ALDEHYDES AND KETONES

CONTENTS

STRUCTURE OF CARBONYL COMPOUNDS

NOMENCLATURE FORMATION OF ALDEHYDES &

KETONES PHYSICAL PROPERTIES CHEMICAL PROPERTIES IDENTIFICATION OF ALDEHYDES &

KETONES

THE CARBONYL GROUP• The carbonyl group (C=O) is found in aldehydes,ketones, so they are closely related to each other.• The carbon in the carbonyl group aresp2-hybridized, with bond angles of 120°.

• In ketones, two carbon groups are attached to thecarbonyl carbon, while in aldehydes at least onehydrogen is attached to the carbon

NOMENCLATUREALDEHYDES

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 possible minimum number.

IUPACLONGEST CHAIN WITH ALDEHYDEDROP “e” AND ADD “-al”ALDEYHYDE TAKES PRECEDENCE OVER ALL OTHER GROUPS SO FAR.EXAMPLES

ALDEHYDES

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

Ketones with Common Names

IUPAC retains well-used but unsystematic names for a few ketones

FORMATION OF ALDEHYDES1.Oxidation of Primary Alcohols

R CH2 OH R C H

O

R C OH

O

[O] [O]

The aldehyde can be oxidized in a second step

[O] represent an oxidation

Aldehydes are formed during many chemical reactions. Of these we would describe only those important reactions which are actually used for preparative purposes.

OXIDATION OF PRIMARY

ALCOHOLS TO ALDEHYDES

Requires less vigorous oxidation conditions.

We can try to remove the aldehyde from the reaction medium as quickly as it is formed

Generally, the aldehyde has a lower boiling point than either the corresponding alcohol or

carboxylic acid

We can remove the aldehyde from the reaction mixture by distillation as soon as it formed.

Oxidation of Primary Alcohols with K2Cr2O7

R CH2 OH R C H

O

R C OH

OK2Cr2O7

H2SO4

K2Cr2O7

H2SO4

+ Cr3+

•This reaction can also be done using CrO3 (chromic oxide) in sulfuric acid.

•The aldehyde is distilled away from the reaction vessel as quickly as it is formed. If the aldehyde is not removed, it will suffer a second oxidation, and the product will be the carboxylic acid.

OXIDATION WITH CHROMIC OXIDE AND

PYRIDINE

R CH R

OH

R C R

O

N

CrO3.

CH2Cl2

Sarett Oxidation

2.OZONOLYSIS OF ALKENES

In this method, the ozonide formed by reaction between an alkene and ozone is decomposed with a reducing agent (Zn+CH3COOH; or H ,Pd) to yield aldehydes and ketones.

1-Butene

3.Stephen’s reaction:

Alkyl cyanides on reduction with stannous chloride and hydrochloric acid give rise to aldimine. The aldimine gets hydrolysed on warming with water to yield an aldehyde.

4. Catalytic dehydrogenation of primary alcohols: The oxidation of primary alcohols to yield aldehydes can also be brought about by passing the alcohol vapours over copper heated to 475-575K. The process is generally reffered to as dehydrogenation as it involves direct elimination of hydrogen.

The dehydrogenation process does not involve the risk of further oxidation of aldehyde formed.

5.Gattermann-Koch reaction: This reaction constitute a convenient method for preparing aromatic aldehydes by treating benzene (or other aromatic compounds) with a mixture of CO and HCl in the presence of a lewis acid and copper (I) chloride.

In a similar reaction as Gattermann reaction, HCN is used in place of CO. The aldimine initially formed is hydrolysed to get the corresponding aromatic aldehyde.

6.Reimer –Tiemann reaction: This is a convenient method for the preparation of phenolic aldehydes, particularly those which the aldehyde group is present in ortho position to the phenolic group. The reacton is carrie out by treating a phenol with chloroform and aqueous sodium hydroxide solution.

Salicylaldehyde(Main product)p-Hydroxybenzaldehyde

(Formed in small quantities)

FORMATION OF KETONES The important methods of preparing ketones are described below:1.OXIDATION OF SECONDARYALCOHOLS

R C R

OH

H

[O]R C R

O

OXIDATION OF TERTIARY ALCOHOLS

R C R

OH

R

[O]NO REACTION

2.From 1,3-dithanes:This reaction involves the conversation of 1,3-dithane into their dialkyl derivatives (in two stages) and the subsequent hydrolysis of the dialkyl derivativeto form ketones.

