Chemical Properties :

1. Polymerization:

a) Paraldehyde:

It is formed when anhydrous acetaldehyde is treated with concentrated H2SO4.

3CH3CHO    <—–> (CH3CHO)3  [Paraldehyde]


paraldehyde is used as hypnotic and soporific (sleep producing).

b) Metaldehyde:

CH3CHO on treatment with HCl is converted into metaldehyde.


2. Addition reactions of Acetaldehyde :

a) Addition of H2—-

i) Aldehydes are reduced to primary alcohol.Reduction is carried out with LiAlH4/ether, H2/Pd,H2/Ni etc.

>C=O +H2 —->   >CHOH

CH3CHO + H2——> CH3CH2OH (primary alcohol)

ii) Clemenson’s reduction-  When aldehydes are reduced with Zn-Hg/HCl then alkanes are formed.In this reaction >C=O is reduced to >CH2 group.

>C=O +4[H] —->   >CH2 +H2O

CH3CHO +4[H]——–> CH3CH3 +H2O

b) Addition of HCN — Aldehydes on addition with  [NaCN & H2SO4] or HCN give cyanohydrins.

>C=O +HCN—–>    >C(OH) CN

CH3CHO + HCN—–>    CH3CH(OH) CN [acetaldehyde cyanohydrin]

c) Addition of NaHSO3— form crystalline Sodium bisulphite  compound.

>C=O + NaHSO3 —–>    >C(OH) SO3Na

CH3CHO +NaHSO3—-> CH3CH(OH) SO3Na [acetaldehyde sodium bisulphite compd]

d) Addition of Grignard reagent– Form addition product  which on hydrolysis give secondary alcohol (only formaldehyde gives primary alcohol).

>C=O +RMgX —–>   >C(OMgX) R——–>   >C(OH) R [sec.alcohol]

CH3CHO + CH3MgBr —–>   CH3CH (OMgBr) CH3——–>   CH3CH(OH) CH3 [sec.alcohol]

3) Substitution reaction of Acetaldehyde-

a) Reaction with Hydroxylamine – Oximes are formed.

>C=O + H2NOH——->       >C=NOH [oxime] +H2O

CH3CH=O + H2NOH——->       CH3CH=NOH [Acetaldoxime]+H2O

b) Reaction with Hydrazene –  Hydrazones are formed.

>C=O + H2NNH2——->       >C=NNH2 [Hydrazone] +H2O

CH3CH=O + H2NNH2——->       CH3CH=NNH2[ Acetaldehyde hydrazone] +H2O

c)  Reaction with Phenyl Hydrazene –  Phenyl Hydrazones are formed.

>C=O + H2NNHC6H5 ——->       >C=NNHC6H5 [  Phenyl Hydrazone] +H2O

CH3CH=O + H2NNHC6H5——->  CH3CH=NNHC6H5 [Acetaldehyde Phenyl  hydrazone] +H2O

d)  Reaction with Semicarbazide –  Semicarbazones are formed.

>C=O + H2NNHCONH2——->       >C=NNHCONH2 [  Semicarbazone] +H2O

CH3CH=O + H2NNHCONH2  —–>  CH3CH=NNHCONH2 [Acetaldehyde Semicarbazone] +H2O

e) Reaction with Phosphorous pentachloride—  Alkylidene dichloride is formed.

>C=O + PCl5  ——>   >CCl2 +POCl3

CH3CH=O + PCl5  ——>   CH3CHCl2 [Etylidene dichloride] +POCl3

f) Reaction with Thioalcohol—  Mercaptal  is formed.

>C=O + 2RSH ——>   >C(SR)2 + H2O

CH3CH=O + C2H5SH ——>   CH3CH(SC2H5)2 [Mercaptal ] +H2O