⇉ 031212.formulae_carbonyl.jpg ⇔
In the previous Unit, you have studied organic
compounds with functional groups containing carbonoxygen
single bond. In this Unit, we will study about the
organic compounds containing carbon-oxygen double
bond (>C=O) called carbonyl group, which is one of the
most important functional groups in organic chemistry.
In aldehydes, the carbonyl group is bonded to a
carbon and hydrogen while in the ketones, it is bonded
to two carbon atoms. 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 acids, while in compounds where
carbon is attached to carbon or hydrogen and nitrogen
of -NH2 moiety or to halogens are called amides and
acyl halides respectively. Esters and anhydrides are
derivatives of carboxylic acids. The general formulas of
these classes of compounds are given below:
⇉ 031212.formulae_carbonyl.jpg ⇔
Aldehydes, ketones and carboxylic acids are widespread in plants
and animal kingdom. They play an important role in biochemical
processes of life. They add fragrance and flavour to nature, for example,
vanillin (from vanilla beans), salicylaldehyde (from meadow sweet) and
cinnamaldehyde (from cinnamon) have very pleasant fragrances.
⇉ 031212.fragrances_carbonyl.jpg ⇔
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.
Nomenclature
Aldehydes and ketones
Aldehydes and ketones are the simplest and most important carbonyl
compounds.
There are two systems of nomenclature of aldehydes and ketones.
Common names
Aldehydes and ketones are often called by their common names
instead of IUPAC names. The common names of most aldehydes are
derived from the common names of the corresponding carboxylic
acids by replacing the ending –ic of acid with aldehyde.
At the same time, the names reflect the Latin or Greek term for the
original source of the acid or aldehyde. The location of the substituent
in the carbon chain is indicated by Greek letters α, β, γ, δ, etc. The
α carbon being the one directly linked to the aldehyde group, β and so on
e.g.
⇉ 031212.common_names.jpg ⇔
IUPAC names
The IUPAC names of open chain aliphatic aldehydes and ketones
are derived from the names of the corresponding alkanes by
replacing the ending –e with –al and –one respectively. In case of
aldehydes the longest carbon chain is numbered starting from the
carbon of the aldehyde group while in case of ketones the
numbering begins from the end nearer to the carbonyl group. The
substituents are prefixed in alphabetical order along with numerals
indicating their positions in the carbon chain. The same applies to
cyclic ketones, where the carbonyl carbon is numbered one. 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 start from the carbon atom attached to the
aldehyde group. The name of the simplest aromatic aldehyde
carrying the aldehyde group on a benzene ring is
benzenecarbaldehyde. However, the common name benzaldehyde
is also accepted by IUPAC. Other aromatic aldehydes are hence
named as substituted benzaldehydes.
⇉ 031212.iupac_names.jpg ⇔
Structure of the Carbonyl Group
The carbonyl carbon atom is sp2-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.
⇉ 031212.carbonyl_structure.jpg ⇔
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 substantial dipole moments and are
polar than ethers. The high polarity of the carbonyl group
is explained on the basis of resonance involving a neutral
(A) and a dipolar (B) structures.
Preparation of Aldehydes and Ketones
1. By oxidation of alcohols
Aldehydes and ketones are generally prepared by oxidation of primary
and secondary alcohols, respectively (Unit 11, Class XII).
Oxidation of alcohols involves the formation of a carbonoxygen
double bond with cleavage of an O-H and C-H bonds.
Such a cleavage and formation of bonds occur in oxidation
reactions. These are also known as dehydrogenation reactions as
these involve loss of dihydrogen from an alcohol molecule. Depending
on the oxidising agent used, a primary alcohol is oxidised to an
aldehyde which in turn is oxidised to a carboxylic acid.
⇉ 031212.preparation_from_alc.jpg ⇔
2. By dehydrogenation of alcohols
This method is suitable for volatile alcohols and is of industrial
application. In this method alcohol vapours are passed over heavy
metal catalysts (Ag or Cu). Primary and secondary alcohols give
aldehydes and ketones, respectively (Unit 11, Class XII).
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 while tertiary
alcohols undergo dehydration.
⇉ 031212.preparation_dehydrogenation_alc ⇔
3. From hydrocarbons
(i) By ozonolysis of alkenes: As we know, ozonolysis of alkenes
followed by reaction with zinc dust and water gives aldehydes,
ketones or a mixture of both depending on the substitution
pattern of the alkene (Unit 13, Class XI).
⇉ 031212.preparation_from_alkene.jpg ⇔
(ii) By hydration of alkynes: Addition of water to ethyne in the
presence of H2SO4 and HgSO4 gives acetaldehyde. All other
alkynes give ketones in this reaction (Unit 13, Class XI).
⇉ 031212.preparation_from_alkynes.jpg ⇔
Preparation of Aldehydes
1. From acyl chloride (acid chloride)
Acyl chloride (acid chloride) is hydrogenated over catalyst, palladium
on barium sulphate. This reaction is called Rosenmund reduction.
⇉ 031212.rosenmund_reduction.jpg ⇔
2. From nitriles and esters
Nitriles are reduced to corresponding imine with stannous chloride
in the presence of hydrochloric acid, which on hydrolysis give
corresponding aldehyde.
This reaction is called Stephen reaction.
Alternatively, nitriles are selectively reduced by
diisobutylaluminium hydride, (DIBAL-H) to imines followed by
hydrolysis to aldehydes:
⇉ 031212.stephan_dibalh.jpg ⇔
3. From hydrocarbons
Aromatic aldehydes (benzaldehyde and its derivatives) are prepared
from aromatic hydrocarbons by the following methods:
(i) By oxidation of methylbenzene
Strong oxidising agents oxidise toluene and its derivatives to
benzoic acids. However, it is possible to stop the oxidation at
the aldehyde stage with suitable reagents that convert the methyl
group to an intermediate that is difficult to oxidise further. The
following methods are used for this purpose.
(a) Use of chromyl chloride (CrO2Cl2): Chromyl chloride oxidizes methyl group to a chromium complex, which on hydrolysis
gives corresponding benzaldehyde.
This reaction is called Etard reaction.
(b) Use of chromic oxide (CrO3):
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.
(ii) 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.
(iii) 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.
⇉ 031212.aldehyde_from_hc.jpg ⇔
Preparation of Ketones
1. From acyl chlorides
Treatment of acyl chlorides with dialkylcadmium, prepared by the reaction of cadmium chloride with Grignard reagent, gives ketones.
⇉ 031212.ketones_from_acyl_chlorides.jpg ⇔
2. From nitriles
Treating a nitrile with Grignard reagent followed by hydrolysis yields
a ketone.
⇉ 031212.ketones_from_nitriles.jpg ⇔
3. From benzene or substituted benzenes
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.
⇉ 031212.ketones_from_ benzene/substituted benzenes.jpg ⇔
Physical Properties
The physical properties of aldehydes and ketones are described as
follows.
Methanal is a 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 interactions. Also, their boiling points are lower than those
of alcohols of similar molecular masses due to absence of intermolecular
hydrogen bonding. The following compounds of molecular masses 58
and 60 are ranked in order of increasing boiling points.
The lower members of aldehydes and ketones such as
methanal,
ethanal and propanone are miscible with water in all proportions,
because they form hydrogen bond with water.
⇉ 031212.carbonyp_physical_prop.jpg ⇔
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.
As
the size of the molecule increases, the odour becomes less pungent
and more fragrant. In fact, many naturally occurring aldehydes and
ketones are used in the blending of perfumes and flavouring agents.
Chemical Reactions
Since aldehydes and ketones both possess the carbonyl functional
group, they undergo similar chemical reactions.
1. Nucleophilic addition reactions
Contrary to electrophilic addition reactions observed in alkenes, the
aldehydes and ketones undergo nucleophilic addition reactions.
(i) Mechanism of nucleophilic addition reactions
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 hybridisation 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.
⇉ 031212.carbonyp_nucleophilic_addition.jpg ⇔
(ii) Reactivity
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 carbon more effectively than in former.
(iii) Some important examples of nucleophilic addition and
nucleophilic addition-elimination reactions: 031212.carbonyl_addition_elimination.jpg
031212.carbonyl _addition_elimination
(a) Addition of hydrogen cyanide (HCN):
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 the generated cyanide ion (CN-)
being a stronger nucleophile readily adds to
carbonyl compounds to yield corresponding
cyanohydrin.
Cyanohydrins are useful synthetic
intermediates.
(b) Addition of sodium hydrogensulphite:
Sodium
hydrogensulphite adds to aldehydes and ketones to form the addition products. 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.
(c) Addition of Grignard reagents:
Alcohols are produced by the reaction of Grignard reagents (Unit 10,
Class XII) 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 an alcohol.
(d) 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.
(e) 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 = Alkyl, aryl, OH, NH2, C6H5NH, NHCONH2, etc.
2. Reduction
sodium
borohydride (NaBH4) or lithium aluminium hydride (LiAlH4)
as
well as by catalytic hydrogenation (Unit 7, Class XII).
(ii) Reduction to hydrocarbons: The carbonyl group of aldehydes
and ketones is reduced to CH2 group on treatment with zincamalgam
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).
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 a-hydrogen
Acidity of a-hydrogens of aldehydes and ketones: The aldehydes
and ketones undergo a number of reactions due to the acidic nature
of a-hydrogen.
The acidity of a-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
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.
(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 a-hydrogen
atoms, it gives a mixture of four products. This is illustrated
below by aldol reaction of a mixture of ethanal and propanal.
5. Other reactions
(i) Cannizzaro reaction:
Aldehydes which do not have an
a-hydrogen atom, undergo self oxidation and reduction
(disproportionation) reaction on heating 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
a. In chemical industry aldehydes and ketones are used as solvents,
starting materials and reagents for the synthesis of other products.
b. 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.
c. Acetaldehyde is used primarily as a starting material in the manufacture
of acetic acid, ethyl acetate, vinyl acetate, polymers and drugs.
d. Benzaldehyde is used in perfumery and in dye industries.
e. 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.
Carboxylic Acids
Carbon compounds containing a carboxyl functional group, –COOH are
called carboxylic acids.
The carboxyl group, consists of a carbonyl group
attached to a hydroxyl group, hence its name carboxyl.
Carboxylic acids
may be aliphatic (RCOOH) or aromatic (ArCOOH) depending on the group,
alkyl or aryl, attached to carboxylic carbon. Large number of carboxylic
acids are found in nature.
Some higher members of aliphatic carboxylic
acids (C12 – C18) known as fatty acids,
occur in natural fats as esters of glycerol.
Carboxylic acids serve as starting material for several other important organic compounds such as anhydrides, esters, acid chlorides, amides, etc.
Nomenclature and Structure of Carboxyl Group
Since carboxylic acids are amongst the earliest organic compounds to
be isolated from nature, a large number of them are known by their
common names. The common names end with the suffix –ic acid and
have been derived from Latin or Greek names of their natural sources.
For example, formic acid (HCOOH) was first obtained from red ants
(Latin: formica means ant), acetic acid (CH2COOH) from vinegar (Latin:
acetum, means vinegar), butyric acid (CH3CH2CH 2COOH) from rancid
butter (Latin: butyrum, means butter).
In the IUPAC system, aliphatic carboxylic acids are named by
replacing the ending –e in the name of the corresponding alkane with –
oic acid.
In numbering the carbon chain, the carboxylic carbon is
numbered one.
For naming compounds containing more than one
carboxyl group, the alkyl chain leaving carboxyl groups is numbered
and the number of carboxyl groups is indicated by adding the
multiplicative prefix, dicarboxylic acid, tricarboxylic acid, etc.
to the name
of parent alkyl chain. The position of –COOH groups are indicated by the
arabic numeral before the multiplicative prefix. Some of the carboxylic
acids along with their common and IUPAC names are listed below.
Structure of Carboxyl Group
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:
Methods of Preparation of Carboxylic Acids
1. From primary alcohols and aldehydes
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 (Jones reagent).
Carboxylic acids are also prepared from aldehydes by the use of
mild oxidising agents.
2. From alkylbenzenes
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.
3. From nitriles and amides
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.
From Grignard reagents
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.
As we know, the Grignard reagents and nitriles can be prepared
from alkyl halides. The above methods (3 and 4) are useful for converting alkyl halides into corresponding carboxylic acids having one carbon atom more than that present in alkyl halides (ascending the series).
5. From acyl halides and anhydrides
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.
6. From esters
Acidic hydrolysis of esters gives directly carboxylic acids while basic
hydrolysis gives carboxylates, which on acidification give
corresponding carboxylic acids.
Physical Properties
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.
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.
The reaction of carboxylic acids are classified as follows:
Acidity
Reactions with metals and alkalies
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.
Carboxylic acids dissociate in water to give resonance stabilised
carboxylate anions and hydronium ion.
(031212_strong/weak_acids)
Smaller the pKa, the stronger the acid ( the better it is as a proton
donor). Strong acids have pKa values < 1, the acids with pKa values
between 1 and 5 are considered to be moderately strong acids, weak
acids have pKa values between 5 and 15, and extremely weak acids
have pKa values >15.
Carboxylic acids are weaker than mineral acids, but they are stronger
acids than alcohols and many simple phenols (pKa is ~16 for ethanol
and 10 for phenol).
In fact, carboxylic acids are amongst the most acidic
organic compounds you have studied so far. You already know why
phenols are more acidic than alcohols. The higher acidity of carboxylic
acids as compared to phenols can be understood similarly.
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
(Unit 7,
Class XII). Thus, the carboxylate ion is more stabilised than phenoxide
ion, so carboxylic acids are more acidic than phenols.
Effect of substituents on the acidity of carboxylic acids:
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.
Electron withdrawing group (EWG)
stabilises the carboxylate anion
and strengthens the acid
Electron donating group (EDG)
destabilises the carboxylate
anion and weakens the acid
(031212_increasing_order_acids)
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 sp2 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.
Reactions Involving Cleavage of C–OH Bond
1. Formation of anhydride
Carboxylic acids on heating with mineral acids such as H2SO4 or with
P2O5 give corresponding anhydride.
2. Esterification
Carboxylic acids are esterified with alcohols or phenols in the presence
of a mineral acid such as concentrated H2SO4 or HCl gas as a catalyst.
Mechanism of esterification of carboxylic acids:
(031212_esterification_acids_mechanism)
3. Reactions with PCl5, PCl3 and SOCl2
(031212_acids_chlorides)
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.
4. Reaction with ammonia
(031212_acids_ammonia)
Carboxylic acids react with ammonia to give ammonium salt which
on further heating at high temperature give amides. For example:
Reactions Involving –COOH Group
1. Reduction
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.
2. Decarboxylation
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.
Alkali metal salts of carboxylic acids also undergo decarboxylation
on electrolysis of their aqueous solutions and form hydrocarbons having
twice the number of carbon atoms present in the alkyl group of the acid.
The reaction is known as Kolbe electrolysis (Unit 9, Class XI).
Substitution Reactions in the Hydrocarbon Part
(031212.Substitution_Reactions_Hydrocarbon Part)
1. Halogenation
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.
2. Ring substitution
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).
Uses of Carboxylic Acids
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.
Esters of benzoic
acid are used in perfumery.
Sodium benzoate is used as a food preservative.
Higher fatty acids are used for the manufacture of soaps and detergents.
