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CCl

OCl

CCl

OCl

CCl

OCl

CO Cl2 COCl2+

CCl4 + 2SO3 COCl2 + S2O5Cl2H2SO4

11

CARBONIC ACID DERIVATIVES

The carbonic acid, , which has never been

freely isolated,

but proved its existance only in solution, is the root for its salts and covalent derivatives that theoretically result by replacing the OH with other groups.

Monofunctional derivatives (monochloride, monoester etc.) are unstable and therefore are not known; however, bifunctional derivatives are stable and have been isolated: dicloride, diesters, diamides.

11.1. CARBONYL CHLORIDE

The carbonyl chloride (carbon oxichloride) or phosgen, COCl2 is obtained photochemically starting from carbon oxide and chlorine, following a radical or catalityc mechanism (in presence of active coals).

or, starting from carbon tetrachloride and fuming sulfuric acid.

Phosgen is a colourless, (bp 7.6 oC), tear and choking gas.

The length of the carbon-oxygen bond is longer than the expected length for C=O bond and the lenghts of the carbon-chlorine bonds are shorter than te usual simple bonds, therefore a resonance state is assumed to exist.

COOH

OH

550 Organic Chemistry – Mircea Iovu

Phosgen undergoes reactions similar to acid chlorides. In presence of water, hydrolysis occurs yielding carbon dioxide and hydrogen chloride.

Phosgen forms in reaction with alcohols a carbonic acid ester-chloride (chloroformate). Excess of alcohol leads to formation of the neutral ester, namely the alkyl carbonate. For HCl neutralisation, a tertiary amine is usually used. Reaction with phenols or alkaline phenoxides occurs similarly.

Organic carbonates can be quantitatively obtained starting from epoxides and carbon dioxide.

Carbonates are good solvents for polymers like polyacrylonitrile, polyethylenterephthalate, polyamides and PVC.

Phosgen forms carbamyl (“urea”) chloride in reaction with ammonia.

Reaction with primary amines leads to intermediate formation of isocyanates, which subsequently (after cooling) add hydrogen chloride and yield N-substituted carbamic acid chloride derivatives

However, reaction with primary amines may be stopped to isocyanates.

O CCl

Cl

H2O O COH

ClCO2 HCl

CH3OH O COCH3

ClO C

OCH3

OCH3

NH3 O CNH2

ClNH4 Cl

RNH2 R N C O O CNHR

Cl

+

+

+

+

+

+HCl

HCl HCl

CH3OH

500

C2

HCl

-

-

-

HCl+

Methylchloroformate

carbonate

Methyl

Carbamylchloride

Isocyanate

+ C6H5 N C OC6H5 NH COClC6H5 NH2 COCl2 HCl HCl- -

+ -HClCH3 N C OCH3 NH COClCOCl2CH3 NH2 HCl-

ONH2

NH2

ONH2

OH

Carbonic acid derivatives

551

Methyl

isocyanate

Phenyl isocyante

Organic isocyanates are considered to be esters of isocyanic acid HNCO. Isocyanates have a “very unsaturated group” which allows a large and various reactivity. They are mainly used in addition reactions to active hydrogen containing compounds.

Reaction of methyl isocyanate with various alcohols and amines serves to large scale preparation of efficient herbicides and insecticides.

For example, addition of methyl isocyanate to 1-naphtol leads to 1-naphtyl-N-methylcarbamate, which is an insecticide called Sevin.

CH3 N C O +

OH OCNHCH3

O

In 1984 a leakage of methyl isocyanate, usually used to fabrication of Sevin, took place in the Indian town Bhopal leading to death of more than 2000 people and contamination of other 300000. This has been the most severe incident in the hystory of the chemical industry and determined authorities in the whole world to take serious actions of handling large quantities of toxic compounds.

Toxicity of isocyanates is caused by their attack on the hydroxy, amino and thio groups from proteins molecules.

11.2 CARBONIC ACIDE AMIDES

Carbonic acid (bibazic) may form two amides: a monoamide called carbamic acid and a diamide called urea:

Carbamic acid Urea

O CO NH4

NH2

O CNH2

NH2

H2O+

+ R NH CO

CH2C6H5RNH2 C6H5CH2OCClO

HO

Pd

H2

CH3CO2H

HBr+ +

+ +

RNH3 CO2 C6H5 CH2BrR NH C

OCH2C6H5

RNH2 CO2 C6H5 CH3


Free carbamic acid has never been isolated, due to its high tendency to decompose to CO2 and NH3; however, its salts and functional derivatives are stable compounds.

