Aldehydes, ketones and carboxylic acids 3 Flashcards
describe the structure of carboxylic acid
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:
physical state of carbo acids
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.
bpt of carbo acids
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
solubility of carbo acid in water
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.
how is the strength of acid denoted?
For convenience, the strength of an acid is generally indicated by pKa. pKa
of hydrochloric acid is –7.0, where as pKa of trifluoroacetic
acid (the strongest carboxylic acid), benzoic acid and acetic acid are
0.23, 4.19 and 4.76, respectively.
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.
two reasons why carboxylic acids are more acidic than phenols
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).
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 11,
Class XII). Thus, the carboxylate ion is more stabilised than phenoxide
ion, so carboxylic acids are more acidic than phenols.
effect of substituents
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. They decrease the e- density of the oxygen and increase the polarity of OH bond so h+ is lost easily
Conversely, electron donating groups
decrease the acidity by destabilising the conjugate base.They increase the e- density of the onxygen and hence H+ is lost in difficulty.
why is benzoic acid stronger than normal aliphatic acids
The conjugate base of benzoic acid is stabilized by electron-withdrawing groups (EWG). This makes the acid more acidic by delocalizing the charge of the carboxylate ion. 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
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.
why is benzoic acid weaker than formic acid
Benzoic acid is weaker than formic acid because the phenyl has an overall releasing effect ( which is smaller than that of methyl group ). When the carboxyl group is directly attached to the nucleus ,the resonance effect (+R)overcomes the (-I) effect .This prevents to a large extent the lone pair on the O atom of the -OH group from entering into resonance with the -CO group. The result is smaller positive charge on the O atom of the -OH group and so proton release is more difficult than in formic acid.
uses of these carbo acids:
i) methanoic acid
ii) ethanoic acid
ii) hexanedioic acid
iv)benzoic acid
v) higher fatty acids
i) Methanoic acid is used in rubber, textile, dyeing, leather and electroplating
industries.
ii) Ethanoic acid is used as solvent and as vinegar in food industry. Used as a coagulant for manufacture of rubber from latex and casein from milk.
iiii)Hexanedioic acid is used in the manufacture of nylon-6, 6.
iv) 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.
