NCERT Solution / Notes Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes

NCERT Solutions, Question Answer and Mind Map for Class 12 Chemistry Chapter 10, “Haloalkanes and Haloarenes,” is a study material package designed to help students understand the chemistry of organic compounds containing halogens, including their nomenclature, physical and chemical properties, and reactions.

NCERT Solutions provide detailed explanations and answers to the questions presented in the chapter. The solutions cover all the topics in the chapter, including the classification and nomenclature of haloalkanes and haloarenes, the physical and chemical properties of these compounds, and their reactions with nucleophiles and bases. They also provide tips on how to answer different types of questions, including short answer, long answer, and multiple-choice questions.

The question-answer section of the chapter covers a wide range of topics, from the reactions of haloalkanes and haloarenes with nucleophiles and bases to their uses in everyday life. It also includes questions on the effects of halogens on the physical and chemical properties of organic compounds and the environmental impact of halogenated compounds.

The mind map provides a visual representation of the key topics covered in the chapter, allowing students to understand the connections between different concepts and ideas. The mind map covers the nomenclature of haloalkanes and haloarenes, their physical and chemical properties, and their reactions with nucleophiles and bases.

NCERT Solution / Notes Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes with Mind Map PDF Download

HALOALKANES AND HALOARENES

Based on the structure i.e depending upon the number of halogen atoms in a compound,

Alkyl/ Aryl halides are classified as mono, di, and polyhalogen. When compared to carbon,

halogen atoms are more electronegative. Therefore

• The bond (carbon-halogen bond) of alkyl halide is polarised.
• Halogen atom carries a partial negative charge
• Carbon atom carries a partial positive

Preparation Of Alkyl Halides

Alkyl halides are produced by the free radical halogenation of alkanes-

Step 1 – Adding halogen acids to alkenes

Step 2 – Replacing –OH group of alcohols by halogens with the use of phosphorus halides or

halogen acids or thionyl chloride.

Aryl halides are prepared with the help of electrophilic substitution to arenes. Iodides and

Fluorides are prepared with halogen exchange method. Organohalogens have a higher boiling

point when compared hydrocarbons due to strong van der Waals forces and dipole-dipole

forces. They partial dissolve in water but completely dissolve in organic solvents.

Organometallic compounds are formed by the nucleophilic substitution, elimination, and

reaction with metal atoms which occurs due to the polarity of a carbon-halogen bond of alkyl

halides. Based on the kinetic properties Nucleophilic substitution reactions are classified as

S_N1 and S_N2. Chirality plays a very important in S_N2 reactions of understanding the reaction

mechanisms of these reactions. S_N2 reactions are characterized by inversion configuration

whereas SN1 reactions are characterised by racemisation.

Substitution Reaction

The substitution reaction is defined as a reaction in which the functional group of one chemical compound is substituted by another group or it is a reaction which involves the replacement of one atom or a molecule of a compound with another atom or molecule.

Substitution Reaction Example

These type of reactions are said to possess primary importance in the field of organic chemistry. For example, when CH₃Cl is reacted with the hydroxyl ion (OH-), it will lead to the formation of the original molecule called methanol with that hydroxyl ion. The following reaction is as shown below-

CH₃Cl + (OH⁻) → CH₃OH (methanol) + Cl^–

One more example would be the reaction of Ethanol with the hydrogen iodide which forms iodoethane along with water. The reaction is as shown-

CH₃CH₂OH + HI → CH₃CH₂I + H₂O

Substitution Reaction Conditions

In order to substitution reaction to occur there are certain conditions that have to be used. They are-

• Maintaining low temperatures such as room temperature.
• The strong base such as NaOH has to be in dilute form. Suppose if the base is of a higher concentration, there are chances of dehydrohalogenation taking place.
• The solution needs to be in an aqueous state such as water.

Substitution Reactions – Types

Substitution Reactions are of two types naming nucleophilic reaction and electrophilic reactions. These two types of reactions mainly differ in the kind of atom which is attached to its original molecule. In the nucleophilic reactions the atom is said to be electron-rich species, whereas, in the electrophilic reaction, the atom is an electron-deficient species. A brief explanation of the two types of reactions is as given below.

nucleophiles

Nucleophiles are those species in the form of an ion or a molecule which are strongly attached to the region of a positive charge. These are said to be fully charged or have negative ions present on a molecule. The common examples of nucleophiles are cyanide ions, water, hydroxide ions, and ammonia.

