//M3//QN1//SUB//DL0//EQ

What is carbonyl group? Give information about the functional groups containing carbonyl group with their structures.

//X

Carbonyl group: The organic compounds in which carbon-oxygen double bond (>C=O) containing functional group is present is called Carbonyl compounds and >C=O group is known as Carbonyl group.
Functional groups containing carbonyl group :

- Aldehydes and ketones: In aldehydes, the carbonyl group is bonded to a carbon and hydrogen while in the ketone, it is bonded to two carbon atoms.

Carboxylic acids: The carbonyl compounds in which carbon of carbonyl group is bonded to carbon or hydrogen and oxygen of hydroxyl moiety (–OH) are known as carboxylic acid.
Amides and acyl halides: The carbonyl compounds in which carbon of carbonyl group is bonded to carbon or hydrogen and nitrogen of –NH 2 moiety or to halogen are called amides and acyl halides respectively.
Esters and anhydrides: The compounds in which carbonyl carbon is bonded to a carbon or hydrogen and oxygen of alkoxy moiety (–OR’) and carbon of acyl moiety (–COR’) are known as esters and anhydrides respectively.

//M2//QN2//SUB//DL0

Give importance of aldehydes, ketones and carboxylic acids.

//X

Aldehydes, ketones and carboxylic acids are widespread in plants and animal kingdom.
They play an important role in biochemical process of life.
They add fragrance and flavour to nature, for example, vanillin (from vanilla beans), salicylaldehyde (from madow sweet) and cinnamaldehyde (from cinnamon) have very pleasant fragrance.
They are used in many food products and pharmaceuticals to add flavours.
Some of these families are manufactured for use as solvents (i.e. acetone) and for preparing materials like adhesives, paints, resins, perfumes, plastics, fabrics, etc.

//M1//QN3//SUB//DL0//EQ

Write common names of following compounds.

(i) CH 3 CHO
(ii)
(iii)
(iv) (v)
(vi)
(vii)
(viii)
(ix) CH 2 = CHCHO
(x) (xi) (CH 3)2 CHCHO
(xii) (CH 3)2 CHCOCH(CH 3)2
(xiii)
(xiv) (CH 3)2 C=CHCOCH 3

//X

(i) Acetaldehyde
(ii) Benzaldehyde
(iii) b-Bromobutyraldehyde
(iv) Acetone
(v) Acetophenone
(vi) Propiophenone
(vii) Benzophenone
(viii) γ-Methylcyclohexanecarbaldehyde
(ix) Acrolein
(x) Phthaldehyde
(xi) Isobutyraldehyde
(xii) Diisopropyl ketone
(xiii) a-Methylcyclohexanone
(xiv) Mesityl oxide

//M3//QN4//SUB//DL0//EQ

Discuss structure of the carbonyl group.

//X

The carbonyl carbon atom is sp 2-hybridised and forms three sigma (s) bonds.
The fourth valence electron of carbon remains in its p-orbital and forms a p-bond with oxygen by overlap with p-orbital of an oxygen.
In addition, the oxygen atom also has two non bonding electron pairs. Thus, the carbonyl carbon and the three atoms attached to it lie in the same plane and the p-electron cloud is above and below this plane.
The bond angles are approximately 120° as expected of a trigonal coplanar structure (Fig.).
The carbon-oxygen double bond is polarised due to higher electronegativity of oxygen relative to carbon. Hence, the carbonyl carbon is an electrophilic (Lewis acid), and carbonyl oxygen, a nucleophilic (Lewis base) centre. Carbonyl compounds have significant dipole moments and are polar than ethers. The high polarity of the carbonyl group is explained on the basis of resonance involving neutral (A) and a dipolar (B) structures as shown.

//M2//QN5//SUB//DL0//EQ

Explain the preparation of aldehydes and ketones by oxidation of alcohols with suitable examples.

//X

RCH 2 OH RCHO
A better reagent for oxidation of primary alcohols to aldehydes in good yield is pyridinium chlorochromate (PCC), a complex of chromium trioxide with pyridine and HCI.
CH 3 – CH = CH – CH 2 OH
CH 3 – CH = CH – CHO
Secondary alcohols are oxidised to ketones by chromic anhydride (CrO 3)

//M2//QN6//SUB//DL0//EQ

Explain the preparation of aldehydes and ketones by dehydrogenation of alcohols with suitable examples.

