Transcript chapter 8-carboxyl compounds

ORGANIC CHEMISTRY
CHM 207
CHAPTER 8:
CARBOXYL COMPOUNDS
(ALIPHATIC AND
AROMATICS)
NOR AKMALAZURA JANI
• Functional group: carboxyl group, -COOH
O
C OH

General formulae:
O
R C OH
R = alkyl group
O
Ar C OH
Ar = aryl group
NOMENCLATURE OF CARBOXYLIC ACIDS
• The carbonyl group (-C=O) is always at the
beginning of a carbon chain.
• The carbonyl carbon atom is always designated
as C-1.
3
2
1
• The IUPAC name of a carboxylic acid is derived from the
name of the alkane corresponding to the longest carbon
chain that contains the carboxyl group.
• The parent name is formed by changing the –e ending of
the alkane to –oic acid.
methane
methanoic acid
Examples of carboxylic acids
O
3
2
CH3 CH C OH
1
Br
2-bromopropanoic acid
4
3
CH3 CH
O
2
1
CH C OH
CH3 CH3
2,3-dimethylbutanoic acid


Organic acids are usually known by common
names.
These names usually refer to a natural source
of the acid.
ethanoic
acetic acid
acid
common
IUPAC name
methanoic
formic acid
acid
common
IUPAC name
CARBOXYLIC ACID DERIVATIVES
Group replacing
the –OH group
of RCOOH
Classes of
compound
General
formula
-X (halogen)
Acyl
halide/acid
chlorides
O
-OR’
-NH2
Ester
Amide
O
R C X
O
O C R'
Acid
anhydride
R C OR'
O
O
H3C C CI
ethanoyl chloride
O
R C NH2
O
Example
O
R C O C R'
H3C C OC2H5
ethyl ethanoate
O
H3C C NH2
ethanamide
O
O
H3C C O C CH3
ethanoic anhydride
NOMENCLATURE OF AROMATIC
CARBOXYLIC ACIDS
• General formula for the aromatic carboxylic acids:
ArCOOH, where Ar is aryl group (aromatic group).
• Examples:
O
C OH
COOH
OH
COOH
1
6
5
benzoic acid
2-hydroxybenzoic acid
2
3
4
NO2
3-nitrobenzoic acid
NOMENCLATURE OF ALIPHATIC
DICARBOXYLIC ACIDS
• Carboxylic acids containing two carboxyl groups are called
dicarboxyl acids.
• Their systematic names have the suffix ‘dioic’.
COOH
COOH
CH2
COOH
COOH
ethanedioic acid
(oxalic acid)
propanedioic acid
(malonic acid)
COOH
(CH2)4
COOH
hexanedioic acid
(adipic acid)
PHYSICAL PROPERTIES OF CARBOXYLIC ACIDS
• Methanoic acid has a pungent odour.
• Ethanoic acid and propanoic acids have strong vinegar smell.
• The higher members of the homologous series (acids with
four to eight carbon atoms) have a very strong unplesent
odour of rancid butter.
• Butanoic acid is present in human sweat and in butter.
BOILING POINTS OF ALIPHATIC CARBOXYLIC
ACIDS
• Aliphatic acids with one to 10 carbon atoms are liquids.
• The boiling points increase with increasing relative molecular mass.
• Carboxylic acids have higher melting and boiling points than alkanes of
similar relative molecular mass.
- reason: carboxylic acid can form hydrogen bonds with one another in
the solid and liquid states.
• Boiling points of carboxylic acid is higher than alcohols, aldehyde or
ketone with similar relative molecular mass.
O
O
CH3 C OH
CH3CH2CH2-OH
CH3 C CH3
acetic acid
1-propanol
propanone
(RMM 60)
(RMM 60)
(RMM 58)
bp 118oC
bp 97 C
o
bp 56oC
Reason:
i) Carboxylic acids form stronger hydrogen bonds than that alcohols.
