Question

Dihalogen derivative (A) of a hydrocarbon having two carbon atoms reacts with alcoholic potash and forms another hydrocarbon which gives a red precipitate with ammoniacal cuprous chloride. Compound A gives an aldehyde when treated with aqueous KOH. Write down the name and formula for the organic compound.

Answer 1

When we boil the geminal dihalides with $aq.{\text{ }}KOH$ or $aq.NaOH$. It gives aldehydes and ketones as a product. So we can use the above information to solve this question.

Complete step by step answer:
Following is the summary of question,
$Dihalogen{\text{ }}derivative\left( {2C} \right) + alcoholic\;potash \to hydrocarbon{\text{ }}product$ ………………………………. (1)
From equation 1,
Hydrocarbon product + Ammoniacal cuprous chloride → substance that forms Red precipitate
Again from equation 1,
$Dihalogen{\text{ }}derivative\left( {2C} \right) + aq.KOH \to Aldehyde$
And it has been asked to find the name and formula of dihalogen derivative.
So, the compound A is the dihalogen derivative of hydrocarbon. Again it has mentioned that the compound is a 2-carbon compound. So, let’s assume that the two carbon containing hydrocarbons with two halogen group are geminal dihalides $\left( {C{H_3} - CH - {X_2}} \right)$ having two halogen atoms situated on a terminal carbon atom.
So, considering halogen X as chlorine, the $C{H_3} - CH - {X_2}$ is 1-1- dichloroethane, which is geminal dihalides that react with KOH and produce an unstable hydroxyl compound. This hydroxyl compound on water molecule loss yields acetaldehyde $\left( {C{H_3} - CHO} \right)$

This geminal dihalide of 1-1- dichloroethane when reacts with alcoholic $KOH$ or potash undergoes the dehydrohalogenation process and yields $CH \equiv CH$, acetylene or ethyne
Acetylene or Ethyne in presence of ammoniacal cuprous chloride forms a red-brown colored precipitate of 1,3-Butyne.
So, Compound A is 1,1 dichloroethane whose chemical formula is $C{H_3} - CH - C{l_2}$

Note:
We must know that the Glaser coupling test is the identification test for terminal alkynes, where terminal alkyne reacts with ammoniacal cupric chloride which on subsequent oxidation in air gives di-yne.