//X

When the vapours of a primary or a secondary alcohol are passed over heated copper at 573 K, dehydrogenation takes place and an aldehyde or a ketone is formed.

//M2//QN7//SUB//DL0//EQ

Explain the preparation of aldehydes and ketones by ozonolysis of alkenes with suitable example.

//X

Ozonolysis: Ozonolysis of alkenes involves the addition of ozone molecule to alkene to form ozonide by Zn-H2O to smaller molecules. This reaction is highly useful in detecting the position of the double bond in alkenes or other unsaturated compounds.

//M2//QN8//SUB//DL0//EQ

Explain the preparation of aldehydes and ketones by hydration of alkynes with suitable example.

//X

Like alkanes and alkenes, alkynes are also immiscible and do not react with water. However, one molecule of water adds to alkynes on warming with mercuric sulphate and dilute sulphuric acid at 333 K to form carbonyl compounds.

//M2//QN9//SUB//DL0//EQ

Explain Rosenmund reduction in brief.

//X

Acyl chloride (acid chloride) is hydrogenated over catalyst palladium on barium sulphate. This reaction is called Rosenmund reduction.

//M2//QN10//SUB//DL0//EQ

Explain Stephen reaction in brief.

//X

Nitriles are reduced to corresponding imine with stannous chloride in the presence of hydrochloric acid, which on hydrolysis give corresponding aldehyde.
RCN + SnCl 2 + HCl RCH=NH RCHO
This reaction is called Stephen reaction.

//M2//QN11//SUB//DL0//EQ

Explain the reduction of nitriles and esters by diisobutylaluminium hydride (DIBAL-H) with suitable example.

//X

Nitriles are selectively reduced by diisobutylaluminium hydride, (DIBAL-H) to imines followed by hydrolysis to aldehydes:
CH 3–CH=CH–CH 2 CH 2–CHO
Similarly, esters are also reduced to aldehydes with DIBAL-H.

//M4//QN12//SUB//DL0//EQ

Explain with chemical equations, various methods for the preparation of benzaldehyde by oxidation of methylbenzene.OR Explain the preparation of benzaldehyde by Etard and Gatterman-Koch reaction.

//X

(a) Use of chromyl chloride (CrO 2 Cl 2): Chromyl chloride oxidises methyl group to a chromium complex, which on hydrolysis gives corresponding benzaldehyde.
This reaction is called Etard reaction.
(b) Use of chromic oxide (CrO 3): Toluene or substituted toluene is converted to benzylidene diacetate on treating with chromic oxide in acetic anhydride. The benzylidene diacetate can be hydrolysed to corresponding benzaldehyde with aqueous acid.
By side chain chlorination followed by hydrolysis: Side chain chlorination of toluene gives benzal chloride, which on hydrolysis gives benzaldehyde. This is a commercial method of manufacture of benzaldehyde.

By Gatterman - Koch reaction: When benzene or its derivative is treated with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride or cuprous chloride, it gives benzaldehyde or substituted benzaldehyde.

//M2//QN13//SUB//DL0//EQ

Explain the preparation of ketones from acyl chlorides.

//X

Treatment of acyl chlorides with dialkylcadmium, prepared by the reaction of cadmium chloride with Grignard reagent, gives ketones.

//M2//QN14//SUB//DL0//EQ

Explain the preparation of ketones from nitriles.

//X

Treating a nitrile with Grignard reagent followed by hydrolysis yields a ketone.

//M2//QN15//SUB//DL0//EQ

Explain the preparation of ketone from benzene or substituted benzenes.

//X

When benzene or substituted benzene is treated with acid chloride in the presence of anhydrous aluminium chloride, it affords the corresponding ketone. This reaction is known as Friedel-Crafts acylation reaction.

//M2//QN16//SUB//DL0

Discuss the physical properties of aldehydes and ketones in term of their boiling point.

