Organic Chemistry I & II
Organic Chemistry I & II

Organic Chemistry I & II

Lead Author(s): Steven Forsey

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Organic Chemistry I & II is designed for instructors who want an active, dynamic, and understandable approach to support their own efforts in the classroom. This ever-evolving textbook includes auto-graded questions, videos and approachable language in order to make difficult concepts easier to understand and implement.

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Top Hat

Steven Forsey, “Organic Chemistry”, Only one edition needed

Up to 40-60% more affordable

Lifetime access on any device

McGraw-Hill

Carey & Giuliano, “Organic Chemistry”, 10th Edition

$219

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Solomons et al., “Organic Chemistry”, 12th Edition

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David R. Klein, “Organic Chemistry”, 3rd Edition

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Always up-to-date content, constantly revised by community of professors

Constantly revised and updated by a community of professors with the latest content

Top Hat

Steven Forsey, “Organic Chemistry”, Only one edition needed

McGraw-Hill

Carey & Giuliano, “Organic Chemistry”, 10th Edition

Wiley

Solomons et al., “Organic Chemistry”, 12th Edition

Wiley

David R. Klein, “Organic Chemistry”, 3rd Edition

In-book Interactivity

Includes embedded multi-media files and integrated software to enhance visual presentation of concepts directly in textbook

Top Hat

Steven Forsey, “Organic Chemistry”, Only one edition needed

McGraw-Hill

Carey & Giuliano, “Organic Chemistry”, 10th Edition

Wiley

Solomons et al., “Organic Chemistry”, 12th Edition

Wiley

David R. Klein, “Organic Chemistry”, 3rd Edition

Customizable

Ability to revise, adjust and adapt content to meet needs of course and instructor

Top Hat

Steven Forsey, “Organic Chemistry”, Only one edition needed

McGraw-Hill

Carey & Giuliano, “Organic Chemistry”, 10th Edition

Wiley

Solomons et al., “Organic Chemistry”, 12th Edition

Wiley

David R. Klein, “Organic Chemistry”, 3rd Edition

All-in-one Platform

Access to additional questions, test banks, and slides available within one platform

Top Hat

Steven Forsey, “Organic Chemistry”, Only one edition needed

McGraw-Hill

Carey & Giuliano, “Organic Chemistry”, 10th Edition

Wiley

Solomons et al., “Organic Chemistry”, 12th Edition

Wiley

David R. Klein, “Organic Chemistry”, 3rd Edition

About this textbook

Lead Authors

Dr. Steven Forsey, Ph.D.University of Waterloo

Steven Forsey is currently a Professor at University of Waterloo, teaching a variety of organic chemistry courses to Chemistry, Science, Chemical Engineering, Nanotechnology and distance education students. He received his Ph.D. (2004) for Synthetic Organic Chemistry from University of Waterloo, Ontario. He is a recipient of the Excellence of Science Teaching Award and has acted as the Teaching Fellow for the Department of Chemistry since 2016.

Contributing Authors

Felix NgassaGrand Valley State University

Neil GargUCLA

Jennifer ChaytorSaginaw Valley State University

Greg DomskiAugustana College

Christian E. MaduCollin Community College

Christopher NicholsonUniversity of West Florida

Franklin OwEast Los Angeles College, UCLA

Robert S. PhillipsUniversity of Georgia

Grigoriy SeredaUniversity of South Dakota

Simon E. LopezUniversity of Florida

Brannon McCulloughNorthern Arizona University

Jason JonesKennesaw State University

José BoquinAugustana College

Stephanie BrouetSaginaw Valley State University

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Chapter 22: Aldehydes, Ketones, and their Anomeric Derivatives

Formaldehyde, the most commonly used aldehyde, is used as a preservative of tissue. [1]


