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Solubility
Like dissolves like
Organic compounds are not expected to be soluble in water
Rule of Thumb: C/O ratio
CH3OH
CH3CH2OH
CH3CH2CH2OH
CH3CH2CH2CH2OH
C/O ratio
1
2
3
4
soluble
insoluble
However,
If the organic compound can be ionized
It is more likely to be water-soluble
Requirement: the presence of an ionizable atom/group
since ionic compounds are polar, and thus water-soluble
What is an ionizable atom/group
one which can form a conjugate acid or conjugate base upon treatment with an acid or base
Classification of Organic Compounds
from a solubility point of view
will be soluble upon reaction with
Examples
basic acid (HCl –aq) Amines
weakly acidic strong base (NaOH –aq) Phenols
strongly acidic weak base (NaHCO3 –aq) Carboxylic Acids
neutral strong acid (H2SO4 –conc)
Alcohols, Aldehydes, Ketones, Esters, etc
inert always insoluble No ionizable FGe.g. alkyl halides
will be soluble upon reaction with
Examples
basic acid (HCl –aq) Amines
NH H
H Cl
HCl
NH H
H
will be soluble upon reaction with
Examples
weakly acidic strong base (NaOH –aq) Phenols
OH
NaOH (aq)
O H O
H2O+
OH
NaOH (aq)
O H O
H2O+
phenolate
NaOH (aq)
O H
OH
No conjugate base formed
alcohol
But
NaOH (aq)
O H
OH
No conjugate base formed
alcohol
Phenols have very different acid-base propertiescompared to alcohols
O
phenolate
a phenolate, the conjugate base of a phenol, is much more stable than an alkoxide
O
alkoxide
the conjugate base of an alcohol
O O OO
extensive charge delocalization into the aromatic ring
O
no charge delocalization here
O O OO
extensive charge delocalization into the aromatic ring
electron/charge delocalization is a stabilizing factormore stable weaker conjugate base
the conjugate base of a stronger acid is a weaker conjugate base
thus a phenols is a stronger acid than an alcohol
will be soluble upon reaction with
Examples
strongly acidic weak base (NaHCO3 –aq) Carboxylic Acids
+OH
O
NaHCO3 (aq)
OC
OH
O
O
O
HOC
OH
O
HOC
OH
O
CO2 + H2O
will be soluble upon reaction with
Examples
neutral strong acid (H2SO4 –conc)
Alcohols, Aldehydes, Ketones, Esters, etc
OH R H
O
R R'
O
R O
O
R'
alcohol aldehyde ketone ester
all insoluble with water if C/O ratio > 4not strongly basic enough to ionize in the presence of NaOH (aq) (remember why not)But oxygen will protonate upon treatment with H2SO4 (conc)
OH O
SO
O OHH
OH
H
will be soluble upon reaction with
Examples
neutral strong acid (H2SO4 –conc)
Alcohols, Aldehydes, Ketones, Esters, etc
R R'
OO
SO
O OHH
R R'
OH
will be soluble upon reaction with
Examples
inert always insoluble No ionizable FGe.g. alkyl halides
Br
Cl
Cl
Samples to be tested
N
H
N-ethylanilineOH
a-naphthol (a phenol)
OH
benzoic acid
butanol
Cl
Cl
para-dichlorobenzene
OH
O