//X

Methanal is gas at room temperature.
Ethanal is a volatile liquid.
Other aldehydes and ketones are liquid or solid at room temperature.
The boiling points of aldehydes and ketones are higher than hydrocarbons and ethers of comparable molecular masses.
It is due to weak molecular association in aldehydes and ketones arising out of the dipole-dipole interaction.
Also, their boiling points are lower than those of alcohols of similar molecular masses due to absence of intermolecular hydrogen bonding.
Hence, the order of boiling point of the organic compounds with similar molecular masses is: Hydrocarbons < Ether < Aldehyde or Ketone < Alcohol < Carboxylic acid

//M2//QN17//SUB//DL0//EQ

Write a note on solubility of aldehydes and ketones.

//X

The lower members of aldehydes and ketones such as methanal, ethanal and propanone are miscible with water in all proportion, because they form hydrogen bond with water.
However, the solubility of aldehydes and ketones decreases rapidly on increasing the length of alkyl chain. All aldehydes and ketones are fairly soluble in organic solvents like benzene, ether, methanol, chloroform, etc. The lower aldehydes have sharp pungent odours.

//M3//QN18//SUB//DL0//EQ

Explain mechanism of nucleophilic addition reaction of aldehydes and ketones with figure.

//X

A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of
sp2 hybridised orbitals of carbonyl carbon (Fig.).
The hybridization of carbon changes from
sp2 to sp3 in this process, and a tetrahedral alkoxide intermediate is produced. This intermediate captures a proton from the reaction medium to give the electrically neutral product. The net result is addition of Nu– and H+ across the carbon oxygen double bond as shown in Fig.

//M2//QN19//SUB//DL0

Discuss the reactivity of aldehydes and ketones towards the nucleophilic addition reaction.

//X

Aldehydes are generally 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 more effectively than in former.

//M3//QN20//SUB//DL0//EQ

Explain nucleophilic addition reaction of aldehydes and ketones with hydrogen cyanide (HCN).

//X

Aldehydes and ketones react with hydrogen cyanide (HCN) to yield cyanohydrins.
This reaction occurs very slowly with pure HCN.
Therefore, it is catalysed by a base and generated cyanide ion (CN –) being a stonger nucleophile readily adds to carbonyl compounds to yield corresponding cyanohydrin.
Cyanohydrins are useful synthetic intermediates.
Examples:
(1)
(2)

//M2//QN21//SUB//DL0//EQ

Explain nucleophilic addition reaction of aldehydes and ketones with sodium hydrogensulphite (NaHSO3). OR Explain the nucleophilic addition reaction which is useful for separation and purification of aldehydes.

//X

Addition of sodium hydrogensulphite: Sodium hydrogensulphite adds to aldehydes and ketones to form the addition products.
(1)
(2)
The position of the equilibrium lies largely to the right hand side for most aldehydes and to the left for most ketones due to steric reasons.
The hydrogensulphite addition compound is water soluble and can be converted back to the original carbonyl compound by treating it with dilute mineral acid or alkali.
Therefore, these are useful for separation and purification of aldehydes.

//M4//QN22//SUB//DL0//EQ

Explain with equations, the nucleophilic addition reaction of aldehydes and ketones with Grignard’s reagent (R’-Mg-X).OR Write only equations to obtain 1°, 2° and 3° alcohols from aldehydes and ketones.

//X

Alcohols are produced by the reaction of Grignard reagents with aldehydes and ketones.
The first step of the reaction is the nucleophilic addition of Grignard reagent to the carbonyl group to form an adduct. Hydrolysis of the adduct yields and alcohol.
The overall reactions using different aldehydes and ketones are as follows:
You will notice that the reaction produces a primary alcohol with methanal, a secondary alcohol with other aldehydes and tertiary alcohol with ketones.

//M4//QN23//SUB//DL0//EQ

Explain with equations, the nucleophilic addition reaction of aldehydes and ketones with alcohols. And also explain that why this reaction is carried out in the presence of dry hydrogen chloride?

//X

Addition of alcohols: Aldehydes react with one equivalent of monohydric alcohol in the presence of dry hydrogen chloride to yield alkoxyalcohol intermediate, known as hemiacetals, which further react with one more molecule of alcohol to give a gem-dialkoxy compound known as acetal as shown in the reaction.
Ketones react with ethylene glycol under similar conditions to form cyclic products known as ethylene glycol ketals.
Dry hydrogen chloride protonates the oxygen of the carbonyl compounds and therefore, increases the electrophilicity of the carbonyl carbon facilitating the nucleophilic attack of ethylene glycol. Acetals and ketals are hydrolysed with aqueous mineral acids to yield corresponding aldehydes and ketones respectively.