Contents

22.1 Aldehydes and Ketones in Nature
22.2 Structure and Properties of Aldehydes and Ketones
     22.2.1 Bonding and Resonance in Carbonyls
22.3 Synthesis of Aldehydes and Ketones
     22.3.1 Synthesis from Alkenes (Review Section 12.13)
     22.3.2 Synthesis from Terminal Alkynes (Review Section 13.5.5)
     22.3.3 Synthesis from Aromatic Compounds (Review Section 21.1.5)
     22.3.4 Synthesis from Alcohols, Dess-Martin Periodinane (DMP) and Swern Oxidation (Review Section 14.6)
     22.3.5 Synthesis from Esters
     22.3.6 Synthesis from Nitriles
22.4 Nucleophilic Addition to Carbonyls
     22.4.1 General Mechanism of Nucleophilic Addition
     22.4.2 Reactivity of Aldehydes vs. Ketones Towards Nucleophilic Addition
     22.4.3 Nucleophilic Addition in Acidic Medium
     22.4.4 Nucleophilic Addition in Basic or Neutral Medium
22.5 Reactions of Aldehydes and Ketones
     22.5.1 Reduction to Alcohols (Review Section 14.5.4)
     22.5.2 Addition of Grignard Reagents (Review Chapter 14)
     22.5.3 Hydration of Ketones and Aldehydes
          22.5.3.1 Mechanism of Hydration of Ketones & Aldehydes under Acidic and Basic Conditions
     22.5.4 Cyanohydrin Formation
     22.5.4.1 Mechanism of Cyanohydrin Formation
     22.5.5 Acetal and Hemiacetal Formation
          22.5.5.1 Mechanism of Acetal Formation
Keeping it Real
          22.5.5.2 Cyclic Acetals
          22.5.5.3 Mechanism of Cyclic Acetal
          22.5.5.4 Anomeric Effect
          22.5.5.5 Acetals as Protecting Groups
     22.5.6 Aldehyde Oxidations
          22.5.6.1 Tollen's Test
     22.5.7 Reactions with 1° Amines 
     22.5.8 Reactions with 2° Amines
     22.5.9 Reduction to Alkanes
     22.5.10 Wittig Reaction
          22.5.10.1 Mechanism of Wittig
     22.5.11 Conjugate Addition
22. 6 Summary of Key Reactions

Learning Objectives

  • Describe and employ methods to synthesize aldehydes and ketones.
  • Choose the best oxidizing agent to form a given aldehyde or ketone from an alcohol starting material.
  • Choose the best reagent to synthesize an aldehyde or ketone depending on the starting material provided.
  • Predict the product of reactions of aldehydes or ketones and provide a reasonable mechanism for formation of the proposed product.
  • Discuss the characteristics of nucleophilic addition to aldehydes and ketones.
  • Compare acid-catalyzed and base-catalyzed addition of water to aldehydes and ketones and provide a mechanism for each.
  • Justify the relative reactivity and electrophilicity of aldehydes vs. ketones.
  • Describe the anomeric effect and identify when it would likely be observed based on the structure of a given acetal.
  • Choose the best reagent to reduce aldehydes and ketones to alcohols and alkanes.
  • Differentiate between imine and enamine formation and choose which will be formed under the given conditions, giving a mechanism for each formation.
  • Compare and contrast direct (1,2) addition and conjugate (1,4) addition.
  • Employ acetals and thioacetals as protecting groups in order to synthesize aldehydes and ketones.


22.1 Aldehydes and Ketones in Nature

Aldehydes and ketones are characterized by the carbonyl (C=O) moiety. They occur widely in nature as intermediates in metabolism and biosynthesis. Aldehydes and ketones are particularly prevalent in carbohydrates and nucleic acids. The aldehyde functional group can be clearly seen in the open-chain form of D-glucose. The closed-chain form of D-glucose and D-ribose (in adenosine, a deoxyribonucleoside) contain a carbonyl derivative functional group called a hemiacetal. This derivative will be discussed further in a later section of this chapter.

