P- Aldehydes And Ketones.pdf

AISSCE -19

Chemistry Project  Topic – Aldehydes and Ketones  Roll Number –  Class – XII (PCM)

Submitted by – Priyanshu Patel Submitted to – Sir Kamal Singh

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This is to certify that _________________of class XII PCM has successfully completed the investigatory project on the topic ____________________ under my guidance during the year 2018-2019 in the partial fulfillment of the chemistry practical examination conducted by CBSE

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Acknowledgement I would like to sincerely and profusely thank my chemistry teacher Mr. Kamal Singh, for his able guidance and support in completing my project.

I would also like to extend my gratitude to the principal for providing me with all the facility that was required Last but not the least, I would extend my gratitude towards all teaching and non-teaching staff of St. Anthony’s senior secondary school and towards my friends who has supported me to complete this project.

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Aldehydes and Ketones  What are aldehydes? An aldehyde is a common functional group in organic chemistry. Aldehydes can be found in perfume fragrances as well as natural and synthetic hormones. In an aldehyde, a carbonyl group is singlebonded to a hydrogen atom. A carbonyl is a carbon that is double bonded to an oxygen atom. The carbonyl carbon is also bonded to another hydrogen atom or a carbon/hydrogen chain, typically known as an R group. The general formula of aldehyde is

 What are ketones? Ketones are Compounds in which a carbonyl group is bonded to two carbon atoms. The general formula of ketone is

Where R maybe H, alkyl or aryl group. If R and R are same, the ketone is called simple ketone and if R and R are different then the ketone is called mixed ketone.

Nomenclature of aldehyde and ketones

a) Aldehyde Common system In the common system, aldehydes are named according to the name of the corresponding carboxylic acid which they form on oxidation. The suffix –ic acid of the name of the acid is replaced by aldehyde. For example, CH3CHO derived from acetic acid (CH3COOH) is named as acetaldehyde.

We use letters such as α, β, γ, and δ, and so on to indicate the location of the substituent present in the carbon chain. The α-carbon is the one to which the aldehyde group is attached. β- Carbon is the carbon next to the α-carbon, and so on.

β-Bromobutyraldehyde α-Methylpropionaldehyde

IUPAC System In the IUPAC system, the aldehydes are known as alkanals. The name of aldehyde is derived by replacing the terminal –e of the name of corresponding alkane by al. For example,

The branched chain aldehydes are named by the following rules:  The longest chain containing the –CHO group is considered as the parent chain and the name is derived by replacing the terminal –e of the name of the corresponding alkane by the suffix –al.  In case of substituted aldehydes, the parent chain is numbered in such a way that the aldehydic group (–CHO) gets lowest number i.e., 1.  The positions of the other substituents are indicated by numbers.  When the aldehyde group is attached to a ring, the suffix carbaldehyde is added after the full name of the cycloalkane. The numbering of the ring carbon atoms starts from the carbon atom attached to the aldehyde group. For example,

OHCCH2CH2CH2CHO 4-Bromo-3-Methylheptanal

Pentanedial

C6H5CH2CH2CHO 3-Phenylpropanal 2-Ethylbutanal

CH3CH=CHCH2CH2CHO Hex-4-enal

In Aromatic aldehydes, –CHO group is directly attached to the benzene ring. The name of the simplest aromatic aldehyde carrying aldehyde group on benzene is benzene carbaldehyde. However the common name benzaldehyde is also accepted by IUPAC. The other aromatic aldehydes are therefore, named as substituted benzaldehydes. In case of substituted aromatic aldehydes, the positions of the substituents in benzene ring with respect to –CHO group are indicated either by suffixes ortho, meta or para or by numbers 1, 2, 3… etc. with the carbon bearing the –CHO group as number 1. For example -

3-Hydroxy 4-Methoxybenzaldehyde

2-Methylbenzaldehyde

Benzaldehyde

4-Nitrobenzaldehyde

The aldehydic group may also be present in the side chain. For example –

3-Phenylprop-2-enal

2- Phenylethanal

b) Ketone Common system In the common system, ketones are named by using the name of alkyl groups present in the molecule. For example –