//M0//QN24//SUB//DL0//EQ

Write detailed note on the addition reaction of aldehydes and ketones with ammonia derivatives .

//X

Addition of ammonia and its derivatives: Nucleophiles, such as ammonia and its derivatives H2N–Z add to the carbonyl group of aldehydes and ketones. The reaction is reversible and catalysed by acid.
The equilibrium favours the product formation due to rapid dehydration of the intermediate to form >C=N–Z.

Z | Reagent name | Carbonyl derivative | Product name
–H | Ammonia |  | Imine
–R | Amine |  | Substituted imine (Schiff’s base)
–OH | Hydroxylamine |  | Oxime
–NH 2 | Hydrazine |  | Hydrazone
| Phenylhydrazine |  | Phenylhydrazone
| 2,4-Dinitrophenyl- hydrazine |  | 2,4 Dinitrophenyl- hydrazone
| Semicarbazide |  | Semicarbazone

//M2//QN25//SUB//DL0//EQ

Explain with equations, the reaction to obtain alcohols by reduction of aldehydes and ketones.

//X

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.
RCHO + H 2 RCH 2 OH
Examples:
CH 3 CHO + H 2 CH 3 CH 2 OH
Ethanal Ethannol

//M2//QN26//SUB//DL0//EQ

Explain with equations, the reactions to obtain hydrocarbons by reduction of aldehydes and ketones.OR Write a note on Clemmensen and Wolff-Kishner reduction.

//X

Reduction to hydrocarbons: The carbonyl group of aldehydes and ketones is reduced to CH2 group when treated 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 (Wolff-Kishner reduction).
Examples:
Examples:

//M2//QN27//SUB//DL0//EQ

Explain oxidation of aldehydes and ketones.

//X

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.
R – CHO R–COOH
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.

//M2//QN28//SUB//DL0

Explain the Tollens’ test to distinguish aldehydes from ketones with equation.OR Write a note on silver mirror test.

//X

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.
RCHO + 2[Ag(NH 3)2]+ + 3OH– → RCOO– + 2Ag + 2H 2 O + 4NH 3

//M2//QN29//SUB//DL0

Explain the Fehling’s test to distinguish aldehydes from ketones with equation.

//X

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 tartrate (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.
R – CHO + 2Cu 2+ + 5OH– → RCOO– + Cu 2 O ↓ + 3H 2 O Red-brown ppt

//M3//QN30//SUB//DL0//EQ

Discuss in details the oxidation of methyl ketones by haloform reaction. And also mention that for detection of which type of group this reaction is useful?

//X

Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon atom (methyl ketones) are oxidised by sodium hypohalite to sodium salt of corresponding carboxylic acid 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.

//M3//QN31//SN//DL0//EQ

Explain aldol condensation with suitable example.OR Write a short note: Aldol condensation.

//X

Aldol condensation: Aldehydes and ketones having at least one a-hydrogen undergo a reaction in the presence of dilute alkali as catalyst to form b-hydroxy aldehydes (aldol) or b-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 a, b-unsaturated 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.

//M4//QN32//SN//DL0//EQ

Explain Cross aldol condensation with suitable example.OR Write a short note: Cross aldol condensation.

//X

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 a-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.

//M2//QN33//SUB//DL0//EQ

Write a note on Cannizzaro reaction.

//X

Cannizzaro reaction: Aldehydes which do not have an a-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.

//M2//QN34//SUB//DL0

Explain the electrophilic substitution reaction of benzaldehyde with suitable example.

//X

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.

//M3//QN35//SUB//DL0

Give Uses of aldehydes and ketones

//X

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 phenol-formaldehyde 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.

//M2//QN36//SUB//DL0

Give examples of the common names of carboxylic acid which are derived from Latin or Greek names of their natural sources.