Figure 22.1. Structure of the ketone and aldehyde functional groups​
Figure 22.2. Aldehyde and hemiacetals in nature. Aldehyde or hemiacetals noted in red​ ​

Carbonyls are also found in many natural products and are commonly used in fabrics, flavorings, plastics, and drugs. For example, aldehydes are responsible for the flavor of cinnamon and vanilla, and ketones are the source of spearmint flavoring.

Figure 22.3. Natural products contain ketone and aldehyde functional groups​​


22.2 Structure and Properties of Aldehydes and Ketones

22.2.1 Bonding and Resonance in Carbonyls

The carbon of the carbonyl is sp2 hybridized, giving the carbonyl a trigonal planar geometry with bond angles of 120°. When compared to a C=C bond, the C=O bond is stronger, shorter, and polarized.

Figure 22.4. The polarized C=O bond is shorter and stronger than a C=C bond​


Due to the polarization of the C=O bond, two resonance forms can be drawn. The major form gives all atoms an octet, while the minor form shows the location of charges, with a negative charge preferring the electronegative oxygen atom. This minor form shows the typical reactivity of the carbonyl group. The carbonyl carbon, being positively charged, is electrophilic with the negatively charged oxygen being nucleophilic.

Figure 22.5. a) The electrophilic carbon and nuceleophilic oxygen of a carbonyl shown by resonance contributors. b) net molecular dipole and electrostatic potential energy diagram

The polarization of the carbonyl bond causes dipole-dipole interactions to occur. These affect the boiling points of carbonyl compounds, making the boiling point higher than those of hydrocarbons or ethers of similar molecular weight. However, the boiling point of carbonyl compounds is lower than those of alcohols, as aldehydes and ketones cannot act as hydrogen bond donors. The oxygen atom does allow aldehydes and ketones to act as hydrogen bond acceptors when in the presence of alcohols (OH) or amines (NH). Polarization of the carbonyl group causes aldehydes and ketones (in particular, acetone) to be used as polar aprotic solvents, particularly in SN2 reactions.

Q22.1 - Level 2

Rank these compounds in order of increasing boiling point, with the highest boiling point at the top and the lowest at the bottom.

question description
A

Compound D

B

Compound A

C

Compound B

D

Compound C


22.3 Synthesis of Aldehydes and Ketones:

22.3.1 Synthesis from Alkenes (Review Section 12.13)

Ozone (O3) adds to alkenes and cleaves them, resulting in aldehydes and/or ketones, depending on the substitution of the alkene.

The overall reaction is as follows.

Figure 22.6. Schematic of the ozonolysis reaction​

Example:

Figure 22.7. Ozonolysis of 2-methylbut-2-ene gives acetone and acetaldehyde as products​

The second step is called a reductive workup and uses a reducing agent (commonly Me2S or Zn metal) to cleave the intermediate resulting in the given products.


Q22.2 - Level 2

Predict the product(s) of the following reaction.

question description
A

a

B

b

C

c

D

d

E

e


22.3.2 Synthesis from Terminal Alkynes (Review Section 13.5.5)

Aldehydes and ketones can be prepared by hydration of terminal alkynes. In the hydration reaction, water is added across the triple bond. Methyl ketones are prepared by the mercury-catalyzed hydration of terminal alkynes, and hydroboration-oxidation gives an aldehyde product. These reactions are related to the oxymercuration-demercuration and hydroboration-oxidation reactions of alkynes, discussed in an earlier chapter.

Mercury–catalyzed hydration of alkynes is as follows.

Figure 22.8. Mercury-catalyzed hydration of terminal alkynes gives methyl ketones. Note that as in the oxymercuration-demercuration reaction of alkenes, the alcohol group goes to the more substituted position of the enol intermediate to give a product with Markovnikov regiochemistry.​


Hydroboration-oxidation of alkynes is as follows.

