Ncert Solutions Chemistry Class 12th

7.37 

Chemical reactivity of group 15 elements towards hydrogen, oxygen, halogens, and metals are discussed below.

Reactivity towards hydrogen: Group 15 elements react with H to form hydrides of type EH3 where E=N, P, As, Sb or On moving down from NH₃ to BiH₃, the stability of the hydrides decreases. For example, the P-H bond in PH₃ is less stable than the N-H bond in NH₃. The strength of the E-H bond gets weaker as the size of the central atom increases.

Stability order of E-H bond (where E is group 15 elements) can be represented as

N-H > P-H > As-H >Sb-H > Bi-H

Reactivity towards oxygen: Group 15 elements react with O to form oxides of the type E₂O₃ and E₂O₅ where E = N, P, As, Sb or Bi. The oxide of the element with higher oxidation state is more acidic than those with lower oxidation Down the group, the acidic character of the oxides decreases.

Reactivity towards halogens: Group 15 elements form halides of the type EX₃ and EX₅ where E = N, P, As, Sb or Bi. However, N does not form pentahalides EX₅ as it lacks d-orbital. All group 15 elements form trihalides but trihalides of N are

Reactivity towards metals: Group 15 elements react with metals to form compounds of general formula M₃E₂ where E= N, P, As, Sb or The binary compounds formed by the reaction of group 15 elements with metal show oxidation state -3.

The reaction involves the approaching of the nucleophile to the carbon atom to which the leaving group is attached. When the nucleophile is sterically hindered (does not have any free space or very crowded), then the reactivity towards S_N2 displacement decreases. Due to the presence of substituents, hindrance to the approaching nucleophile increases in the following order.

• Bromopentane < 2-bromopentane < 2-Bromo-2-methylbutane. The structures are shown below:

Hence, the increasing order of reactivity towards S_N2 displacement is:

• Bromo-2-methylbutane < 2-Bromopentane < 1-Bromopentane
• The stearic hinderance in alkyl halides increases in the order of 1° < 2° < 3°, the increasing order of reactivity towards SN2 displacement is given as-

3° < 2° < 1°.

The structures are given below:

Hence, the given set of compounds can be arranged in the increasing order of their reactivity towards S_N2 displacement as:

2-Bromo-2-methylbutane < 2-Bromo-3-methylbutane < 1-Bromo-3-methylbutane

• The stearic hinderance to the nucleophile in the SN2 mechanism increases with a decrease in the distance of the substituents from the atom containing the leaving group. Further, the stearic hinderance increases with an increase in the number of Therefore, the increasing order of steric hindrances in the given compounds is as below:

Hence, the increasing order of reactivity of the given compounds towards SN2 displacement

1-Bromo-2, 2-dimethylpropane < 1-Bromo-2-methylbutane < 1-Bromo-3- methylbutane < 1- Bromobutane

7.1

The general characteristics of Group 15 elements are:

Electronic configuration: All Group 15 elements have 5 electrons in their valence The general electronic configuration of these elements is ns² np³

Oxidation state: Group 15 elements have 5 valence electrons and they require 3 more electrons to complete their However, the gaining of 3 electrons is difficult

Atomic size: Atomic size increases as we move down the group due to increase in the number of

Ionisation enthalpy: Ionisation enthalpy decreases as we move down the group because of increase in atomic

Electronegativity: Electronegativity decreases on moving down the group due to increase in atomic

Since ethanol is a alcohol, so in presence of alcoholic KOH, alkyl chloride undergo elimination reaction, which results in the removal of proton and being substituted by a bromide ion.

It is also a simple substitution reaction in which under the presence of aqueous NaOH, bromide ion is replaced by hydroxide ion as it is a better leaving group than hydroxide ion.

 

It is a simple substitution reaction in which a better leaving group leaves and is being substituted by an ion.

CH₃CH₂Br + KCN - CH₃CH₂CN + kBr

5-C₆H₅ONa + C₂H5Cl

It is a simple substitution reaction in which a better leaving group leaves and is being substituted by an ion.

C₆H₅ONa + C₂H₅Cl? C₆H₅ – O – C₂H₅ + NaCl

6-CH₃CH₂CH₂OH + SOCl₂

This reaction is called as wurtz reaction in which in which clubbing together of a alkyl halide group with a sodium or potassium salt of an ether group.

 

7-CH₃CH₂CH = CH₂+ HBr

It is an antimarkovnikoff reaction in which under presence of a peroxide an alkene undergoes substitution, wherein the halo group is attached to that carbon which has least no. Of alkyl groups attached to it.

 

8-CH₃CH = C (CH₃)₂ + HBr

It is a markovnikoff reaction in which an alkene undergoes substitution reaction with hydrohalide, wherein a halo group is attached to that carbon atom which has the largest no. Of alkyl groups attached to it.