acid base indicators, carbon acid, pH scale., carbanions, acids and conjugate bases, reactions of carbon acids, phosphonium ions as carbon acids , Carbon acids in synthesis, Super acid,

1. CARBON ACIDS
Roshen Reji Idiculla
IDMS14/11
IIRBS

2. What is carbon + acid?
• As the name suggests , carbon acid is an acid
having carbon.
• Then what is acid ?
Definiton Acid Base
Arrhenius H+ donor OH donor
Bronsted Lowry H+ donor H+ acceptor
Lewis Electron pair acceptor Electron pair donor

3. Acids and Bases
• Bronsted definition of Acids and Bases
• If a high concentration of hydrogen ions is observed when an acid is added
to water (an aqueous solution of the acid), that acid must have an
ionizable hydrogen called proton.
• weak acids and weak bases will ionize to a lesser extent in water relative
to the strong acids or bases.

4. How to quantify acidic or basic nature
• Adding an acid to water increases the H3O+ ion
concentration and decreases the OH- ion
concentration.
• Adding a base to water decreases the H3O+ ion
concentration and increases the OH- ion
concentration.

5. • In 1909, S. P. L. Sorenson used logarithmic
mathematics to condense the range of H3O+
and OH- concentrations to a more
convenient scale.
• Concentrations of the H3O+ and OH- ions in
aqueous solutions are smaller than 1 M
• So the logarithms of these concentrations
are negative numbers.
• As positive numbers more convenient, the
sign of the logarithm was changed after
calculation by introducing the negative of
the logarithm of a number.
Concentration (mol/L)
[ H3O+] [OH-] pH = -
log[H3O+]
1 1 x 10-14 0
1 x 10-1 1 x 10-13 1
1 x 10-2 1 x 10-12 2
1 x 10-3 1 x 10-11 3
1 x 10-4 1 x 10-10 4
1 x 10-5 1 x 10-9 5
1 x 10-6 1 x 10-8 6
1 x 10-7 1 x 10-7 7
1 x 10-8 1 x 10-6 8
1 x 10-9 1 x 10-5 9
1 x 10-10 1 x 10-4 10
1 x 10-11 1 x 10-3 11
1 x 10-12 1 x 10-2 12
1 x 10-13 1 x 10-1 13
1 x 10-14 1 14

6. • Based on extent of ionization in water, the
mineral acids HCl, HBr, HI, H2SO4, HNO3, and
HClO4 are all strong acids.

7. Can we measure pH without finding
hydronium ion concentration?
• pH indicator - acid-base indicators.
• It change color according to the concentration
of the hydrogen ion
• They are usually weak acids
• In− is the conjugate base of the pH indicator,
which gives colour

8. Curcumine as acid-base indicator
A. Curcumin in an acidic solution (pH 3.5)
B. curcumin in a neutral solution (pH 7.4);
C. Curcumin in a basic solution
• The colour of curcumine remains yellow in neutral and
acidic medium
• Colour changes in basic medium
• So basic medium affects colour of curcumine
• So basic medium affects structure of curcumine

9. Curcumine as a carbon acid
• All organic compounds containing a C-H bond
can formally be classed with СН-acids

10. Stability of a carbanion
• Inorder for the molecule to remain in the
required colour of pH, the carbanion must be
stable based on following
• Resonance stabilisation
• inductive effect
• hybridisation

11. Curcumine as a carbon acid stabilised
by resonance
• In the presence of carbonyl functional group,
the alpha-hydrogens of a molecule exhibit
acidity
• Beta-diketones have extremely acidic alpha-
hydrogens (pKa = 9),

12. • inductive effect - if electron withdrawing
groups are directly attached to the alpha-
carbon, it is more acidic
• Aldehydes are more acidic than ketone.

13. Competition for resonance
stabilisation
• Ketones are more acidic than esters
• 1st reason : oxygen acts as an electron
donating group via resonance

14. • ketones are more acidic than esters
• So β-Diketones more acidic than β-Ketoesters
• β-Ketoesters are more acidic than β-Diesters

15. Effect of hybridisation
• the electrons are stabilized by being closer to
the nucleus
• s orbitals are closer to the atom’s nucleus than
p orbitals,
• Conjugate base has negative charge, which is
stabilized by orbitals with greater s character

16. a charged molecule is more acidic than
a neutral molecule
• K = 2.5 * 10-5 , pKa = -log(2.5 * 10-5) = 4.6
• K = 2.5 * 10-5 , pKa = -log(2.5 * 10-5) = 4.6

17. • Electronegativity of the atom attached to H
• the more EN the attached atom, the more
acidic the molecule

18. Functional
group
pKa Conjugate base
alkane 50
alkene 43
hydrogen 36 H-
amine 35 NH2-
sulphoxide 31
alkyne 25
ester 25
nitrile 25
ketone 24
aldehyde 20
alcohol 17
water 16
diester 13
thiol 13
Functional
group
pKa Conjugate base
Protonated
amine
9-11 NH3
Carboxylic acid 4
Hydrofluoric
acid
3.2 F-
Sulfonic acid -1
Hydronium ion -1.7 H2O
Sulphuric acid -3 HSO4-
Hydrochloric
acid
-6 Cl-
Hydrobromic
acid
-9 Br-
Hydroiodic acid -10 I-