Figure 22.9. Hydroboration-oxidation of terminal alkynes gives aldehydes. As in the related hydroboration-oxidation reaction of alkenes, the hydroxyl group goes to the less substituted side of the enol intermediate, giving a product with anti-Markovnikov regiochemistry.​

Using BH3 in the hydroboration-oxidation reaction of alkynes can give a mixture of products, so disiamylborane or 9-BBN are often used.

Figure 22.10. Structure of disiamylborane and 9-BBN, alternate reagents for hydroboration-oxidation reactions​

In both of these reactions, the intermediate is called an enol, which tautomerizes into a ketone or aldehyde. The ketone and enol forms are called tautomers, which are isomers that rapidly interconvert by the movement of a proton. Tautomerism is discussed in detail in Chapter 26.

Figure 22.11. Keto-enol tautomerism generally favors the keto tautomer


Q22.3 - Level 1

Choose the correct final product of the following reaction.

question description
A

a

B

b

C

c

D

d

E

e


22.3.3 Synthesis from Aromatic Compounds (Review Section 21.1.5)

The Friedel-Crafts acylation of aromatic rings provides aromatic ketones. In this reaction, an acid chloride reacts with an aromatic ring in the presence of a Lewis acid catalyst (usually AlCl3) to introduce an acyl group.

Figure 22.12. A general Friedel-Crafts acylation to give an aryl ketone

The reactive intermediate in this reaction is a resonance-stabilized acyl cation, which does not undergo rearrangement, unlike the carbocation intermediate in the related Friedel-Crafts alkylation reaction described in a previous chapter. Furthermore, multiple acylations do not typically occur as the product is less reactive than the starting material. However, this reaction does suffer from another limitation of Friedel-Crafts reactions in that it does not occur when the aromatic ring contains an amino group or a strongly electron-withdrawing group (e.g. NO2). In the acylation reaction of substituted benzenes, the directing effects of the substituents must be considered (review Sections 21.3.1 and 21.3.2).


Q22.4 - Need Level

Identify the sequence of reactions that will accomplish the following transformation in high yield.

question description
A

a

B

b

C

c

D

d


22.3.4 Synthesis from Alcohols, Dess-Martin Periodinane (DMP) and Swern Oxidation (Review Section 14.6)

Alcohols can be oxidized to form aldehydes and ketones. Primary alcohols are oxidized to aldehydes using specialized reagents (typically Dess-Martin periodinane, DMP, or pyridinium chlorochromate, PCC or Swern oxidation). Ketones are prepared from secondary alcohols using a variety of oxidizing agents, while tertiary alcohols cannot be oxidized.

Figure 22.13. Oxidation of primary alcohols with DMP, PCC or Swern oxidation reagents gives aldehydes as products​


Dess-Martin Periodinane (DMP) contains a hypervalent iodine atom that imparts its oxidizing ability. This reagent works well under mild conditions and is a stable, commercially-available solid. In the mechanism, the oxygen of the primary or secondary alcohol attacks the iodine atom, displacing an acetate anion, which can be protonated to give acetic acid (AcOH). In the second step, a second acetate anion removes a proton from the carbon bearing the hydroxyl group, eventually forming a C=O double bond, as shown in Figure 22.14a.

22.14.png
Figure 22.14a. Mechanism for DMP oxidation

​Swern oxidation of alcohols to aldehydes can be carried out under very mild conditions and like DMP avoids the use of the toxic metal, chromium. Dimethyl sulfoxide (DMSO) and oxalyl chloride are added at low temperatures (-78 °C) to form an alkyoxysulfonium ion which with the addition of a base, produces a sulfur ylide. The sulfur ylide undergoes intramolecular deprotonation and fragmentation to yield an aldehyde and the foul-smelling gas, dimethyl sulfide (DMS). In the first step of the mechanism, DMSO reacts with oxalyl chloride to produce a sulfur compound which is attacked by a chloride ion to form a chlorosulfonium salt, CO and CO2. The alcohol then reacts with the chlorosulfonium cation to form an alkoxysulfonium ion. After the addition of a base (Et3N) the salt is deprotonated to form an alkoxysulfonium ylide. The redox reaction is completed after a proton is transferred to the anionic carbon of the ylide via a five-membered transitions state to produce an aldehyde and DMS.

