In this reaction: $$\ce{H2C=O + H2O -> H2C-(OH)2}$$ My textbook says it's a nucleophilic addition reaction.

But in this reaction: $$\ce{H2C=CH2 + HBr -> H3C-CH2Br}$$ is an electrophilic addition reaction. How do we know when a reaction is via an electrophilic or nucleophilic addition?

• Additon to aldehydes or ketones are always Nucleophilic. I have rarely seen any electrophilic addition to aldehydes or ketones. But addition to C=C , i.e. alkenes are mostly electrophilic. Feb 26, 2018 at 14:01
• I agree. The C=O bond is strongly polarised making the C receptive to nucleophiles Feb 26, 2018 at 14:21
• Why can't we say "making the O receptive to electrophiles ". @Waylander Please help me out and thank you so much for your time! Feb 26, 2018 at 16:59
• You can say that. There is a well-established procedure for making carbonyl groups more reactive by premixing them with Lewis acids e.g. BF3 before adding a nucleophile to attack the C Feb 26, 2018 at 17:36

As Soumik Das correctly pointed out , "Additon to aldehydes or ketones are always Nucleophilic."

In your case , this can be viewed as ,

$$\ce{H2C=O}$$ has a carbon attached to oxygen , which is fairly electronegative . As a result , a partial positive $(\delta^+)$ charge gets created on the carbon.As a result , the lone pairs on the oxygen atom of $\ce{H2O}$ get attracted towards the slightly positive carbon and carries out nucleophilic addition reaction.

I hope you can carry it from here now.

• So does a partial negative charge develop on the oxygen and could attract the H+ from the water molecule, making it an electrophilic addition reaction. Feb 26, 2018 at 16:38
• @raajsuriya it could definitely take place, but the step here which ultimately leads to the product formation of the more stable product is the nucleophilic addition.Please upvote and accept the answer if you found it helpful. Feb 26, 2018 at 16:42
• Both products are the same, aren't they? @Tanuj Feb 26, 2018 at 16:46
• I gave it an upvote alright but I can't accept the answer because it doesn't really help or answer my question. Thank you so much for your time though. @Tanuj Feb 26, 2018 at 16:56
• @raajsuriya both products are the same but the product is formed through a transition state which is more stable (in which activation energy is less) and that is nucleophilic addition and not eletrophilic addition. Feb 26, 2018 at 23:46

Learning organic chemistry is about learning key modes of reactivity and being about to identify key features of reactants that may trigger relevant modes.

Relevant examples here are:

1. Nature of orbitals: Carbonyl groups have a low lying $\sigma^{*}$ orbital that is quite susceptible to nucleophilic attack. Alkenes, on the other hand, do not. What alkenes do have is a lot of electron density, so they are susceptible to protonation to form cations.

2. Acidity. Water is very mildly acidic while hydrogen bromide is quite acidic. Therefore different modes of reactivity should be considered.

The real question here is: Why are you trying to compare these two reactions at all? They're using different reagents with very different functional groups. Naturally, very different mechanisms were involved between the two reactions.

• I came across these two questions in my textbook and I just wasn't happy with the answer my teacher gave me. Thank you for your time! Feb 27, 2018 at 6:33