//X

Following are examples of the common names of carboxylic acid which are derived from Latin or Greek names of their natural sources.
Formic acid (HCOOH) was first obtained from red ants (Latin: formica, means ant).
Acetic acid (CH3COOH) was first synthesized from vinegar (Latin: acetum, means vinegar).
Butyric acid (CH3CH2CH2COOH) was first obtained from rancid butter (Latin: butyrum, means butter)

//M1//QN37//SUB//DL0//EQ

Write the IUPAC names of the following compounds: (i) CH 3 CH 2 COOH (ii) (CH 3)2 CHCOOH (iii) HOOC–COOH (iv) HOOC–CH 2–COOH (v) HOOC–(CH 2)2–COOH (vi) HOOC–(CH 2)4–COOH (vii) HOOC–(CH 2)4–COOH (viii) HOOC–CH 2–CH(COOH)–CH 2–COOH (ix) (x)

//X

No. | Structure | IUPAC name | Common name
(i) | CH 3 CH 2 COOH | Propanoic acid | Propionic acid
(ii) | (CH 3)2 CHCOOH | 2-Methyl- propanoic acid | Isobutyric acid
(iii) | HOOC–COOH | Ethanedioic acid | Oxalic acid
(iv) | HOOC–CH 2 –COOH | Propanedioic acid | Malonic acid
(v) | HOOC–(CH 2)2 – COOH | Butanedioic acid | Succinic acid
(vi) | HOOC–(CH 2)3 –COOH | Pentanedioic acid | Glutaric acid
(vii) | HOOC–(CH 2)4 –COOH | Hexanedioic acid | Adipic acid
(viii) | HOOC–CH 2 –CH(COOH) –CH 2–COOH | Propane-1, 2, 3-tricarboxylic acid | Tricarballylic acid OR carballylic acid
(ix) |  | Benzene-1,2-dicarboxylic acid | Phthalic acid
(x) |  | 2-Phenyl- ethanoic acid | Phenylacetic acid

//M2//QN38//SUB//DL0//EQ

Discuss the structure of carboxyl group.

//X

In carboxylic acids, the bonds to the carboxyl carbon lie in one plane and are separated by about 120°. The carboxylic carbon is less electrophilic than carbonyl carbon because of the possible resonance structure shown below:

//M2//QN39//SUB//DL0//EQ

Explain the preparation of carboxylic acid from primary alcohols.

//X

Primary alcohols are readily oxidised to carboxylic acids with common oxidising agents such as potassium permanganate (KMnO4) in neutral, acidic or alkaline media or by potassium dichromate (K2Cr2O7) and chromium trioxide (CrO3) in acidic media.

//M2//QN40//SUB//DL0//EQ

Explain the preparation of carboxylic acid from alkylbenzenes with necessary equations.

//X

Aromatic carboxylic acids can be prepared by vigorous oxidation of alkyl benzenes with chromic acid or acidic or alkaline potassium permanganate. The entire side chain is oxidised to the carboxyl group irrespective of length of the side chain. Primary and secondary alkyl groups are oxidised in this manner while tertiary group is not affected. Suitably substituted alkenes are also oxidised to carboxylic acids with these oxidising reagents.

//M2//QN41//SUB//DL0//EQ

Explain the preparation of carboxylic acid from nitriles and amides with necessary equations.

//X

Nitriles are hydrolysed to amides and then to acids in the presence of H+ or OH− as catalyst. Mild reaction conditions are used to stop the reaction at the amide stage.

//M2//QN42//SUB//DL0

Explain the preparation of carboxylic acid from Grignard reagents with necessary equations.

//X

Grignard reagents react with carbon dioxide (dry ice) to form salts of carboxylic acids which in turn give corresponding carboxylic acids after acidification with mineral acid.
Ex.:

//M2//QN43//SUB//DL0//EQ

Explain the preparation of carboxylic acid from acyl halides and anhydrides with necessary equations.

//X

Acid chlorides when hydrolysed with water give carboxylic acids or more readily hydrolysed with aqueous base to give carboxylate ions which on acidification provide corresponding carboxylic acids. Anhydrides on the other hand are hydrolysed to corresponding acid(s) with water.

//M2//QN44//SUB//DL0//EQ

Explain the preparation of carboxylic acid from esters with necessary equations.

//X

Acidic hydrolysis of esters gives directly carboxylic acids while basic hydrolysis gives carboxylates, which on acidification give corresponding carboxylic acids.

