Organic chemistry for medicine and biology students

Organic B

Chapter 18Aldehydes and Ketones

By Prof. Dr.Adel M. Awadallah

Islamic University of Gaza

Nomenclature of Aldehydes and KetonesCommon aldehydes

H H

O

CH3 H

O

CH3CH2CH2 H

O

CH3CH2 H

O

H

O

HO HO

OH

HO

OH

OMe

Methanl ethanl propanal butanal(formaldehyde) (acetaldehyde) (propionaldehyde) (n-butyraldehyde)

benzaldehyde salicylaldehyde(2-hydroxybenzenecarbaldehyde) Vanillin

cyclopentanecarbaldehyde

Common Ketones

CH3 CH3

O

CH3

O

CH3

O

CH3CH3

OOCH3

O

propanone (acetone)

2-butanone(ethyl methyl ketone)

3-pentanone(diethyl ketone)

cyclohexanone acetophenone(methyl phenyl ketone)

benzophenone(diphenyl ketone)

Nomenclature of aldehydes and ketones

(al) aldehyde, (one) ketonealkanes < alkenes < OH < ketone < aldehyde < acid < esterExamples

OOH

H

OO

CH3

CH3

O

CH3 O

HCH3

Cl O

CH3CH3

12

3451

23

4

123

456

4-hydroxy-2-pentanone 3-oxobutanl 3-hexen-2-one

1

23

41 2

3

45

6

2-cholro-3-methylbutanal 2,4-dimethyl-3-hexanone

Acyl groups

Common aldehydes and ketones

CH3OH CH2 = O + H2

Ag

600 - 700 oC

Formaldehyde

Formaldehyde is a gas (b. p. -21 oC) Formalin (37% aqueous solution of formaldehyde)

Acetaldehyde (Wacker synthesis)

2 CH2 = CH2 + O22 CH3CH = O

Pd - Cu

100 - 130 oC (bp 20 oC)

O

OH

CH3 CH3

O

Acetone (Wacker synthesis)

2 CH3CH2 = CH2 + O22 CH3 C CH3

Pd - Cu

100 - 130 oC (bp 56 oC)

From isopropylbenzene

1) O2

2) dil H2SO4

+

Synthesis of aldehydes and Ketones

1) Oxidation of Alcohols

primary gives aldehydes using PCC

secondary gives ketones

2) Friedel-Crafts

3) From Alkynes

OCH3

CH3COCl

AlCl3

4- Aldehydes from Acid Chlorides

5 -Ketones from Acid Chlorides

Synthesis of Alkylbenzenes

Reduction of Acids and their derivatives by Lithium Aluminum hydride (LAH)

Naturally occuring aldehydes and Ketones

The carbonyl group

Reactions of the carbonyl group

Remember: Generation of a chiral center

A. Hydration and Hemiacetal Formation• Water adds rapidly to the carbonyl function of aldehydes and ketones. In most

cases the resulting hydrate (a geminal-diol) is unstable relative to the reactants and cannot be isolated.

• Exceptions to this rule exist, • one being formaldehyde (a gas in its pure monomeric state). • Thus, a solution of formaldehyde in water (formalin) is almost exclusively the

hydrate, or polymers of the hydrate. • Another is chloral hydrate

CCl3 OH

OH

HChloral hydrate

Addition of Alcohols

Examples

Addition of hydrogen cyanide to aldehydes and ketones

Reducing Agents

The reduction of an aldehyde

You get exactly the same organic product whether you use lithium tetrahydridoaluminate or sodium tetrahydridoborate.

For example, with ethanal you get ethanol:

The reduction of a ketone

• Again the product is the same whichever of the two reducing agents you use.

• For example, with propanone you get propan-2-ol:• Reduction of a ketone leads to a secondary alcohol.

REACTION OF ALDEHYDES AND KETONES WITH GRIGNARD REAGENTS

The reaction between Grignard reagents and methanal

1) EtMgBr

2) H2O, H+

1) EtMgBr

2) H2O, H+

The reaction between Grignard reagents and other aldehydes

The reaction between Grignard reagents and ketones

1) EtMgBr

2) H2O, H+

Reaction with Acetylides

OXIDATION OF ALDEHYDES AND KETONES

6) Cannizzaro reaction. (self oxidation/reduction)

a reaction of aldehydes without α-hydrogens

CHO

Br

conc. NaOH

CH2OH COO-

Br Br

+

CH3OH + HCOO-H2C=Oconc. NaOH

The Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro, is a chemical reaction that involves the base-induced disproportionation of an aldehyde.[1][2] Cannizzaro first accomplished this transformation in 1853, when he obtained benzyl alcohol and potassium benzoate from the treatment of benzaldehyde with potash (potassium carbonate). More typically, the reaction would be conducted with sodium or potassium hydroxide:

Mechanism of the Cannizzaro Reaction

Crossed Cannizzaro:

Formaldehyde is the most easily oxidized aldehyde. When mixed with another aldehyde that doesn’t have any alpha-hydrogens and conc. NaOH, all of the formaldehyde is oxidized and all of the other aldehyde is reduced.

CH=O

OCH3

OH

vanillin

+ H2C=Oconc. NaOH

CH2OH

OCH3

OH

+ HCOO-

ADDITION-ELIMINATION REACTIONS OF ALDEHYDES AND KETONES

with hydroxylamineThe product is an "oxime" - for example, ethanal oxime.

Formation of Imines and Related Compounds

The reaction of aldehydes and ketones with ammonia or 1º-amines forms imine derivatives, also known as

Schiff bases, (compounds having a C=N function).

Wittig Reaction

Keto-Enol Tautomerism• Keto-enol tautomerism refers to a chemical equilibrium between a

keto form (a ketone or an aldehyde) and an enol. The enol and keto forms are said to be tautomers of each other. The interconversion of the two forms involves the movement of a proton and the shifting of bonding electrons; hence, the isomerism qualifies as tautomerism.

• A compound containing a carbonyl group (C=O) is normally in rapid equilibrium with an enol tautomer, which contains a pair of doubly bonded carbon atoms adjacent to a hydroxyl (−OH) group, C=C-OH. The keto form predominates at equilibrium for most ketones. Nonetheless, the enol form is important for some reactions. Furthermore, the deprotonated intermediate in the interconversion of the two forms, referred to as an enolate anion, is important in

carbonyl chemistry, in large part because it is a strong nucleophile.

Acidity of α-Hydrogen

• Examples of -Hydrogen exchange

The Aldol Condensation

The name aldol is derived from "aldehyde" and "alcohol". An aldol is a β-hydroxycarbonyl compound.