The carbonyl, C=O group in a carboxylic acid is an electron
withdrawing group. This causes the –OH group in a carboxylic acid to
be more polarised than that the –OH group in an alcohol.
HH
δ- δ+
R R C CO OH H
OO
R R CC
δ-
O
O
δ+
H
H
HH
ii) Formation of dimers between two molecules of carboxylic acids to form
FORMATION OF DIMERS
DIMERS
a single molecule viaFORMATION
hydrogenOF
bonding.
hydrogen bond
hydrogen bond
O
R
C
R
H
O
CO H
OH
H
O
O
C R
OC R
O
hydrogen bond
SOLUBILITY OF ALIPHATIC CARBOXYLIC ACIDS
• Carboxylic acids of fairly low relative molecular mass (one to four carbon
atoms) – completely miscible in water.
- reason: the –COOH group is able to form hydrogen bonds with water
molecules.
• The solubility of carboxylic acids in water decreases as the relative molecular
mass increases. For examples, propanoic acid is very soluble in water,
butanoic acid and pentanoic acid are soluble in water, but hexanoic acid is
only slightly soluble in water.
• The solubility of carboxylic acids in non-polar solvents such as hexane
increases as the carbon chain gets longer.
δ+
H
δ-
O
H
δ+
H
δ-
R
C O
O δ-
H δ+
O
H
hydrogen bonds
H
H
O
PHYSICAL PROPERTIES OF BENZOIC ACIDS
• Crystalline compound
• Melting points: 122 oC
• Slightly soluble in water at room temperature but
dissolve readily in hot water.
• Soluble in benzene and other organic solvents.
• In organic solvent, it exists as a dimer through
hydrogen bonding.
ACIDITY OF CARBOXYLIC ACIDS
1)
-
-
The acidity of carboxylic acids compared with alcohols and
phenols.
Carboxylic acids are acidic because they dissolve in water to
give hydronium ions (H3O+).
RCOOH + H2O
RCOO- + H3O+
Carboxylic acids are stronger acids than alcohols and phenols.
Reasons:
i) the negative inductive effect of the carbonyl group
ii) the resonance effect of the carbonyl group.
Strong negative inductive effect of the carbonyl
oxygen
• Inductive effect: the shift in electron density from one atom to another
to form a polar bond.
• Indicated by an arrow showing the direction of the shift of the electronic
charge.

δ+
δ-
C
Cl
The arrow in the representation of the inductive effect shows
that
a) the carbon atom repels electrons
b) the chlorine atom attracts electrons because of its higher
electronegativity
• The oxygen atom in –C=O group is electronegative and
acts as a powerful electron-withdrawing atom.
• The withdrawal of electrons away from the carboxyl
hydrogen atom weakens the O-H bond. The carbonyl
group can lose a proton readily. This means that a
carboxylic acid is much stronger acid than an alcohol.
δ-
CH3
δ+
O
C
O
H
Resonance effect of the carbonyl group
• The carboxylate anion is a resonance hybrid of two resonance structures.
O
R
C
R
O





O
C
O
In the carboxylate anion, the negative charge is delocalised over two
carbon-oxygen bonds.
The delocalisation or resonance stabilises the carboxylate anion.
The carboxylate anion has less tendency to accept H3O+ ions and the
equilibrium
RCOOH + H2O
RCOO- + H3O+
tends to the right.
Delocalisation of electrons in the carboxylate anion promotes the
release of a proton and makes the carboxylic acid a stronger acid
than alcohols.
The carboxylate anion is delocalised to a far greater extent than the
corresponding phenoxide ion. Carboxylic acid is a stronger acid than
phenol.
2) Effects of substituent groups on the acidity of
carboxylic acids.
i)
electron-withdrawing groups increase acidity
- any factor that stabilises the carboxylate anion relative to
undissociated carboxylic acid will shift the equilibrium to
the right and result in increased acidity.
- any factor that destabilises the carboxylate anion relative
to the undissociated acid will result in decreased acidity.
