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10 May 2020 1

SARASWATI COACHING CLASSES Chemistry – II

Aldehydes and Ketones

1) Introduction : H R Aldehydes (R – C = O) and ketones (R – C = O) both contains carbonyl group ( C = O) so they are called as carbonyl compounds. In aldehydes at least one hydrogen atom is directly attached to carbonyl group while in aliphatic ketones, two similar or different alkyl groups are directly attached to carbonyl group.

2) Definition : A) Aldehydes (Aliphatic saturated) : The carbonyl compounds in which at least one hydrogen atom is directly attached to carbonyl group are called as aldehydes. OR Aldehydes are the first oxidation products of primary alcohols.

H H H | Oxidation | |

R – C – OH +(O) R – C – OH R – C = O + H2O I |

H OH 1° alcohol Unstable Aldehyde

Their general molecular formula is CnH2n+1 – CHO (n = 0, 1, ----) OR CnH2nO (n = 1, 2 ------) and aldehydes are represented as R – CHO (R = H or alkyl group). (The functional group of aldehyde is always present at the end of chain (Actually beginning of chain). H |

( – C = O Aldehydic or formyl group or methanoyl group)

NOTE: Aldehydes may be aromatic Ex. : C6H5 – CHO (Benzaldehyde)

B) Ketones (Aliphatic saturated) : "The carbonyl compounds in which two similar or different alkyl groups are directly attached to carbonyl group are called as ketones. OR Ketones are first oxidation product of secondary alcohols. R R R | Oxidation | | R – C – OH + (O) R – C – OH R – C = O + H2O | | H OH

2° alcohol Unstable Ketone Their general molecular formula is CnH2nO (where n = 3, 4 ----) and ketones are represented as R–CO–R (symmetrical ketones) and R–CO–R|

(Unsymmetrical ketones) (where R & R| H but any alkyl group).

The functional group is divalent in –CO– (keto / ketonic / oxo group) and which is present somewhere between two ends of carbon chain.

NOTE: Ketones may be aromatic Ex. i) CH3 – CO – C6H5 Methyl phenyl ketone (Acetophenone) ii) C6H5 – CO – C6H5 Diphenyl ketone OR (Benzophenone)

3) Structure (Orbital concept) : i) The carbonyl carbon is in SP2 hybridised state. Oxygen is also SP2 hybridised ii) The C–C– O and H –C–O bond angle is of 1200. and geometry is trigonal planer.


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iii) Due to electronegativity difference of 'C’ & 'O' atoms the C = O group is polar

+ –

i.e. C =O. iv) Carbonyl group is a resonance hybrid of following two structure.

+ – + –

C = O C – O

4) Nomenclature : a) Nomenclature of aldehydes (R – CHO) : 1) Common system : The common name of aldehyde is obtained from the name of corresponding acid by changing suffix ' ic acid' of common name of acid into aldehyde.

Greek word nomenclature or - system of aldehydes : In this system carbon atom

adjacent to carbonyl carbon is indicated by letter and the next carbon atoms in chain

are indicated by and so on. i) CH3

|

CH3 – CH – CHO Methyl propionaldehyde Derived system of aldehydes : Higher aldehydes are considered as derivatives of acetaldehyde. ii) CH3

|

CH3 – CH – CHO Di-methyl acetaldehyde iii) CH3

|

CH3 – C – CHO Tri- methyl acetaldehyde | CH3

2) IUPAC system :

i) Select the longest continuous chain of carbon atom containing functional group (–CHO) as parent alkane.

ii) The carbon atom of – CHO group is always numbered as 1. iii) Write alkyl groups alphabetically with proper number, iv) In IUPAC system aldehyde Is named as "Alkanal".

Formula Common Name IUPAC Name

1) H-CHO Formaldehyde Metnanal

2) CH3-CHO Acetaldehyde Ethanal

3) CH3-CH2-CHO Propionaldehyde Propanal

4) C3H7-CHO Butyraldehyde Butanal

5) CH3-CH2-CH2-CHO n-Butyraldehyde Butanal

6) CH3 – CH – CHO | CH3

iso-Butyraldehyde 2-Methylpropanal

7) CH3–CH2–CH2–CH2–CHO n-Valeraldehyde Pentanal

8) CH3 –CH– CH2 – CHO | CH3

iso-Valeraldehyde 3-Methylbutanal

9) CH3 | CH3 – C – CHO | CH3

Tri-methyl acetaldehyde

2,2-dimethyl propanal


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B) Nomenclature of ketones : Common system : in common system, simple ketone is named as dialkyl ketone, while mixed ketone is named as alkyl aIkyI ketone. In this ketone write name of alkyl groups alphabetically. 2) lUPAC system :

i) Select the longest chain of carbon atoms containing C = O group as parent alkane.

ii) Number the carbon atoms of the chain, beginning from the end which is nearer to functional group.

iii) Write alkyl groups alphabetically with proper number. iv) In IUPAC system ketone is name as "Alkanone".

