7.aldehydes and ketones new

8/11/2019 7.Aldehydes and Ketones New

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The carbonyl group is formed by joining together the

C atom and the O atom by a double bond.

Compounds having the aldehyde and ketonefunctional groups are named carbonyl compounds.

Differences among aldehydes and ketones are due to

the positioning of the carbonyl group in their

molecules.

The C=O group in aldehydes is always located at the

end of the alkyl stem, so aldehydes can be represent

as

R C H

O


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Unlike aldehydes, the ketone carbonyl group is situatedaway from the ends of the parent carbon chain.

This means that the carbonyl group in ketones is

sandwiched between two alkyl stems and isrepresented as

The group stamped a polar region in the aldehyde andketone molecules. Most reactions of aldehydes andketones will be centred at the carbonyl group.

R C R

O


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Nomenclature:

The names of aldehydes and ketones aresimply derived by dropping "-e" from the root

and adding "-al" or "-one" respectively. A

position number is needed for ketones sincethe carbonyl group may be on any number of

several carbons in the "middle" of a chain. The

carbonyl on the aldehyde is always on the

number one carbon so no position number is

needed.


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Aldehyde -al

Propanal

Ketone -one Propanoneor

acetone

(common

name)


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Preparation of Aldehydes and Ketones

Aldehydes can be prepared by the oxidation of

primary alcohols with hot, acidified potassium

manganate (VII) solution or potassium

dichromate(VI) solution.

R-C-OH + (O) R-C=O + H2O

H

H

H

aldehyde


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Ketones are prepared by the oxidation of

secondary alcohols by hot acidified KMnO4

solution. Ketones resist oxidation, thus

distillation is not required.

R1-C-OH + (O) R1-C=O + H2O

R2

H

R2

Ketone


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Physical Properties of

Aldehydes and Ketones


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Physical Properties of Aldehydes and Ketones

a). Solubility in water

The small aldehydes and ketones are freely soluble in

water but solubility falls with chain length. For

example, methanal, ethanal and propanone - thecommon small aldehydes and ketones - are miscible

with water in all proportions.

The reason for the solubility is that althoughaldehydes and ketones can't hydrogen bond with

themselves, they canhydrogen bond with water

molecules.


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As chain lengths increase, the hydrocarbon "tails" of the

molecules start to get in the way.

By forcing themselves between water molecules, they

break the relatively strong hydrogen bonds between

water molecules without replacing them by anything as

good. This makes the process energetically less

profitable, and so solubility decreases.


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b). Boiling points

The other aldehydes and the ketones are liquids, with

boiling points rising as the molecules get bigger. The size

of the boiling point is governed by the strengths of the

intermolecular forces.

The other aldehydes and the ketones are liquids, with

boiling points rising as the molecules get bigger. The sizeof the boiling point is governed by the strengths of the

intermolecular forces.


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c). Density of aldehyde and ketones is lesser than water.

d). Aldehydes and ketones are polar molecules. The bond

angles and hybridization are the same. They also havedipole-dipole intermolecular bonding. Because of this,

they are held together with greater attraction than you

would find with alkanes of similar size and molecular

weight.e). They dont have hydrogen bonding but form hydrogen

bonding with the compound which have hydrogen

bonding.

f). They are excellent organic solvents.


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Chemical Properties of

Aldehydes and Ketones


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Chemical Properties of Aldehydes and

KetonesReduction

Variety of agents can reduce aldehydes and ketones to

alcohols

NaBH4 and H2 commonly used


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Condensation Reactions

Aldehydes and ketones react with derivatives of

ammonia in condensation reactions. Aldehydes and ketones react with hydroxylamine at

room temperature to form an oxime.

C= O+ H2N- ON C=N-OH+H2O

hydroxylamine oxime

Aldehydes and ketones react with hydrazine at room

temperature to form a hydrazone.

C= O+ H2N- NH C=N-OH+H2Ohydrazine hydrazone


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O2N O2N

C= O+ H2N- H- NO2 C=N-N- NO2+ H2O

H H

A solution of 2, 4- dinitrophenylhydrazine in

methanol and sulphuric acid is called Bradysreagent.

The formation of a yellow- orange precipitate is

a chemical test for the presence of the carbonylgroup in aldehydes and ketones.

2, 4-dinitrophenyldrazine 2, 4- dinitrophenylhydrazone


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Hydration

Formaldehyde dissolves readily in water

Acetaldehyde somewhat also Form hydrates


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Addition Reactions

Aldehydes and ketones react with hydrogen

cyanide in sodium hydroxide solution at 10-20celcius to form cyanohydrin.

H H

CH3-C=O+H-C= N CH3-C-OH

C=

2- hydroxypropanenitrile

(ethanal cyanohydrin)


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The reaction can also be carried put by adding

dilute sulphuric acid to a mixture of carbonyl

compound and potassium cyanide at 10-20celcius.

TheCN group in cyanohydrin can be

hydrolysed by heating with a dilute acid to forma hydroxyacid. This is a method to prepare 2-

hydroxycarboxylic acid.