Nucleophilic substitution reaction

A Nucleophilic substitution reaction in organic chemistry is a type of reaction where a nucleophile gets attached to the positive charged atoms or molecules of the other substance.

Nomenclature Of Haloalkanes And Haloarenes

Initially, there was no proper system for the naming of compounds. Mostly there were trivial names that were used depending upon the country and region. These trivial names were based on the discoverer or the nature of the compound or its place of discovery.

The system of trivial names was not standard and led to much confusion, thus raising the need for a standard system for the naming of organic compounds. IUPAC came up with a set of rules that are used universally for the naming of organic compounds.

There are two names associated with every compound:

Common name – It is different from a trivial name in the sense that it also follows a rule for its nomenclature.

IUPAC name – The IUPAC (International Union of Pure and Applied Chemistry) naming system is the standard naming system that chemists generally use.

Rules of Nomenclature

• Find the longest carbon chain.
• Number the longest carbon chain such that the carbon atom(s) to which the halogen(s) is/are attached get the lowest number(s).
• Multiple halogen atoms are labelled with the Greek numerical prefixes such as di, tri, tetra, to denote the number of identical halogen atoms attached to a carbon atom. If more than one halogen atoms attached to the same carbon atom, the numeral is repeated that much time.
• In case, different types of halogens are attached, they are named alphabetically.
• The position of the halogen atom is indicated by writing the position and name of the halogen just before the name of the parent hydrocarbon.

The Methodology of Writing Name

• First, write the root word for the parent hydrocarbon (depending upon the no. of carbon atoms in the longest carbon chain).
• Secondly, calculate the number of halogen atoms present. If there are multiple halogen atoms present, then arrange the halogens alphabetically in the prefix, labelling them with their respective positions. But, if the same halogen atom is present more than once then use the prefixes di, tri, tetra, etc.

Nomenclature of Haloalkanes

Alkyl halides are named in two ways. In the common system, the alkyl group is named first followed by an appropriate word chloride, bromide, etc. The common name of an alkyl halide is always written as two separate words. In the IUPAC system, alkyl halides are named as haloalkanes. The other rules followed in naming compounds is that

• Select the longest chain of carbon atoms containing the halogen atom.
• Number the chain to give the minimum number to the carbon carrying halogen atom.
• If multiple bonds (double or triple bond) is present, then it is given the preference in numbering the carbon chain.
• The IUPAC name of any halogen derivative is always written as one word.

Nomenclature of Haloarenes

• Aryl halides are named by prefixing “halo” to the name of the parent aromatic hydrocarbon.
• If there is more than one substituent on the ring then the relative positions of the substituents are indicated by mathematical numerals.
• In the common system, the relative position of two groups is shown by prefixes ortho, meta or para.

The common and IUPAC names of some representative haloarenes are given below.

Haloarenes: Nature of C-X bond

Haloarenes are the chemical compounds containing arenes, where one or more hydrogen atoms bonded to an aromatic ring are replaced with halogens. The nature of C-X bond depends on both the nature of carbon in the aromatic ring and the halogen attached. Halogens are generally denoted by “X”.

As we know halogens are group 17 elements having high electronegativity namely, fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At). Out of them, Fluorine has the highest electronegativity. The elements in this group are just one electron short of completing their nearest noble gas configuration.

Carbon in haloarenes is a 14th group element with comparatively lesser electronegativity in comparison to halogen molecules. This is due to the fact that electronegativity increase across a period from left to right.

Salient Points on the Nature of C-X Bond in Haloarenes are:

• The C-X bond in haloarenes is polarized, as halogens are more electronegative than carbon. Due to the high electronegativity of halogen, it attracts the electron cloud more towards itself and thus gains a slight negative charge, on the other hand, carbon obtains a slight positive charge.
• As halogens need only one electron to achieve their nearest noble gas configuration, only one sigma bond is formed between one carbon and one halogen atom.
• Due to the increase in atomic size from fluorine to astatine, the C-X bond length in haloarenes increases from fluorine to astatine and bond dissociation strength decreases.
• Dipole moment depends on the difference in electronegativity of carbon and halogens (group 17 trends properties) and as we know that the electronegativity of halogens decreases down the group, the dipole moment also decreases. There is an exception to C-Cl and C-F dipole moments. Though the electronegativity of Cl is less than F, the dipole moment of a C-Cl bond is more than C-F.

