Download ALDEHYDES , KETONES AND CARBOXYLIC ACIDS

ALDEHYDES , KETONES AND CARBOXYLIC ACIDS
Organic compounds containing carbon-oxygen
double bond (>C=O) called carbonyl group
Amine
O
C---OH → Carbonyl + Hydroxyl = Carboxylic
Acid
Ester
Acid chloride
Anhydride
Nomenclature:
ALDEHYDES => Alkanal
KETONES => Alkanone
Amides => Alkanamide ,
Esters => Alkylalkanoate
Anhydride = > (e.g) (CH3CO)2O => Acetic anhydride.
Acids => Alkanoic Acid
Acid Chlorides = > Alkanoyl Chloride
Structure of the Carbonyl Group:
The carbon-oxygen bond is partially
polarized. In which carbon has partial +ve
charge and oxygen has partial –ve charge.
Preparation of Aldehydes and Ketones:
1. By oxidation of alcohols: 10 alcohol on oxidation gives aldehydes and 20 alcohol on oxidation gives
ketones.
R-CH2OH+ [O] → R-CHO (Aldehyde)
R2CHOH+ [O]→
→ R2CO (Ketone)
2. By dehydrogenation of alcohols: Alcohols when heated with Cu or Silver catalyst at 573K, we get
carbonyl compounds.
R-CH2OH Cu/573 K R-CHO
R2CHOH Cu/573 K R2CO
3. From hydrocarbons: Ozonolysis of alkenes followed by reaction with zinc dust (Zn) and water gives
aldehydes, ketones or both.
4. By hydration of alkynes: hydration of alkynes in presence of H2SO4 and HgSO4 to give carbonyl
compounds.
Preparation of Aldehydes:
1. From acyl chloride:(Rosenmund’s Reduction)
Acid chlorides react with hydrogen in presence of Pd supported on BaSO4, we get aldehydes. This reaction is
called Rosenmund’s reduction.
R-COCl + H2 Pd/BaSO4 R-CHO + HCl
Benzoyl chloride
Benzaldehyde
2. From nitriles and esters: Nitriles when reduced with stannous chloride in the presence of
hydrochloric acid, we get imines, which on hydrolysis give aldehyde. This reaction is called Stephen
reaction.
Nitriles can also be selectively reduced by diisobutylaluminium hydride (DIBAL-H) to imines
followed by hydrolysis to aldehydes. DIBAL-H does not affect other functional group.
Aromatic aldehydes are prepared from aromatic hydrocarbons by the following methods.
1. Etard`s reaction: Methyl benzenes when oxidised by using mild oxidising agents like chromyl
chloride (CrO2Cl2) to give a chromium complex. Which on hydrolysis gives corresponding
benzaldehyde.
2. By side chain chlorination and hydrolysis: Side chain chlorination of toluene gives benzal chloride,
which on hydrolysis gives benzaldehyde. This method is used for the manufacture of benzaldehyde.
Toluene
Benzal chloride
Benzaldehyde
3. By Gatterman – Koch reaction :When benzene is treated with a mixture of carbon monoxide and
hydrogen chloride in the presence of anhydrous aluminium chloride or cuprous chloride, we get
benzaldehyde.
Preparation of Ketones:
From acyl chlorides: Acid chlorides react with dialkyl cadmium gives ketones. R2Cd is prepared by the
reaction of CdCl2 with Grignard reagent.
From nitriles: Nitriles on reacting with Grignard reagent followed by hydrolysis gives ketones.
By Friedel – Crafts acylation:
Physical properties:
• The boiling points of aldehydes and ketones are higher than hydrocarbons and ethers due to dipole
dipoe interaction and lower than those of alcohols due to absence of intermolecular hydrogen
bonding.
• The lower members of aldehydes and ketones are miscible with water due to hydrogen bonding.
1. Chemical Reactions : Nucleophilic addition reactions
(i) Mechanism of nucleophilic addition reactions
In this process hybridisation of carbon changes from
sp2 to sp3 and a tetrahedral alkoxide intermediate is
formed, which captures a proton from the reaction
medium to give the product.