H + H

CH3-C=OH+H-H2O CH3-C-OH

C= N COOH

2-hydroxycarboxylic acid


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This is a useful way to add a carbon atom to the

carbon chain.

TheCN group can also be reduced to an aminegroup using hydrogen gas in the presence of hot

nickel catalyst or by using lithium

tetrahydridoaluminate (III)


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Oxidation

Aldehydes are oxidised rather easily to

carboxylic acids by heating with acidifiedpotassium manganate (VII) or potassiumdichromate (VI) solution.

H OH

R-C=O+[O] R-C=OAldehyde carboxylic acid

In alkaline conditions, the aldehyde is oxidised to

the salt of the corresponding carboxylic acid.The carboxylic acid is formed upon addition of adilute acid.


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H O-

R-C=O+[O]+OH- R-C=O+H2O

Aldehyde salt of carboxylic acid

O OH

R-C=O+H+ R-C=O

carboxylic acid

Benzaldehyde is oxidised to benzoic acid, which

is soluble in hot water, but when cooled, will

precipitate out as a white solid.H OH

-C=O+[O] -C=OBenzaldehyde benzoic acid


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Ketones resist oxidation under normal conditions.However they undergo oxidative cleavage whenboiled for a long time to form a mixture of products.

There will be a loss of carbon atoms in the form ofcarbon dioxide.

An important industrial application of the oxidationof ketones is the oxidation of cyclohexanone by nitric

acid to hexanedioic acid, which is a monomer in thesynthesis of nylon-6,6.

O O OHO-C-CH2CH2CH2CH2-C-OH

cyclohexanone hexanedioic acid


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Tollens reagent

When adding the aldehyde or ketone to Tollens'

reagent, the test tube is put in a warm water bath. If

the reactant under test is an aldehyde, Tollens' test

results in a silver mirror. If the reactant is a ketone, it

will not react because a ketone cannot be oxidized

easily. A ketone has no available hydrogen atom on thecarbonyl carbon that can be oxidized - unlike an

aldehyde, which has this hydrogen atom.


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Fehling reagent

An aldehyde is added to the Fehling solution and the

mixture is heated. The aldehyde oxidizes to an acid

and cupric ions, which were complexed with the

tartrate ion, are reduced to cuprous ions, which

precipitate as red cuprous oxide. Ketones (except

alpha hydroxy ketones such as are present in ketosesand other metabolites) and aromatic aldehydes do

not respond to the Fehling test.


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Benedict's test

Benedict's reagent will detect the presence of

aldehydes.

Benedict's reagent contains blue copper(II) ions (Cu2+)

which are reduced to copper(I) (Cu+). These are

precipitated as red copper(I) oxide which is insoluble in

water.


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Uses of Aldehydes and

Ketones

H d d f ld h d d k t d


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Hundreds of aldehydes and ketones are used as

flavoring, odouring and preserving agents e.g.,

vanillin and cinnamonaldehyde (flavors).

Glyceraldehyde is an intermediate in carbohydrate

metabolism and testosterone and progesterone are

hormones that relate to the biological role of these

compounds.

Aldehydes and ketones are being used by chemists

daily basically to synthesize other compounds.

In industry these compounds are used as solvents.


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Uses of Aldehydes

A 40% formaldehyde known as formalin solution


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A 40% formaldehyde known as formalin solution

can be used to preserve dead animals.

It is also used in the manufacture of polymeric

plastics like bakelite, melamine and urea-

formaldehyde glues.

Formaldehyde is used in the tanning of leather,

decolouring of vat dyes and as a reducing agent in

silvering of mirrors.

It is also used in manufacture of plywood.

Paraldehyde is a sweet smelling liquid used in


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Paraldehyde is a sweet smelling liquid used inmedicine as a hypnotic.

Acetaldehyde is used primarily as a startingmaterial in the manufacture of acetic acid, ethylacetate, vinyl acetate, polymers and drugs.

It is also used in silvering of mirrors.

Mataldehyde a polymer of acetaldehyde is usedas a solid fuel and for killing slugs when mixedwith bran.


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Benzaldehyde is an almond extract used as a

flavoring agent as well as in perfumery.

It is used in the manufacture of dyes like

malachite green.

In the chemical industry it is also used as a

starting material for making a number of

organic compounds such as cinnamic acid,benzoic acid, benzoyl chloride etc.


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Uses of ketones

Acetone is a common fingernail polish remover and


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Acetone is a common fingernail polish remover and

is a solvent for lacquers and varnishes.

Acetone is used as a cleaning fluid and is very

flammable.

It is also used to prepare a number of compounds

such as chloroform, ketene acetic anhydride etc.

It finds further use in the manufacture of smokeless

powders cordite, celluloid.

2-Butanone (MEK, methyl ethyl ketone) finds use as

a solvent and paint stripper.

Carvone is used as spearmint flavoring. It is used as

caraway seed flavoring.


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7.aldehydes and ketones new

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