S_N1 and S_N2 Reaction of Haloalkanes

Haloalkanes are converted into alcohols using hydroxide ion in aqueous media through SN1 and SN2 Reactions. Alcohols can efficiently be prepared by substitution of haloalkanes and sulfonic esters with good leaving groups. The choice of reagents and reaction conditions for the hydrolysis is important because competitive elimination reactions are possible especially at high temperatures leading to alkenes.

The hydrolysis of haloalkanes depends on the structure of the haloalkanes, primary haloalkanes typically undergo S_N2 reactions whereas tertiary haloalkanes react an S_N1 mechanism for tertiary haloalkanes or tertiary alkyl halides. There are two kinds of reactions of haloalkanes naming S_N1 And S_N2 Reaction.

S_N1 Reaction 

The S_N1 reaction is a substitution nucleophilic unimolecular reaction. It is a two-step reaction. In the first step, The carbon-halogen bond breaks heterolytically with the halogen retaining the previously shared pair of electrons. In the second step, the nucleophile reacts rapidly with the carbocation that was formed in the first step.

This reaction is carried out in polar protic solvents such as water, alcohol, acetic acid etc. This reaction follows first order kinetics. Hence, this is named as substitution nucleophilic unimolecular. This reaction takes place in two steps as described below.

Step-1:

• The bond between carbon and halogen breaks due to the presence of a nucleophile and formation of carbocation takes place.
• It is the slowest and the reversible step as a huge amount of energy is required to break the bond.
• The bond is broken by solvation of the compound in a protic solvent, thus this step is slowest of all.
• The rate of reaction depends only on haloalkane, not on nucleophile.

Step-2:

• The nucleophile attacks the carbocation formed in step 1 and the new compound is formed.
• Since, the rate defining step of the reaction is the formation of a carbocation, hence greater the stability of formation of an intermediate carbocation, more is the ease of the compound undergoing substitution nucleophilic unimolecular or S_N1 reaction.
• In the case of alkyl halides, 3^o alkyl halides undergo S_N1 reaction very fast because of the high stability of 3^o carbocations.
• Hence allylic and benzylic halides show high reactivity towards the S_N1 reaction.

S_N2 Reaction

This reaction follows second order kinetics and the rate of reaction depends upon both haloalkane and participating nucleophile. Hence, this reaction is known as substitution nucleophilic bimolecular reaction. In this reaction, the nucleophile attacks the positively charged carbon and the halogen leaves the group.

It is a one-step reaction. Both the formation of carbocation and exiting of halogen take place simultaneously. In this process, unlike the S_N1 mechanism, the inversion of configuration is observed. Since this reaction requires the approach of the nucleophile to the carbon bearing the leaving group, the presence of bulky substituents on or near the carbon atom has a dramatic inhibiting effect.

So opposite to S_N1 reaction mechanism, this is favoured mostly by primary carbon, then secondary carbon and then tertiary carbon. Nucleophilic substitution reaction depends on a number of factors. Some important factors include.

• Effect of the solvent
• Effect of the structure of the substrate
• Effect of the nucleophile
• Effect of leaving – group.

Comparing S_N1 and S_N2 Reactions

The solvent in which the nucleophilic substitution reaction is carried out also has an influence on whether an S_N2 or an S_N1 reaction will predominate. Before understanding how a solvent favours one reaction over another we must understand how solvents stabilize organic molecules.

Polyhalogen Compounds

“Polyhalogen compounds: Carbon compounds containing more than one halogen atom per molecule.”

