[Martin10928]

Why is water addition on ethene an electrophilic addition and hemiacetal formation a nucleophilic addition, when both reactions start by the attack of proton which creates a positive charge that is attacked by a nucleophile? Hence, shouldn't be both reactions classified as nucleophile or both as electrophile?

Thank you very much for the answers  .

[pgk]

Although this is a very good question, please, do not confuse the catalytic role of the proton with the real mechanism of the reaction!
1). Alkene hydration:
Water is firstly protonated, followed by electrophilic attack of the double bond to the so formed hydronium cation and thus, leading to the corresponding carbocation intermediate, which is then attacked by water and accompanied by liberation of a proton that plays a catalytic role that way.
2). Acetal formation:
The carbonyl is firstly protonated, followed by nucleophilic attack of the alcohol and accompanied by liberation of a proton that plays a catalytic role that way.

[hollytara]

Also..

For the carbonyl, you CAN make a hemiacetal by dissolving the aldehyde in a basic solution: say sodium methoxide in methanol.  You will have nucleophilic attack of methoxide anion to give the tetrahedral intermediate as an alkoxide.  Since this is in a pool of methanol solvent, it will react with the methanol to make the neutral hemiacetal and a new methoxide.  There will be an equilibrium mixture of the two alkoxides (methoxide and hemiacetal alkoxide) based on pKa and concentration.  So in this case it IS a nucleophilic reaction - yes it can also go by an acid catalyzed mechanism in some conditions.

For most alkenes, if you dissolve them in a solution of sodium methoxide in methanol  - No Reaction  This is ONLY electrophile (acid) catalyzed.  It is similar to reactions of other electrophiles with alkenes (addition of Br2 or Cl2, oxymercuration, etc.).  So it is treated as electrophilic.

[Martin10928]

Thank you very much, I get it now.