- for example, an electron-withdrawing atom (such as
halogen atom) or an electron-withdrawing group (such as
–NO2) in the carboxylic acid molecule will withdraw
electron density from carboxylate anion and delocalise the
negative charge.
- the carboxylate anion is stabilised and acidity increases.
EFFECT OF ELECTRON-WITHDRAWING GRIUPS ON ACID STRENGTH
Formula
pKa
CH3COOH
4.74
I
CH2COOH
3.12
Br
CH2COOH
2.90
Cl
CH2COOH
2.86
F
CH2COOH
2.66
O2 N
CH2COOH
ACID
STRENGTH
INCREASES
1.67
• Fluorine is more electronegative than chlorine and therefore
has a stronger electron-withdrawing effect.
• Fluoroethanoic acid is a stronger acid than chloroethanoic
acid
ii) Number of halogen atoms and acid strength
- the acid strength will increases when the number of
halogen atoms increases.
- trichloroethanoic acid (Cl3C-COOH) is more acidic than
ethanoic acid and dichloroethanoic acid (Cl2CHCOOH).
Formula
pKa
CH3COOH
4.74
Cl
2.86
CH2COOH
Cl
Cl
Cl
Cl
Cl
CH2COOH
CH2COOH
1.29
0.65
ACID
STRENGTH
INCREASES
iii) Effect of position of halogen atom on acid strength
- The magnitude of the inductive effect is dependent on its
distance from the carboxyl group.
- substituents on the α-carbon (the carbon atom next to the –
COOH group) are the most effective in increasing acid
strength.
- the effect of a chlorine substituent decreases rapidly as the
substituent moves further from the carboxyl group.
- the inductive effect is negligible after the second carbon.
Formula
pKa
Cl
CH3CH2CH
Cl
CH3CH
Cl
CH2CH2
COOH
CH2
CH2
COOH
COOH
2.84
4.06
4.52
ACID
STRENGTH
DECREASES
- the aromatic nucleus (benzene ring) and multiple bonds are
electron-withdrawing groups and possess negative inductive
effects.
- benzoic acid is a stronger acid than ethanoic acid and the
unsaturated acid (CH2=CHCOOH) is a stronger acid than the
corresponding saturated acid, CH3CH2COOH.
Formula
CH3CH
COOH
COOH
3
COOH
COOH
pKa
4.74
4.19
2COOH
CH3CH
CH32CH
COOH
4.87
2=CHCOOH
CH2CH
=CHCOOH
4.26
iv) electron-donating groups decrease acidity
- an electron-donating group destabilises the
carboxylate anion by increasing the charge density of
the oxygen atom in the C-O bond.
CH3
CH3
CH3
C
OIncrease in charge density
- The increase in charge density strengthens the –OH bond.
- This makes proton loss more difficult.
- Thus, the presence of electron-donating groups decreases the
strength of an acid.
- Example of electron-donating groups: alkyl and ethoxy (-OR)
Effect of electron-donating groups on acid strength
Formula
H-COOH
pKa
3.77
CH3
COOH
CH3
CH2
4.74
COOH
4.88
CH3
CH
COOH
4.85
CH3
CH3
CH3
CH3
C
O-
5.07
ACID
STRENGTH
DECREASES
Trends in acidity of substituted benzoic acids
COOH
COOH
COOH
OCH3
4-methoxybenzoic acid
pKa = 4.46
NO2
benzoic acid
pKa = 4.19
4-nitrobenzoic acid
pKa = 3.41
acid strength increases
* methoxy substituent (CH3O-) : electron-donating and decreases the acid strength
* -NO2 : electron-withdrawing and increases the acid strength
REACTIONS CARBOXYLIC ACIDS
• Salt formation
- neutralisation
- reactions with electropositive metals
• Reduction to alcohols
• Formation of Acyl Chlorides
• Formation of Esters
• Formation of Acid Anhydrides
• Formation of Amides
SALT FORMATION
1) Neutralisation:
- carboxylic acids undergo neutralisation reactions with bases to
form carboxylate salts of carboxylic acids and water.
