Aldehyde Functional Group


 
 
Concept Explanation
 

Aldehyde Functional Group

Aldehyde: 

Aldehydes are characterised by the presence of carbonyl group > C = O in their molecules. Aldehydes contain group RCHO . It is formed by the oxidation of alcohol. 

Formaldehyde or Methanal (HCHO) : -

Formaldehyde is the first member of the aldehyde series. It is present in green leaves of plants where its presence is supposed to be due to the reaction of CO_2 with water in presence of sunlight and chlorophyll.

Preparation :

- It can be prepared by the following methods :

(i)  By oxidation of methly alcohol : - The oxidation is done either by oxygen in presence of a catalyst like platinised asbestos or copper at 300-400^oC or by acidified potassium dichromate.

                                      2CH_3OH+O_2 xrightarrow[300-400^oC]{Copper} 2HCHO+O_2

                                   CH_3OH=[O] xrightarrow[H_2SO_4]{K_2Cr_2O_7} HCHO+H_2O

(ii) By dehydrogenation of methyl alcohol : Vapours of methyl alcohol are passed over copper or silver at 300-400^oC when hydrogen is eliminated with the formation of formaldehyde.

                                 CH_3OH xrightarrow[300-400^oC]{Cu; or ; Ag}HCHO+H_2

(iii)  By heating calcium formate : Dry calcium formate is strongly heated to obtain formaldehyde.

                                                 Ca(HCOO)_2 overset{Heat}{rightarrow}CaCO_3+HCHO

                                              Calcium formate                                 Formaldehyde

(iv)  By ozonolysis of ethylene : Ethylene is dissolved in an organic solvent such as chloroform or carbom tetrachloride and a stream of ozonised oxygen is passed. The ozonide formed is decomposed by passing hydrogen in presence of palladium as catalyst.

                                          CH_2=CH_2+O_3 rightarrow Ozonide xrightarrow[Pd]{H_2}2HCHO+H_2O

Physical Properties

  • It is colourless, pungent smelling gas.
  • It i extremely soluble in water.
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    Sample Questions
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    Question : 1

    _________________________ is the first member of the aldehyde series.

    Right Option : D
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    Explanation
    Question : 2

    ________________________ are characterised by the presence of carbonyl group > C = O in their molecules.

    Right Option : B
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    Explanation
    Question : 3

    The oxidation of methly alcohol is done either by __________________________ in presence of a catalyst like copper at 300-400^oC.

    Right Option : B
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    Explanation
     
     


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