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Wednesday, 26 June 2013

Organic Chemistry Part 3 : Carboxylic Acids

Different Types of  Simple Carboxylic Acids

Carboxylic Acids


Naming Carboxylic Acids:

Carboxylic Acids Slide 1

The parent chain must include the carboxyl carbon, which is given position number 1. The name of the alkane attached is changed by replacing the -e with -oic acid. For example:
HCOOH methanoic acid
CH3COOH ethanoic acid (acetic acid)
CH3CH2CH2COOH butanoic acid
Because carboxylic acids have both a lone oxygen and an OH group, they are strongly hydrogen-bonded to each other, therefore having high boiling points. 

Carboxylic Acids Slide2

Ethyl Ethanoate

The carboxyl group is weakly acidic and all carboxylic acids neutralize OH-.

Esters of Carboxylic Acids
Carboxylic acids are used to synthesize two important derivatives, esters and amides. In esters, the OH of the carboxyl group is replaced by OR.
To name esters, first have the name of the akyl group attached to the O, followed by the name of the parent carboxylic acid after its name is altered by changing -ic acid to -ate. For exmample:
octyl ethanoate

Naming of Esters

In an mixture with carboxylic alcohol, an equilibrium reaction forms ester and water in the presence of heat and a strong acid catalyst.
RCOOH + HOR' <==> RCOOR' + H2O
For example:
butanoic acid + ethanol <==> ethyl butanoate + water

Hydrolysis and Saponification of Esters

Hydrolysis of Ester

An ester is hydrolyzed to its parent acid and alcohol when it is heated together with a stoichiometric excess of water (with acid catalyst)--this is the reverse of the reaction above. Saponification, is the reaction of an ester with a strong base, like sodium hydroxide. For example:
CH3COOCH2CH3 + NaOH ==> CH3COO- + Na+ + HOCH2CH3
ethyl ethanoate + sodium hydroxide ==> ethanoate ion + sodium ion + ethanol
                                         sodium ethanoate salt
The mixtures of salt of long-chain carboxylic acids are the ingredients of ordinary soap. 

Functional Groups

Amides of Carboxylic Acids
In amides, the OH of the carboxyl group is replaced by nitrogen holding any combination of H atoms or hydrocarbon groups.
The names of simple amides are devised by writing the name of the parent carboxylic acid and replacing the ending, -oic acid, with -amide For example:
ethanamide (acetamide)
The preparation of simple amides are similar to those of esters, but they are in an excess of ammonia. For example:
RCOOH + NH3 ==> RCONH2 + H2O
carboxylic acid + ammonia ==> simple amide + water
When amides are in water, they are changed back to the carboxylic acid and ammonia.
Unlike amines, the amides are nonbasic even though they have the NH2 group in simple amides. This is because of the O atom in the carbonyl group, which is very electronegative. This tightens the electrons on N so that it is unable to accept a proton.
Methods of Preparation:

Methods of Preparation

1) Oxidation of primary alcohols:
          R---CH2OH ------------> R---COOH
2) Oxidation of alkylbenzenes:
           C6H5---R -----------> C6H5---COOH
3) Carbonation of Grignard reagent:
            R---MgX + O = C = O --------> R---COO(-)MgX(+) ------> R---COOH + Mg(2+) + X(-)

4) Hydrolysis of nitriles:
     R---C = N + 2H2O ------------> R---COOH + NH3
Nitriles may be prepared by adding NaCN to the corresponding halides. for example:
                                                                                                                 H2O, H(+)
            CH3CH2CH2Br + CN(-) --------------------> CH3CH2CH2CN --------------> CH3CH2CH2COOH

Hydrolysis of nitriles:

The above substitution of X with CN is applicable only when the halide is a primary halide.Tertiary halides yield alkene and for secondery halide, the yield of substitution product is poor.

Physical Properties:
The molecules of carboxylic acids are polar and exibit hydrogen bonding.The bonding point of a carboxylic acid is higher than that of an alcohol of comparable molar mass.This is due to the fact that the carboxylic acids exit as dimer.
Chemical Reactions:
1) Acidity: Carboxylic acids are weak acids and their carboxylate anions are strong conjugate bases.The aqueous solutions of carboxylate salts are slightly alkaline due to the hydrolysis of carboxylate anion. Compared to other species, the orders of acidity and basicity of corresponding conjugate bases are as follows.
     Acidity : RCOOH > HOH > ROH >HC = CH > NH3 > RH
     Basicity:RCOO(-) < HO(-) < RO(-) < HC = C(-) < NH2 (-) < R(-)
Carboxylic acids react with metals to liberate hydrogen and are soluble in both NaOH and NaHCO3 solutions. For example:
                  2CH3COOH + 2Na -------> 2CH3COO(-)Na(+) + H2
                  CH3COOH + NaOH-------->CH3COO(-)Na(+) + H2O
                   CH3COOH + NaHCO3 ----------> CH3COO(-)Na(+) + H2O + CO2

2) Conversion to acid chloride:This may be carried out by using thionyl chloride(SOCl2), Phosphorus trichloride(PCl3)and phosphorus pentachloride(PCl5). Thionyl chloride is more convenient as the side products are only gaseous and thus the acid chloride can be easily separated; any excess of SOCl2 can be easily removed as its boiling point is low (79`C).
                       RCOOH + SOCl2 ----------------> RCOCl + SO2 + HCl
                       RCOOH  + PCl5 ----------------> RCOCl + POCl3 + HCl
3) Conversion into esters:
                                    SOCl2                   R`OH   
                 RCOOH ------------> RCOCl-----------> RCOOR
                   acid                                  acid chloride                     ester
A direct reaction between acid and alcohol is a reversible one. Ester can be obtained either by using one of the reactants in excess or b y removing one of the products.
                 RCOOH + R`OH  <------> RCOOR` + H2O
H(+) acts  as a catalyst.The presence of bulky groups near the site of reaction, whether in the alcohol or in the acid, slows down esterification as well as its hydrolysis.The relative order of esterification is.
      CH3OH > 1` > 2` > 3`
The accepted mechanism is as follows.

Mineral acid speeds up both processes by protonating carbonyl oxygen and thus rendering carbonyl carbon more susceptible to nucleophilic attack.In esterification, the nucleophile is R`OH and leaving group is water and inthe hydrolysis, the roles are reversed.
4) Reduction of acids to alcohol: Lithium aluminium hydride, LiAlH4, is used to convert acids into alcohols. The initial product is an alkoxide which on hydrolysis gives an alcohol.

        4R---COOH + 3LiAlH4 -----------> 4H2 + 2LiAlO3 + (RCH2O)4AlLi
Alternatively, an ester may be converted into alcohol by the use of sodium metal and alcohol or LiAlH4 for example:
                  CH3(CH2)14COOC2H5 ----------------> CH3(CH2)14CH2OH + C2H5OH

5) Halogenation of aliphatic acids(Hell-Volhard-Zelinsky reaction): In the pesence of phosphorus, chlorine or bromine replaces a hydrogen of an acid by halogen atom for example

(Hell-Volhard-Zelinsky reaction)

6) Decarboxylation: Heating of sodium salt of of carboxylic acid with soda lime (NaOH + CaO) produces alkane
                           RCOONa ----------------> RH + Na2CO3
Heating of calcium salt of carboxylic acid producs the compound containing CO group.
                          RCO2(Ca/2) + R`CO2(Ca/2) -------------> RCOR` + Ca2CO3
   If R is H, then aldehyde R`CHO is produced.

Chemical Reactions of  Carboxylic Acids

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