​Figure 22.14b. Mechanism for the Swern Oxidation


An example of the oxidation of secondary alcohols is shown below.

Figure 22.15. Oxidation of secondary alcohols gives ketones as products, while tertiary alcohols cannot be oxidized​


22.3.5 Synthesis from Esters

Aldehydes can be prepared by careful reduction of an ester with one equivalent of a reducing agent such as diisobutyl aluminum hydride (DIBAL-H or DIBAH). Cold temperatures help to prevent over-reduction.

Figure 22.16 Reduction of esters with DIBAL-H gives aldehydes

Example:

Figure 22.17. Reduction of ethyl pentanoate with DIBAL-H gives pentanal​


Q22.5 - Level 2

Match the correct reagent to each reaction.

question description
Premise
Response
1

A

A

Reagent I

2

B

B

Reagent II

3

C

C

Reagent III


22.3.6 Synthesis from Nitriles

Reaction of nitriles with Grignard reagents gives imines (discussed later in this chapter), which can be hydrolyzed to give ketones.

Figure 22.18. Grignard reaction of nitriles gives ketones as products​


Q22.6 - Level 3

Match the starting material to the necessary reagent to produce acetophenone. Not all reagents will be used.

question description
Premise
Response
1

1

A

Reagent F

2

2

B

Reagent D

3

3

C

Structure A

4

4

D

Reagent E

E

Structure B

F

Reagent C


Watch the video below to see an explanation of the answer to Question 22.6.



Q22.7 - Need Level

Predict the correct final product of the following sequence of reactions.

question description
A

a

B

b

C

c

D

d

E

e


22.4 Nucleophilic Addition to Carbonyls

22.4.1 General Mechanism of Nucleophilic Addition

As mentioned in Section 22.2.1, the carbon atom of the carbonyl is electrophilic and reacts with nucleophiles. An alkoxide tetrahedral intermediate results, which can then be protonated to give an alcohol product.

Figure 22.19. Reaction of carbonyls with negatively charged nucleophiles forms a tetrahedral intermediate which is then protonated to yield an alcohol.​


The nucleophilic addition of a nucleophile to an electrophile occurs between the HOMO of the nucleophile, the orbital that possesses the electrons to be donated, and the LUMO of the electrophile, an empty orbital that can accept the electrons from the nucleophile.

When a nucleophile attacks the electrophilic carbonyl carbon, it may be surprising to discover that the angle of attack is approximately 107° from the carbonyl oxygen, which is almost the same angle as the newly formed sp3 σ bond. The angle of attack is referred to as the Bürgi-Dunitz trajectory, named after the scientists who used crystallographic methods to elucidate the angle of attack. This angle of attack can be thought of as a compromise between two effects: first, the maximum orbital overlap of the HOMO of the nucleophile with the π* orbital, and second, the minimum repulsion of the nucleophile and the HOMO of the C=O bond.

Figure 22.20. Nucleophilic attack on the carbonyl carbon occurs at 107°.​

As seen in the diagram, as the nucleophile approaches the electrophilic carbon, electrons are being donated from the HOMO of the nucleophile into the LUMO (antibonding π*) of the C=O bond, but because of the electron repulsion of the HOMO of the C=O bond, the nucleophile must attack at an oblique angle. The filling of the antibonding orbital with the electrons from the nucleophile forms a new σ bond and causes the π bond to break, with the electrons from the π bond moving onto the electronegative oxygen. Anything that sterically interferes or gets in the way of allowing the nucleophile to attack at this bond angle greatly reduces the rate of addition.