Ammonium carbamate may be prepared by direct synthesis starting from carbon dioxide and ammonia in an inert solvent (alcohol) or under solvent free conditions, under pressure:

Reaction is reversible and exothermal and occurs with large volume contraction. That is why, reaction must be handled under pressure at 100 at and at a relatively low temperature, 150o C. Under these conditions, it occurs almost spontaneously.

The ammonium carbamate transforms into urea at heating and pressure drop:

Urethanes are esters of carbamic acid which may be obtaied starting from carbamyl chloride (carbamic acid chloride), alkyl alcohols or chloroformates and ammonia:

O CCl

NH2

HO C2H5 O COC2H5

NH2

HCl+ +

Benzyl chloroformate serves to protection of the amimo groups in protein and peptide synthesis. This protecting group has the great advantage of being able to be replaced in mild conditions, simply by treating with hydrogen in the presence of a catalyst or using HBr in acetic acid.

Amine protection:

Deprotection:

NH3 CO2 O CO NH4

NH2

H = kJ-163

+2

O COC2H5

NHCH3

O COC2H5

ClCH3NH2+

C6H5 N C O ROH O COR

NHC6H5

+


553

Urethanes are cristalline, volatile compounds, slighltly soluble in water, but soluble in ethanol and ethyl ether. They have a low basicity due to conjugation of -NH2 p electrons with C=O electrons. That is why, treatment with sodium in ether solution leads to sodium derivaties (at the nitrogen atom).

Ethyluretane is used as cytostatic with hypnotic and sedative effect. N-alkylurethanes are prepared by reaction of chloroformates with amines:

Reaction of alkyl or aryl isocyanates with alcohols yields N-alyllurethanes and N-arylurethanes:

Nitrogen substituted urethanes are cristalline solids and for this reason phenyl isocyanate serves to identification of alcohols.

Polyurethanes are obtained by a polyaddition reaction of a glycol or a macroglycol to a diisocyanate. For example, using hexamethylenediisocyanate and 1,4-butanediol:

C NO

(CH2)6 N CO

HOH2C (CH2)2 CH2OHn n+

CO

NH

(CH2)6 NH

CO

O CH2 (CH2)2 CH2 O

nor ethyleneglycol and 4,4’-diphenylmethanediisocyanates:

Intensively used is toluenediisocyanate, TDI, with its two isomers 2,4 and 2.6 in 80 : 20 ratio. TDI is obtained by toluene nitration, followed by hydrogenation and reaction with phosgene.

Polyurethanes are used as adhesives and serve to preparation of cellular materials (used for example as cushions).

+H2OO CNH2

NH2

O CNH2

ONH4

O CNH2

ONH4

H2O (NH4)2CO3+


Urea, the carbonic acid carbamide or diamide, was the first organic compound synthetised in the lab by Wöhler in 1828, by heating ammonium isocyanate at 100 oC. An equilibrium is reached between the isocyanate and urea, the latter being the main component.

95%5%

H2N CO NH2NH4OCN)(

Urea is industrially prepared by reaction of carbon dioxide and ammonia. Urea is one of the main metabolism products of nitrogen containing organic compounds in mammals organisms.

Urea is a cristalline compound (mp=132.7 0C), colourless, tasteless, water soluble, but slightly soluble in ethyl ether and hydrocarbons.

At melting, it decomposes in ammonia, biuret, cyanuric acid, amelide and triuret.

HN NH

N OO

NH2

HN NH

N

O

OO

HH2N

O

N

O

NH2

H H

HH2N

O

N

O

N

O

NH2H

biuret cyanuric acid amelide triuretBiuret is the main impurity and it is the most undesirable

one for commercial urea. It is a weak base due to conjugation between nitrogen

nonbonding electrons and carbonyl double bond electrons.Under acid or basic conditions, or even in aqueous

medium, under temperature (100 0C), urea undergoes hydrolysis:

O CNH2

NH2

H2O CO2 NH3+ +2

It serves as fertilizer and in soil it undergoes hydrolysis to nitrogen containing compounds (ammonia-like) under the action of urobacteria.

(NH4)2CO3 H2O NH4HCO3 NH4OH++

These transformations ensures the fertilizer value of urea. However, it may be used as extraradicular fertilizer, as it

O CNH2

NH2

CH2O O CNH2

NH CH2OH+

+ O CNH

NH CH2OH

CH2OHCH2OO C

NH2

NH2

...CH2 NH CO NH CH2 NCONH2

CH2 NH CO N CH2

CH2OHN CO NH CH2OHCH2OH


555

is an organic compound and easily penetrates the vegetal tissue (rather than the inorganic nitrogen containing fertilizers).