Dimethyl ketone

Ethyl methyl ketone

CH3CH2COCH2CH3 Diethyl ketone

Methyl n-propylketone

IUPAC system In IUPAC system, the ketones are known as alkanones. The individual members are named by following the general rules as:  The longest chain carrying the carbonyl group is considered as the parent chain and the name is derived by replacing the terminal –e of the name of corresponding alkane by the suffix –one.  In case of substituted ketones, the parent chain is numbered in such a way that the keto group gets the lowest number.  The position of the carbonyl group and the substituents is indicated by numbers. For example,

Propanone

Pentan-2-one

CH3 -CH2 –CO-CH2 -CH3 Pentan-3-one 4-Methylpent-3-en-2-one

2-Methylcyclohexane

2, 4 Dimethylpentan-3-one

Important note – If the compound contains both aldehyde and ketonic groups, then aldehyde group is considered as principal functional group ketonic group is regarded as substituent. It is named as prefix oxo– along with a number to indicate its position.

(3-Oxopentanal)

Preparation of both aldehyde and ketones I. FROM OXIDATION OF ALCOHOLS Oxidation of primary and secondary alcohols leads to the formation of aldehydes and ketones. The oxidation is possible with the help of common oxidizing agents are KMnO4, K2Cr2O7, and CrO3. Aldehyde and Ketone preparation is possible by oxidation of primary and secondary alcohol by agents such as PCC (pyridinium chlorochromate), Collins reagents (Chromium trioxide-pyridine complex), and Cu at 573 K.

II. BY DEHYDROGENATION OF ALCOHOLS This preparation method applies in case of conversion of volatile alcohols to aldehydes. It is generally used in industrial application. Vapours of alcohol are passed through heavy metal catalysts such as Cu or Ag in this technique. Primary alcohol produces aldehyde whereas secondary alcohol produces ketones, respectively.

III. FROM HYDROCARBONS This method is further divided into two separate methods. They are  By ozonolysis of alkenes  By hydration of alkynes

Ozonolysis of Alkenes Formation of aldehyde and ketone is possible by ozonolysis of alkenes. Ozonolysis is a reaction method in which addition of ozone molecules or O3 to an alkene compound leads to the formation of ozonide. Reduction of the ozonide compound with the help of zinc dust and water produces the smaller molecules, which in this case will be the respective aldehydes and ketones. The reaction produces aldehydes, ketones and in some cases both the compounds on the basis of the substitution arrangement of the alkene compounds.

Hydration of Alkynes Alkynes follow Markovnikov’s rule in the presence of a proper catalyst to produce ketones. All alkynes react with water in the presence of HgSO4 and H2SO4 to form ketones. However, the reaction of ethyne with water in the presence of the catalyst (HgSO4 and H2SO4) leads to the formation of acetaldehyde. This is an only exception where alkyne on hydration produces acetaldehyde. Rest all the alkyne on hydration produces ketones.

IV. WACKERS PROCESS Alkenes can be converted to aldehydes and ketones by treating with an acidified aqueous solution of palladium chloride (PdCl2) containing a catalytic amount of cupric chloride (CuCl2) in the presence of air or oxygen. This method is known as Wacker’s process.

Preparation of Aldehyde I. From acyl chloride (acid chloride) Acyl chloride/acid chloride undergoes hydrogenation in the presence of a catalyst such as barium sulfate (BaSO4) or Palladium (Pd) to form aldehydes. Aldehyde formation with this process is possible after the partial poisoning of the reaction by the addition of compounds such as sulfur or quinolone. This is an important step for the formation of aldehydes. This is also known as Rosenmund’s Reaction. In this reaction, Sulphur or quinolone behaves as poison for catalysts and causes partial poisoning to stop further reduction of aldehydes into alcohols. However, it is not possible to prepare formaldehyde from this reaction because the acyl chloride form, formyl chloride, is not stable at room temperature. It is not possible to prepare Ketones by this reaction.

