Fluorine is more electronegative than chlorine even then, p-flurobenzoic acid is weaker acid than p-chlorobenzoic acid explain ?

Since halogens are more electronegative than carbon and also possesses lone pair electrons , therefore they exert both -I and +R effects . Now in F , the lone pair of electrons are present in 2p-orbitals but in Cl , they are present in 3p-orbitals . Since 2p- orbitals of fluorine and carbon are almost  equal size , therefore , the +R effect is more pronounced in p- flurobenzoic acid than p-chlorobenzoic acid.

 
Thus in p- flurobenzoic acid +R effect is out weight the -I effect but in p-chlorobenzoic acid, it is the -I effect which is out weight  the +R effect. Hence p- flurobenzoic acid is weaker acid than p-chlorobenzoic acid.

Trifluoroethyl carbocation is less stable than trifluromethyl carbocation why ?

In case of trifluroethylcarbocation the highly electronegative fluorine atom , withdraws shared pair electrons between C-F bonds towards itself to great extent and intensifies positive charge on carbon. Greater the intensification of charge more is the instability.theirfore this carbocation is destabilised.

In case of trifluroethylcarbocation carbocation, the unshared paired of electrons in P - Orbital of each of the fluorine can be shifted into vacant Orbital of carbon atom of carbocation via P-P overlap.

It's leads to dispersal of charge and provide stability to the carbonation.

Stability of Heteroatomic carbocations:

The carbocaton contaning hetero atoms adjacent to positive carbon of cation such as oxygen, nitrogen and sulphur etc which are more electronegative than carbon, you might expect that they would by definition be electron withdrawing groups that destabilize carbocations.

But there is opposite effect, if the oxygen, nitrogen or sulphur atom is present at adjacent to carbocation, the overall effect is carbocation stabilization.

This is due to the fact that although these heteroatoms are electron withdrawing groups by induction, they are electron donating groups by resonance, and it is this resonance effect which is more powerful.

ILLUSTRATIVE EXAMPLE(1): Give the correct Stability of given carbocation .

SOLUTION: Sulphur containing carbocation is more stable because lone pair of sulphur atom show more resonating effect due to least electronegativity as compared to nitrogen as well as oxygen atom similarly  nitrogen show more resonance than oxygen atom

Conceptual Facts:

(1) If number of conjugation increases stability of carbocation increases ...

(2) We known that on increasing conjugation stability increases. their is exception in option (A) their is three nitrogen may involving in resonance but  actually not, 

 Because positive carbon does not involved in resonance due to bridge head carbon. we known according to Bredt,s rule bridge carbon cannot for duble bond.

Hence correct stability order is  (B) > (C) >> (A): 

(3)  Similarly 

Hence correct stability order is  (B) > (C) >> (A): 

ILLUSTRATIVE EXAMPLE(3): Give the correct Stability of given carbocation.

SOLUTION: (1):  (C) > (B) > (A)     and    (2):   (C) > (B) > (A)  

ILLUSTRATIVE EXAMPLE(4): Give the correct Stability of given carbocation.

SOLUTION: (1):  (D) > (B) > (C) > (A)     and    (2):  (D) > (B) > (C) > (A)  

Other important examples:

IIT UPDATE:

QUESTION(1)

SOLUTION: (D)

Dancing resonance or Sigma Resonance:

Dancing resonance is a special stability mechanism which increases stability of carbocations attached directly to the three membered rings. For example Cyclopropylmethyl Carbocation.

In cyclopropane all the carbon is sp3 hybridized and the bond angle for the same should be 109 degree 28′ but the actual bond angle is 60 due to which angle strain develops. So in order to minimize the strain p orbital bents due to which it acquires partial sigma and partial pi bond character which behave like pi bond. And resonance take place between sigma and vacant p-orbitals hence called P-orbitals overlapping or sigma resonance.

We know by Drago’s rule bond angle is directly proportional to the s-character while inversely proportional to p-character.  

Dancing resonance is a hypothetical phenomenon which is reduces strain of the ring, hence the carbocation is more stable.  CH₂⁺ has a vacant p orbital and a very effective overlapping takes place between p-orbital and electron density of cyclopropane, due to this its stability is very high. There is a conjugation between the sigma bond and positive charge.

The exceptional stability of cyclopropane methyl cation can be explained by the concept of dancing resonance concept. The stability of additional cyclopropyl group , is result of more conjugation between the bent orbital of cyclopropyl ring and cationic carbon.

The most stable carbocation known till date in organic chemistry is explain by Dancing resonance.

ILLUSTRATIVE EXAMPLE: Why is the Tropylium carbocation less stable than the tricyclopropylmethyl carbocation?

Why is the Tropylium carbocation less stable than the tricyclopropylmethyl carbocation?

Tropylium is highly stable due to conjugated system, that being resonance stabilized and the number of canonical forms of tropylium is more. But it is less stable than tricyclopropylmethyl carbocation because tricyclopropylmethyl carbocation undergoes a strong  stability mechanism factor that is sigma resonance or dancing resonance  (sigma-tropic rearrangement ).

What is Bredt's rule ?

The Bredt’s rule is relates to bridgeheads with carbon-carbon and carbon-nitrogen double bonds. This rule is  first discussed by german Scholar  Bredt’s  in 1902.

According to Bredt’s rule a bridgehead carbon atom of bicyclo compound cannot be sp2 hybridised or in other word a bridgehead carbon atom cannot be form double bond. unless the ring that contains at least eight atoms 

In Other word Bredt's rule state that planarity is not possible at bridge head carbon of bicyclo compound till one of the ring size is (8) eight member or more.

