stereo specific and stereo selective reaction

What is stereo specific reaction ?

A reaction may be stereo specific, if stereo chemically different materials give stereo chemically different products.  

For example, the addition of carbene to cis and trans but-2-ene is a cis addition. Thus the addition is stereo specific, that is, each geometrical isomer forms one product, cis isomer gives cis-product and trans isomer gives trans product .

The configuration of the two products are different.  In both addition reaction , the carbene used is singlet carbene.

What is stereo selective reaction ?

A stereo selective reaction is that where between two or more possible stereo isomeric product , one is formed predominance. For example, addition of triplet carbene toalkene.

Stereo chemistry of halogen addition to alkenes

Addition of halogen to alkene is usually predominantly trans that is , the addition is stereo selective.

For example the addition of bromine or chlorine to maleic acid gives (+,-) dibromosuccinic acid, which can result only from trans addition.

In this reaction, the step is the formation of a bridged or non classical carbonium ion, followed by  attact by the bromide or chloride ion .

The bromide ion attact from behind and along the bonding C – Br+ line , and this results in a walden inversion at this carbon atom.

Since the bromonium ionis symmetrical, each carbon atom is attracted equally well ,and consequently, resulting the formation of a pair of enantiomers and hence the product is a racemic modification.

Another example  of halogen addition to alkenes is addition of  bromine to cis and trans but-2-ene.

The reaction occurs through the formation of a bridged or non classical carbonium ion, followed by attact by the bromide ion.  

Evidence for the formation of cyclic bromide ion come from the lower temperature and NMR  studies.

Summary

• What is stereo selective reaction ?
• What is stereo specific reaction ?
• Stereo chemistry of halogen addition reaction to alkenes .

Click here: Huckel's Rule of aromaticity

Huckel’s rule of aromaticity and anti aromatic compounds

Huckel’s rule of aromaticity 

Huckel’s stated that the presence of (4n+2) numbers of delocalized π electrons in a flat or nearly flat cyclic system is the cause of aromaticity and it is called Huckel’s rule of aromaticity .

Where ,n = 1,2,3 …..etc, the whole number.

What is the condition of aromaticity ?

An organic species will be aromatic if the species satisfy the following conditions.

(I)The species  must be  planar 

(II)It  must  have  cyclic  structure 

(III)  The cyclic  structure must be stable 

(Iv) The  species must contain Huckel’s number of delocalized π electrons .

For example, benzene.

Benzene is a cyclic compound. It is a planar and stable structure. 

Benzene  contains of (4n+2) numbers of delocalized π electrons, which is a Huckel’s number. Hence , benzene is aromatic.

Besides benzene, there are a large number of benzenoid compounds , they are also obey  the Huckel’s rule  and hence aromatic.They are called benzenoid aromatic

What is non benzenoid aromatics ?

There are few conjugated monocyclic and poly cyclic systems  which contains  (4n+2) numbers  of delocalized π electrons and not benzenoid are known as non-benzenoid aromatic compounds.

Huckel’s rule predicts the [14] [18] and [22] annulene with (4n+2) π electrons when  n= 3 ,4, 5 respectively  have been found to be aromatic.

Aromatic compounds in biochemistry

Two amino acids phenylalanine and tyrosine  necessary for protein synthesis contain the benzene ring  are aromatic.

What is anti aromatic ?

Planar or non planar cyclic conjugated molecule  with 4n number of  π electrons and  related with high energies are called anti aromatic.

For example:

[4] annulene  that is cyclo butadiene  is a 4n molecule . It is a highly unstable compound and it is anti aromatic compound. 

Besides butadiene, [8], [16]  and [24] annulenes are anti aromatic . [10] annulene is  also anti aromatic , although it contains (4n+2) π electrons, because the cyclic [10] annulene is not planar.

Summary:

What is aromaticity ? what is Huckel's rule of aromaticity ?
What is anti aromatic ?
Aromatic compound in biochemistry .

Why phenol is acidic in nature-click here .

How electronegativity depends on oxidation number-hybridization and what is group electronegativity ?

How electronegativity depends on oxidation number,hybridization and other factors?

Although the electronegativity of an element is conventionally assigned a definite value, yet it may differ from one molecule to another.

The ability of an atom in a molecule  to attracts electron towards itself depends upon its environment in the molecule.

The electronegativity of an atom would thus depend upon the nature of the other atom with which it is attached in the molecule.

Besides the atomic size , the  factors which largely determine the elctronegativity  of an atom are  hybridization , oxidation state, substituents, ionization energy and effective nuclear charge.

Charge on the atom 

An atom which acquires a positive charge would tend to attract electrongs more strongly than a neutral atom.

Thus, a cation will be more electronegative than the parent atom. But an anion would be less electronegative than the neutral atom.

Why HClO₃ behaves as a stronger acid than HClO?

The  electronegativity of an atom depends on  its oxidation state of the concern element.The oxidation state of central chlorine atom is +1 in HClO acid and +5 in HClO₃ acid.Hence, the Cl atom in HClO₃ acid will be more electronegative than  in HClO acid.