II. From nitriles and esters Preparation of Aldehydes is possible with the help of nitriles. Reduction of nitriles with the compound Stannous Chloride (SnCl2) in the presence of HCl leads to the formation of the nitrile compound’s corresponding imine form. The imine compound undergoes hydrolysis to yield the corresponding aldehydes. The reaction is known as Stephen’s Reduction. Ketones cannot be prepared by this method.

Moreover, nitriles can undergo reduction by the compound DIBAL-H or di-isobutyl aluminum hydride for the formation of imines. The imines further undergo hydrolysis thereby forming aldehyde compounds.

Similarly, esters are also reduced to aldehyde with DIBAL-H

III. From hydrocarbons Formation of Aromatic Aldehyde, benzaldehydes and the derivatives of benzaldehyde, is possible with the help of aromatic hydrocarbons primarily by two methods.

a) Oxidation of methylbenzene Toluene and the derivatives of toluene undergo oxidation with the help of a strong oxidizing agent to form benzoic acids. However, it is possible to stop the reaction at the aldehyde stage with the help of proper reagents. The reagents can convert the methyl group to an intermediate that cannot undergo further oxidation easily. Oxidation of methylbenzene or toluene falls under two categories on the basis of reagents used in the reaction

 Use of chromyl chloride Oxidizing agent chromyl chloride can oxidize and convert methyl group to a chromium complex. The chromium complex undergoes hydrolysis to produce benzaldehyde. We refer to this reaction as Etard Reaction. In this reaction, methylbenzene/toluene undergoes oxidation process with the reagent of chromyl chloride

(CrO2Cl2) present in solution form in CCl4 or in CS2 thereby forming chromium complex.

 Use of chromic oxide It is possible to oxidize toluene or substituted toluene to aldehydes on treatment with reagents such as Chromium oxide, chromium trioxide, with acetic anhydride. This reaction leads to the formation of benzylidene diacetate. The intermediate or in this case benzylidene diacetate can undergo further hydrolysis to corresponding benzaldehyde with aqueous acid.

b) By side chain chlorination followed by hydrolysis Preparation of aldehydes is possible by side chain halogenation, more specifically side chain chlorination, followed by hydrolysis. Side chain chlorination of toluene yields benzal chloride which undergoes hydrolysis leads to the formation of benzaldehyde. The preparation technique is also the commercial way of benzaldehyde manufacture.

c) By Gatterman-koch reaction When benzene and its derivatives undergo treatment with carbon monoxide and HCl in the presence of a Lewis acid such as cuprous chloride/ anhydrous aluminum chloride leads to the formation of benzaldehyde or substitution of benzaldehyde

compounds. This reaction method refers to as Gatterman-Koch Reaction

Preparation of ketones  From acyl chlorides Ketone formation is possible by the treatment of acyl chloride with dialkyl cadmium [(R)2 Cd]. Cadmium chloride reacts with the Grignard reagent to form ketones.

 From nitriles Nitrile undergoes treatment with nitrile with Grignard reagent and further undergoes hydrolysis to form ketones.

 From benzene or substituted benzene Aromatic ketone formation is possible from benzene or substituted benzenes. The most suitable preparation technique for an aromatic aldehyde is Friedel-Crafts acylation reaction. In this reaction benzene

or substituted benzenes undergoes treatment with an acid chloride or acid anhydride to form ketones. The reaction occurs in the presence of a catalyst such as Lewis acid such as anhydrous AlCl3 (anhydrous aluminium chloride)

Physical properties of aldehydes and ketones 1) Boiling Point At room temperature, methanol behaves as a gas whereas ethanol is in liquid form that is volatile in nature. The boiling point of methanol and ethanol is -19o C and +21o C. Thus, the boiling point of ethanol is nearly at room temperature. Moreover, all other aldehydes and ketones are either liquid or solid at room temperature. The boiling point of these compounds increases with increase in molecular weight. Additionally, the strength of intermolecular forces is also responsible for the boiling point of aldehydes and ketones. However, the boiling points of these organic compounds are higher in comparison to hydrocarbons or ethers having nearly similar molecular masses. The reason for such behavior is the weak molecular association of these compounds occurring due to dipole-dipole interactions. Similarly, the boiling of aldehydes and ketones are lower than alcohol of nearly same molecular masses. The reason is lack of intermolecular hydrogen bonding. The size of the boiling point is governed by the strengths of the intermolecular forces.