- examples:
CH3COOH (aq) + NaOH (aq) → CH3COONa (aq) + H2O (l)
sodium ethanoate
2CH3COOH(aq) + CuO (s) → (CH3COO)2Cu(aq) + H2O (l)
copper(II) ethanoate
CH3CH2COOH + NaOH → CH3CH2COONa + H2O
propanoic acid
sodium propanoate
* carboxylate salts are soluble in water


an aqueous solution of benzoic acid turns blue litmus paper to
red.
Benzoic acids dissolves readily in alkalis to form salts
(benzoates) and water.
COOH
NaOH
COO-Na+
H2O
sodium benzoate
• Carboxylic acids react with carbonates and hydrogen carbonates to form CO2,
water and salts of carboxylic acids.
• Examples:
2HCOOH (aq) + Na2CO3 (aq) → 2HCOONa (aq) + CO2 (g) + H2O (l)
sodium
methanoate
CH3CH2COOH(aq) + NaHCO3(aq) → CH3CH2COONa (aq) + CO2(g)+ H2O(l)
sodium
propanoate
• Phenol is a weak acid compared to carboxylic acids.
• Phenol did not react with NaHCO3 and only react with strong base such
as NaOH to form salt.
• Reactions with NaHCO3 can be used to distinguish carboxylic acid with
phenol and other organic compounds.
• Comparison of the solubility of organic compounds are listed in table
below:
Solubility of carboxylate salts from base
Organic compounds
NaOH
NaHCO3
Neutral organic
compounds
Insoluble
Insoluble
Phenol
Soluble
Insoluble
Carboxylic acids
Soluble
Soluble
2) Reaction with electropositive metals
- reactive metals (i.e. metals that are very
electropositive) react with carboxylic acids to form
hydrogen gas and salts of carboxylic acids.
- examples of metals: calcium, magnesium and iron.
2CH3COOH (aq) + Mg → (CH3COO)2Mg(aq) + H2 (g)
magnesium ethanoate
REDUCTION TO ALCOHOLS
• Reducing agents: LiAlH4 in dry ether
• Carboxylic acids reduced primary alcohols
H
O
R C OH
(1) LiAlH4 / ether
(2) H2O
R C OH
H
carboxylic acids
primary alcohols
examples:
H
O
CH3
C OH
ethanoic acid
(1) LiAlH4 / ether
(2) H2O
CH3
C OH
H
ethanol
CH3 C OH
(1) LiAlH4 / ether
(2) H2O
CH3 C OH
H
 Benzoic acid can be reduced to phenylmethanol
ethanolby using LiAlH 4
in ether
at low
temperatures.
- Benzoic
acid
can be reduced to phenylmethanol by using LiAlH4 in ether at
 An alkoxide
intermediate is formed first.
low temperatures.
- An alkoxide
is formed
 On adding
water,intermediate
hydrolysis
of the first.
intermediate yields the
- On alcohols.
adding water, hydrolysis of the intermediate yields the primary alcohols
primary
O
H
ethanoic acid
C OH
(1) LiAlH4 / ether
(2) H2O
C OH
H
benzoic acid
phenylmethanol
 LiAlH4 has no effect on the benzene ring or the double bond.
 -COOH is reduced to –CH2OH but the C=C bonds remains
unchanged.
CH3CH2CH=CHCOOH
1) LiAlH4
2) H2O
CH3CH2CH=CHCH2OH
FORMATION OF ACYL CHLORIDES / ACID CHLORIDE
• Carboxylic acids reacts with phosphorus (v) chloride or sulphur
dichloride oxide (thionyl chloride) or phosphorus trichloride
(PCl3) at room temperature to form acyl chloride.
• In the case of benzoic acid, the reaction mixture is heated.
