Nucleophilic substitution reaction in alkyl halide

Nucleophilic Substitution Reaction of Alkyl Halides (R – X)

First let us understand what a nucleophilic substitution reaction is:

A nucleophile is:       nucleo + philic

                 means       nucleus loving 

That means a compound which loves nucleus i.e. A^-  (- because a negative charge attracts toward nucleus)

In substitution reaction, one functional group ( X ) is substituted (replace) by another group (Y)

So, when a nucleophile reacts with haloalkane (R- X) , substitution reaction takes place and halogen atom (X) is called leaving group departs as halide ion. 
Since the substitution reaction is initiated by a nucleophile, it is called nucleophilic substitution reaction.

Mechanism:

Substitution reactions are categories into unimolecular substitution reaction (S_N1) and bimolecular substitution reaction (S_N2)

a)    S_N2 (Bimolecular substitution reaction):

                                                  

·       It is bimolecular reaction means rate of reaction depends upon the concentration of both reactants (B- and RA)

·       No intermediate [X] is formed in this reaction because formation of bond and breaking of bond take place simultaneously.

·       In this reaction, configuration of carbon atom inverted while pushing the leaving group out, so this process is called inversion of configuration.

·       Order of reactivity of alkyl halide decreases with:

1⁰ > 2⁰ > 3⁰

As in primary halide, carbon atom (which bears halide group) has only small H atoms so Nu- easily approach to C atom while in secondary and tertiary halide, bulky R groups present which create hindrance to Nu- to approach C atom. 

b)                   S_N1 (Unimolecular substitution reaction):

·       This reaction is carried out in polar solvent (R⁺A^-)

                                                  

·       It is unimolecular reaction means rate of reaction depends upon the concentration of only one reactant (which is RA).

·       This reaction occurs in two steps:
In step I, a carbocation is formed while in II step, Nu- attacks to carbocation and alkyl halide is formed.

·       Greater the stability of carbocation (which is R – CH₃⁺), greater will be its ease of formation from alkyl halide and faster will be rate of reaction. So order of reactivity increases with:

1⁰ < 2⁰ < 3⁰

Rate of reaction is very fast in tertiary (3⁰) carbocation because of its high stability

So we can say order of reactivity of alkyl halide:

Reactivity of alkyl halide will be:

R – I  >  R – Br  >  R – Cl  >  R – F

Close Packed Structure in Solid

Close Packed Structure

In solid, constituent particles are so closely packed that there is very low space between them.  

Let us consider the constituent particles as identical hard spheres and build a three dimension structure in three steps:

1)    Close packing in one dimension:

If we arrange spheres in a row touching each other, a one dimension structure is formed.

The number of nearest neighbours of a particle is called its coordination number. In one dimension structure a particle is in contact of two of its neighbours. Thus, its coordination number is 2.

2)    Close packing in two dimensions:

It can be generated by placing two rows of closed packed spheres together. It formed in two ways.

a)     When second row is placed just above the first row, a two dimension structure is formed.

If the centres of these 6 immediate neighbouring spheres are joined, a

square is formed hence this is called square close packing in two dimensions.

In this arrangement, each sphere is in contact with four of its neighbours. Thus, the two dimensional coordination number is 4.

If we called first row ‘A’ type, then second row will also called “A’ because second row is exactly same as the first row. So this type of arrangement is called ‘AAA’ type.

b)    When second row is placed above the first one in a staggered manner so that second row’s sphere fit in the depression of first one, a two dimension structure is formed.

              

In this arrangement, each sphere is in contact with six of its neighbours. Thus, the two dimensional coordination number is 6.

If the centres of these 6 immediate neighbouring spheres are joined, a

regular hexagon is formed hence this is called hexagonal packing in two dimensions.

If we called first row ‘A’ type, then second row will called “B’ because second row is different from the first row. Similarly if we place third row above the second one in staggered manner then its spheres are aligned with those of the first layer Hence this layer is also of ‘A’ type. The spheres of similarly placed fourth row will be aligned with those of the second row (‘B’ type). Hence this arrangement is of ‘ABAB’ type.

3)    Close packing in three dimensions:

They can be obtained by placing two dimensional layers one above the another. Let us find out.

a)     Three dimensional close packing from two dimensional square close packed structure:

It is formed when one row of square close packed is placed just above the other.

In this arrangement spheres of both layers are perfectly aligned horizontally as well as vertically.

If we called first row ‘A’ type, then second row will also called “A’ because second row is exactly same as the first row. So this type of arrangement is called ‘AAA’ type.

b)    Three dimensional close packing from two dimensional hexagonal close packed structure:

It is formed when one row of hexagon close packed is placed just above the other.

