Carbon Compounds

CARBON COMPOUNDS


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ALDEHYDES



CARBONYL COMPOUNDS
       The compounds containing carbonyl group () are called carbonyl compounds. The majority of biologically important compounds contain carbonyl group. For example,


Name Structural Formula Structural Formula
Aldehydes Structure of Aldehydes or RCHO
Ketones Structure of Ketones or RCOR'
Carboxylic acids Structure of Carboxylic acids
or RCOOH
Esters Structure of Esters or RCOOR

       Where, R represents the alkyl group.


ALDEHYDES

       Aldehydes are the compounds which contain carbonyl group. That is why; these compounds are also called carbonyl compounds. The structure of Aldehyde group is:

Structure of Aldehydes


       Where R represents the alkyl group and carbonyl group is attached to one hydrogen atom. The aldehyde ( CHO ) functional group is also called as aldehydic group. General formula of aldehydes is CnH2n+1CHO.

Nomenclature of Aldehydes

       According to IUPAC nomenclature aldehydes are named by replacing ‘e’ in the name of parent alkane by ‘al’. For example,




Molecular Formula Common Name IUPAC Name
HCHO Formaldehyde Methanal
CH3CHO Acetaldehyde Ethanal
C2H5CHO Propionaldehyde
Propanal
C3H7CHO n – Butryaldehyde Butanal


       Formaldehyde is the simplest aldehyde and it is produced on commercial scale from methane gas which is main component of natural gas.



Production Of Formaldehyde


Preparation of Methanal

       Molecular formula of methanal is HCHO. It can be prepared by following two methods.

1. From Methanol


  1. Methanal can be prepared by dehydrogenation process by passing methanol over copper or silver at temperature 573 K.

    Formation Of Methanal Using Silver or copper

  2. Methanal can also be prepared by oxidation of methanol with potassium dichromate and sulphuric acid.

    Formation Of Methanal By Oxidation Of Methanol


2. From Distillation of Calcium Salts of Acids

       Methanal can be prepared by distilling the calcium salts of the acids. For example,

Formation Of Methanal By Distilling The Calcium Salts


Properties of Methanal

Physical Properties

  1. Methanal is a colourless gas with pungent odour.

  2. Its boiling point is 252 K.

  3. Formaldehyde is soluble in water. Its 40% solution in water is called formalin. Formalin is very important preservative in biology because it is used for preserving biological specimens.

Chemical Properties

       Following are the important chemical reactions of methanal:

1) Oxidation Reaction

       Methanal can be easily oxidized into methanoic acid on treatment with common oxidizing agents like KMnO4.


Formation Of Methanoic Acid


2) Reduction Reaction

       Methanal gives methanol on catalytic hydrogenation in the presence of catalysts such as Ni, Pt or Pd.


Formation Of Methanol From Methanal


3) Reaction with Hydrogen Cyanide

       Methanal reacts with hydrogen cyanide to form methanal cyanohydrin.


Reaction Of Methanal With Hydrogen Cyanide


4) Reaction with Tollen’s Reagent

Preparation of Tollen’s Reagent

       Tollen’s reagent is ammoniacal solution of silver nitrate. Its chemical formula is [Ag(NH3)2]OH. Tollen’s reagent can be prepared by adding ammonium hydroxide (NH4OH) to silver nitrate solution (AgNO3).


Tollen's Reagent Reaction 1


       The black precipitates of AgOH are dissolved in excess of NH4OH by adding more NH4OH in it, to produce [Ag(NH3)2]OH molecule which is Tollen’s reagent.


Tollen's Reagent Reaction 2


Reaction of Methanal with Tollen’s Reagent (Silver Mirror Test)

       When methanal is heated with Tollen’s reagent then a bright silver mirror is produced on the inner side of the test tube.



Silver Mirror Test


5) Reaction with Fehling’s Solution

Preparation of Fehling’s Solution

       Fehling’s solution is an alkaline solution of copper sulphate (also called Fehling solution A) and sodium potassium tartarate (also called Fehling solution B). The chemical formula of Fehling’s solution is [Cu(OH)2 + NaOH]. It is prepared by mixing Fehling solution A and Fehling solution B in equal amount.

Reaction of Methanal with Fehling’s Solution

       When methanal reacts with blue coloured Fehling’s solution, red precipitates of cuprous oxide (Cu2O) are formed and the colour of Fehling’s solution changes from blue to red.

Formation Of Methanal By Distilling The Calcium Salts

Uses of Methanal


  1. Formaldehyde is used in the manufacture of synthetic polymers like bakelite and synthetic dyes.

  2. Formaldehyde is used to preserve biological specimens in the form of formalin.

  3. Formaldehyde is also used as an insecticide.

  4. Formaldehyde is used in leather industry.

  5. Formaldehyde is used for silver polishing of mirrors.



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