Vander Waals Dispersion Force The boiling point of aldehydes and ketones depends on the numbers of the carbon atom. It increases with increase in the number of atoms of carbon. The longer the molecules become and with the increase in the number of electrons, the attraction between the compounds increases.

Vander Waals Dipole-Dipole Attraction Aldehydes and ketones are polar in nature due to the presence of the carbon-oxygen double bond. This creates an attraction between the permanent dipoles and with the nearby present molecules. Hence, the reason why this compound has a higher boiling point in comparison to the hydrocarbons of similar size. Refer to the table below to note the arrangement of boiling points in the increasing order of the compounds having molecular masses from 58 to 60. Name of the compound

Molecular mass

Boiling point (k)

n- Butane

58

273

Methoxyethane

60

281

Propanal

58

322

Acetone

58

329

Propan-1-ol

60

370

2) Solubility Generally, these aldehydes and ketones are soluble in nature with respect to water. However, the solubility gradually decreases with the increase in the alkyl chain length. Therefore, lower members such as methanal, ethanal, and propanone demonstrate miscible nature with all proportions of water.

This happens due to the ability of the lower members of the aldehydes and ketones to develop hydrogen bong with water. However, these compounds are unable to form hydrogen bonds with themselves. The reason for such behavior is dispersion forces and dipole-dipole interaction. Usually, all aldehydes and ketones are relatively soluble in organic solvents such as ether, methanol, benzene, chloroform, etc. The lower members of these classes of compounds demonstrate the characteristic sharp pungent odours but the odour converts to more fragrant smell with an increase in the size of molecules. Hence, aldehydes and ketones are used in different industrial applications. In fact, there are certain naturally occurring aldehydes and ketones that help in the blending of perfumes and also act as flavouring agents.

Chemical reactions 1) Nucleophilic addition reactions We will be able to convert multiple bonds into different functional groups with the help of addition reactions. The reaction will help to convert the unsaturated compounds to saturated and more functional species. Contrary to this, aldehydes and ketones undergo nucleophilic addition reaction.

 Mechanism of Nucleophilic Addition Reaction We know that carbonyl carbon demonstrates sp2 hybridization and together the structure is coplanar. A nucleophile acts on the polar carbonyl’s electrophilic carbon atom perpendicular to the orbital demonstration sp2 hybridization of the carbonyl carbon structure. However, on the attack of the nucleophile, the hybridization of the

carbon atom changes from sp2 hybridization of sp3 hybridization thereby forming tetrahedral alkoxide intermediate complex. This intermediate complex will take a proton from reaction medium to produce an electrically neutral compound. Hence, the reaction results in the addition of nucleophile and hydrogen in the carbon-oxygen double bond.

 Reactivity of Aldehydes and Ketones Aldehydes are more reactive and readily undergo nucleophilic addition reactions in comparison to ketones. Aldehydes demonstrate more favourable equilibrium constants for addition reactions than ketones because of electronic and steric effect. In the case of ketones, two large substituents are present in the structure of ketones which causes steric hindrance when the nucleophile approaches the carbonyl carbon. However, aldehydes contain one substituent and thus the steric hindrance to the approaching nucleophile is less. Moreover, electronically aldehydes demonstrate better reactivity than ketone.

This is because ketones contain two alkyl groups which decrease the electrophilicity of carbonyl carbon atom more than aldehydes.

