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.

Silicon Carbide(SiC): Carborundum:

Silicon carbide (SiC) is a compound of silicon and carbon. It is extremely rare on Earth in mineral form (moissanite) and it has semiconductor properties. It has a bluish-black appearance. It has a large number of crystalline forms.

Preparation: When Silica treated with carbon at around 2000℃ to 2500℃ give carbide

Properties:(1) SiC also known as carborundum which is very hard .(2) Silicon carbide is inert and not attack by even HF.
(3) Silicon carbide when treated with NaOH give silicates and carbonate .

(4) Silicon carbide is covalent carbide.

(5) Silicon carbide has a high sublimation temperature and is a good heat conductor. It's expansion with increase in temperature is low. Also, it has high electric field breakdown strength.

(6) Silicon carbide exhibits the phenomenon of polymorphism, and hence has a large number of crystalline forms. Alpha silicon carbide is the most common polymorph and has a hexagonal crystal structure, while Beta Silicon Carbide has zinc blende structure.

Silicon Oxide : Silica (SiO2):

Silicon is unable to form pp - pp bond with oxygen atom due to its relatively large size. Thus it satisfies its all four valency with four oxygen atoms and constitutes three - dimensional network. In this structure each oxygen atom is shared by two silicon atoms. Three crystalline modification of SiO₂ are quartz, cristobalite and tridymite of which quartz and cristobalite are important.

Quartz (rock crystal) is the purest form of silica. It is used in preparation of costly glasses and lenses. It is also used as piezoelectric material (crystal oscillators and transducers).

Several amorphous forms of silica such as silica gel and fumed silica are known. Silica gel in made by acidification of sodium silicate and when dehydrates, is extensively used as a drying agent in chromatographic and catalyst support.

STRUCTURE OF SILICA:

PREPARATION OF SILICA:

Artificially silica can be obtained by following methods

PROPERTIES OF SILICA:

Pure silica is colourless but sand is brownish or yellowish due to presence of impurities of iron oxide.

Question: Why SiO₂ is solid while CO₂ is a gas?

Answer: This is because SiO₂ exists in silicate form where Si forms tetrahedron SiO₄^2- ions which are regularly attached with each other and form a giant network structure hence remains solid.

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.