Acetaldehyde : properties Part 1

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 & H₂SO_4] 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