Ncert Solutions Chemistry Class 12th

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.

(i) Freon 12:- the chlorofluorocarbon compounds of methane and ethane are collectively known as freons. They are extremely stable, unreactive, non-toxic and non-corrosive. Example of Freon 12 is CCl₂F₂. It is mainly used in refrigeration and eventually makes its way into the atmosphere where it diffuses unchanged into the stratosphere. In stratosphere, Freon is able to initiate the radical chain reactions that can upset the natural ozone balance.

• DDT: - it stands for p, p'-Dichlorodiphenyltrichloroethane (DDT). It is mainly used as an
• Iodoform: - it was earlier used as an antiseptic but the antiseptic properties are due the liberation of free iodine and not due to the iodoform itself. Due to its objectionable smell, it has been replaced by other formulations containing iodine.
• Carbon tetrachloride (CCl₄):- it is used as an industrial solvent for oils, fats, resins etc. and also in dry Used as a cleaning fluid both in industries, as a degreasing agent and in the home, as a spot remover and as a fire extinguisher.

Let us explain each question from Haloalkane and Haloarenes chapter one by one. Students who are currently in school and plan to take the CBSE board exam for class 12th soon, need to prepare all these questions. Let us get started.

(i) The dipole moment of chlorobenzene is lower than that of cyclohexyl chloride.

Sp² hybrid carbon (s-character=33.33%) in chlorobenzene is more electronegative than a sp³-hybrid carbon (s-character=25%) in cyclohexylchloride, due to greater s-character. Thus, Carbon atom of chlorobenzene has less tendency to release electrons to Cl than carbon atom of cyclohexylchloride.

Chlorine atom in chlorobenzene is attached to the benzene ring. Here, the carbon-chlorine bond is polar. However, electron delocalization is possible in the benzene ring. Due to the electron delocalization, the overall dipole moment reduced since the polarity is distributed over the entire ring structure.

On the other hand, the chlorine atom in cyclohexyl chloride is attached to a saturated carbon atom in the cyclohexane ring. Because of this, there is no electron delocalization in the saturated structure. Hence, the dipole moment is more localized and stronger as compared to chlorobenzene.

 (ii) Alkyl halides, though polar, are immiscible with water.

Alkyl halides are polar because of the difference in electronegativity between carbon and halogen atoms. Here, there is not enough polarity to make them soluble in water.

The alkyl part of molecule is non-polar as well as hydrophobic in nature. This portion is large enough to dominate the solubility characteristics of the molecule. The entire molecule becomes insoluble in water due to this.

Water molecules create extensive hydrogen bonds with each other. However, Alkyl halides cannot form hydrogen bonds with water, which reduces their solubility in water. Haloalkane and Haloarene NCERT excercise discusses this question in further detail.

(iii) Grignard reagents should be prepared under anhydrous conditions.

Grignard reagents are very reactive organometallic compounds and they react readily with water to produce hydrocarbons. This destroys the Grignard reagent and makes it ineffective for its original purpose.

The reaction with water not only destroy Grignard reagent but also it produces by products that can complicate the synthetic pathway.

Preparing Grignard reagents under anhydrous conditions makes sure that they remain stable and effective for use in further reactions. Moisture-free conditions are important for the formation and utilization of reagents.

R-Mg-X + H-OH → R-H + Mg (OH)X

Do note that questions based on such equations from organic chemistry are also be asked in the CUET exam which makes it important for students to remember it.

Thus, Grignard reagents must be prepared under anhydrous conditions.