3.Friedel-Crafts acylation:This method is specifically used for the preparation of ketones in which the ketonic group is attached to at least one aromatic ring. It consist in treating an aromatic compound with an acid halide in the presence of a Lewis acid such as anhydrous aluminium chloride.ArH

Aromatic compound

RCOCl

acid chloride

ArCOR

ketone

HCl (R may be alkyl or aryl or aryl group)

It is also possible to use an acid anhydride in place of acid chloride. For instance:

4.Hydration of alkynes:Hydration of homologues of acetylene leads to the formation of methyl ketones. For example:

5.Ozonolysis of alkenes:Ozonolysis of alkenes in which the double bond is next to a position of branching of the carbon chain also leads to the formation of ketones. For example:

6.Acetoaetic ester synthesis:Methyl ketones can be conveniently obtained from alkyl derivatives of ethyl acetoacetate, popularly known as acetoacetic ester. The method involves the conversion of acetoacetic ester into sodioacetoacetic ester by treatment with sodium ethoxide. Subsequent treatment with alkyl halide yields alkyl acetoacetic ester. This is subjected to hydrolysis and the β-keto acid thus obtained is heated to get methyl ketone. For example:

PHYSICAL PROPERTIES OF ALDEHYDES AND

KETONES1.Physical state: - Most of common aldehyde &

ketones are liquids at ordinary temperature , except formaldehyde which is gas at room temperature. The lower molecular weight aldehyde have sharp & unpleasant smells. But high molecular weight aldehyde & ketones have pleasant smell .In Fact, some ketones are used in perfumes.

2.Boiling point:- The b.p. of aldehyde & ketones are lower than those of alcohols & carboxylic acids of approx. equal molecular weight. This is due to the fact that , unlike alcohols & acids, there are no effective hydrogen bonds between aldehydes & ketones molecules. However, due to their highly polar nature, there is intermolecular attraction in aldehydes or ketones. As such their boiling points are higher than those of hydrocarbon & ether of compareable molecular weight.

3. Melting point :- Melting point of aldehyde & ketones do not show any regural trend with increase in molecular weight.

4.Solubility :- Aldehyde & ketones containing upto five carbon atoms are , more or less , soluble in water , but the solubility falls rapidly with increasing molecular weight. Higher members as well as aromatic aldehyde and ketones are insoluble . However all aldehydes & ketones are readily soluble in organic solvents.

The solubility of lower aldehyde & ketones in water is due to the formation of intermolecular hydrogen bonds involving the carbonyl group as shown:

c o H O

H O C

But increase in the size of the non- polar group attached to carbonyl carbon does not allow hydrogen bonding to take place so that solubility decrease with increase in molecular weight.

CHEMICAL PROPERTIES OF ALDEHYDES AND KETONESAldehydes and ketones are highly reactive compounds

due to the presence of carbonyl group in both types.

1. REACTIVITY OF CARBONYL GROUP TOWARDS NUCLEOPHILLIC ATTACK:-The carbonyl group contains a double bond b/w two hetero atoms, i.e, carbon and oxygen. Since oxygen is more electonegative than carbon, the π- e‾ are pulled stongly towards oxygen.As a result, the carbonyl group is very polar, and carbonyl carbon is e‾ deficient and carbonyl oxygen is e rich. Consequently, carbonyl group is higly reactive.

Most of the reactions of aldehydes and ketones involve the initial attack of nucleophillic reagent on the relatively less stable e‾ deficient carbonyl carbon and ultimately lead to the formation of addition products.MECHANISM:-It is initiated by the attack of nucleophile(:Nu‾) on the carbonyl carbon. This leads to the formation of a transition state in which the hybridisation of carbon changes sp² to sp³. At the same time,oxygen begins to acquire electrons of C=O bond as shown below:

As the πelectrons of C= O are completely transferred to oxygen, a tetrhedral intermediate is formed in which carries –ve charge. This intermediate picks up a proton to form the addition product.