22.4.2 Reactivity of Aldehydes vs. Ketones Towards Nucleophilic Addition

In general, aldehydes are more reactive than ketones towards nucleophilic additions because the transition state for the addition is less crowded and lower in energy for an aldehyde than for a ketone. This is due to both steric and electronic factors. In terms of sterics, ketones have two large alkyl substituents bonded to the carbonyl carbon, while aldehydes only have one.

Aldehydes are also more electrophilic due to electronic factors, because they only have one inductively electron-donating alkyl group to stabilize the partial positive charge on the carbonyl carbon while ketones have two.

Figure 22.21. Aldehydes are more reactive towards nucleophiles (i.e. more electrophilic) than ketones. Blue arrows indicate electron donation through hyperconjugation.


Q22.8 - Level 2

Rank the following compounds in order of decreasing reactivity towards nucleophilic addition, from fastest at the top to slowest at the bottom.

question description
A

Compound A

B

Compound E

C

Compound D

D

Compound B

E

Compound C


Watch the video below to see an explanation of the answer to Question 22.8.


22.4.3 Nucleophilic Addition in Acidic Medium

Nucleophilic additions to carbonyls take place under both acidic and basic conditions. When carbonyls are in acidic media, the first step is protonation of the carbonyl oxygen by the acid. This makes the carbonyl more electrophilic, so that weak nucleophiles such as water, alcohols, thiols, and amines are then able to react with the electrophilic carbon. The nucleophile generally needs to be deprotonated after the attack.

Figure 22.22. Mechanism for nucleophilic attack under acidic conditions​​


22.4.4 Nucleophilic Addition in Basic or Neutral Medium

Nucleophilic attack will also occur under basic conditions. In this case, the nucleophile is a strong nucleophile and the carbonyl does not need to be protonated first to make it more electrophilic.

Figure 22.23. Mechanism for nucleophilic attack under basic or neutral conditions


Q22.9 - Level 3

Predict the correct sequence of curved arrows for the acid-catalyzed nucleophilic addition of water to 2-butanone by clicking on the appropriate arrows.


Watch the video below to see an explanation of the answer to Question 22.9.


22.5 Reactions of Aldehydes and Ketones

22.5.1 Reduction to Alcohols (Review Section 14.5.4)

Treatment of aldehydes and ketones with reducing agents gives alcohol products. Lithium aluminum hydride (LiAlH4) and sodium borohydride (NaBH4) are nucleophiles and react as donors of a hydride (H-) ion. LiAlH4 is also a very strong base that reacts violently with water, so the reaction must be carried out in aprotic solvents like anhydrous THF. Otherwise, the reagent is destroyed. Water is then added in the second step to protonate the alkoxide. NaBH4 is less reactive and can be used in protic solvents, followed by an acidic workup. Aldehydes react with these reagents to give primary alcohols, and secondary alcohols are formed from the reaction of these reagents with ketones. A general reaction is shown below with NaBH4 as the nucleophile.

Figure 22.24. Simplified mechanism for reduction of a ketone with sodium borohydride, followed by acidic workup​


Figure 22.25. Reduction of aldehydes gives primary alcohols while secondary alcohols are formed by reduction of ketones.​


22.5.2 Addition of Grignard Reagents (Review Chapter 14)

Alcohols are formed by reaction of aldehydes and ketones with Grignard reagents. The Grignard reagent acts as a source of an alkyl anion, which is a strong nucleophile that adds to the carbonyl carbon. Aldehydes and ketones react with Grignard reagents to form secondary and tertiary alcohols, respectively.

Figure 22.26. Simplified mechanism for addition of a Grignard reagent to a ketone​ ​


Figure 22.27. Grignard reactions with aldehydes and ketones give secondary and tertiary alcohols, respectively.


Q22.10 - Level 2

Match the correct product to each reaction.

question description
Premise
Response
1

1

A

Product B

2

2

B

Product D

3

3

C

Product C

4

4

D

Product A


22.5.3 Hydration of Ketones and Aldehydes

Ketones and aldehydes undergo nucleophilic addition with water under both acidic and basic conditions. The hydrated product of the reaction is a geminal diol.