 Some important examples of nucleophilic addition

and nucleophilic addition-elimination reactions: a) Addition of hydrogen cyanide (HCN): Aldehydes and ketones undergo reaction with HCN to produce cyanohydrins. The reaction progresses very slowly by using pure hydrogen cyanide. Hence, base as a catalyst helps to speed up the reaction. This is because catalysis helps in the generation of cyanide ion (CN) which acts as a stronger nucleophile and adds to carbonyl compounds to produce the corresponding cyanohydrin. Cyanohydrins are important synthetic intermediates.

b) Addition

of sodium hydrogensulphite: Sodium hydrogensulphite adds to aldehydes and ketones to form the addition products. Addition of Sodium Hydrogen Sulphite to aldehydes and ketones will result in the formation of the addition of products. The equilibrium position of the reaction for aldehydes will be on the right-hand side but the equilibrium position of the reaction for will be on the left-hand side because of the steric effect. The hydrogen sulphite compound form from the sodium hydrogen sulphite addition is water soluble. Therefore, it can be

converted back to parent carbonyl compound by treatment of the compound with dilute mineral acid or alkali. The reaction is also useful for the purification and separation processes of aldehydes.

Addition of Grignard reagents

c)

Grignard Reagents or R-MgX demonstrates polar nature. In this compound, the carbon atom is electronegative in nature and the Mg atom is electropositive in nature. The polar nature of the Grignard Reagents helps the compound reacts with aldehydes and ketone to produce additional products. The addition products undergo decomposition reaction to give alcohol with water or dilute sulphuric acid.

Necessary Points to Note in this Reaction 

If Grignard Reagent reacts with formaldehyde (HCHO), the reaction will form primary alcohol as the product.



If the reagent reacts with aldehydes other than HCHO, the reaction will produce secondary alcohols.



Ketone reaction with the reagent will produce tertiary alcohols.

d) Addition of alcohols Aldehydes undergo reaction with the monohydric alcohol to produce hemiacetals or alkoxyalcohol intermediate. The hemiacetal will further undergo reaction with an alcohol to produce gem-dialkoxy compound or acetal. The reaction is carried out in the presence of dry hydrogen chloride. On application of similar conditions, ketone undergoes reaction with ethylene glycol to produce cyclic compounds or ethylene glycol ketals.

The dry hydrogen chloride present in the reaction protonates the oxygen atom present in the carbonyl structure thereby increasing the electrophilicity of the carbonyl carbon. Thus, it helps in the nucleophilic attack of ethylene glycol. Further hydrolysis of acetals and ketals with mineral acids (aqueous) will help in retrieval of respective aldehydes and ketones.

e)

Addition of ammonia and its derivatives Many nucleophiles like ammonia and derivatives of ammonia (H2N-Z) can also be added to the carbonyl group of aldehydes and ketones. The reaction of ammonia and its derivatives is reversible and the reaction happens in the presence of acid to form addition products. The reaction equilibrium will help the product formation because of fast dehydration of the intermediate complex. Thus, the reaction finally forms the compound >C=N-Z. In the structure >C=N-Z, Z can be alkyl, OH, aryl, NH2, NHCONH2, C6H5NH, etc.

2) Reduction  Reduction to alcohols: Aldehydes and ketones are reduced to primary and secondary alcohols respectively by sodium borohydride (NaBH4) or lithium aluminium hydride (LiAlH4) as well as by catalytic hydrogenation.

 Reduction to hydrocarbons: The carbonyl group of aldehydes and ketones is reduced to CH2 group on treatment with zinc amalgam and concentrated hydrochloric acid [Clemmensen reduction] or with hydrazine followed by heating with sodium or potassium hydroxide in high boiling solvent such as ethylene glycol (WolffKishner reduction).

3) Oxidation Aldehydes differ from ketones in their oxidation reactions. Aldehydes are easily oxidised to carboxylic acids on treatment with common oxidising agents like nitric acid, potassium permanganate, potassium dichromate, etc. Even mild oxidising agents, mainly Tollens’ reagent and Fehlings’ reagent also oxidise aldehydes.

Ketones are generally oxidised under vigorous conditions, i.e., strong oxidising agents and at elevated temperatures. Their oxidation involves carbon-carbon bond cleavage to afford a mixture of carboxylic acids having lesser number of carbon atoms than the parent ketone.