Formula Common name IUPAC Name

1) CH3 – CO – CH3 Dimethyl ketone or Acetone Propanone

2) CH3 –CO – CH2 – CH3 Ethyl methyl ketone Butanone

3) CH3 –CH2 –CO – CH3 Ethyl methyl ketone Butanone

4) CH3–CH2–CO–CH2–CH3 Diethyl ketone 3-Pentanone or pentan - 3 – one

5) CH3–CO–CH2–CH2–CH3 Methyl n-propyl ketone 2-Pentanone or pentan-2-one

6) CH3 –CO – CH – CH3

| CH3

Methyl Iso-propyl ketone 3-methyl butanone

7) CH3–CH2–CO–CH–CH3

| CH3

Ethyl Iso-propyl ketone 2-Methyl-3--pentanone

8) CH3–CH–CO–CH–CH3

| | CH3 CH3

Di-isopropyl ketone 2,4-Dimethyl-3-pentanone

5) Isomerism : 1) Aldehydes : They shows chain isomerism, optical isomerism among themselves & functional Isomerism with ketones. Ex. A) Chain isomers : CH3

| i) CH3-CH2-CH2 – CHO ii) CH3 – CH – CHO

n-butyraldehyde isobutyraldehyde B) Functional isomers:

i) CH3-CH2-CHO ii) CH3 – CO – CH3 Propanal Propanone

NOTE : Aldehydes also shows tautomerism with unsaturated alcohols . O OH || | Ex. H – CH2 – C – H CH2 = C – H i.e. CH3 – CHO Vinyl alcohol

Acetaldehyde 2) Ketones : They shows optical, metamerism, among themselves and functional isomerism with aldehydes also shows tautomerism. Ex. A) Metamers : CH3 |

i) CH3 – CO–CH2CH2CH3 ii) CH3 – CO–CH – CH3 Methyl, n–propyl ketone Methyl, isopropyl ketone

Or 2–pentanone or 3–Methyl – 2–butanone


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B) Metamers : i) CH3–CH2–CO–CH2–CH3 ii) CH3 – CO – CH2 –CH2 – CH3

3-pentanone 2-pentanone NOTE : 3-pentanone and 2-pentanone are metamers and not believed to be position isomers. C) Tautomers : O OH || | i) CH3 – C – CH2 – H CH3 – C = CH2 Keto form Enol form D) Functional isomers : i) CH3 – CH2 – CHO ii) CH3 – CO – CH3 Propionaldehyde Dimethyl ketone (Acetone) Note : 1) Formula C3H6O represents 2 isomers i.e. 1 aldehyde and 1 ketone. 2) Formula C4H8O represents 3 isomers i.e. 2 aldehydes and 1 ketone. 3) Formula C5H10O represents 7 isomers i.e. 4 aldehydes and 3 ketones.

6) PREPARATION : 1) By oxidation of alcohols: A) Preparation of aldehyde (R – CHO) 1° alcohol on oxidation with acidified K2Cr2O7 gives aldehyde containing same number of carbon atoms. H H H | dil. H2SO4 | |

R – C – OH + (O) R – C – OH R – C = O | K2Cr2O7 | – H2O Aldehyde H H

1°Alcohol Unstable NOTE: To stop the reaction of aldehyde stage, aldehyde should be immediately distilled. So the temp. should be maintained at boiling point of aldehyde. Another way to overcome this problem is to use special reagent like PCC for the oxidation of alcohols PCC (Pyridinium Chloro Chromate) i.e. C5H5NH+CrO3Cl– B) Preparation of ketone (R – C O –) 2° alcohol on oxidation with acidified K2Cr2O7 gives ketone containing same number of carbon atoms. R R R | dil. H2SO4 | |

R – C – OH + (O) R – C – OH R – C = O | K2Cr2O7 | – H2O Ketone H OH

2°Alcohol Unstable 2) By alkaline hydrolysis of geminal dlhalides : A) Preparation of aldehyde (R – CHO) When terminal geminal dihalide (The geminal dihalide in which two halogen atom are attached to terminal carbon atom i.e. 1° geminal dihalide i.e. RCHX2) boiled with aq. KOH or aq. NaOH alkaline hydrolysis takes place & aldehyde is obtained.

H H H | Alkaline | |

R – C – X + 2KOH R – C – OH R – C = O + H2O | Hydrolysis | – H2O Aldehyde X – 2 KX OH

i.e. 1°geminal Unstable dihalide


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Preparation of ketone (R – CO – R) : When non-terminal geminal dihalide (The dihalide in which two halogen atoms are attached to non terminal carbon atom i.e. 2° geminal dihalide i.e. RCX2R) boiled with aq. KOH or aq. NaOH alkaline hydrolysis takes place and ketone is obtained.