O
R C OH
O
PCl5
R C Cl
carboxylic acids
acid chlorides
O
O
R C OH
carboxylic acids
SOCl2
R C Cl
acid chlorides
POCl3
SO2
HCl
HCl
Examples:
O
O
CH3 C OH
SOCl2
O
benzoic acid
SO2
HCl
ethanoyl chloride
ethanoic acid
C OH
CH3 C Cl
O
SOCl2
C Cl
benzoyl chloride
SO2
HCl
FORMATION OF ESTERS
• When a carboxylic acid is heated with an alcohol in the presence
of a little concentrated sulphuric acid, an ester is formed.
• Simple esters have fragrant odours. They are used as flavouring
agents in the food industy.
O
CH3 C OH
O
H OC2H5
O
O
benzoic acid
H2O
ethyl ethanoate
ethanoic acid
C OH
CH3 C OC2H5
H OC2H5
C OC2H5
ethyl benzoate
H2O
FORMATION OF ACID ANHYDRIDES
• Preparation of acid anhydrides:
- reaction of sodium carboxylate with an acid chloride.
O
O
R C O- Na+
Cl
sodium carboxylate
C R'
acid chloride
O
O
NaCl
R C O C R'
acid anhydrides
examples
O
O
-
+
CH3 C O Na
sodium ethanoate
Cl
ethanoyl chloride
O
-
CH3 C O Na
sodium ethanoate
+
C CH3
Cl
O
O
CH3 C O C CH3
acetic anhydride
O
O
O
C
CH3 C O C
benzoyl chloride
NaCl
acetic benzoic anhydride
NaCl
• Acid anhydride is also formed when a carboxylic acid is heated
with phosphorus pentoxide (P2O5) – dehydration reaction.
• The water is absorbed by P2O5 to form H3PO4.
O
O
CH3 C OH
HO C CH3
two molecules of acetic acids
P2O5
O
O
CH3 C O C CH3
acetic anhydride
H2O
FORMATION OF AMIDES
• Amides can be synthesised directly from carboxylic acids, but the
yield is poor.
• A better method of synthesising amides is by using acid chlorides.
• When ammonium carboxylates are heated in the presence of the
free acid, dehydration occurs to form the primary amide.
• Ammonium carboxylates are obtained by the reaction of carboxylic
acids with ammonia.
RCOO-NH4+
Excess RCOOH
Heat (100-200 °C)
RCONH2 + H2O
1° amide

For example:
CH3COOH + NH3 → CH3COONH4
ammonium
ethanoate
heat
CH3CONH2 + H2O
ethanamide
• Secondary and tertiary amides can be synthesised by using primary
amines and secondary amines respectively.
O
O H
H
R C OH
H N R'
o
1 amine
examples:
O
CH3 C OH
o
O H
H N CH3
O
o
heat (100-200 C)
o
2 amine
CH3 C N CH3
H2O
N-methylethanamide
O R"
R"
H N R'
H2O
2 amide
H
methylamine
R C OH
R C N R'
heat (100-200 oC)
heat (100-200 oC)
R C N R'
o
H2O
3 amide
examples:
O
CH3
CH3 C OH
H N CH3
dimethylamine
O CH3
o
heat (100-200 C)
CH3 C N CH3
N,N-dimethylethanamide
H2O
THE IMPORTANCE OF CARBOXYLIC ACIDS AND THEIR DERIVATIVES
• Methanoic acid and ethanoic acid: coagulate rubber latex.
• Ethanoic acid:
- used in the food industry as vinegar.
- making cellulose ethanoates for producing artificial fibres.
• Hexanedioic acid, HOOC(CH2)4COOH:
- manufacture of nylon 6,6
• Benzoic acid and sodium benzoate:
- as preservatives in foodstuff.
• 2-hydroxybenzoic acid:
- making aspirin
• 1,4-benzenedicarboxylic acid:
- making PET plastic
• Coumarin (C9H6O3) and its derivative, coumarinic acid (C9H8O3):
- anti-coagulants in medicine.
• Esters:
- responsible for the smell and flavour of many fruits and flowers.
• Vinyl acetate:
- formation of polyvinyl acetate (PVA) plastic.