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  • (i)    Give reasons :     (a) HCHO is more reactive than CH3 -CHO towards addition of HCN.     (b) pKa of O2 N–CH2 –COOH is lower tha

(i)    Give reasons :
    (a) HCHO is more reactive than CH3 -CHO towards addition of HCN.
    (b) pKa of O2 N–CH2 –COOH is lower than that of CH3 –COOH.
    (c) Alpha hydrogen of aldehydes & ketones is acidic in nature.

(ii)    Give simple chemical tests to distinguish between the following pairs of compounds :
    (a) Ethanal and Propanal
    (b) Pentan-2-one and Pentan-3-one

 

 

 

 
 
 
 
 

Answers (2)

(i)    (a)    HCHO is more reactive towards additions of HCN than CH3CHO because of the electron releasing effect of CH3 in CH3CHO, electrophilicity of carbonyl carbon decreases.

        (b)    pKa of CH3COOH is more than NO20CH2COOh because CH3 is an electron releasing group, it decreases the acidic strength of carboxylate ion due to the increase of negative charge.

        (c)    \alpha-Hydrogen of aldehyde and ketone is acidic in nature due to e- withdrawing effect of -CO- group and resonance stabilization of releasing carbanion.

(ii)    (a)    Ethanal to Propanal

                Ethanal on heating with NaOI gives yellow precipitate of iodoform.

                \mathrm{CH_3CHO + NaOI\xrightarrow { \Delta} \underset{\text{Yellow}}{CHI_3}\downarrow + HCOONa}

        (b)    Pentan-2-one and Pentan-3-one

                Pentan-2-one is methyl ketone. Therefore on reaction with sodium hypoiodite gives yellow ppt of CHI3.

                \mathrm{CH_3CH_2CH_2\overset{O}{\overset{||}{C}}CH_3 + NaOI\xrightarrow { \Delta} \underset{\text{Yellow}}{CHI_3}\downarrow + CH_3CH_2CH_2COONa}

Posted by

Sumit Saini

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Due to no effect in HCHO and +I effect in CH3CHO, HCHO is more reactive towards HCN.

Posted by

Moni

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