Amidines
Amidines are a class of oxoacid derivatives.
The oxoacid from which an amidine is derived must be of the form RnE(=O)OH, where R is a substituent. The −OH group is replaced by an −NH2 group and the =O group is replaced by =NR, giving amidines the general structure RnE(=NR)NR2.
When the parent oxoacid is a carboxylic acid, the resulting amidine is a carboxamidine or carboximidamide (IUPAC name), and has the following general structure:
Carboxamidines are frequently referred to simply as amidines, as they are the most commonly encountered type of amidine in organic chemistry. The simplest amidine is formamidine, HC(=NH)NH2.
Examples of amidines include:
The most common way to make primary amidines is by the Pinner reaction. Reaction of the nitrile with acidic alcohol gives an iminoether; treatment of this with ammonia then completes the conversion to the amidine.
Amidines are much more basic than amides and are among the strongest neutral bases.
Protonation occurs onto the sp² hybridized nitrogen. This occurs because the positive charge can be delocalized onto both nitrogen atoms. The resulting cationic species is known as a amidinium ion and possesses identical C-N bond lengths.
An notable subclass of amidinium ions are the formamidinium cations; which can be represented by the chemical formula [R2N-CH=NR2]+. Deprotonation of these gives stable carbenes which can be represented by the chemical formula R2N-C:-NR2.
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