//M2//QN45//SUB//DL0//EQ

Discuss the physical properties of carboxylic acids in terms of their physical state and boiling point.

//X

Aliphatic carboxylic acids upto nine carbon atoms are colourless liquids at room temperature with unpleasant odours.
The higher acids are wax like solids and are practically odourless due to their low volatility.
Carboxylic acids are higher boiling liquids than aldehydes, ketones and even alcohols of comparable molecular masses.
This is due to more extensive association of carboxylic acid molecules through intermolecular hydrogen bonding. The hydrogen bonds are not broken completely even in the vapour phase. In fact, most carboxylic acids exist as dimer in the vapour phase or in the aprotic solvents.

//M2//QN46//SUB//DL0//EQ

Write a note on solubility of carboxylic acids.

//X

Simple aliphatic carboxylic acids having upto four carbon atoms are miscible in water due to the formation of hydrogen bonds with water.
The solubility decreases with increasing number of carbon atoms. Higher carboxylic acids are practically insoluble in water due to the increased hydrophobic interaction of hydrocarbon part. Benzoic acid, the simplest aromatic carboxylic acid is nearly insoluble in cold water.
Carboxylic acids are also soluble in less polar organic solvents like benzene, ether, alcohol, chloroform, etc.

//M2//QN47//SUB//DL0//EQ

Explain reaction of carboxylic acids with metals and alkalies. OR Write the equations for the reactions showing that carboxylic acids are acidic in nature and state the usefulness of this reactions.

//X

The carboxylic acids like alcohols evolve hydrogen with electropositive metals and form salts with alkalies similar to phenols. However, unlike phenols they react with weaker bases such as carbonates and hydrogencarbonates to evolve carbon dioxide. This reaction is used to detect the presence of carboxyl group in an organic compound.
2R–COOH + 2Na → 2R – CONa+ + H 2
Sodium carboxylate
R–COOH + NaOH → R–CONa+ + H 2 O
R–COOH + NaHCO 3 → R–CO Na+ + H 2 O + CO 2

//M3//QN48//SUB//DL0//EQ

Give reason: carboxylic acids are more acidic than alcohols and phenols.OR Although phenoxide ion has more number of resonating structures than carboxylate ion, carboxylic acid is a stronger acid than phenol. Why?

//X

Carboxylic acids dissociate in water to give resonance stabilised carboxylate anions and hydronium ion.
Phenols dissociate in water to give resonance stabilized phenoxide anions and hydronium ion.
The conjugate base of carboxylic acid, a carboxylate ion, is stabilised by two equivalent resonance structures in which the negative charge is at the more electronegative oxygen atom.
The conjugate base of phenol, a phenoxide ion, has non-equivalent resonance structures in which the negative charge is at the less electronegative carbon atom.
Therefore, resonance in phenoxide ion is not as important as it is in carboxylate ion. Further, 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.
Thus, the carboxylate ion is more stabilised than phenoxide ion, so carboxylic acids are more acidic than phenols.

//M4//QN49//SUB//DL0//EQ

Explain the effect of substituents on the acidity of carboxylic acids.

//X

Substituents may affect the stability of the conjugate base and thus, also affect the acidity of the carboxylic acids. Electron withdrawing groups increase the acidity of carboxylic acids by stabilising the conjugate base through delocalisation of the negative charge by inductive and/or resonance effects. Conversely, electron donating groups decrease the acidity by destabilising the conjugate base.
The effect of the following groups in increasing acidity order is
Ph < I < Br < Cl < F < CN < NO 2 < CF 3.
Thus, the following acids are arranged in order of decreasing acidity (based on pKa values):
CF 3 COOH > CCl 3 COOH > CHCl 2 COOH > NO 2 CH 2 COOH > NC–CH 2 COOH >
FCH 2 COOH > ClCH 2 COOH > BrCH 2 COOH > HCOOH > ClCH 2 CH 2 COOH >
[Continue]
C 6 H 5 COOH > C 6 H 5 CH 2 COOH > CH 3 COOH > CH 3 CH 2 COOH
[Continue]
Direct attachment of groups such as phenyl or vinyl to the carboxylic acid, increases the acidity of corresponding carboxylic acid, contrary to the decrease expected due to resonance effect shown below:
This is because of greater electronegativity of sp 2 hybridised carbon to which carboxyl carbon is attached. The presence of electron withdrawing group on the phenyl of aromatic carboxylic acid increases their acidity while electron donating groups decrease their acidity.