R R R | Alkaline | |

R – C – X + 2KOH R – C – OH R – C = O + H2O | Hydrolysis | – H2O Aldehyde X – 2 KX OH i.e. 2°geminal Unstable dihalide

3) By dry distillation OR pyrolysis OR Thermal decomposition OR decarboxylation of calcium salt of carboxylic acid ((RCOO)2Ca) i) Dry distillation : It means heating under dry condition pyrolysis or thermal decomposition. ii) Decarboxylation : It means removal of CO2 (This reaction involves removal of CaCO3 which is combination of CaO & CO2. Or (Removal of metal carbonate from the salt of acid is called as decarboxylation) A) Preparation of aldehyde (R-CHO) : When calcium formate or mixture of calcium formate and calcium salt of any other organic acid is dry distilled, decarboxylation takes place and aldehyde is obtained. NOTE : Thus calcium formate is necessary for the preparation of aldehyde. HCOO Dry distilled i) Ca H – CHO + CaCO3 HCOO Decarboxylation Formaldehyde HCOO OOC CH3 Dry distilled ii) Ca + Ca 2CH3 – CHO + 2CaCO3 HCOO OOC CH3 Acetaldehyde Calcium formate Calcium acetate Preparation of Ketone ( R – CO – R ) : When calcium salt of organic acid (except calcium formate) is dry distilled, decarboxyiation takes place & ketone is obtained. CH3COO Dry distilled i) Ca CH3 – CO – CH3 + CaCO3 CH3COO Acetone Calcium acetate CH3COO OOC C2H5 Dry distilled ii) Ca + Ca 2CH3 – CO–C2H5 + 2CaCO3 CH3COO OOC C2H5 Ethyl methyl ketone Calcium acetate Calcium propionate Advantage : In this method, calcium carbonate remains in the flask & pure aldehyde or ketone distills over. Limitation : Yield of mixed ketone is poor, because in addition to mixed ketone, the other two simple ketones are also formed.


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4) From Alkyl cyanide : (R – CN) Preparation of ketone (R – CO – R) : R Dry ether |

R – Mg – X + R – C N R – C = N – Mg – X Alkyl mag. Alkyl Addition Addition product boil halide cyanide dil HCl R Hydrolysis X | Mg + NH3 + R – C = O OH i.e. R – CO – R Ketone Statement : When alkyl magnesium halide reacts with alkyl cyanide or (RCN) in presence of dry ether addition takes place and addition product is obtained, which when boil with dil. HCI hydrolysis takes place & ketone is obtained. [Note : When Grignard reagent (R Mg X) reacts with hydrogen cyanide (HCN) gives alkanes.]

7) Physical Properties of aldehydes & ketones : 1) State :

i) Formaldehyde is a gas of room temp. Its 40% solution in water is called formalin. When dry formaldehyde is needed, it is generated by heating its solid linear polymer. (para formaldehyde) or solid trimer, (trioxane).

ii) Acetaldehyde and acetone are colourless liquids. Acetaldehyde is also used as a trimer (paraldehyde) and a tetramer (metaldehyde).

iii) Lower aldehydes & ketones (upto C11) colourless volatile liquids white higher members are solids.

2) Odour : Lower aldehydes have pungent smell but higher aldehydes and all ketones have fruity smell.

3) Nature : Aldehydes & ketones are polar in nature due to polar carbonyl group

+| – – C O

–|| || +

O C – |

4) Boiling point : i) Aldehydes & ketones (polar compound) have higher B.P. than the

corresponding alkane (non polar compounds) due to dipole - dipole attractions between two polar carbonyl groups.

ii) But aldehydes & ketones have lower B. P. than the corresponding alcohols or acids which forms intermolecular hydrogen bonding.

5) Solubility : Lower aldehydes & ketones are soluble In water because they form hydrogen bonding with water (but not among themselves). But solubility decreases with increase in molecular weight It is due to increase in hydrophobic nature of carbon chain. 6) Aldehydes and ketones have lower density than water. 7) They are neutral compounds. CH3 CH3 – CH – O – CH – CH3 | | CH O O O O | | CH3 – CH – O – CH – CH3 CH CH Metaldehyde

CH3 O CH3 Paraldehyde

This polar character of the molecule gives intermolecular attraction called as dipole-dipole attraction which are weaker (less stronger) than hydrogen bonding.


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8) Chemical reactions of aldehydes and ketones : There are seven reactions of aldehydes & ketones 1) Addition reactions 2) Condensation reactions 3) Reduction 4) Pinacol formation (Not given by aldehyde) 5) Aldol condensation 6) Cannizzaro's reaction 7) Reducing properties of aldehydes (Not given by ketones) 1) Addition reactions Q. 1 Why aldehydes & ketones undergo nucleophilic addition reaction? Ans :

+ – i) In aldehydes and ketones carbonyl group is polar group i.e. C = O

ii) This polar group may be attacked by nucleophile or by electrophile. iii) But attack of nucleophile on positive carbon of carbonyl group gives alkoxide ion

as intermediate while attack of electrophile on negative oxygen of carbonyl group gives carbonium ion as intermediate.