19. Usual acid-base reactions
• Carboxylic acids converted to corresponding
carboxylate anions
• Carboxylic acids converted to corresponding
sodium salt of acids in presence of NaOH
• Carboxylic acids converted to corresponding
sodium salt of acids in presence of NaHCO3 or
Na2CO3, giving CO2 and H2O
• Carboxylic acids converted to corresponding
potassium salt of acids in presence of aqueous
KOH

20. Reactions involving carbon acids
• Many carbonyl compounds electrophilic
substitution at hydrogen on an alpha carbon.
• enolate anion formed as conjugate bases react
further with electrophiles
• Examples : Aldol reaction

21. Aldol reaction
• Claisen condensation is the ester analogue of the
Aldol condensation.

22. Alcohols as carbon acid
• Chlorohydrins are converted into epoxides
with base.
• https://www.chemthes.com/rxn.php?type=15
preoton transfer bronsted reaction

23. phosphonium ions as carbon acids
• Sodium hydride, a strong base, takes hydrogen
from phosphonium ions to produce the
corresponding ylide.
• This ylide is used in Wittig reaction

24. Carbon acids in synthesis
• Organic Syntheses, Coll. Vol. 3, 213 ,1955

25. Super acid
• HCl in aqueous solvent, has only 5-10%
dissociation.
• more acidic combination than HCl.
• Bronsted acid + lewis acid -> super acid
• Eg. Triflic acid CF3SO3H, F –electron withdrawing,
increase acidity
• HFSbF5 (non aqueous acid - donot work in water)
– more acidic,

26. Super acids
• super acid has very +ve S, next to highly
electronegative +ve chaged F.
• the charged complexation in super acid give
more stable conjugate base.

27. • Here σ bond is lewis base, attacked by H+

28. CARBON ACIDS
• Concept of carbon acid
• All organic compounds containing a C-H bond can formally be classed with СН-acids
• X, Y, Z - Electron withdrawing Group (EWG), OR Carbanion stabilizing group( Ph)
B: +BH+
• The acidity of carbon acids is proportional to the stability of the carbanions that are
their conjugate bases.
• In general, they are weaker than oxygen acids because carbon is less electronegative.
If the negative charge can be delocalized on to more electronegative atoms such as
oxygen or nitrogen, the conjugate base will be stabilized and hence the acid will be
stronger.
C
X
Y
Z
H
C
X
Y
Z

29. Ex-Aldol Condensation
• enolates are good nucleophiles and carbonyl C are good electrophiles.
• Enolate formation :base removes a proton from ɑ carbon of one molecue of
acetalehyde to give a resonance stabilised enolate
• Addition of enolate:the enolate then acts as a nucleophile and attacks the carbonyl
carbon of a second molecule of acetaldehyde
• Protonation of alkoxide

30. FACTORS AFFECTING STRENGTH OF ACID
1. Resonance
If the conjugate base is charge stabilized, more strong is the acid.
acidity : phenol> carboxylic acids> alcohols
• Phenoxide ions are stabilized by the delocalization of negative charge
through the interaction with π orbitals of the aromatic nucleus.
• The delocalization of negative charge of carboxylate anion makes
carboxylic acid stronger acid than alcohol

31. 2. Effect Of Hybridization:
With hybrid orbitals having more s character means that the electrons of
the anion will be lower in energy ,and the anion will be more stable.
An sp(50% s character) carbon atom is more electronegative than sp2
(33.3% s character) which in turn is more electronegative than sp3 (25% s
character) carbon.
pKa=25 pKa =44 pKa =50

32. 3. Solvent Effect
• In the absence of a solvent (in gas phase), most acids are far weaker than
they are in solution. For example; in gas phase, acetic acid has a pKa=130.
• When an acetic acid molecule donates a proton to a water molecule in the
gas phase, the ions that are formed are oppositely charged particles, and
they must become separated.
• In solution, solvent molecules surrounds the ions, insulating them from
one another, stabilizing them and making it far easier to separate them
than in the gas phase.
• Solvation decreases the entropy of the solvent - solvent molecules
become much more ordered.

33. 4. Inductive Effect
• One end of the bond near the fluorine atom is more electronegative than the
other.
• This polarization of the C-C bond results from a intrinsic electron attacking ability
of the fluorine; that is transmitted through space and through the bonds of the
molecule.
• It can be of an electron donating or electron withdrawing.
• Dispersal of charge always makes species more stable, and any factor that
stabilizes the conjugate base of an acid increases the strength of the acid.
• The three fluorine atoms in CF3H reduce the pKa to 26 from the 48 of methane,
while the nine fluorines in (CF3)3CH reduce the pKa still further to 10.

34. REFERENCE
• Organic Chemistry, 11th Edition ;T. W. Graham Solomons, Craig B. Fryhle, Scott
A. Snyder December 2012, ©2014
• A Guide Book To Mechanism In Organic Chemistry;Peter sykes ;sixth editon
• March’s Advanced Organic Chemistry,reactions mechanism and structure –
sixth edition
• Chemistry Chronicles : A Basic History of Acid—From Aristotle to Arnold ;
MARK S. LESNEY