//M3//QN50//SUB//DL0//EQ

Explain with chemical reactions, the formation of anhydrides and esters from carboxylic acids.

//X

Carboxylic acids on heating with mineral acids such as H 2 SO 4 or with P 2 O 5 give corresponding anhydride.
Carboxylic acids are esterified with alcohols or phenols in the presence of a mineral acid such as concentrated H 2 SO 4 or HCl gas as a catalyst.
RCOOH + R′OH RCOOR′ + H 2 O

//M2//QN51//SUB//DL0

Explain with chemical reactions, the formation of acyl chlorides from carboxylic acids.OR Write a note on reaction of carboxylic acids with PCl3, PCl5 and SOCl2.

//X

The hydroxyl group of carboxylic acids, behaves like that of alcohols and is easily replaced by chlorine atom on treating with PCl5, PCl3 or SOCl2. Thionyl chloride (SOCl2) is preferred because the other two products are gaseous and escape the reaction mixture making the purification of the products easier.
RCOOH + PCl 5 → RCOCl + POCl 3 + HCl
3RCOOH + PCl 3 → 3RCOCl + H 3 PO 3
RCOOH + SOCl 2 → RCOCl + SO 2 + HCl

//M3//QN52//SUB//DL0//EQ

Explain with chemical reactions, the reaction of carboxylic acids with ammonia.

//X

Carboxylic acids react with ammonia to give ammonium salt which on further heating at high temperature give amides. For example:

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Explain reduction reaction of carboxylic acids.

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Carboxylic acids are reduced to primary alcohols by lithium aluminium hydride or better with diborane. Diborane does not easily reduce functional groups such as ester, nitro, halo, etc. Sodium borohydride does not reduce the carboxyl group.
R–COOH R–CH 2 OH
e.g.:

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Explain decarboxylation reaction of carboxylic acids.

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Carboxylic acids lose carbon dioxide to form hydrocarbons when their sodium salts are heated with sodalime (NaOH and CaO in the ratio of 3: 1). The reaction is known as decarboxylation.
R–COONa R–H + Na 2 CO 3

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Explain Hell-Volhard-Zelinsky reaction of carboxylic acids.

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Carboxylic acids having an a-hydrogen are halogenated at the a-position on treatment with chlorine or bromine in the presence of small amount of red phosphorus to give a-halocarboxylic acids. The reaction is known as Hell-Volhard-Zelinsky reaction.
R–CH 2–COOH

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Explain electrophilic substitution (ring substitution) reactions of aromatic carboxylic acid. Why aromatic carboxylic acids do not undergo Friedel-crafts reaction?

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Aromatic carboxylic acids undergo electrophilic substitution reactions in which the carboxyl group acts as a deactivating and meta-directing group. They however, 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).

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Complete the following reaction. (i) C 6 H 5 COOH + SOCl 2 → (ii) (iii) CH 3 CH 2 COOH + NH 3

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(i) C 6 H 5 COCl + SO 2 + HCl
(ii)
(iii) CH 3 CH 2 CONH 2

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Write chemical reactions to affect the following transformations. (i) Butanal to butanoic acid (ii) Acetone to propane (iii) Benzaldehyde to m-nitro benzaldehyde

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Write reactions to obtain acetic acid from ethanenitrile, acetyl chloride and ethyl ethonoate.

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State the reaction equation to prepared benzoic acid from the following compounds.

(i) Ethyl benzene (ii) Benzoic anhydride

(iii) Benzamide

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(i) Ethyl benzene
(ii) Benzoic anhydride
(iii) Benzamide

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An organic compound with the molecular formula C 8 H 8 O forms 2, 4 - DNP derivative, reduce Tollen's reagent and undergoes cannizzaro reaction. On vigorous oxidation it gives 1, 4 benzene di-carboxylic acid. Identify the compound and write the equation for the reactions involved.

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Organic compound is
(i)
(ii)
(iii)
(iv)
Class 12 Chemistry (Part 2) 012