Ex. a) C = O + Nu C – O | Nu Alkoxide ion (More stable) + +

b) C = O + E C – O – E Carbonium ion (less stable) iv) Hence in carbonyl group, nucleophilic attack come before electrophilic attack. So

aldehydes & ketones undergoes nucleophilic addition reaction. Reactivity of aldehydes & ketones : i) The reactivity of carbonyl compounds depends upon + ve charge on carbonyl

carbon atom. ii) More is the + ve charge more is the reactivity of carbonyl compound. iii) Electron releasing (ER) group such as CH3, C2H5 (which shows +I effect)

decreases the + ve charge on carbonyl group and thus decreases the reactivity of carbonyl compound.

iv) Electron withdrawing (EA) group such as –Cl, –Br, –NO2 (which shows –I effect) increases the + ve charge on carbonyl group & thus increases the reactivity of carbonyl compound.

v) Reactivity also depends upon stearic factor (Crowded group). vi) Hence the reactivity order of aldehydes & ketones is –

H – CHO > R – CHO > Ar – CHO > R – CO – R

1) Addition reactions of aldehydes & ketones : There are 5 addition reactions.

I) Addition of hydrogen cyanide (H – CN) : A) Reaction with aldehyde :When aldehyde reacts with HCN In presence of base as catalyst addition takes place and aldehyde cyanohydrin is obtained. H H | Base | R – C = O + H – CN R – C – OH Aldehyde Addition | CN

Cyanohydrin 2) Reaction with ketone :When ketone reacts with HCN in presence of base as catalyst, addition takes place and ketone cyanohydrin is obtained R R | Base | R – C = O + H – CN R – C – OH | Ketone CN

Ketone cyanohydrin


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Note : The base removes a proton from hydrogen cyanide and readily produces CN ion and makes the reaction fast)

HCN + OH H2O + CN II) Addition of sodium bisulphite (NaHSO3) in the form of saturated solution A) Reaction with aldehyde : When aldehyde reacts with NaHSO3, addition takes place and crystalline aldehyde sodium bisulphite is obtained. H H | Addition | R – C = O + NaHSO3 R – C – OH | Aldehyde SO3Na Aldehyde sodium bisulphite B) Reaction with ketone : When ketone reacts with NaHSO3, addition takes place and crystalline ketone sodium bisulphite is obtained. R R | Addition | R – C = O + NaHSO3 R – C – OH | Ketone SO3Na Ketone sodium bisulphite Mechanisms : Na

C = O + SO3 C – O– Proton

transfer C – OH

H | | SO3H SO3Na NOTE : The bisulphite compounds are water soluble can be easily decomposed by dil. acids or bases to regenerate aldehydes & ketones. So this reaction is useful for, separation and purification of aldehyde & Ketones from a mixture. III) Addition of ammonia (NH3) :Ammonia reacts with – A) Formaldehyde (H – CHO) B) Aldehyde (Except formaldehyde) and C) Ketone in different manner A) Reaction with formaldehyde : When formaldehyde reacts with ammonia, hexamethylene tetraamine or urotropine (trade name) Is obtained. 6H – CHO + 4NH3 (CH2)6N4 + 6H2O Formaldehyde Urotropine NOTE : 1) Urotropine is used for gout, rheumatism

and as urinary antiseptic. 2) Urotropine is condensation product of

formaldehyde & ammonia. 3) Formaldehyde reacts with ammonia in

different manner because formaldehyde is most reactive aldehyde & the reaction does not stop at the first stage.

4) Urotropine when nitrated, gives a high explosive known as cyclonite (RDX).

B) Reaction with aldehyde (other than formaldehyde) : When aldehyde reacts with ammonia addition takes place and aldehyde ammonia / amine is obtained,

Hexamethylene tetramine has a cage like structure with three six membered rings, each in a chair conformation.

Urotropine

i) C – C bond = zero ii) N – N bond = zero iii) C – N bond = 12 iv) C – H bond = 12


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H H | Addition | R – C = O + H–NH2 R – C – OH | Aldehyde i.e.NH3 NH2 Aldehyde ammonia/amine If reaction does not stop at aldehyde ammonia. It loses water to form an imine, which further trimerizes to give a heterocylic compound. H H R

| –H2O | trimerizes NH – CH

R – C – NH2 [ R – C = NH ] R – CH NH | aldimine NH – CH OH R C) Reaction with ketone : When acetone reacts with ammonia, two molecules of acetone condensed to give mesityl oxide which further reacts with ammonia, Markownikoffs addition takes place and diacetone ammonia or diacetonamine is obtained. CH3 H CH3 CH3 CH3 | | | NH3 | | i) CH3 – C = O + H2C – C = O CH3 – C = CH – C = O + H2O Acetone Mesityl oxide

CH3 CH3 CH3 CH3 | | M.R. | | ii) CH3 – C = CH – C = O + H – NH2 CH3 – C – CH2 – C = O Mesityl oxide | 4-Methyl pent-3-en-2-one NH2 Diacetonamine (4 – Amino – 4 – methyl 2- pentanone) IV) Addition of Grignard’s reagent (R – MgX) : Formation of 10, 20 and 30 alcohols (For details see chapter Grignard’s reagent) H | Dry A) R – Mg – X + H – C = O R – CH2 OH + MgXOH Grignard’s formaldehyde Ether 10 alcohol reagent (This reaction is used to convert C = O group into –CH2OH group) H | Dry B) R – Mg – X + R – C = O R2 CHOH + MgXOH Grignard’s Aldehyde Ether 20 alcohol reagent (Except formaldehyde) (This reaction is used to convert C = O group into CHOH group)

R R | Dry | C) R – Mg – X + R – C = O R – C – OH + MgXOH Grignard’s Ketone Ether | reagent R 30 alcohol

(This reaction is used to convert C = O group into – C – OH group) V) Addition of alcohols : Aldehyde and ketones reacts with excess of alcohols to form acetals. Two molecules of alcohol are added to carbonyl group to form acetal by elimination of one water molecule.


