The nucleophile attacks the partially positive carbon, and the high electron-density double bond gives an electron to the partially negative oxygen. The negative ion atom gives an electron to an H+ ion (given from the solution) to form an alcohol with a nucleophile attached.
Using hydrogen cyanide as the nucleophile allows the carbon chain length to increase by one. this produces a hydroxynitrile.
Aldehydes can be oxidised to carboxylic acids, but ketones cannot because a C--C bond must be broken. A common oxidising agent is potassium dichromate (vi).
Fehlings test --> made from Cu2+ ion, an alkali and a complexing agent. The aldehyde creates a brick red precipitate of copper (i) oxide whereas no reaction is seen with a ketone.
Silver mirror test --> Tollen's reagent is used, which is made of [Ag(NH3)2]+. The complex ion is reduced to Ag + NH3 and the aldehyde is oxidised to a carboxylic acid.
NaBH4 is a reducing agent, used in the reduction of aldehydes and ketones to alcohols.The hydride ion (H-) is the nucleophile.
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