Accordingly, the release of hydrogen as H+ ion in HClO₃ would be much easier than a similar change in HClO. In other words , HClO₃ behaves as a stronger acidthan HClO.

Electronegativity and Periodic variation of electronegativity

What is electronegativity ?

The tendency or power of an atom to attract electrons towards itself, when combined in a covalent compound is termed as electronegativity of atom .

Unit of electronegativity.

Electronegativity is the tendency of an atom to attract covalently bonded electrons toward itself. So electronegativity has no unit.

The most electronegative element in the world is fluorine ( F ) and the less electronegative element in the world is cesium ( Cs ) .

The most electronegative compound in the world is sodium perxenate,  Na₄[XeO₆ ].The term electronegativity has been defined differently by differently investigators .

 The approaches of some of the investigators are discussed  below.

Pauling ‘s Approaches :

Pauling defined electronegativity as the power of an atom in a molecule to attract electrons to itself. He calculated electronegativities of different elements from thermodynamics data.

Pauling consider a reaction of the given type for calculating of electronegativity of elements are shown below,

 1/2( A – A ) +  1/2 ( B – B ) → ( A – B )………………………..( I )

The bond dissociation energy of (A – B)  is higher than the mean of the bond dissociation energies of  ( A – A )  and ( B – B ) bonds and that their

difference  Δ  is related to the difference in the electronegativities of A and B, according to the following empirical equation,

Δ  = E _{A – B} ­­ -  1/2( E  _{A – A}  +  E  _{B – B}  )  = 23 ( χ_A - χ_B )2 ……………………….( II )

Where   E _{A – B} , E _{A – A}  and  E _{B – B} represents the bond dissociation energies of ( A –B ) ,(  A – A ) and ( B – B ) bonds respectively and  χ_A  and χ_B denote the  electronegativities of A and B respectively.

Thus,   Δ = 23 ( χ_A - χ_B )2 ………………………………( III )

Or  0.208 √Δ  =  χ_A - χ_B ……………………………………( IV )

Pauling later took the geometric mean instead of the arithmetric mean of E _{A – A}  and  E _{B – B} and suggested the following empirical relation,

  E _{A – B} ­­ -   ( E  _{A – A}  x  E  _{B – B}  )1/2  = Δ’ = 30( χ_A - χ_B )2 …………………….( V ) 

Or  0.182√Δ’ = χ_A - χ_B  ……………………………( VI )

In Pauling’s equation , the dissociation energies are taken in Kcal per mol.

Assuming arbitrarily  the electronegativity of hydrogen to be 2.1 , Pauling determined electronegativities of other elements with the help of equation    (VI ) .

Allred and Rochow's Approach :

Allred and Rochow defined electronegativity as the electrostatic force of attraction exerted by the nucleus of an atom on the valency electrons.

Allred and Rochow , by applying effective nuclear charge, Zeff , gave an empirical relation for calculating the electronegativity,

Where χ is the electronegativity and  r is the covalent radius of the atom .

Mulliken's Approach:

Mulliken suggested that the mean of the ionization potential and electron affinity of an atom should be a measure of the electronegativity of the atom.

Mulliken proposed the following empirical relation for determining   electronegativities  of various elements are ,

Where χ is the electronegativity and  r is the covalent radius of the atom .

Mulliken's Approach:

Mulliken suggested that the mean of the ionization potential and electron affinity of an atom should be a measure of the electronegativity of the atom.

Mulliken proposed the following empirical relation for determining electronegativities  of various elements are ,

In this equation, ionization potential as well as  electron affinity are in volts.

Periodic variation of electronegativity:

The electronegativity of elements gradually increases from left to right along a period and electronegativity decreases from top to bottom along a group. 

The change of electronegativities of elements according to Pauling's scale are as follows,

 Summary : 



What is electronegativity ? unit of electronegativity.
Periodic variation of electronegativity.
The most and the least electronegative elements in the world. 

Why phenol is stronger acid than aliphatic alcohol

Why phenol is stronger acid than alcohol and salicylic acid is a stronger acid than p-hydroxy benzoic acid ?

Why phenol is stronger acid than aliphatic alcohol ?

With aliphatic alcohols there is no factor stabilizing group which stabilize the alkoxide anion RO ^– relative to the alcohol itself.

On the other hand, with phenols there is possibility of relative stabilization of the anion by delocalization of its negative charge through interaction with the pi orbitals of the aromatic nucleus.

Delocalization also occurs in the undissociated phenol molecules. For this reason phenols are stronger acids than aliphatic alcohols.

Why aliphatic alcohol  is less acidic than aliphatic carboxylic acid ?

In  case of aliphatic alcohol there is no factor stabilizing group which stabilize the alkoxideanion, RO ^– relative to the alcohol itself.

On the other hand, delocalization of the negative charge in the carboxylate anion involves structure of identical energy content and their centres involved two highly electronegative oxygen atoms.

That is why, aliphatic alcohols are very less acidic than carboxylic acid.

