Formaldehyde : properties Part 1

Formaldehyde

source : wild mint shop

source: This land

Chemical Properties of Formaldehyde –

1) Oxidation –

Aldehydes are easily oxidised & thus act as reducing agents . Aldehyde on oxidation form corresponding acid containing same number of carbon atoms.

-CHO +[O]—–>  -COOH {acid}

HCHO + [O] —–>  HCOOH [Formic acid]

Oxidising agents may be strong such as acidified K₂Cr₂O₇ or acidified KMnO₄ or mild such as Tollen’s reagent or Fehling solution or Benedict solution.

Tollen’s Reagent-

It is an ammonical solution of Silver nitrate. When aldehyde is added to tollen’s reagent, Ag₂O is reduced to silver which deposits as black ppt. or silver mirror.

-CHO + Ag₂O—->  -COOH + 2Ag (silver mirror)

HCHO + Ag₂O—->  HCOOH + 2Ag (silver mirror)

Fehling solution-

It is an alkaline solution of cupric ion complexed with sodium potassium tartarate . Aldehyde reduces cupric ion (blue ) to cuprous oxide (red).

Fehling solution I – dilute solution of CuSO₄
Fehling solution II – Sodium potassium tartarate & NaOH

HCHO + 2 CuO —–>   HCOOH + Cu₂O(red ppt. of cuprous oxide)

Benedict’s solution- It is a solution of CuSO₄, Sodium citrate & Na₂CO₃. When heated with an aldehyde , a reddish brown ppt. appears.

2) Schiff’s Test-

Schiff’s reagent is a dilute solution of rosaniline hydrochloride whose red or pink colour disappears by passing SO₂. Aldehydes when treated with Schiff’s reagent, restore its pink colour.

3) Reaction of Formaldehyde with alkali –

Formaldehyde forms a complex product with ammonia.The product is hexamethylene tetramine  or Urotropine.

6HCHO + 4NH₃—->(CH₂)₆N₄ +6 H₂O

Urotropine is used as medicine in case of urinary troubles.

4) Cannizarro’s reaction-

In this reaction one molecule is oxidised & other  molecule is reduced to alcohol.

2HCHO + NaOH ——> CH₃OH +HCOONa

5) Condensation Reaction-

HCH=O + HCH=O  —–>HCH(OH)CH=O [Glycollic aldehyde] ——–> HCH(OH)CH(OH)CH=O

——->  HCH(OH)CH(OH)CH(OH)CH(OH)CH(OH)CH=O [Formose ]

6) Polymerization:

a) Para formaldehyde :

when aqueous solution of  formaldehyde is evaporated to dryness, a white crystalline solid.

nHCHO <—–> (CH₂O)n (paraformaldehyde) (n = 6 to 50)

b) Meta formaldehyde :

On allowing  formaldehyde  gas to stand at room temperature, it polymerises to form  meta formaldehyde i.e (HCHO)₃. It is a white solid.

3 HCHO <—-> (HCHO)₃

Uses:

• Medical specimens.
• It is used in the preparation of urotropine which is used as medicine.
• It is used for making synthetic plastics like bakellite, urea formaldehyde, resin etc.
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• source :safer chemicals.org