2.ROLE OF ACID CATALYSIS IN NUCLEOPHILIC RXNS

The hydrogen ion is generated by the acid rapidlly combines with the e‾ rich oxygen of the carbonyl group. This leads to a decrease in the act. Energy for the nucleophillic attack as it enables oxygen to acquire the πelectrons without having to accommodate a -ve charge, as shown below:It may be noted , however that if the rxn soln is made too acidic, the H⁺ ions would combine with the attacking nucleophillic reagent as well. This would suppress the conc. of attacking nucleophillic species and thus adveresely affect the nucleophiilic attack. Therefore it is important to control the pH of rxn mixture.Addition of powerful nucleophiles doesnot require any acidic catalyst at all.

3.Relativic are reactivities of aldehyde and ketones in nucleophilic reaction

In nucleophilic substitution reactions, aldehyde are more reactive than ketone and aliphatic aldehyde or ketones are more reactive than the aromatic ones.

1.Steric factors : In transition state of nucleophilic attack on carbonyl carbon, the carbon starts forming a bond with the attacking species & tends to change from trigonal to tetrahedral state.Now, larger the size of alkyl or aryl group attached to the carbonyl group, the more crowded will be the transition state. As the transition state gets more and more crowded the steric hindrance of the groups increases & the formation of the transition state gets more & more difficult .Thus the formation of transition state would be more difficult in case of ketones than in case of aldehydes since ketones contain 2 alkylor aryl group attached to the carbonyl group while aldehydes have 1 hydrogen & only 1 alkyl or aryl group. As such ketones would be less reactive than aldehydes.

2. Electronic factors: (1.)Aliphatic aldehydes & ketones :Alkyl group have electron releasing inductive effect & their presence tends to destabilise the transition state by enhancing the negative charge developing on oxygen.

Nu Nu

C C

R O R O

H R

Since aldehydes contain only one alkyl group while ketones have 2 such groups, the transition state resulting from ketone is destabilised to greater extent than that resulting from an aldehyde. As such the transition state is more difficult to form in case of ketones, they are less reactive.

(2.) Aromatic aldehydes and ketones: An aromatic nucleus directly attached to carbonyl group may be expected to increase the reactivity of aldehyde or ketones & to stabilise the transition state due to its electron withdrawing inductive effect. However, aromatic nucleus stabilises the reacting aldehyde or ketone even more due to resonance interaction with the π electrons. As such aldehyde or ketones having aryl group directly attached to the carbonyl group are less reactive than the aliphatic counter parts.

O + O‾ O‾

C C + C R(or H) R(or H) R(or H)

O‾

C + R(or

H) It may be noted that presence of electron-

withdrawing substituents at ortho & para position on benzene ring inc. the reactivity of aromatic aldehyde & ketone. Electron donating have opp. effect

NUCLEOPHILIC ADDITION REACTIONS

A large variety of nucleophilic reagents can attack the carbonyl group to yield addition compounds in which the carbonyl group is bonded to some atom of attacking reagent.

The addition compounds thus formed, may

itself represent the final product or it may undergo a subsequent elimination or substitution reaction to furnish a more stable product.

1.ADDITION OF SODIUM BISULPHITE:

Nearly all aldehydes and many ketones (particularly methyl ketones) reacts with a saturated sodium bisulphite solution to form crystalline solids referred to as bisulphite addition compounds. In these compounds , carbonyl carbon get’s bonded to sulphur of the attacking reagent.

O

R

H (R')

+ NaHSO3

R

H (R')

OH

SO3Na

Sodium bisulfite Bisulfite addition product

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2.ADDITION OF ALCOHOLS-

Aldehydes and ketones react with alcohols in the presence of anhydrous acid catalyst to form ethereal compounds called ACETALS.

MECHANISM- To start with, a molecule of alcohol adds

to the carbonyl group of the aldehyde to form a hemiacetal. On the other hand under the catalytic influence of the acid, the hemiacetal is readily converted into the stable ACETAL.

ACETALS

R

H

O +

R

H

OH

OR

Nucleophilic addition

Nucleophilic reagent

H-OR

Hemiacetal °ëËõÈ©

H+

R

H

OH

OR

(unually too unstable to isolate)Aldehyde

HCl

ROH

R

H

OR

OR

an acetal ËõÈ©

3.ADDITION OF GRIGNARD REAGENT Grignard reagents reacts with aldehydes and

ketones to form ddditional products which are readily decomposed by water, preferably in the presence of an acid such as HCl or H2SO4 to yield alcohols.