Figure 22.28. Hydration of an aldehyde or ketone

Under aqueous conditions, aldehydes exist as partial hydrates. Ketones generally do not favor the hydrated product. The mechanism is important to consider under both acidic and basic conditions.

22.5.3.1 Mechanism of Hydration of Ketones and Aldehydes under Acidic and Basic Conditions

Under basic conditions, the nucleophile is strong. The basic mechanism begins with attack of the hydroxide ion at the carbonyl carbon, leading to the formation of  the tetrahedral intermediate. The alkoxide species is protonated by water forming the hydrate and hydroxide.

Figure 22.29. Mechanisim of hydration under basic conditions. ​

Under acidic condition, the electrophile (either the aldehyde or the ketone) is first made more electrophilic by protonating the carbonyl oxygen. This allows the weak nucleophile (water) to attack, once again, leading to the formation of a tetrahedral species. This species forms the geminal diol after deprotonation with water.

Figure 22.30. Mechanism for hydration under acidic conditions​ ​

Notice that under basic conditions, the protonation occurs with water. This leads to the formation of hydroxide, which is acceptable under basic conditions. Under acidic conditions, water is also used to deprotonate and leads to the formation of the hydronium ion. Water, in its original form, will exist under both sets of conditions. A common mistake while writing mechanisms is proposing unlikely species inadvertently. For example, hydroxide should not be proposed or generated under acidic conditions and the hydronium ion should not be generated under basic conditions
 

Q22.11 - Level 2

Which of these species are likely to exist under only basic conditions, only acidic conditions, or both acidic and basic conditions?

Premise
Response
1

MeOH

A

Acidic

2

H2_{2}O

B

Basic

3

HO^{-}

C

Basic

4

EtO^{-}

D

Acidic and basic

5

H3_{3}O+^{+}

E

Acidic

F

Acidic and basic


In an aqueous solution, a ketone or aldehyde is in equilibrium with its hydrate and in most cases the hydrate is impossible to isolate.

Figure 22.31 Equilibrium between ketone or aldehyde and its hydrate

The equilibrium will shift depending on the reactivity of the carbonyl compound and there are two main factors that determine the electrophile’s reactivity:

Steric effects: Groups attached to the carbonyl carbon sterically hinder the attack by a nucleophile and decreases the rate of reaction.

Electronic effects:

  • Electron-withdrawing groups: Electronegative atoms like halogens adjacent to the carbonyl carbon increase the partial positive charge on the carbon making it more electrophilic.
  • Electron-donating groups: Alkyl groups stabilize the partial positive charge on the carbonyl carbon making it less electrophilic.

Thus formaldehyde is extremely reactive to hydration because it contains no alkyl groups and does not sterically hinder an attack by a nucleophile. In fact, it is so reactive that there is essentially no free formaldehyde; it is completely hydrated.

Q22.12 - Level 2

Order the following aldehydes and ketone from most reactive to least reactive towards hydration, with the most reactive at the top.

question description
A

3

B

2

C

1


Trichloroacetaldehyde (Cl3CHO) as seen from the above question is extremely reactive and forms a stable hydrate. It can be isolated as crystals and it is used as an anesthetic. Most hydrates cannot be isolated as pure substances.  However, they do play a role in the oxidation of alcohols to ketones in aqueous chromic acid.

22.5.4 Cyanohydrin Formation

An important reaction of aldehydes and ketones involves the addition of a cyanide anion, a strong nucleophile.

Figure 22.32. Reaction of an aldehyde or ketone with the cyanide ion​


Figure 22.33. Cyanohydrin product of the reaction of potassium cyanide with benzaldehyde​


The reaction results in the formation of a carbon-carbon bond, which is important synthetically. It adds only one carbon, which is an important result to note. Further synthetic transformations are possible, such as hydrolyzing the cyano group to a carboxylic acid or reduction to form an amine.