   

ILLUSTRATIVE EXAMPLE: Bredt's use for the predicting major and minor  product of elimination reactions.

What is the Inversion of Amines ?

Inversion of amines takes place at room temperature and required 64 k j per mole energy.

An amine such as ethyl methyl amine has a tetrahedral (sp3) nitrogen atom, and the nitrogen lone pair in an sp3 orbital. However this tetrahedral amine structure is not static. It is found that the nitrogen atom inverts. The lone pair disappears from one face, moves through the nucleus, and reappears on the opposite face. The lone pair repels the ethyl group, the hydrogen atom, and the methyl group, so as the lone pair reappears, these groups move away. As the lone pair is tunneling through the nucleus, the nitrogen has just three attachments so this nitrogen atom is trigonal planar with sp2 hybridization. Overall flipping of amines is called as inversion of Amine. This magical act is also known as quantum mechanical tunneling.

The tetrahedral amine structures are in equilibrium. Furthermore, these structures are enantiomers. All physical properties of enantiomers are identical except for the direction in which they rotate plane polarized light). From this we can conclude that the two enantiomeric amines are present in equal amounts and have equal stability .

There are two conditions in which a inversion of nitrogen atom does not take place.

1: No Lone Pair:

2: Ring Strain:

When the amine nitrogen is not part of a ring, this bond angle change is easily and inversion takes place. However, if the nitrogen atom is part of a three-membered or a four-membered ring then inversion is significantly retarded by strain.

Effect of Inversion on basicity of amines:

The rate of nitrogen inversion also correlates with, among other things, hybridzation. But in cases where these "other things", such as ring constraints, limit nitrogen inversion, then the rate of nitrogen inversion may not correlate with hybridization. So the rate of nitrogen inversion does not always correlate with hybridization and is therefore not a good indicator of lone pair "availability".

 It is clear that basicity of amines reduced by inversion of amines.

ILLUSTRATIVE EXAMPLE (1): 1-Azabicyclo[2,2,1]heptane is more basic than triethylamine why?.

 SOLUTION:  1-Azabicyclo[2,2,1]heptane is more basic than triethylamine . Because in case of triethylamine the lone pair of electrons is less available in the latter due to rapid nitrogen inversion. Nitrogen inversion is not possible in the bicyclic amine.

ILLUSTRATIVE EXAMPLE (2): Which of the following compound is more basic?

SOLUTION: Option (A) is least basic because Option (B) and (C) both have restriction of inversion hence more basic while option (C) is least basic than option (B) due to –I effect of N-atom to another reduce availability of lone pair of nitrogen atom.

ILLUSTRATIVE EXAMPLE (3):The correct order of decreasing basicity of the compounds is :

SOLUTION:  III > II > IV > I

In Case of Option (III) their is restriction of inversion take place at bridge head nitrogen atom hence it is most basic than (II), while (II) is more basic then (VI) because its lp does not delocalized  while lp of (IV) is delocalized with ring hence less basic  but (I) is least basic to all due to presence of three strong  -I group.

What is Steric Inhibition in Resonance of SIR effect ?

SIR stand for Steric Inhibition in Resonance, means as per name steric means size, inhibition means some kind of hindrance, little hurdle, and resonance means delocalisation of conjugate points.

We know that planarity is the main condition for resonance that means resonance can occur only when all the atom involved in resonance lie in the same plane or nearly in the same plane. Any change in structure which destroys planarity of molecule will restrict or inhibit resonance, this phenomenon is known as steric inhibition of resonance.

SIR effect finds immense use in explaining Stability of carbocation, acidity and basicity of organic compounds:

For example(1): Ortho substituted Benzoic acid is more acidic then meta or para substituted benzoic acid irrespective of nature of group (electron donating group or electron withdrawing group) 

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 Ortho effect.

Note:  However for groups like -NH₂ or -OH does not experience SIR effect due to small size and –CN is linear group so SIR effect not applicable.

For example (2):  N,N,2,6-Tetramethyl aniline is more basic than  N,N-Dimethyl aniline. In cas of N,N-Dimethyl aniline its dialkyl derivatives the NMe₂ group is coplanar with benzene ring.  So p-electron (lone pair) on N atom and pi orbitals are remain in the same plane. For this the p-electron on N-atom can delocalized via pi orbital in the benzene ring and its result the electron availability in para position is high and less available at nitrogen atom and hence less basic.

On the other hand , in case of N,N,2,6 tetramethyl aniline having two bulky methyl group in ortho position of the benzene ring , the NMe₂ group can not remain in the same plane. That is why the p-electron on N-atom cannot delocalized through pi orbital in para position. (due to steric inhibition of resonance)  It is localized at nitrogen atom Thus 2,6-Dimethyl aniline  derivative is more basic.

Basicity of Guanidine :

Guanidine is strongest organic nitrogenous compound with the formula HNC(NH₂)₂. Guanidine is analogue of carbonic acid. That is, the C=O group in carbonic acid is replaced by a C=NH group, and each OH is replaced by a NH₂ group.

A guanidine group also appears in larger organic molecules, including on the side chain of arginine (a basic amino acid).

It is a colourless solid that dissolves in polar solvents. It is a strong base that is used in the production of plastics and explosives. It is found in urine as a normal product of protein metabolism.

Basicity of Guanidine:

Guanidine is the strongest base among neutral compounds:

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.
IIT UPDATE:
QUESTION:

SOLUTION: (B)

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).