C

O+ RMgX C

O

R

MgBr

C

O

R

MgBr+ H2O C

OH

R

+ Mg(OH)Br

larger alcohol

MECHANISMIn the formation of additional product,the nucleophillic carbanion portion of the grignard reagent adds to cabonyl carbon while the remaining portion gets attached to oxygen as depicted below. This is followed by hydrolysis of addiion product.

C

ORMgBr+

RDSC

O

R

+ MgBr

C

O

R

+ MgBr C

OMgBr

R

mechanism = nucleophilic addition

In the presence of a dilute base, two molecules of an aldehyde or ketone having alpha hydrogen atom combine to form β- hydroxyaldehyde or β-hydroxyketone. The hydroxyaldehyde obtained from acetaldehde was originally named aldol and consequently the reaction came to be known as aldol condensation. For example:

ALDOL CONDENSATION

CH3CH=Odil. NaOH

CH3CHCH2CH O

OH

acetaldehyde 3-hydroxybutanal

CH3CCH3

Odil. NaOH

CH3CCH2CCH3

OOH

CH3acetone

4-hydroxy-4-methyl-2-pentanone

MECHANISM:

The base removes an alpha- hydrogen from aldehyde or ketone molecule to form resonance stabilised carbanion or enolate ion. This ion, being strongly nucleophillic, attacts the carbonyl carbon of a second molecule of aldehyde or ketone to yield an anionic intermediate. The intermediate then abstracts a proton from water to form the aldol.The complete mechanism is illustrated below:

CH3CH=Odil. NaOH

CH3CHCH2CH O

OH

acetaldehyde 3-hydroxybutanal

OH

CH2CH=O CH3CH+ O CH3CHCH2CH O

O

+ H2O

+ H2O

nucleophilic addition by enolate ion

CROSSED ALDOL CONDENSATION:

If you react two aldehydes or ketones together in an aldol condensation, you will get four products. However, if one of the reactants doesn’t have any alpha hydrogens it can be condensed with another compound that does have alpha hydrogens to give only one organic product in a “crossed” aldol.

CH3CH2CH + H2C OO CH3CHCH2 OH

CH O

The is the reaction between aldehydes or ketons and a type of compound known as phosphorus ylides to form substituted alkenes. The phosphorus ylides generally used are alkylidene triphenylphosphorus. The overall reaction may be represented as:

THE WITTIG REACTION

C O + Ph3P=C R'

R

ylide

C

O

C R'

R

PPh3

C C

R

R' + Ph3PO

CH2CH=O + Ph3P=CH2 CH2CH=CH2 + Ph3PO

TESTS OF ALDEHYDES AND KETONES AND DISTINCTION B/W THEM

• TESTS OF ALDEHYDES AND KETONES: 1. They form crystalline addition products with sodium bisulphate

solution.2. They form yellow or orange coloured derivatives with 2,4-

dinitrophenyl hydrazine.

• DISTINCTION B/W ALDEHYDES AND KETONES:1. Aldehydes form a silver mirror on warning with TOLLEN’S

REAGENT but ketones do not.2. Aldehydes give red ppt. on warning with FEHLING SOLUTION but

ketones do not.3. Aldehydes give a pink colour with schiff’s reagent but ketones

do not. ( schiff’s reagent is a solution of p-rosaniline hydrochloride decolourised by passing SO₂ into it.)

• DISTINCTION B/W METYL KETONES AND OTHER KETONES: (2-pentanone and 3-pentanone)

when methyl ketone is treated with sodium hypoiodite,

a yellow ppt. of iodoform is produced. Other ketones donot give this test.this test is known as iodoform test.2-pentanone produces yellow ppt. while there is no reaction in case of 3-pentanone. O ||CH₂─C ─CH₂ CH₂ CH₃ + 3NaOI → CHI₃ + CH₃ CH ₂CH₂COONa O +2NaOH ||CH₃─CH₂─C─CH₂ CH₃ + NaOI → No reaction

• DISTINCTION B/W ACETALDEHYDE AND BENZELADEHYDE:

When treated with a little of iodine and afew drops of sodium hydroxide, acetaldehyde gives yellow ppt. of iodoform but there is no reaction in case of benzaldehyde or formaldehyde. O ||CH₃─C ─H + 3NaOI → CH₃ + HCOONa + 2NaOH yellow ppt.

THE END