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i)

ii)

Acetal is a geminal dialkoxy compound (an ether). Note : Aldehydes and ketones react with one equivalent of 1, 2, or 1, 3–diols in presence of dry hydrogen chloride to give cyclic acetals or ketals, respectively.

2) Condensation reactions :

The reaction in which two molecules are joined together with elimination of small molecules like H2O, HX etc is known as condensation reaction.

There are six condensation reaction of aldehydes & ketones.

A) Reaction with hydroxyl amine (NH2OH) : When aldehyde or ketone reacts with hydroxyl amine in weakly acidic medium, condensation takes place and corresponding aldoxime or ketoxime is obtained.

i) H H | Weakly acidic I R – C = O + H2NOH R – C = NOH – H2O Aldehyde Hydroxyl amine medium Aldoxime ii) H H | Weakly acidic I R – C = O + H2NOH R – C = NOH – H2O Ketone Hydroxyl Ketoxime amine B) Reduction with hydrazine (H2N – NH2) : When aldehyde or ketone reacts with hydrazine in weekly acidic medium, condensation takes place a corresponding aldehyde hydrazone or ketone hydrazone is obtained. H H | H+ | i) R – C = O + H2N – NH2 R – C = N – NH2 + H2O Aldehyde Hydrazine Aldehyde hydrazone R R | H+ | ii) R – C = O + H2N – NH2 R – C = N – NH2 + H2O Ketone Hydrazine Ketone hydrazone C) Reaction with phenyl hydrazine (C6H5NHNH2) : When aldehyde to ketone reacts with phenyl hydrazine in weakly acidic medium, condensation takes place and corresponding aldehyde phenyl hydrazone or ketone phenyl hydrazone is obtained.

Ketal

Hemiacetal or alkoxy alcohol


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i) H H | Weakly acidic I R – C = O + H2N – NH – C6H5 R – C = N – NH – C6H5 + H2O Aldehyde Phenyl medium Aldehyde phenyl hydrazone hydrazine ii) R R | Weakly acidic I R – C = O + H2N – NH – C6H5 R – C = N – NH – C6H5 + H2O Ketone Phenyl Ketone phenyl hydrazone hydrazine D) E) (Brady’s reagent) (Orange ppt) F)

NOTE : 1) The ammonia derivatives like hydroxyl amine or phenyl hydrazine NH2 – Z are weak nucleophile. Therefore reaction is catalysed by weak acidic medium. pH = 3 to 4 is taken. Otherwise it would reacted strong acid instead of aldehyde and ketone. 2) In acidic medium, the carbonyl oxygen gets protonated. Due to presence of positive charge on carbonyl carbon, the weak nucleophile like ammonia derivatives attack the carbonyl group very easily. 3) Compound forms by the condensation are often solids and have sharp melting points, hence are used for characterization and identification of aldehydes and ketones.

3) Reduction of aldehydes and ketones : I] Reduction by using i) NaHg and H2O or by using NaBH4, LiAlH4, OR ii) H2 and Ni / Pt/ Pd/ Ru/ Rhodium. A) Reduction of aldehydes (R – CHO) H H | NaHg + H2O | R – C =O + 2 (H) R – C – OH Aldehyde Reduction | H 10 alcohol

(2 NaHg + 2 H2O 2 NaOH +2 Hg + 2H) sodium amalgam Nascent OR H H | Ni/Pt/ Pd at 453 to 473 K | R – C = O + H2 R – C – OH Ru/ rhodium | Aldehyde H 10 alcohol


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Statement : When aldehyde reacts with reducing agent like sodium amalgam and water or heated with hydrogen gas in presence of catalyst like Ni/Pt/Pd at 453 to 473 K reduction takes place and 10 alcohol is obtained. B) Reduction of ketone (R – CO – R) : R R | NaHg + H2O | R – C =O + 2 (H) R – C – OH Ketone Reduction | H 20 alcohol OR R R | Ni/Pt/ Pd at 453 to 473 K | R – C = O + H2 R – C – OH Ru/ rhodium | Ketone H 20 alcohol Statement : When ketone reacts with reducing agent like sodium amalgam and water or heated with hydrogen gas in presence of catalyst like Ni/Pt/Pd at 453 to 473 K reduction takes place and 20 alcohol is obtained.

