[nadjal]

Hi all,
This is my first topic in this forum and hope someone help me with my problem. I have to write something about formic acid properties - phisical and chemical. From google i found many wikipedia articles which helped me well. A problem for me is one quote about the chemical properties of formic acid which is: "Formic acid is unique among the carboxylic acids in its ability to participate in addition reactions with alkenes. Formic acids and alkenes readily react to form formate esters". This is strange for me because i cant find in any of my organic books such properties cited...Also if could give me mechanism explanation for such type of addition.
If someone could give me reference and short explanation about this properties i'll be very thankful!

[HP]

I am not very familiar with this problem but what i can share as knowledge on your problem is the following: Alkenes react by electrophilic addition reaction with many strong mineral acids like HCl, HBr, HI, H2SO4 and H3PO4 resulting alkylhalides or alkyl sulfates and phosphates which is classical example. With carboxylic acid as acetic acid for example (weak acids) this addition is possible only in the presence of strong acids like H2SO4 or tosylates. Formic acid is the simplest and the strongest unsubstitueted carboxylic acid with pKa = 3.75 which makes it about 1000X stronger acid than acetic though it is still a weak acid. This may be is one of the reasons for its properties for easy alkenes addition. I also can't find any book source giving explanation for detailed mechanism of this reaction and make an appeal for further assistence if anyone could enrich this discussion.

[Dan]

With carboxylic acid as acetic acid for example (weak acids) this addition is possible only in the presence of strong acids like H2SO4 or tosylates. Formic acid is the simplest and the strongest unsubstitueted carboxylic acid with pKa = 3.75 which makes it about 1000X stronger acid than acetic though it is still a weak acid. This may be is one of the reasons for its properties for easy alkenes addition.

I agree completely. The only reason I can see as to why this would work is the fact that formic acid is the strongest unsubstituted carboxylic acid. However, I´ve never heard of this reaction, and I must say I find quite suprising that it would go without a strong acid catalyst. Although there are plenty of things I haven´t heard about, this does sound like something that I would have heard about during my undergrad. Not having free access to all the journals or my books at the moment, I can´t really read up on it right now. I would be very interested to see someone shed some more light on this.

[Custos]

I think you're right Dan this reaction will only go in the presence of strong acids such as sulfuric and hydrofluoric acids.

[Yarr]

Formic acid is a weak acid, and so it is able to protonate the alkene double bond. Then the resulting carbocation is able to react with a formate anion to produce the formate esther.
In the presence of strong acids (such as sulphuric acid) the reaction takes another course, because then carbon monoxide is generated from formic acid/acid catalyst (and also water), which then reacts with the carbocation to produce ultimately a carboxilic acid.