REACTIONS OF CARBOXYLIC ACIDS
Carboxylic acids undergo the following type of reactions.
- The reaction in which the hydrogen atom of the carboxyl group is involved (salt formation).
- The reactions in which the OH group is replaced by another group.
- The reactions involving the carboxyl group as a whole.
(a) Reactions Involving Hydrogen atom of the Carboxyl Group:
Carboxylic acids are weaker acids than mineral acids. They furnish H+ when dissolved in water
1 Reaction with Bases
Carboxylic acids react with bases (NaOH, KOH) to form salts. CH3COOH NaOH CH3COONa + H2O
- Reactions with Carbonates and Bicarbonates
Carboxylic acids decompose carbonates and bicarbonates evolving carbon dioxide gas with effervescence.
2CH3COOH — Na2CO3 2CHIc0 6 :;1 a + CO2 + H20
CFKOOH + NaHCO3 CH3COO N a + CO2 + H2O
- Reactions with Metals
Carboxylic acids react with active metals (Na, K, Ca, Mg etc.) to form their salts with the evolution of hydrogen gas.
2CH3COOH + 2Na 2CH3C00 N a ± H2
(b) Reactions involving the -OH group of carboxylic acid: Carboxylic acids containing – COOH group are susceptible to nucleophilic attack. The addition of a nucleophile to the carboxyl group as always followed by the displacement of the -OH group by some other group the results in the derivative of a carboxylic acid. When hydroxyl group is replaced by X. -OR and -NH, to form acid halides. esters and amides.
- 3CH3COOH + PCI3 —> 3CH3— C —CI + H3P03
CH3COOH + PCI3 CHr C —CI + POCI3 + HCI
Formation of Acid anhydride (Dehydration)
Carboxylic acids are dehydrated on heating strongly in the presence of phosphorous pentaoxide.
Carboxylic acid reactions with lithium aluminium hydride (LiAIH4) are reduced alcohols.
CH, —C—OH– 4[H)L‘A=1 >4 CH; CH:—OH—H:0 acetic acid >> ethyl alcohol
- Complete Hydrogenation (Reduction to alkanes) Carboxylic acids on reduction with HI and red phosphorous give alkanes.
Reactions of alkenes
The addition of hydrogen to an alkene in the presence of a catalyst and at a moderate pressure (1-5 atm) to give a saturated compound is called hydrogenation.
The process is known as catalytic hydrogenation. It is highly exothermic process.
Heat of hydrogenation: The amount of heat evolved when one mole of hydrogen is added to an alkene is called heat of hydrogenation. Its value is about 120 KJ/mol.
Catalysts: The catalyst used are
- Platinum or Palladium at room temperature. These are expensive metals.
- Raney Nickel at 100°C.
A form of Nickel prepared by treating Ni—Al alloy with caustic soda (NaOH) is called Raney Nickel. It is more effective than ordinary Nickel due to presence of pores on the surface which provides large surface area for catalytic activity.
Ni—Al + NaOH + H2O
Most alkenes are hydrogenated over Raney nickel at about 100°C and 3- atmospheric pressure.
Uses of catalytic hydrogenation:
Catalytic hydrogenation of alkenes is used
- in the laboratory as well as in industry.
- for the manufacture of vegetable ghee from vegetable oils.
- to synthesize many chemicals like alkane or cyloalkane.
- as analytical tool to determine the degree of unsaturation of compounds as reaction is generally quantitative
Addition of Hydrogen Halides:
Dry gaseous halogen acid react with alkenes to form alkyl halides. The order of reactivity of halogen acids is Reason for reactivity order: This order of reactivity is based upon hydrogen to halogen bond strength. Greater the bond strength, lesser will be the reactivity. The change in concentration of HI is regularly decreasing for a fixed interval of time of 50 seconds. The curve becomes less and less steep and this steepness can help to give the rate of reaction.
The graph is plotted between time on x-axis and concentration of HI in moles dm-3 on the y-axis as shown in the figure. As HI is reactant so it gives a falling curve. The steepness of the concentration-time curve (slope of the curve) reflects the progress of reaction. Greater the slope of curve near the start of reaction, greater is the rate of reaction. In order to measure the rate of reaction, a tangent is drawn say at 100 seconds on the curve and right angled triangle is completed with tangent as hypotenuse. The slope of that tangent is measured. The slope is equal to tang.
(Hi) Dilatometric Method
- Refractrometric Method
- Optical rotation Method
This method for the measurement of rate of reaction is only applicable if a reactant or a product absorbs ultraviolet, visible or infrared radiations. The rate of reaction is measured by measuring the amount of radiation absorbed.
In this method, rate of change in amount of radiations absorbed is equal to the rate of reaction.
- Electrical Conductivity method:
The rate of reactions which involve ions is measured by the electrical conductivity method. The conductivity of such a solution depends upon the rate of change of concentration of the reacting ions or ions which may form during the chemical reaction. The conductivity will be directly proportional to the rate of change in the concentration of such ions.
In this method, rate of change of conductivity of reaction mixture is equal to the rate of reaction.
- Dilatometric Method:
This method is applicable for the measurement of rate of those reactions which involve small volume change in the solutions. The volume change is directly proportional to the extent of reaction.
In this method, rate of change of volume of reaction mixture is equal to the rate of reaction.
- Refractrometric Method:
The method is applicable to reactions in solutions where there are changes in refractive indices of the substances taking part in the chemical reactions.
In this method, rate of change of refractive indices of reacting substances is equal to the rate of reaction.
Optical Rotation Method:
In this method, the angle through which plane polarized light is rotated by the reaction mixture is measured by a polarimeter. The extent of rotation determines the concentration of optically active substance. If any of the species in the reaction mixture is optically active, then this method can be followed to find out the rate of reaction.
In case of hydrolysis of an ester, acid acts as catalyst. After some time, a sample of reaction mixture is withdrawn by a pipette and run into about four time its volume of ice cold water. The dilution and chilling stops the reaction.
The acid formed is titrated against a standard alkali (NaOH), using phenolphthalein as an indicator.
The analysis is repeated at various time intervals after the start of the reaction. This would provide information about the change in the concentration of acetic acid formed during the reaction.
Initially, the rate of reaction is high but it decreases with passage of time. The hydrolysis of ester is pseudo- first order reaction, water being in large excess does not affect the rate of chemical reaction.