The remarkable basicity of guanidine

The remarkable basicity of guanidine is attributed to the fact that the positive charge on

the guanidinium ion is delocalized equally over the three nitrogen atoms, as shown by

these three equivalent resonating  structures:

Basicity of nitrogen can be increased by attachment to pi-donors (NH₂) group. These two pi-donating NH₂ groups donate electron density to the (pi-accepting) C=NH. Hence, the guanidinium ion is a highly stable cation.

Related Questions:

1. How is base strength related to the availabihty of the electron-pair?
2. Amines are more basic than ammonia why?
3. What is relative basic strength order 1° amines , 2°amines and 3° amines ? Explain:
4. Give an explanation for the fact that Guanidine NH=C(CH3)2 is a stronger base than most of amines?
5. Arrange in correct order of basic Character of aniline, pyrrol, pyridine and piperidine?
6. What is correct basicity order of pyridine, pyridazine, pyrimidine and pyrazine ?
7. Give an explanation for the fact that Guanidine NH=C(CH3)2 is a stronger base than most of amines?
8. Arrange in correct order of basic Character of aniline, pyrrol, pyridine and piperidine?
9. What is correct basicity order of pyridine, pyridazine, pyrimidine and pyrazine ?
10. Why pyridine is more basic than Pyrrole?
11. Why pyrimidine is less basic than pyridine?
12. Imidazole is more basic than pyridine? Why?
13. Biological Important of Imidazole and structure:
14. Pyridine is almost 1 million times less basic than piperidine? Why?
15. Cyclohexylamine amine is the stronger base than Aniline? Why?
16. Tetrahydroquinoline amine is the stronger base than Tetrahydroisoquinoline? Why?
17. Arrange the following in the order of increasing basicity : p-Toluidine, N, N-Dimethyl-p-toluidine, p-Nitroaniline, Aniline. (I.I.T.1986)
18. Arrange the following in the increasing order of their acid strength : Methyl amine, Dimethyl amine, Aniline, N-methyl aniline (I.I.T, 1988).

Basic strength order of (1). aniline 2). 2,6-dimethyl-aniline (3). 1,1-dimethyl aniline (4). 2,6,1,1-dimethyl aniline are?

Ortho effect (SIP) in ortho Substituted aniline:

Ortho effect in case of ortho substituted aniline is also known as steric Inhibition of protanation (SIP).

In ortho substituted aniline after protanation due to crowding  of ortho substituted groups bond strain of H-N-H bond increases and molecule becomes unstable so we can conclude that ortho substituted aniline avoid to protanation hence it's basic strength also decrease than aniline.

Ortho Effect: SIR effect: SIP effect:

The presence of a substituent at Ortho-position of aromatic acid increases the acidic character by steric inhibition of resonance(SIR). This effect is called the ortho effect. 

Hence all ortho substituted benzoic acids are stronger than their meta and para counter parts. 

The ortho effect is not observed in phenols.

The presence of electron withdrawing groups particularly at Ortho and Para positions increases the strength of benzoic acid.

Notes : (1) SIR (ortho) effect observed only in V-shaped groups like -COOH , -NO2 , -N(CH3)2, -COOR, -C+(CH3)2 etc and also aniline.

(2) SIR effect not observed on atomic, linear groups and smaller groups like -X , -CN, -OH, -CH3,  etc.

(1) S.I.P. :Steric Inhibition of protanation (Ortho effect):

Alpha  bond angle is smaller than Beta angle due to bond pairs and lone pair repulsion.

And after protanation, repulsion increases and anilinium ion becomes unstable  due to the Steric effect of ortho substituted groups, hence  aniline always avoid to protanation in presence of ortho substituted groups. so ortho substituted aniline is less basic than aniline . Except -OH and -OCH3.

For example:

.  

 Basic order given as above:

SIR : Steric Inhibition of resonance ( ortho effect):

When a group present in the ortho position with respect to carboxylic group creates steric strain resulting in rotation of the carboxylic group and shifting it out of plane of the benzene ring as a result the carboxylic group can no longer participate in ring resonance and thereby the acidity increases as delocalization of negative charge equally on the both oxygen atom of  conjugate base ( carboxylate ion) of the benzoic acid , hence carboxylate ion more stablised. This is  also called the Ortho effect.

Ortho substituted benzoic acid is stronger acid than benzoic except -NH2 groups.

For example:

The  acidic strength order given as above: