Course : Mcat
Subject : Chemistry
Topic : Carboxylic Acid
H – COOH Formic acid Methanoic acid
CH3COOH Acetic acid Ethanoic acid
CH3 – CH2 – COOH Propanoic acid Propanoic acid
CH3 – CH2 – CH2 – COOH Butyric acid Butanoic acid
CH2 – CHCOOH
|
CH3
Preparation of Carboxylic acid
From primary alcohols and
aldehydes
• In case of alcohol K2Cr2O7 is used as
oxidizing agent.
•In case of aldehyde, Tollens reagent
can also be used as oxidizing agent
•By hydrolysis of ethyl nitrile
Nitriles oxidize in acidic medium which
gives free carboxylic acid
nitrile(With HCI)
PHYSICAL PROPERTIES:
Smell:
C1 to C4 have pungent smell and C4 to
C6 have unpleasant smell.
Reactivity of Carboxyl Group:
• Carbonyl group is electron withdrawing
it increases the polarity of – OH.
• The – OH group of carboxylic acid is more active in chemical reactions than those of
alcohols.
The reactivity order of carboxylic acids and their derivatives in nucleophilic acyl substitution
reactions generally follows this trend:
Acid chlorides (most reactive)>Acid anhydrides>Esters>Amides (least reactive)
Types of Reaction:
• Reaction in which H of carboxylic acid is replaced.
• Reaction in which – OH of carboxylic acid is replaced.
• Reaction involving carboxylic group as whole
CHEMICAL PROPERTIES OF CARBOXYLIC ACID:
REACTIONS INVOLVING – H GROUP OF CARBOXYLIC ACID
Salt formation
•Alkalis form salt and replaces the H of carboxylic
acid
•Carbonates give same result with effervescence of
CO2
• Metals form salt and H2 with carboxylic acid
REACTIONS INVOLVING – OH GROUP OF CARBOXYLIC ACID
Formation of acid halide
• OH group is replaced by X group
Ester formation
RCOO – H + R RCOO – R + H2O
• OH group is replaced by alkoxy radical
• Reaction is reversible
• Reaction is acid catalyzed
Formation of amide With NH3 form ammonium salt on dehydration yield acid amide
• Amides are very stable compound
Formation of acid anhydride
• P2O5 works as dehydrating agent
REACTIONS INVOLVING CARBOXYL GROUP
Partial reduction of carboxylic acid to alcohol
• Reduction of carboxylic group is very difficult
• Strong reducing agent like LiAH4 is used
•Reducing agents which are used in aldehydes or
ketones cannot be used
Complete reduction of carboxylic acid to
alkanes
•Reduction is carried out in the presence of red
phosphorus and HI forms alkane
ACETIC ACID:
• Its dilute solution is called vinegar. (6 – 10 %)
• In combined form It IS present in the form of ester.
• It was first of all derived from vinegar.
• In free form it occurs in a number of fruit, juices, which have undergone fermentation
ESTER FLAVOUR
Amylacetate Banana
Isobutyl formate Rasberry
Benzylcetate Jasmine
Ethyl butyrate Pineapple
Amyl buryrate Apricot
Octyl acetate Orange
PREPARATION OF ACETIC ACID
LABORATORY SCALE
Oxidation of C2H5OH
• Dilute H2SO4 is used
• First acetaldehde is formed which then convert into acetic acid
Hydrolysis of CH3CN
• Dilute HCl is used
• Acetamide is formed, which on hydrolysis gives CH3COOH
INDUSTRIAL SCALE
From Acetylene
• Best source to prepare CH3COOH
•Acetylene is bubbled in 20 % H2SO4 at 80° C and 1 % HgSO4 acting as catalyst
results in acetaldehyde
• Acetaldehyde changes Into acetic acid on oxidation by using V2O5 as catalyst
• 97 % pure acid is obtained Oxidation of ethanol
• 12 % ethanol is obtained by fermentation of molasses
• Oxidation of ethanolis done by K2Cr2O7 and H2SO4
Laboratory preparation:
(i) By the oxidation of ethyl alcohol or acetaldehyde
CH3– CH2 – OH +[O] CH3– CHO
[O[→ CH3– COOH
(ii) By the hydrolysis of methyl nitrile:
CH3– CN CH3 - CO – NH2
HO2H+→ CH3 – COOH + NH4+
PHYSICAL CHARACTERISTICS
PROPERTIES DESCRIPTION
Colour, taste and odour Colourless, sour taste, strong vinegar odour
Melting point, solidifying 16.5 °C is MP, below 16.5° C it solidify
(called glacial acetic temperature acid)
Boiling point 118° C
Specific gravity 1.0 gm/cm3
Solubility Soluble in ether and alcohol
Existence Dimer
USES DESCRIPTION
Chemical industry Dye stuffs, perfumes, rayon, rubber, copper acetate used in paint
Food industry Ingredients of food pickles etc
Medical sciences Preparation of lead acetate for fracture and burn sand aluminum acetate
used as antiseptic
STRENGTH OF ORGANIC ACIDS
In each of the carboxylic acids, the H – O group is attached to a carbonyl C = O group which
is in turn bonded to other atoms, The comparison we observe here is between carboxylic
acid molecules, denoted as RCOOH, and other organic molecules containing the H – O
group, such as alcohols denoted as ROH (R is simply an atom or group of atoms attached to
the functional group). The former are obviously acids whereas the latter group contains
molecules which are generally extremely weak acids. One interesting comparison is tor the
acid and alcohol when R is the benzene ring (C6H5), benzoic acid (C6H5COOH), has pKa =
4.2, whereas phenol (C6H5OH) has pKa = 9.9. Thus, the presence of the doubly bonded
oxygen atom on the carbon atom adjacent to the O – H clearly increases the acidity of the
molecule, and thus increases ionization of the O – H bond.
Strength of Chloro-Substituted Acids:
Carboxylic acids are converted to acid chlorides by a range of reagents: SOCI2, PCI5 or
PCI3 are the usual reagents. Other products are HCI & SO2, HCI & POCI3 and H3PO3
respectively. Theconditions must be dry, as water will hydrolyze the acid chloride in a
vigorous reaction.
Hydrolysis forms the original carboxylic acid.
CH3COOH + SOCO2→ CH3COCI + HCI + SO2
C6H5COOH + PCI5→ C6H5COCI + HCI + POCI3
3CH3CH2COOH + PCI3→ 3CH3CH2COCI + H3PO3
Substituents of high electronegativity (especially – OB, – CI and – NH3+) near the carboxyl
group increase the acidity of carboxylic acids, often by several orders of magnitude
compare, for example, the acidities of acetic acid and the chlorine-substituted acetic acids.
Both dichloroacetic acid and trichloroacetic are stronger acids than acetic acid (pKa 4.75)
and H3PO4(pKa 2.12).Formula: CH3COOH CICH2COOH Cl2CHCOOH CI3CCOOH
Name: acetic acid chloroacctic acid dichloroacetic acid trichloroacetic acid
pKa: 4.76 2.86 1.48 0.70
