Acetaldehyde
source :sobermd.blogspot.com
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