(ii) Reactivity: Aldehydes are more reactive than
ketones in nucleophilic addition reactions due to
steric and electronic reasons. Sterically, the
presence of two relatively large substituents in
ketones hinders the approach of nucleophile to
carbonyl carbon than in aldehydes having only one
such substituent. Electronically, aldehydes are more
reactive than ketones because two alkyl groups
reduce the electrophilicity of the carbonyl carbon
more effectively than in former.
Addition of HCN on carbonyl compound
gives cyanohydrins.
Addition of sodium hydrogensulphite gives a white
crystalline bisulphate product.
Addition of Grignard reagent gives alcohols
Addition of alcohol on aldehyde / ketone, first gives
hemicacetal and then acetal.
Addition of ammonia and its derivatives on
aldehyde and ketone gives different products.
Z = Alkyl, aryl, OH, NH2, C6H5NH, NHCONH2.
Addition of ammonia and its derivatives:
• Addition of hydroxyl amine (NH2-OH) gives oxime
R-CHO + NH2OH → R-CH=N-OH + H2O
•
Addition of hydrazine (NH2-NH2) gives hydrazone. R-CHO + NH2-NH2 → R-CH=N-NH2 + H2O
•
Addition of phenyl hydrazine (NH2-NH-C6H5)gives phenyl hydrazone
R-CHO + NH2-NH-C6H5 → R-CH=N-NH-C6H5 + H2O
•
Addition of 2,4-dinitrophenyl hydrazine (2,4-DN) gives 2,4-dinitrophenyl hydrazone. This
reagent is called Borsche`s reagent.
• Addition of Semicarbazide (NH2-NH-CO-NH2) gives semicarbazone
R-CHO + NH2-NH-CO-NH2 → R-CH=N-NH-CO-NH2 + H2O
2. Reduction: To alcohols (Reducing agents are NaBH4, LiAlH4 andH2/ catalyst)
R-CHO +[H]→ R-CH2OH
R2CO+ [H] → R2CHOH
Reduction to hydrocarbons:
Clemmensen reduction: The carbonyl group of
Wolff-Kishner reduction: The carbonyl group of
aldehydes and ketones is reduced to CH2 group on
aldehydes and ketones is reduced to CH2 group on
treatment with zinc-amalgam and concentrated
treatment with hydrazine followed by heating with
hydrochloric acid.
sodium or potassium hydroxide.
3. Oxidation:
CH3CHO +[O]
CH3COOH
Common oxidizing agents are HNO3, KMnO4,
K2Cr2O7, etc.
CH3COCH3+[O]
CH3COOH + HCOOH
Ketones are generally oxidised under vigorous
Conditions with strong oxidising agents.
4. Haloform Reaction:
Aldehydes or ketones having CH3-CO- group or CH3-CHOH- group, when treated with sodium
hypohalite or halogen in presence of NaOH, we get a precipitate haloform (CHX3). This reaction is
called haloform reaction.
R-CO-CH3 + NaOX
R-COONa + CHX3 (where X = Cl, Br or I)
The reaction with sodium hypoiodite gives a yellow precipitate of iodoform and this reaction is used
for the detection of CH3-CO- group or CH3-CHOH- group in a compound.
Important reactions:
1. Aldol condensation Reaction: Aldehydes and ketones having at least one α-hydrogen atom when
treated with dilute alkali, we get β-hydroxy aldehydes (aldol) or ketones (ketol) which on heating,
undergo dehydration to give α,β-unsaturated aldehyde or ketone.
2. Cross aldol condensation: When aldol condensation is carried out between two different aldehydes or
ketones, it gives a mixture of products is called cross aldol condensation.
CH3-CHO + CH2-CH2-CHO
Ethanal
CH3-CH=CH-CHO + CH3-CH2-CH=C(CH3)-CHO +
NaOH/∆
Propanal
But-2-enal
2-methylpent-2-enal
CH3-CH=C(CH3)-CHO + CH3-CH2-CH=CH-CHO
2-methylbut-2-enal
Pent-2-enal
Benzalacetophenone
3. Cannizzaro Reaction: Aldehydes having no α-hydrogen atom when treated with Conc. Alkali undergo
self oxidation and reduction (disproportionation) to form one molecule of the alcohol and one
molecule of carboxylic acid salt. This reaction is called Cannizzaro reaction.