Figure 22.34. Two examples of functional group transformations of the nitrile functionality to access carboxylic acids and amines



Q22.13 - Level 2

Order the following compounds from most reactive to least reactive towards cyanohydrin formation, with the most reactive at the top and the least reactive on the bottom.

question description
A

2

B

3

C

1


22.5.4.1 Mechanism of Cyanohydrin Formation

Figure 22.35. Mechanism of cyanohydrin formation

The mechanism begins with attack on the carbonyl carbon by the cyanide anion followed by protonation of the resulting alkoxide with hydrogen cyanide, a weak acid.

The cyanohydrin reaction can be reversed by dissolving the cyanohydrin in water.

Figure 22.36​


Q22.14 - Level 1

Which of the following would not be a valid mechanism step for the reverse reaction of the cyanohydrin reaction?

question description
A

a

B

b

C

c


Cyanohydrin formation is a reversible reaction. It is readily converted to the aldehyde or ketone in aqueous base. Under these conditions the cyanohydrin is usually converted completely to the aldehyde or ketone. 

Discussion Question 22.14a

Figure 22.37. Can you draw the mechanism for the converstion of a cyanohydrin to a ketone?​
Q22.14a

Can you draw the mechanism for the above reaction?

Click here for the answer.

22.5.5 Acetal and Hemiacetal Formation

Hemiacetal and acetal moieties are commonly encountered in organic synthesis and in nature. Most notably, sugar molecules contain an acetal moiety. A derivative of cellulose also contains a hemiacetal functionality that is commonly observed.

Figure 22.38. Examples of acetals and hemiacetals in nature

The acetal is formed when two equivalents (two moles) of an alcohol combines with an aldehyde or ketone under acidic conditions. It is interesting to note that the oxidation state of the initial carbonyl compound and the acetal are the same this is despite the fact that  the carbonyl carbon has been converted from  sp2  into a sp3 carbon.

The reaction takes place only under acidic conditions.

Figure 22.39. General acetal formation reaction

Example:

Figure 22.40. Example of acetal formation from aldehyde with alcohol under acidic conditions


22.5.5.1 Mechanism of Acetal Formation

The reaction of aldehydes and ketones with alcohols parallels their reaction with water. The reaction requires activation of the carbonyl to form a stronger electrophile so that the weak nucleophile (the alcohol) can attack at the carbon atom. The protonated carbonyl is then attacked by the alcohol species. Deprotonation of the newly attached alcohol leads to the formation of a hemiacetal. This is the halfway point of the reaction.

Figure 22.41. Mechanism of formation of hemiacetals (this is the first half of the mechanism for ketal formation).


Once the hemiacetal is formed, the addition of the second equivalent of alcohol is effected by protonation of the hydroxyl group to create a good leaving group in the form of water. It is very important to avoid proposing an SN2-like displacement at this step of the mechanism. Instead, a lone pair on the first equivalent of alcohol pushes out the water, forming a thermodynamically more stable sp2 hybridized intermediate that looks a lot like our initially activated carbonyl compound. This species is attacked by the second equivalent of alcohol and an acetal results after a final deprotonation step.

Figure 22.42. Second half of the mechanism: hemiacetal to acetal.​

Each step in the mechanism is reversible in the presence of acid. The equilibrium can be shifted towards the acetal by using excess alcohol or removing water. Conversely, you can reverse the reaction towards the aldehyde or ketone by adding excess water. This is called acetal hydrolysis. Acetals, unlike most hydrates and hemiacetals, may be isolated as pure substances by neutralizing the acid used as the catalyst.


Q22.15 - Level 3

Matching: Which ketone or aldehyde must have been used to form the acetals shown? Alcohol partners are not shown.

question description
Premise
Response
1

1

A

Structure D

2

2

B

Structure B

3

3

C

Structure C

4

4

D

Structure A

E