Moreover, urea is useful as additive to ruminates food; the duct microflora helps preserving the nitrogen required to protein formation. Smaller quantities of urea are also used to fabrication of ureaplasts (carbamide resins) by polycondensation with formaldehyde. The first step of the reaction, occuring under basic or acid conditions, yields intermediate products that by further condensations, yield resins. Thus, under weak basic conditions, at maximum 40 0C, monomethylurea or dimethylolurea forms, depending on the reagents ratio.

Under basic or weak acid conditions a filiform macromolecular resin results which is structurally similar to resitol:

It is a soluble product and serves as adhesive. By heating (when mixed with filling materials) and pressing, it transforms in infusible tridimenssional resins, similar to resite.

Ureides. Urea may be acylated by acid chlorides or anhydrides. Urea N-acylated derivatives are called ureides.

There are some -brominated acids ureides that have sedative and weakly hypnotic activity and serve as medicines. For example:

CaN CN H2SO4 H2N CN CaSO4+ +

1000 ++ CaNCN CCaC2 N2

H2N C N H2S S CNH2

NH2

+H2N C N NH3 H N CNH2

NH2+

H2N CNH2

NH2

H2N CNH2

NH2

H2N CNH2

NH2

HHN CNH2

NH2


Moreover, there are well-known the cyclic ureides, called barbiturates, with strong hypnotic activity (2.5.2.).

Cyanamide, H2N-CN, which may be considered as the carbamic acid nitrile, is industrially prepared as calcium salt, as product of the reaction between nitrogen and carbide, at high temperature:

Cyanamide is released from its calcium salt by treatment with acids that form insoluble salts.

The particularity of this method is the capture of air nitrogen A long time, cyanamide served as fertilizer; nowadays, it serves to fabrication of melamine and thiourea.

Cyanamide is a weak acid, cristalline (mp 460C), very soluble in water, soluble in polar organic solvents and insoluble in non-polar organic solvents. At melting (430C), it undergoes dimerisation to cyanguanidine (dicyanamide),

while at higher temperature, under pressure, it undergoes trimerisation to melamine.

Melamine, which is a solid, crystalline white compound that is insoluble in organic solvents and has a high melting point (approximately 3500C recrystalised from water), serves to fabrication of melamine resins, by condensation with formaldehyde.

Cyanamide adds to its triple bond acidic hydrogen containing compounds such as hydrogen sulfide, yielding thiourea and ammonia, yielding guanidine.

Guanidine is a cristalline compound (mp 500C) which binds the atmospheric carbon dioxide. It is highly basic, similar to alkaline hydroxides (conjugated acid pKa 13), which may be explained by an isovalente conjugation (uniform distribution of electrons) as a result of proton binding:

H2N CN

H2N CN H2N C N CNNH2

+Cyanguanidine


557

It was extracted from turnips, mushrooms and wheat germs.

There are some guanidine derivatives that occur naturally as aminoacids, like arginine (15.1.6.), or as antibiotics, like streptomycine (fermentation product).

PROBLEMS

11.1. Give the name for each of the following carbonic acid

derivatives:

ClCOOCH2CH(CH3)2 , OC(O-CH2-CH3)2 , H2NCOCl, H2N-COOCH3,

(CH3)2 NCOCl, C6H5 -N=C=O, CH3NH-COOCH2-CH3, O=C=N-C6H4-

N=C=O, H2N-CO-NH-OC-CH2 -CH3 , H2N-CS-NH2, HN=C(NH2)2 ,

NC-NH2 .

11.2. Fill in the following reactions equations:

R-NH2 + S=CCl2 ?

R-NH2 + R-N=C=S ?11.3. The NMR spectra of urea cristalline salts state monoprotonation of the oxygen atom. Draw the resonance hybrid with the charged oxygen or nitrogen atom.11.4. Suggest a synthetic pathway of benzylchlorocarbonate, benzyl chloroformate, C6H5-CH2-O-CO-Cl, used in polypeptides synthesis.11.5. Write the reaction equation of polyurethane formation starting from 2,4 - diisocyano-1-methylbenzene and polyethyleneglycol, used for industrial purposes. 11.6. Write the reaction equations for preparation of toluenediisocyanates.11.7. Write the polymerization reactions of melamine with formaldehyde using as model reactions of urea with formaldehyde. 11.8. Give the strcutural formulas of alkylchloroformates, dialkylchloroformates, carbamates, hydrogencarbamates and carbamic acid.

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