Benzaldehyde
benzyl alcohol
sodium salt of carboxylic acid
Electrophilic Substitution Reactions: Aldehydic and ketonic groups are deactivating and meta directing.
Tests to distinguish Aldehydes and Ketones:
Tollens’ test: Tollen’s reagent is freshly prepared ammoniacal Silver nitrate. On warming Tollens’ reagent,
aldehydes give a bright silver mirror due to the formation of metallic Ag.
–
R-CHO + 2[Ag(NH3)2]+ + 3OH
R-COO– + 2 Ag + 2 H2O + 4 NH3
Fehling’s test: Fehling reagent is a mixture 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). On heating with Fehling’s reagent, aldehyde gives a reddish brown precipitate of cuprous
oxide (Cu2O). Aromatic aldehydes do not give this test.
R-CHO + 2Cu2+ + 5OH –
R-COO– + Cu2O + 3H2O
CARBOXYLIC ACIDS
Nomenclature: In the IUPAC system, aliphatic carboxylic acids are named by replacing the ending –e in the
name of the corresponding alkane with – oic acid.
Structure of carboxylic acid:
Carboxilic acids are resonance stabilized. The resonance stabilized structures are
Methods of Preparation of Carboxylic acids:
1. From primary alcohols and aldehydes: Oxidation of alcohol or aldehyde using Acidified K2Cr2O7, alkaline
KMnO4 or cromic oxide.
2. From alkylbenzenes: Aromatic carboxylic acids can be prepared by vigorous oxidation of alkyl
benzenes with chromic acid or acidic or alkaline potassium permanganate.
3. From nitriles and amides: Acid or alkaline
hydrolysis of nitrile first give amide, then
carboxylic acid
4. From Grignard reagents: Grignard reagent on treating with carbon dioxide and followed by
hydrolysis gives carboxylic acid
5. From acyl halide and ester: Acyl halide and ester on acid or alkaline hydrolysis gives
corresponding carboxylic acid.
Physical Properties:
Boiling point: Carboxylic acids shows higher boiling point than aldehydes, ketones and alcohols due to
extensive inter molecular hydrogen bonging. Most carboxylic acids exist as dimer in the vapour phase.
Solubility: Lower member aliphatic acids are soluble in water due to inter molecular hydrogen bonding with
water. The solubility decreases with increase the number of carbon atom. Benzoic acid is nearly soluble in
cold water but soluble in hot water. All carboxylic acids are soluble in less polar solvent.
Chemical Reactions:
1. Reactions Involving Cleavage of O–H Bond:
Acidity: Carboxylic acid reacts with metals and alkali to liberate hydrogen gas. Reaction with sodium
carbonate, produce a brisk effervescence (this test is used for the identification of carboxylic acid)
Comparison of acidic character:Carboxylic acids are stronger acid than phenol and alcohol. The higher
acidity of carboxylic acids as compared to phenols can be explained as follows.
i) The carboxylate ion formed from the ionization of carboxylic acid is stabilised by two equivalent
resonance structures in which the negative charge is at the more electronegative oxygen atom. But the
phenoxide ion formed from the ionization of phenol has non-equivalent resonance structures in which the
negative charge is at the less electronegative carbon atom.
ii) The negative charge is delocalised over two electronegative oxygen atoms in carboxylate ion, whereas
it is less effectively delocalised over one oxygen atom and less electronegative carbon atoms in phenoxide
ion.
Carboxylate anions
Effect of substituents on the acidity of carboxylic acids: Electron withdrawing groups increase the
acidity of carboxylic acids by stabilising the carboxylate ion whereas electron donating group decrease the
acidity by destabilising the carboxylate ion.
