Chapter 13. Aldehydes

and Ketones

Based on McMurry’s Organic Chemistry, 6th edition

1

Aldehydes and Ketones

Aldehydes and ketones are characterized by the carbonyl

functional group (C=O)

The compounds occur widely in nature as intermediates in

metabolism and biosynthesis

They are also common as chemicals, as solvents, monomers,

adhesives, agrichemicals and pharmaceuticals

2

The Carbonyl Group

3

The Carbonyl Group

4

Naming Aldehydes and Ketones

Aldehydes are named by replacing the terminal -e of the corresponding alkane name with –al

The parent chain must contain the CHO group

The CHO carbon is numbered as C1

If the CHO group is attached to a ring, use the suffix -carbaldehyde

5

Naming Ketones

Replace the terminal -e of the alkane name with –one

Parent chain is the longest one that contains the ketone group

Numbering begins at the end nearer the carbonyl carbon

6

Common Names

IUPAC retains well-used but unsystematic names for a few

ketones:

7

Ketones and Aldehydes as Substituents

The R–C=O as a substituent is an acyl group is used with the suffix -yl from the root of the carboxylic acid

CH3CO: acetyl; CHO: formyl; C6H5CO: benzoyl; ArCO: aroyl

The prefix oxo- is used if other functional groups are present and the doubly bonded oxygen is labeled as a substituent on a parent chain

8

Preparation of Aldehydes and Ketones

Preparing Ketones

Oxidize secondary alcohols using (PCC), or CrO3 or Na2Cr2O7

9

Preparing Aldehydes

Oxidize primary alcohols using pyridinium chlorochromate (PCC)

Preparing Ketones: Other Methods

Hydration of terminal alkynes in the presence of Hg2+ catalyst

10

Friedel–Crafts acylation of an aromatic ring with an acyl chloride

in the presence of AlCl3 catalyst

Oxidation of Aldehydes

CrO3 in aqueous acid oxidizes aldehydes to carboxylic acids efficiently

Silver oxide, Ag2O, in aqueous ammonia (Tollens’ reagent)oxidizes aldehydes (no acid)

11

Nucleophilic Addition Reactions of Aldehydes

and Ketones

Nu- approaches 45° to the plane of C=O and adds to C, pushing p electrons onto oxygen

A tetrahedral alkoxide ion intermediate is produced

12

Nucleophiles

Nucleophiles can be negatively charged ( : Nu) or neutral ( : Nu) at the reaction site

The overall charge on the nucleophilic species is not considered

13

14

Relative Reactivity of Aldehydes and Ketones

Aldehydes are generally more reactive than ketones in nucleophilic addition reactions

The transition state for addition is less crowded and lower in energy for an aldehyde (a) than for a ketone (b)

Aldehydes have one large substituent bonded to the C=O: ketones have two

15

Electrophilicity of Aldehydes and Ketones

Aldehyde C=O is more polarized than ketone C=O

As in carbocations, more alkyl groups stabilize + character

Ketone has more alkyl groups, stabilizing the C=O carbon inductively

16

Reactivity of Aromatic Aldehydes

Less reactive in nucleophilic addition reactions than aliphatic aldehydes

Electron-donating resonance effect of aromatic ring makes C=O less reactive electrophilic than the carbonyl group of an aliphatic aldehyde

17

Nucleophilic Addition of H2O: Hydration

Aldehydes and ketones react with water to yield 1,1-diols (geminal (gem) diols)

Hydration is reversible: a gem diol can eliminate water

18

Relative Energies

Equilibrium generally favors the carbonyl compound over

hydrate for steric reasons

Acetone in water is 99.9% ketone form

Exception: simple aldehydes

In water, formaldehyde consists is 99.9% hydrate

19

Base-Catalyzed Addition of Water

Addition of water is catalyzed by

both acid and base

In base-catalyzed hydration

nucleophile is the hydroxide ion,

which is a much stronger

nucleophile than water

20

Acid-Catalyzed Addition of Water

Protonation of C=O makes it

more electrophilic

21

Nucleophilic Addition of Alcohols: Acetal Formation

Two equivalents of ROH in the presence of an acid catalyst add

to C=O to yield acetals, R2C(OR)2

These can be called ketals if derived from a ketone

22

Formation of Acetals

Alcohols are weak nucleophiles but acid promotes addition forming the conjugate acid of C=O

Addition yields a hydroxy ether, called a hemiacetal (reversible); further reaction can occur

Protonation of the OH and loss of water leads to an oxonium ion, R2C=OR+ (an oxygen bearing a positive charge) to which a second alcohol adds to form the acetal

23

Uses of Acetals

Acetals can serve as protecting groups for aldehydes and ketones

It is convenient to use a diol, to form a cyclic acetal (the reaction goes even more readily)

24

Cyclic Structures of Monosaccharides:

Hemiacetal Formation

25

glucose

Hydride Addition: reduction

Convert C=O to CH-OH

LiAlH4 and NaBH4 react as donors of hydride ion

Protonation after addition yields the alcohol

26

Reduction Reagent: Sodium Borohydride

NaBH4 is not sensitive to moisture and it does not reduce other common functional groups

Lithium aluminum hydride (LiAlH4) is more powerful, less specific, and very reactive with water

Both add the equivalent of “H-”

27

28

End of chapter 13