II] Reduction to hydrocarbons (deoxygenation) : A) Clemmenson's reduction : (Conversion of carbonyl group C= O) into methylene group ( CH2) when aldehyde or ketone reacts with reducing agent like ZnHg + conc. HCI, Clemmenson's takes place and corresponding alkane is obtained. (This reaction is most commonly used to convert acyl benzenes to alkyl benzenes). H H i) | ZnHg + conc. HCl | R – C = O + 4(H) R – C – H + H2O | Aldehyde H Alkane R R ii) | ZnHg + conc. HCl | R – C = O + 4(H) R – C – H + H2O | Ketone H Alkane [Note : Ketones readily undergoes Clemmenson’s reduction.]

B) Wolff – Kishner reduction : Aldehydes and ketones when heated with hydrazine and KOH or potassium tert- butoxide in high boiling solvents such as ethylene glycol or diethylene glycol give alkanes. This reduction is known as Wolff – Kishner reduction. Here the carbonyl compounds yield alkane through hydrazone. Alkyl aryl ketones obtained by Friedel – Craft acylation can be reduced by this method to get alkylbenzenes. The importance of the reaction is that straight chain alkyl groups can be attached to benzene which is not possible by Friedel – Craft alkylation. The ketone is first converted to its hydrazone which on heating with strong base gives alkane.

C = O 2 2

2

H N NH

H O

C = N – NH2

/KOH ethyleneglycol

CH2 + N2

This reaction is also called as Hung – Milnon reaction.


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4) Pinacol formation (Reduction by Mg metal i.e. Bimolecular reduction) :

This reaction is given only by ketones and not by aldehydes. When ketone reacts with magnesium metal in presence of benzene,

addition 'takes place & addition product is obtained which when boil with dil. HCI hydrolysis takes place & pinacol is obtained (pinacol is ditertiary diol). Ex. CH3 CH3 CH3 CH3 | | Benzene | | CH3 – C + Mg + C – CH3 CH3 – C – C – CH3 || || Addition | | O O O O Acetone Mg (2 molecules) Addition product dil. HCl hydrolysis CH3 CH3 OH | | Mg + CH3 – C – C – CH3 OH | | OH OH Tetramethyl ethylene glycol

OR 2, 3 – Dimethyl butane – 2, 3 – diol) OR Tetra methylene diol

[Note : MgHg + H2O can also be used for Pinacol formation.]

This pinacol when heated with dil. Acid it undergoes dehydration and rearrangement to give a ketone called as pinacolone. CH3 CH3 CH3 | | | CH3 – C – C – CH3 CH3 – C – C – CH3 | | | || OH OH CH3 O

Pinacol Pinacolone

5) Aldol Condensation : 1) Condition : This reaction is given by those aldehydes and those ketones which have

atleast one – hydrogen atom. 2) Mechanism : When such aldehyde or such ketone reacts with dil. base like dil. NaOH or dil. KOH, or dil. NaoCO3 or dil. Ba(OH)2 (Baryta water) etc. then two

molecule of such aldehyde or such ketone takes part in the reaction and - carbon

atom of one molecule gets added to carbonyl carbon of other molecule & -hydrogen atom get migrated to oxygen atom of carbonyl group of other molecule

to give -hydroxy aldehyde and or -hydroxy ketone. 3) eg. When acetaldehyde reacts with dil. base, then two molecules of acetaldehyde

takes part in the reacts to give -hydroxy buteraldehyde. H H H H | | dil. base | | CH3 – C + H – CH2 – C CH3 – C – CH2 – C || || | | O O OH O

i.e. CH3 – CHO - hydroxyl aldehyde

Acetaldehyde or - hydroxy butyraldehyde or 3 - hydroxyl butanal or Aldol or Acetaldol


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4) The compound -hydroxy aldehyde contains two functional group i.e. aldehyde (– CHO) & alcohol (– OH) and so the compound is called as "Aldol". 5) When this aldol heated alone or heated with dil. acid, it undergoes dehydration to

give , -unsaturated aldehyde. e.g. When acetaldol heated alone or heated with dil. acid, it undergoes dehydration to give cratonaldehyde or 2-Butenal. H H H | | |

CH3 – C – C – C = O CH3 – CH = CH – CHO + H2O | | dil. acid crotonaldehyde or OH H dehydration 2 – butenal

Acetaldol (, unsaturated aldehyde) 6) When acetone reacts with dil. base then two molecules of acetone takes part in the

reaction to give -hydroxy diacetone. CH3 CH3 CH3 CH3 | | dil. base | | i) CH3 – C + H – CH2 – C CH3 – C –CH2 – C || || | || O O OH O

- hydroxy diacetone or 4-Hydroxy-4-Methyl-2-petanone or Ketol/Acetol/ Diacetone alcohol

7) Compound -hydroxy ketone contains two functional groups ketone and alcohol. So, it is called as "Ketol". [Note : The yields of keols are very poor as compared to aldols; steric factors are probably responsible for these results.] 8) When acetol is heated alone or with dil. acid, undergoes dehydration to give

mesityl oxide. i.e. , unsaturated ketone. CH3 CH3 CH3 CH3 | | – H2O | |

ii) CH3 – C – CH – C CH3 –C = CH – C = O + H2O

| | || mesityl oxide (, - unsaturated ketone) OH H O 4 – Methylpent – 3 – en – 2 – one

- hydroxy diacetone

[Note : i) , unsaturated carbonyl compound can be obtained directly from an aldehyde or a ketone by a reaction in the presence of dilute base at slightly elevated

temperature, without isolating intermediate – hydroxyl carbonyl compound.] 9) Crossed aldol or mixed condensation : If two different carbonyl compounds in

which atleast one contain -H atom takes part in the reaction then, the reaction is called as crossed aldol condensation (reaction between two different aldehyde or two different

ketones or reaction between aldehyde & ketone) and in this reaction - hydrogen atom of lower carbonyl compounds (gets migrated) takes part in the reaction.