The effect of the following groups in increasing acidity order is Ph < I < Br < Cl < F < CN < NO2 < CF3
2. Reactions Involving Cleavage of C–OH Bond:
i) Formation of anhydride: Carboxylic acids on heating with mineral acids such as H2SO4 or P2O5
give anhydride.
ii) Esterification: RCOOH + R'OH
H+
RCOOR' + H2O
iii) Reactions with PCl5, PCl3 and SOCl2:
RCOOH
PCl5 / PCl3 / SOCl2
RCOCl
iv) Reaction with ammonia: Carboxylic acids react with ammonia to give ammonium salt which on
further heating gives amides.
Phthalimide
Reactions Involving –COOH Group:
Reduction: with LiAlH4 or diborane, primary alcohol is
formed.
Decarboxylation: with a mixture of Calcium
oxide and alkali, hydrocarbon is formed.
HVZ reaction: Carboxylic acids having an α-hydrogen, when treated with halogen in the presence of red
phosphorus, we get α-halocarboxylic acids. This reaction is known as Hell-Volhard- Zelinsky(HVZ) reaction
CH3-CH2-COOH i) Cl2/ Red P
CH3-CHCl-COOH + HCl
ii) H2O
Electrophilic substitution reactions:
The –COOH group is a deactivating group and meta-directing. So, on electrophilic substitution
reactions, we get meta derivatives. They do not undergo Friedel-Crafts reaction, because the carboxyl group
is deactivating and the catalyst aluminium chloride (Lewis acid) gets bonded to the carboxyl group.
Nitration:
Bromination:
Use of carboxylic acid: Methanoic acid is used in rubber, textile, dyeing, leather and electroplating
industries. Ethanoic acid is used as solvent and as vinegar in food industry. Hexanedioic acid is used in the
manufacture of nylon-6, 6. Sodium benzoate is used as a food preservative.
Practice questions:
1. Give Reasons for the following:
a) Carboxylic acid is stronger acid than phenol.
b)
Aldehydes are more reactive than Ketones towards nucleophilic additions.
c) Carboxylic acids has higher boiling points than alcohols of same no. of carbon atoms.
d) Ethanoic acid has molar mass of 120 in vapour state.
e) Carboxylic acids do not give characteristic reactions of carbonyl group
f)
Formaldehyde does not undergo aldol condensation.
g) Flouro acetic acid is a stronger acid than acetic acid.
2. Convert the following:
i) Toluene to benzaldehyde
ii) Acetaldehyde to Acetamide
iii) Methanol to acetic acid
iv) Methanol to Ethanol
v) Acetic acid to Propionic acid
vi) Ethyl alcohol to acetone
vii) Acetone to tert butyl alcohol
viii) Toluene to m- nitrobenzoic acid
ix) Phenol to acetophenone
x) Acetaldehyde to Acetone
3. A compound ‘A’ with formula C5H10O gives a positive 2, 4 –DNP test but a negative Tollen’s test It can
be oxidizing to carboxylic acid ‘B’ of molecular formula C3H6O2, when treated with alk. KMnO4 under
vigorous conditions. The salt of ‘B’ gives a hydrocarbon ‘C’ on Kolbes’ electrolytic decarboxylation.
Identify A,B.C & write chemical equations.
4. Acompound A with molecular formula C5H12O on oxidation forms compound B with molecular formula
C5H10O. The compound B gives iodoform test but does not reduce ammoniacal silver nitrate. The compound
B on reduction with Zn – Hg/ HCl gives compound C with molecular formula C5H12. Identify A,B.C & give
the chemical reactions involved.
5. C H 3 COOH
6. C6 H5 CONH2
7. CH3 COCH3
Cl2/redP
(A)
A
Br2 /KOH
LiAlH4
alc.NH3
X
SOCl2
(B)
NaNO2 /HCl
Y
B K2 Cr2 O7 /H2 SO4
alc KOH
C
Z
8. What is the chemical name of Tollen’s reagent and Fehling’s solution
9. Write the structure of alkenes that on ozonolysis will give ketone only
10. What is the function of BaSO4 in Rosenmund reaction?
11. Name the isomers with molecular formula C3H6O. Which one will have high boiling point?
12. Write a chemical test to distinguish between aldehyde and ketone
13. What happens when acetaldehyde is kept with a trace of sulphuric acid? Write the structure of product.