CARBOXYLIC ACID Derivatives - Salts and Reactions

CARBOXYLIC ACID DERIVATIVES

A. Salts :

Introduction.

arboxylic acid salts, RCOO-M+, are ionic compounds which arereadily obtained from carboxylic acids by the latters reaction withreactive metals or with metal oxides, hydroxides, carbonates and

hydrogen carbonates. The salts are soluble in water, unless of course, the alkyl chain isso large that solubility is again substantially reduced. As seen earlier, the planarcarboxylate ion is stabilized by resonance.

Salts having a bulky or long-chained alkyl group are soaps and have the importantproperty of showing some compatibility both with polar/aqueous systems, where thehydrophilic (water-loving) ionic -COO- side is involved, as well as withnon-polar/organic systems, where the oleophilic (oil-loving) or hydrophobic (water-hating) alkyl group participates.[Soaps fall out of favour where hard water, containing dissolved Ca2+ ions, is beingused as the calcium salts, (RCOO)2Ca, are insoluble in water and give the disagreeablescum. Detergents, often sulphonate salts, R-SO3-M+, are used instead since thecalcium salts of these compounds are water-soluble.]

Nomenclature

alts of carboxylic acids, are derivatives of the carboxylic acids andare therefore named accordingly i.e. the metal ions are first to bementioned, followed by the name of the parent carboxylic acid which

has the ending of oic acid changed to oate to indicate the replacement of the acidichydrogen(s). Thus salts of the methanoic acid are called methanoates, those ofethanoic acid are ethanoates etc. :

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CARBOXYLIC ACID Derivatives - Salts http://staff.um.edu.mt/ratk1/Salts.htm

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(As usual, substituent positions are given, where relevant, starting the numbering withthe carboxylic group carbon as this generally takes precedence in these compounds- anexample of this is given above showing the methyl substituents in blue.) Note : Compounds having a name ending with oate do not necessarily have to beionic. Indeed, esters are similarly named but their bonding is by no means ionic. Theydo have however an alkyl group instead of an acidic hydrogen.

Preparations

i) Direct reaction of reactive metals with carboxylic acids. Reactive metals such as group I and II metals react with carboxylic acids to give saltsand hydrogen gas; the more reactive the metal the more vigorous the reaction andhence it would be inadvisable to react carboxylic acids directly with potassium!

NB. Evolution of Hydrogen with a metal such as magnesium is often used as a testfor the presence of a free carboxylic acid (-COOH) group

ii) Reactions with metal oxides, hydroxides, carbonates and hydrogen


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carbonatesThe mixing together of a carboxylic acid with metal oxides, hydroxides, carbonates orhydrogen carbonates will give a replacement of the acidic hydrogens with the metalions:

Chemical Properties

(a) Hydrolyses of carboxylic acid salts.As these salts are formed from weak acids, their conjugate bases are strong (defining anequilibrium that lies in favour of the molecular carboxylic acid species).

A solution of these species in water is bound to give an overall removal of hydrogenions from the surrounding water, leaving a surplus of hydroxide ions resulting inalkaline solutions.

or simply :

Release of free carboxylic acid : A more permanent hydrolysis reaction of salts wouldcome about by the addition of mineral acids (such as sulphuric or hydrochloric acids) asthe high level of surrounding H+ ions is so high that a much higher percentage of thecarboxylate anions now become protonated to give the molecular carboxylic acid. Thisprocess is important when one wants to convert a salt to its corresponding acid. (Thenon-volatile sulphuric acid is often the better acid to consider but care will have to betaken when the carboxylate in question is sensitive to the oxidising and dehydratingactions of this aggressive acid. Methanoates can only be treated in this way if the acidis kept cold and fairly dilute) :


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(b) Decarboxylation Reactionsi) Electrolytic : The previously encountered Kolbe reaction was seen to give anelectrolytic decarboxylation reaction where the carboxylate anion, after diffusing to theattracting anode, releases an electron (its charge) and undergoes a decarboxylation togenerate a highly reactive (therefore short-lived) radical. As a considerable number ofthese reactions are occurring simultaneously in close proximity to each other, it is notunexpected to get two such radicals combining together to give an alkane.

ii) Sodalime reaction : Also previously mentioned, the sodalime decarboxylationreaction involves strongly heating the solid carboxylate salt with sodalime. In thisreaction the alkane is a carbon short on that of the parent carboxylate.

iii) Thermal decomposition of Calcium salts : Heating calcium methanoates bythemselves produces calcium carbonate (which may also decompose to CaO and CO2 ifthe heating is strong enough) and methanal.(Cf. Aldehydes and Ketones). Othercalcium carboxylate salts give ketones if heated alone but will also give aldehydes,amongst other things, if heated with calcium methanoate.

iv) Decarboxylation via si lver saltsAddition of bromine liquid to a silver carboxylate salt brings about a decarboxylationreaction and produces a haloalkane (bromoalkane) as the organic product.

(c) Amide formation from ammonium salts

The heating of ammonium salts generally results in the, often temporary, separation ofthe basic ammonia from the parent acid which recombine at a cooler part of theapparatus in a process of sublimation (where the material goes straight to the gaseous


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state from a solid state missing out the liquid phase). The equilibrium leading to NH3and RCOOH formation is suppressed if free carboxylic acid is present and heating sucha mixture then gives the dehydration of the ammonium salt to produce the amide andwater.

(d) Ester formation by heating with haloalkanes

Carboxylic acid salts can conceivably react with haloalkanes to give a metal halide (- astable salt!) and an ester. Such a reaction can be carried out to give some desired ester :

(e) Anhydride formation by heating with acid chlorides

In a reaction, which is akin to that given for ester formation above, carboxylate saltscan be made to react with acid chlorides (and other acid halides) to give acidanhydrides and an inorganic salt.

Tests and identifications

a. Metal ion (Cation)Detection of metal ions is either by flame tests or by coloured/precipitation inorganicreactions. For ammonium salts release of NH3 gas with dil. aqueous NaOH is observed.

b. Carboxylic acid derivative (Anion)i) Reaction with concentrated sulphuric acidMethanoate and ethanedioate salts react with conc. H2SO4 to give a dehydrationreaction resulting in the emission of CO from the former and a mixture of CO and CO2from the latter. All other salts will simply release the free carboxylic acid with H2SO4 ii) Reaction with neutral FeCl3 Reactions with neutral FeCl3 can be useful in identifying some of the carboxylategroups on the basis of the colour and behaviour of the carboxylate anion with the Fe3+


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ion.For methanoic and ethanoic acids (only) Fe3+ gives a red/brown solution which, onboiling precipitates out a brown basic salt. With benzoates, PhCOO-, a buff precipitateis seen while reaction with ethanedioates gives a faint yellow colouration, which is astronger colour if a-hydroxycarboxylate salts are present. iii) Reaction with Manganate(VII), MnO4-Methanoates and ethanedioates will decolorise acidified potassium permanganatesolutions as they are oxidized to CO2 iv) Reaction with Tollen`s reagentMethanoates are the only carboxylate salts to give a silver mirror with Tollens reagent,Ag(NH3)2+ (but they are not strong enough as reducing agents to bring about theformation of Cu2O in Fehling`s though)


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CARBOXYLIC ACID Derivatives - Salts and Reactions

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