Chapter 9

Aldehydes and Ketones:Nucleophilic Addition Reactions

Chemistry of the Carbonyl Group

O

Carbonyl group, C, (C=O)

O

Acyl group, RC, (-COR)

Carbonyl compounds can be classified into two categories:

RCHO; R2CO

RCOOH; RCOX; RCOOR’RCOSR’

RCONH2; RCOOCOR’RCOOPO3

2-

Naming Aldehydes and Ketones

• The functional group of an aldehyde is a carbonyl group bonded to a H atom – in methanal, it is bonded to two H atoms

– in all other aldehydes, it is bonded to one H and one carbon atom

O

Methanal, HCH

O

RCH

• IUPAC names: – select as the parent alkane the longest chain of

carbon atoms that contains the carbonyl group– because the carbonyl group of the aldehyde must be

on carbon 1, there is no need to give it a number– changing the suffix -e to -al

• For unsaturated aldehydes, show the presence of the C=C by changing the infix -an- to -en-

• For cyclic molecules in which the -CHO group is attached to the ring, the name is derived by adding the suffix -carbaldehyde to the name of the ring

• The functional group of a ketone is a carbonyl group bonded to two carbon atoms

• IUPAC names: – select as the parent alkane the longest chain that

contains the carbonyl group – changing the suffix -e to -one – number to give C=O the smaller number

• When it’s necessary to the -COR group as a substituent, the general term acyl is used

• Common names

If the double bonded oxygen is considered a substituent, and the prefix oxo- is used

Synthesis of Aldehydes and Ketones

1. The oxidation of alcohol (chapter 8)

2. The hydration of terminal alkyne (chapter 4)

3. The Friedel-Crafts acylation of an aromatic ring (chapter 5)

Oxidation of Aldehydes

• Aldehydes are easily oxidized to yield carboxylic acids, RCHO → RCOOH

• Ketones are unreactive toward oxidation

• Tollens’ reagent: Silver ion, Ag+, in dilute aqueous ammoniaA simple test to detect the presence of an

aldehyde functional group in a sample of unknown structure

Nucleophilic Addition Reactions: Reduction

• One of the most common reactions of the carbonyl group is addition of a nucleophile to form a tetrahedral carbonyl addition compound

• The reaction can take place under either basic and acidic conditions

+ CR

RO CNu

OH

RR

Nu

:

: ::

:

+ -

Nucleophilic Addition of Hydride and Grignard Reagents: Alcohol Formation

• Addition of Hydride reagents: Reduction• Chapter 8

• The nucleophile is hydride ion (:H-) supplied by NaBH4

• The reaction is irreversible

Addition of Grignard Reagents

• Aldehydes give secondary alcohols on reaction with Grignard reagents in ether solution, and ketones give tertiary alcohols

• The nucleophile is a carbanion (R:-) from the Grignard reagents

• Undergo nucleophilic addition mechanism under basic conditions

• The reaction is irreversible

• Grignard reagent can not be prepared from compounds that have the following functional groups in the molecules:

? →

Nucleophilic Addition of Water: Hydration

• Aldehydes and ketones undergo a nucleophilic addition reaction with water to yield 1, 1-diol, called geminal diol

• The reaction is reversible, and the diol product can eliminated water to regenerate a ketone or aldehyde

Acetone (99.9%) gem diol (0.1%)

Formaldehyde (0.1%) gem diol (99.9%)

• The nucleophilic addition reaction of water to aldehydes and ketones is slow but is catalyzed by both base and acid

• The base-catalyzed reaction takes place rapidly because hydroxide ion is a much better nucleophilic donor than neutral water

• The acid-catalyzed reaction takes place rapidly because carbonyl compound is converted by protonation into a better electrophilic acceptor

Nucleophilic Addition of Alcohols:Acetal Formation

• Aldehydes and ketones react with alcohol in the presence of an acid catalyst to yield acetals, R2C(OR’)2, compounds that have two ether-like –OR groups bonded to the same carbon:

• Acetal formation are reversible• Remove water • Add water

• Acetal formation involves the acid-catalyzed nucleophilic addition of an alcohol to the carbonyl group

• The initial nucleophilic addition step yields a hydroxy ether called a hemiacetal, which reacts further with a second equivalent of alcohol to yield the acetal

• The reactions are reversible, depending on the water

• Acetals are valuable to organic chemistry because they can serve as protecting groups for aldehydes and ketones

Acetal can serve as protecting groups for aldehydes and ketones

Importance of Hemiacetals and Acetals: Nature and the Laboratory

Nucleophilic Addition of Amines: Imine Formation

• Ammonia and primary amines. R’NH2, add to aldehydes and ketones to yield imines, R2C=NR’

(Derivative of vitamin B6)

• Imines are common intermediates in numerous biological pathways and processes, including the route by which amino acids are synthesis and degraded

Conjugate Nucleophilic Addition Reactions

• , -unsaturated aldehyde or ketone– A C=C double bond between the so-called carbon (the C

next to the C=O group) and the carbon (the C away from the C=O group)

– the carbon is more electron-poor and more electrophilic than a typical alkene C=C bond

• The initial product of conjugate addition is resonance-stabilized enolate ion, which typically undergoes protonation on the a carbon to give a saturated aldehyde or ketone product

• Conjugate addition occurs because the electronegative oxygen atom of the -unsaturated carbonyl compound withdraws electrons from the carbon

• Thereby making it more electron-poor and more electrophilic than a typical alkene C=C bond