[Note : If both of them contain –hydrogen atom it gives a mixture of four products. ] e.g. O O OH O || || dil. NaOH | || 1) C2H5 – C + H – CH2 – C – H C2H5 – C – CH2 – C – H | | H H Propanal ethanal 3-hydroxy pentanal


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2) C2H5COCH3 + H – CH2 COCH3 .

NaOH

dil C2H5C(CH3) (OH) CH2COCH3

Butanone Propanone 4- methyl-4-hydroxy-2-hexanone

When aldehyde condenses with ketone. It is the -hydrogen of ketone is transferred to carbonyl oxygen of aldehyde. e.g. when acetaldehyde is heated with acetone in the presence of dil. NaOH gives 4-hydroxyl–2-pentanone.

CH3 – CHO + H – CH2 COCH3 .

NaOH

dil CH3CH(OH)CH2COCH3

Acetaldehyde Acetone 4-hydroxy-2-pentanone NOTE : 1) It is nucleophilic addition reaction. 2) In this reaction carbanion is formed 3) It is also called as 'self condensation' or 'auto condensation' or 'Base catalysed

aldol condensation'.

Limitations : 1) The compounds like formaldehyde (H–CHO); trichloroacetaldehyde (CCI3–CHO),

trimethyl acetaldehyde (CH3)3C–CHO), benzaldehyde (C6H5–CHO), diaryl ketone (Ar – CO – Ar) do hot give aldol condensation reaction due to absence of

-hydrogen atom.

6) Cannizzaro's Reaction (Disproportionation reaction) 1) Condition : This reaction is given by those aldehydes which have no -H atom. e.g. Formaldehyde & benzaldehyde will give this reaction. 2) Mechanism : When such aldehyde reacts with conc. base like conc. NaOH or conc. KOH, then two molecules of such aldehydes takes part in the reaction & one molecule get reduced to 10 alcohol, while other molecule get oxidised to acid (but acid reacts with strong base to give salt). e.g. i) H – CHO + NaOH + H – CHO CH3 – OH + HCOO – Na Formaldehyde conc. Methyl alc. Sodium formate When formaldehyde reacts with conc. NaOH, two molecules takes part in the reaction & one molecule get reduced to methyl alcohol while other get oxidised to sodium formate. ii) C6H5 – CHO + NaOH + C6H5 – CHO C6H5-CH2-OH + C6H5–COO– Na

Benzaldehyde conc. Benzyl alcohol Sodium benzoate When benzaldehyde reacts with conc. NaOH, two molecules of benzaldehyde takes part in the reaction and one molecule gets reduced to Benzyl alcohol while other gets oxidised of sodium benzoate. 4) The 'Self - oxidation, reduction reaction or 'auto redox' reaction of such aldehyde

which have no ‘-H’ atom under the influence of conc. base, is called as 'Cannizzaro's reaction'.

NOTE : If two different aldehydes having no. '-H' atom takes part in the reaction then it is called as 'Crossed Cannizzaro's reaction'. In this reaction lower aldehydes (more reactive] undergoes oxidation reaction. 5) e.g. H – CHO + NaOH + C6H5 – CHO C6H5 – CH2 – OH + HCOONa Formaldehyde conc. Benzaldehyde benzyl alcohol sodium formate When mixture of formaldehyde and benzaldehyde reacts with conc. NaOH, then benzaldehyde reduced to benzyl alcohol and formaldehyde oxidised to sodium formate. NOTE : 1) Cannizzaro's reaction is nucleophilic addition in which there is formation of H– ion (Hydride ion). 2) Iso-butyraldehyde (CH3)2CHCHO gives Cannizzaro reaction even though it contains

– hydrogen atom.


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Mechanism :

H H | |

i) NaOH Na++ OH– ii) H – C + OH– H – C – OH || | O O– H H H H | | | |

iii) H – C + H – C – OH H transfer

H – C – H + C – OH || | | || O O – O – O H H | |

iv) H – C + C – OH CH3OH + HCOO– v) HCOO– + Na+ HCOONa | || O – O

7) Reducing properties of aldehydes and not given by ketones

In aldehydes

H

OCR at least one hydrogen atom is directly attached

carbonyl group. Such H atom is called as 'replaceable H atom or oxidisable H atom. Or active H – atom or carbonyl H atom. So, aldehydes can reduces other substance. In

ketones,

R

OCR there is no replaceable H atom & so, ketones can't reduces other

substances. These reactions are used as identification test for aldehydes & these reactions are used to distinguish aldehydes and ketones. A) Reaction with Fehling's solution : Fehling's solution is the mixture of Fehling's solution 7 % CuSO4 solution i.e. blue vitriol & Fehling's solution 'B' (solution of sodium potassium tartarate i.e. Rochelle salt in dil. NaOH) in equal volume. Fehling’s solution is blue coloured solution.