1)    Placing second layer over the first layer

If we take a two dimensional hexagonal close pack layer and place a similar layer above it such that the spheres of the second layer are placed in the depressions of the first layer, a three dimensional structure is formed.

           

             This type of arrangement is called “ABAB” type

When a sphere of second layer is place above the void of first layer (vice versa) a tetrahedral void “T” is formed. (Because if we joined centres of these four spheres, a tetrahedron is formed)

At other places, the triangular voids in the second layer are above the triangular voids in the first layer, and the triangular shapes of these do not overlap. One of them has the apex of the triangle pointing upwards and the other downwards. These voids are called octahedral void ‘O’. Such voids are surrounded by six spheres and are called octahedral voids.

If there are N number of close packed sphere, then

No. of octahedral void will be: N

No. of tetrahedral void will be: 2N

2)    Placing third layer over the second layer

When we place third layer over the second one, there are possibilities

A)   Covering tetrahedral void:

o   In this case, the spheres of third row are placed over the tetrahedral void of second layer.

o   In this manner, spheres of third layer come in straight line with spheres of first layer. Thus, this pattern is written as “ABAB” pattern.

o   This is called hexagonal close packed structure (hcp).

o   Example: Magnesium, zinc

o   The coordination number of this structure is 12. 

B)    Covering hexagonal void:

o   In this case, the spheres of third row are placed over the octaherdral void of second layer.

o   In this manner, third layer neither come in straight line with first layer nor with second layer. They form “C” type. But when fourth layeris placed, its spheres come in straight line with first layer. Thus, this pattern is written as “ABCABC……” pattern.

o   This structure is called cubic close packed (ccp) or face-centered cubic (fcc) structure.

o   Example: copper, silver etc.

o   The coordination number of this structure is 12.

Lanthanoid Contraction

Lanthanoid Contraction:

In general, the atomic and ionic radii of an element increases with increase of atomic number. But in lanthanoid, with increase in atomic number the atomic and ionic radii of elements gradually decrease. This is called lanthanoid contraction.

Cause of Lanthanoid contraction:

The lanthanoid contraction is the result of a poor shielding effect of 4f electrons.

To understand it better, first let us understand what is shielding effect???

When inner-shell electrons shield the outer-shell electrons from the force of attraction exerted by positively charged nucleus, this phenomenon is called shielding effect.

 And the electrons present in inner-shells (between the nucleus and outer shell) are called shielding electron (because they “shield” the outer-shell electrons from the force of attraction exerted by the positively charged nucleus.)

Due to shielding effect, the outer-shell electrons are not affected by nuclear charge. It means that when the shielding is not as good, the positively charged nucleus has a greater attraction to the electrons.

The shielding effect exerted by the inner electrons decreases in following order:

s > p > d > f

As the atomic number of the members of lanthanoids series increases the positive charge on the nucleus increases by +1 unit and one more electrons enters in the same 4f subshell.

The f subshell has lowest shielding effect. Hence with increase of nuclear charge the electron present in outer-shell (i.e. 5d, 6s) is pulled slightly towards nucleus. As a result of the pull, the size of lanthanoids go on decreasing with increase in atomic number.

 

Though the contraction in size from one element to another is very small, but the net contraction over the fourteen elements from Ce to Lu is appreciable.

Effects of Lanthanoid Contraction:

Due to lanthanoid contraction, following properties of lanthanoids are affected.

1)   Basicity:

Due to lanthanoid contraction the size of tripositive lanthanoid ion (M+) regularaly decreases with increase in atomic number. This results into decrease in basic character.

The ionic character of M-OH bond decreases and covalent character of M-OH bond gradually increases. Therefore the basic strength of corresponding hydroxides decreases from La(OH)₃ to Lu(OH)₃. Thus La(OH)₃   is most basic and Lu(OH)₃  is least basic in nature.

 2)   Ionic radii of post lanthanoids:

The element which follow the lanthanoids in the third transition series are known as post lanthanoids. As a result of lanthanoid contraction, the atomic radii (size) of the electrons which follow lanthanium (Hf, Ta, W etc) are similar to that of the elements of previous period.

There is normal increase in size from Sc to Y to La.

This trend disappears after the lanthanoids and pairs of elements. Zr-Hf (group 4) Nb-Ta, (group 5) Mo-W (group 6) and Tc-Re (group 7) etc. have almost identical sizes. These atoms show similar number of valence electrons and similar properties. These pairs are called “Chemical twins”.

The elements of the second and third transition series resemble each other more closely than the elements of the first and second transition series.