Why salicylic acid is a stronger acid than p-hydroxy benzoic acid ?

The conjugate of salicylic acid that is salicylate ion getsstabilized by the intra ionic hydrogen bonding.

But  the conjugate base of p-hydroxy benzoic acid that is p-hydroxy benzoate ion does not stabilized  by such type H-bonding.

Hence, it is clear that salicylate ion is a weaker base than the p-hydroxy benzoate ion , that is, salicylic acid  is a stronger acid than p-hydroxy benzoic acid.

Why  dimethyl amine is a stronger base than trimethyl amine in aqueous medium ?

Dimethylamine is a stronger  base than  trimethyl amine in aqueous  medium because  the conjugate acid of the former is more stable than that of the later compound due to formation of H-bonding .The stability order of the conjugate acids due to salvation effect  are as follows,

Summary

• Why phenol is stronger acid than aliphatic alcohol ?
• Why dimethyamine is more basic than trimethylamine in aqueous medium ?
• Why salicylic acid is stronger acid than p-hydroxy benzoic acid ? 

Click here: Electromeric effect and mesomeric effect

Electromeric effect and mesomeric effect.

What is electromeric effect ?

This  is a temporary effect involving  the complete transfer of a shared pair of electrons to one or other atom  joined by  a multiple bond that is a double or triple bond.

OR

A temporary but complete displacement of electron pair forming a pi bond between two different atoms, toward the more electronegative atom at the requirement of attacking  species is called   electromeric effect, denoted by the symbol’ E ‘. 

There are two types  of electromeric  effect,  +E  effect   and  - E  effect.

What  is difference  between  electromeric  effect   and  inductive  effect ?

A temporary but complete displacement of electron pair forming a pi bond between two different atoms , toward the more electronegative atom at the requirement of attacking  species is called electromeric  effect.

It is a temporary effect.  It  is the polarizability of a multiple bond. In electromeric  effect , pi bond  bonds are involved. Complete transfer of pi electrons  takes  place  in this effect.

Inductive effect  is  permanent  but partial displacement   of electron along the bond in a molecule  actually toward the more electronegative atom due to electronegative difference of the  atom  forming  the  bond .

It  is a permanent  effect . It is  the  permanent  polarization of  a single bond. In  inductive effect,  sigma  electrons  are involved.  Only  electron  displacement  takes place in this  effect.

What  is  mesomeric  effect ?

The  effect  which helps  redistribution of  electrons  with permanent  polarization in the ground state of an entity  in unsaturated and chiefly in conjugated system  via  pi  orbital  is called  mesomeric  effect , denoted  by  ‘M’ .

There are two type  of  mesomeric  effect ,  +M  effect  and  -M   effect .   

When electron  density of a conjugated system increases by  a group through resonance  then this phenomenon of increasing the electron density is called + M effect.

When electron  density of a conjugated system  decreases by  a group through resonance  then this phenomenon of decreasing  the electron density is called  - M effect.

Summary:



What is electromeric effect ? give example.
What is difference between inductive effect and electromeric effect.
What is mesomeric effect ? 

Click here:Inductive effect and field effect.

What is chemical bonding and What is dative bond ?

What is chemical bonding ?

The force  of attraction by  which two or more atoms  combine  together to  form  stable molecules or ionic  cluster , is called  chemical  bonding .

Chemical bonding involves the union of two or more atoms through redistribution of electrons in their outer shells involving either transference  of electrons from one atom to another or sharing of electrons among themselves so that all the atoms acquire the stable noble gas configuration  of minimum energy .  

What is dative bond ?

A  different type of covalent bond which can be formed when both the electrons for sharing between the atoms are contributed by one atom only. This type of bond is known as dative bond.  Dative bond  is also called co-ordinate covalent bond.

Condition  for the formation of dative bond.

A dative bond is formed between two atoms  one of which has a complete octet  and at the same time possesses at  least one  pair of unshared  electrons while the other is short of the electrons .

 The former atom contributes one such pair of electrons for mutual sharing  between the two atoms as a result of which the second atom also completes its  octet and acquires  a stable noble gas configuration.  

The atom which contributes the pair of electrons is known as the donor  and the atom  which accepts these electrons is called the acceptor. The free pair of of electrons is known as the lone pair.

The dative bond  is conventionally represented by an arrow  pointing from the donor to the acceptor .

Summary

• What is chemical bonding ? 
• What is co-ordinate covalent  bond or dative bond ?
• Condition  for the formation of dative bond.

Click here: Hydrogen bonding 

1,3 proto-tropic shifts and difference between tautomerism and resonance

What is 1,3 proto-tropic shifts?

Isomer which are inter-convertible, are differ from each other only in electron distribution and in the position of a relatively mobile groups or atoms are called tautomerism.

The mobile atom is in the great majority of example is hydrogen and the phenomenon is referred to as proto-tropic shifts and usually the proton in a triad system is from 1 to 3 position in the chain and hence it is called 1,3 proto-tropic  shifts.

Few examples of 1,3  proto-tropic  shifts in triad system  are as follows,