Organic chemistry for medicine and biology students

Chem 2311

Chapter 9Aldehydes and Ketones

By Prof. Dr.Adel M. Awadallah

Islamic University of Gaza1

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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

cyclopentanecarbaldehyde4

Common Ketones

CH3 CH3

O

CH3

O

CH3

O

CH3CH3

O OCH3O

propanone (acetone)

2-butanone(ethyl methyl ketone)

3-pentanone(diethyl ketone)

cyclohexanone acetophenone(methyl phenyl ketone)

benzophenone(diphenyl ketone)

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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

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

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

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Acyl groups

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Common aldehydes and ketones

CH3OH CH2 = O + H2

O

OH

CH3 CH3

O

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)

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

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Synthesis of aldehydes and Ketones1) Oxidation of Alcohols

primary gives aldehydes using PCCsecondery gives ketones

2) Friedel-Crafts

3) From Alkynes

OCH3

CH3COCl

AlCl3

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Naturally occuring aldehydes and Ketones

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The carbonyl group

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Reactions of the carbonyl group

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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 hydrate14

Addition of Alcohols

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Examples

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Addition of hydrogen cyanide to aldehydes and ketones

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Addition of sodium hydrogensulphite to aldehydes and ketones• Uses of the reaction The reaction is usually used during the purification of

aldehydes (and any ketones that it works for). The addition compound can be split easily to regenerate the aldehyde or ketone by treating it with either dilute acid or dilute alkali.

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Reducing Agents

The reduction of an aldehydeYou get exactly the same organic product whether you use lithium tetrahydridoaluminate or sodium tetrahydridoborate.For example, with ethanal you get ethanol:

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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.

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Mechanism of reduction

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.• Using sodium tetrahydridoborate (sodium borohydride)

• Sodium tetrahydridoborate is a more gentle (and therefore safer) reagent than lithium tetrahydridoaluminate. It can be used in solution in alcohols or even solution in water - provided the solution is alkaline

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REACTION OF ALDEHYDES AND KETONES WITH GRIGNARD REAGENTS

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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+

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Reaction with Acetylides

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OXIDATION OF ALDEHYDES AND KETONES

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ADDITION-ELIMINATION REACTIONS OF ALDEHYDES AND KETONES

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

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Formation of Imines and Related CompoundsThe reaction of aldehydes and ketones with ammonia or 1º-

amines forms imine derivatives, also known as

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

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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.

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Acidity of α-Hydrogen

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• Examples of -Hydrogen exchange

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The Aldol Condensation

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

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