacetal? (Chapter 15 )

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Problem 64

2-Methylbutene can be formed by dehydrating two different compounds (A and B) of molecular formula $\mathrm{C}_{5} \mathrm{H}_{12} \mathrm{O} .$ When $\mathbf{A}$ is mildly oxidized, compound $\mathbf{C}$ $\left(\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}\right)$ forms, which can be oxidized further to a carboxylic acid. When compound B undergoes dehydration, 2-methylbutene is formed as the minor product. In addition, compound B cannot be oxidized further. Use this information to determine the identity of compounds $\mathbf{A}, \mathbf{B},$ and $\mathbf{C}$.

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Problem 65

Show how to bring about these conversions. In addition to the given starting material, use any other organic or inorganic reagents as necessary.

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Problem 66

The following molecule is an enediol; each carbon of the double bond carries an - OH group. Draw structural formulas for the $\alpha$ -hydroxyketone and the $\alpha$ -hydroxyaldehyde with which this enediol is in equilibrium.

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Problem 67

Alcohols can be prepared by the acid-catalyzed hydration of alkenes (Section $12.6 \mathrm{B}$ ) and by the reduction of aldehydes and ketones (Section $16.4 \mathrm{B}$ ). Show how you might prepare each of the following alcohols by (1) acid-catalyzed hydration of an alkene and (2) reduction of an aldehyde or a ketone..
(a) Ethanol
(b) Cyclohexanol
(c) 2 -Propanol
(d) 1 -Phenylethanol

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Problem 68

Glucose, $\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6},$ contains an aldehyde group but exists predominantly in the form of the cyclic hemiacetal shown here. We will discuss this cyclic form of glucose in Chapter 20
A cyclic hemiacetal is formed when the $-$ OH group of one carbon bonds to the carbonyl group of another carbon
(a) Which carbon in glucose provides the $-$ OH group and which provides the $-$ CHO group?
(b) Draw the alternative chair conformations of $\beta-\mathrm{D}-\mathrm{g}$ lucose and state which of the two is the more stable.

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Problem 69

Ribose, $C_{5} H_{10} O_{5},$ contains an aldehyde group but exists predominantly in the form of the cyclic hemiacetal shown here. We will discuss this cyclic form of ribose in Chapter 20.
Which carbon of ribose provides the $-$ OH group and which provides the CHO group for formation of this cyclic hemiacetal?

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Problem 70

Name the functional groups in each steroid hormone. Methandrostenolone is a synthetic anabolic steroid; that is, it promotes tissue and muscle growth and development.
(a) What are the differences in the structural formula between testosterone and methandrostenolone?
(b) Mark all stereocenters in each molecule and state the number of stereoisomers possible for each. The stereochemistry shown on the structural formulas is that of the biologically active stereoisomer.

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Problem 71

Sodium borohydride is a laboratory reducing agent. NADH is a biological reducing agent. In what way is the chemistry by which each reduces aldehydes and ketones similar?

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Problem 72

Complete the following equations for these reactions.

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Problem 73

Write an equation for each conversion.
(a) $1-$ Pentanol to pentanal
(b) $1-$ Pentanol to pentanoic acid
(c) 2 -Pentanol to 2 -pentanone
(d) 2 -Propanol to acetone
(e) Cyclohexanol to cyclohexanone

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Problem 74

As noted in Section 16.4 and introduced in Sections 8.1 and $12.5,$ chemists use curved arrows to demonstrate a reaction mechanism, illustrating the flow of electrons to show bond breaking and bond formation. Use curved arrows to show the reaction mechanism for the conversion of the starting material shown to the final acetal product.

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