The mild oxidising agents given below are used to distinguish aldehydes from ketones:

(i)

Tollens’ test: On warming an aldehyde with freshly prepared ammoniacal silver nitrate solution (Tollens’ reagent), a bright silver mirror is produced due to the formation of silver metal. The aldehydes are oxidised to corresponding carboxylate anion. The reaction occurs in alkaline medium.

(ii)

Fehling’s test: Fehling reagent comprises of two solutions, Fehling solution A and Fehling solution B. Fehling solution A is aqueous copper sulphate and Fehling solution B is alkaline sodium potassium tartarate (Rochelle salt). These two solutions are mixed in equal amounts before test. On heating an aldehyde with Fehling’s reagent, a reddish brown precipitate is obtained. Aldehydes are oxidised to corresponding carboxylate anion. Aromatic aldehydes do not respond to this test.

(iii) Oxidation of methyl ketones by haloform reaction Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon atom (methyl ketones) are oxidised by sodium hypohalite to sodium salts of corresponding carboxylic acids having one carbon atom less than that of carbonyl compound. The methyl group is converted to haloform. This oxidation does not affect a carbon-carbon double bond, if present in the molecule. Iodoform reaction with sodium hypoiodite is also used for detection of CH3CO group or CH3CH(OH) group which produces CH3CO group on oxidation.

4) Reactions due to alpha- hydrogen 

Acidity of alpha-hydrogen of aldehydes and ketones The aldehydes and ketones undergo a number of reactions due to the acidic nature of α-hydrogen. The acidity of α-hydrogen atoms of carbonyl compounds is due to the strong electron withdrawing effect of the carbonyl group and resonance stabilisation of the conjugate base.

(i) Aldol condensation Aldehydes and ketones having at least one α-hydrogen undergo a reaction in the presence of dilute alkali as catalyst to form β-hydroxy aldehydes (aldol) or β-hydroxy ketones (ketol), respectively. This is known as Aldol reaction.

The name aldol is derived from the names of the two functional groups, aldehyde and alcohol, present in the products. The aldol and ketol readily lose water to give α,βunsaturated carbonyl compounds which are aldol condensation products and the reaction is called Aldol condensation. Though ketones give ketols (compounds containing a keto and alcohol groups), the general name aldol condensation still applies to the reactions of ketones due to their similarity with aldehydes.

(ii) Cross aldol condensation When aldol condensation is carried out between two different aldehydes and / or ketones, it is called cross aldol condensation. If both of them contain α-hydrogen atoms, it gives a mixture of four products. This is illustrated below by aldol reaction of a mixture of ethanal and Propanal.

Ketones can also be used as one component in the cross aldol reactions.

5) Other reactions I. Cannizzaro reaction Aldehydes which do not have an α-hydrogen atom, undergo self-oxidation and reduction (disproportionation) reaction on treatment with concentrated alkali. In this reaction, one molecule of the aldehyde is reduced to alcohol while another is oxidised to carboxylic acid salt.

II. Electrophilic substitution reaction Aromatic aldehydes and ketones undergo electrophilic substitution at the ring in which the carbonyl group acts as a deactivating and meta-directing group.

Uses of aldehydes and ketones In chemical industry aldehydes and ketones are used as solvents, starting materials and reagents for the synthesis of other products.

Formaldehyde is well known as formalin (40%) solution used to preserve biological specimens and to prepare bakelite (a phenolformaldehyde resin), urea-formaldehyde glues and other polymeric products. Acetaldehyde is used primarily as a starting material in the manufacture of acetic acid, ethyl acetate, vinyl acetate, polymers and drugs. Benzaldehyde is used in perfumery and in dye industries. Acetone and ethyl methyl ketone are common industrial solvents. Many aldehydes and ketones, e.g., butyraldehyde, vanillin, acetophenone, camphor, etc. are well known for their odours and flavours.

Conclusion In this investigatory project, we have examined the properties of aldehydes and ketones. Specifically, we have taken a look at the reactivity of the carbonyl carbon in nucleophilic addition reactions and examined how aldehydes and ketones can be oxidized and reduced. We also came to know about uses of aldehydes and ketones and as well as about their physical properties.

Bibliography