CuSO4 + 2NaOH Cu(OH)2 + Na2SO4

Deep blue coloured It is indicated by 2 [Cu(OH)2] / 2Cu++ + 4OH

R – CHO + 2Cu2++ + 4 OH R – COOH + Cu2O + 2H2O

Aldehyde Fehlings solution acid Cuprous oxide (red ppt) When aldehyde is heated with Fehling's solution, it reduces Fehlings solution to red ppt of cuprous oxide & itself gets oxidised to acid.

e.g. H – CHO + 2Cu++ + 4OH H – COOH + Cu2O + 2H2O

Formaldehyde Fehlings solution Formic Cuprous oxide Acid (red ppt)

When formaldehyde is heated with Fehling's solution, it reduces Fehling’s

solution to red ppt. of cuprous oxide & itself gets oxidised to formic acid: In this reaction cupric ion (Cu++) gets converted into cuprous ion (Cu+) i.e. oxidation state of copper decreases form + 2 to + 1. [Note : Aromatic aldehydes are not oxidized by Fehling’s solution.] B) Reaction with Tollen's reagent / Ammonical silver nitrate solution :

Tollen's regent : It is ammonical silver nitrate solution. It is prepared by adding NaOH solution to AgNO3 solution When a grey ppt of silver hydroxide (AgOH) formed NH4OH is then added dropwise so as to just dissolve the ppt of AgOH

i) AgNO3 + NaOH AgOH + NaNO3

ii) AgOH + 2 NH4OH [Ag(NH3)2]OH + 2 H2O

Silver present in the solution in the form of soluble complex ion [Ag (NH3)2].


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R – CHO + 2[Ag (NH3)2]OH R – COOH + 2 Ag + 4NH3 + H2O

Aldehyde Oxidising agent Acid grey ppt When aldehyde warm with Tollen's reagent, it reduces Tollen's reagent to black/grey ppt of colloidal silver which deposits on inner wall of test tube (Silver mirror) & aldehyde Itself get oxidised to acid. (In this reaction, oxidation state of silver change from + 1 to 0). C) Reaction with Schiff’s reagent :

(Schiff s reagent is colourless solution obtained by passing SO2 gas through para -rosaniline hydrochloride solution i.e. magenta red or pink solution or Fuschin dye).

Aldehyde + Schiff’s reagent Pink colour or Magenta red colour

When aldehyde is reacts with Schiff s reagent in cold condition, pink colour or magenta red colour is obtained. Note : i) As less reactive benzaldehyde reduces only Tollen's reagent. ii) Tollen’s reagent stronger oxidising agent then Fehling’s solution and Schiff’s reagent.

Uses : Formaldehyde : 1) It is used in silvering of mirror in leather industry. 2) Urotropine is used as antiseptic specimen. 3) Nitration of urotropine gives powerful explosive cyclonite RDX (Research

Development Explosive). 4) It is used as disinfectant and as preservative for biological specimens. 5) At present the most important use of formaldehyde is for the production of variety

of plastic and resins, particularly in Bakelite and binders in plywood. Acetone : 1) It is used as solvent for nail polish remover. 2) It is also used in manufacture of explosive, lacquers, paint removers, plastics,

drugs perfumes, adhesive and disinfectant. 3) It is used to prepare chloroform, chloretone (Hypnotic), artificial scent (lonone), unbreakable

glass (Plexi glass), synthetic rubber & cordite (Smokeless power explosive).

Benzaldehyde is used in perfumery and in dye industry.

Some additional Points

i) Acrolein (CH2 = CH – CHO) i.e. propenal is aliphatic unsaturated aldehyde like

crotonaldehyde. ii) Naturally occurring carbonyl compound are present in nucleic acid, carbohydrate and

proteins in plant and animal. iii) They play an important role in biochemical processes to sustained life and some

carbonyl compounds (like acetone uses as solvents and for preparing materials like adhesive, paints, resins, perfumes, plastics, fabrics etc.

iv) They add fragrance and flavour to the nature and also components of several pharmaceutical.

v) Some of the common natural substances in which important carbonyl compound occur are almonds (benzaldehyde), cinnamon (cinnamaldehyde), the vanillabean (vanillin), salicylaldehyde (from meadow sweet) they have pleasant fragrances.

vi) The sex hormones (testosterone and progesterone), camphor (from the camphor tree),

oil of citronella (citronellal), vitamin K, the perfume derived from the musk deer (muscone) etc.

Benedict's Solution : It is alkaline solution of cupric ion complexed with citrate ions. (i.e. the mixture of solution of copper sulphate, sodium hydroxide & sodium citrate). It reacts with aldehyde as the same way as Fehling's solution & not with ketones.

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