Polyhalogen compounds are useful in various industries and in griculture. Some important polyhalogen compounds:

Dichloromethane (Methylene chloride)

Uses:

Dichloromethane (methylene chloride) is used as a:

1. Solvent for paint removers
2. Propellant in aerosols
3. Process solvent in the manufacture of drugs
4. Metal cleaning and finishing solvent

Harmful effects:

1. It endangers the human central nervous system.
2. Exposure to lower levels of methylene chloride in air can lead to slightly impaired hearing and vision.
3. High levels of methylene chloride in air cause dizziness, nausea, tingling and numbness in the fingers and toes.
4. In humans, direct skin contact with methylene chloride causes intense burning and mild redness of the skin.
5. Direct contact with the eyes can burn the cornea.

Trichloromethane (Chloroform)

Uses:

1. Chemically, chloroform is used as a solvent for fats, alkaloids, iodine and other substances.
2. The major use of chloroform today is in the production of the freon refrigerant R-22.
3. It was once used as a general anaesthetic in surgery but has been replaced by less toxic, safer anaesthetics such as ether.

Harmful effects:

It is therefore stored in closed dark-coloured bottles which are completely filled so that air is kept out.

Triiodomethane (Iodoform)

Uses:

• It was used earlier as an antiseptic, but the antiseptic properties are due to the liberation of free iodine and not due to iodoform itself.

Drawback:

• Because of its objectionable smell, it has been replaced by other formulations containing iodine.

Tetrachloromethane (Carbon tetrachloride)

Uses:

1. It is produced in large quantities for use in the manufacture of refrigerants and propellants for aerosol cans.
2. It is also used as feedstock in the synthesis of chlorofluorocarbons and other chemicals, in pharmaceutical manufacturing and general solvent use.
3. Until the mid-1960s, it was also widely used as a cleaning fluid, both in industry, as a degreasing agent, and in the home, as a spot remover and fire extinguisher.

Harmful effects:

1. There is evidence that exposure to carbon tetrachloride causes liver cancer in humans.
2. The most common effects are dizziness, light headedness, nausea and vomiting, which can cause permanent damage to nerve cells.
3. In severe cases, these effects can lead rapidly to stupor, coma, unconsciousness or death. Exposure to CCl₄ can make the heart beat irregularly or stop.
4. The chemical may irritate the eyes on contact. When carbon tetrachloride is released into the air, it rises to the atmosphere and depletes the ozone layer.
5. Depletion of the ozone layer is believed to increase human exposure to ultraviolet rays, leading to increased skin cancer, eye diseases and disorders, and possible disruption of the immune system.

Freons

• The chlorofluorocarbon compounds of methane and ethane are collectively known as freons.
• They are extremely stable, unreactive, non-toxic, non-corrosive and easily liquefiable gases.
• They are manufactured from tetrachloromethane by Swarts reaction.
• By 1974, the total freon production in the world was about 2 billion pounds annually.

Uses:

1. These are usually produced for aerosol propellants, refrigeration and air conditioning purposes.
2. Freon 12 (CCl₂F₂) is one of the most common freons in industrial use.
3. Most freons, even those used in refrigeration, eventually make their way into the atmosphere where it diffuses unchanged into the stratosphere.

Harmful Effect:

• In stratosphere, freons can initiate radical chain reactions which can upset the natural ozone balance.

p,p′-Dichlorodiphenyltrichloroethane (DDT)

DDT, the first chlorinated organic insecticide, was originally prepared in 1873.

However, it was not until 1939 that Paul Muller of Geigy Pharmaceuticals in Switzerland discovered the effectiveness of DDT as an insecticide.

Paul Muller was awarded the Nobel Prize in Medicine and Physiology in 1948 for this discovery.

   Paul Muller

Uses:

• The use of DDT increased enormously worldwide after World War II, primarily because of its effectiveness against the mosquito which spreads malaria and lice which carry typhus.

Harmful Effects:

Problems related to extensive use of DDT began to appear in the late 1940s.

1. Many species of insects developed resistance to DDT.
2. It has a high toxicity towards fish.
3. The chemical stability of DDT and its fat solubility compounded the problem. DDT is not metabolised very rapidly by animals. Instead, it is deposited and stored in the fatty tissues. If ingestion continues at a steady rate, DDT builds up within the animal over time.

The use of DDT was banned in the United States in 1973, although it is still in use in some other parts of the world.

S_N¹ mechanism

The tertiary alkyl halides react by S_N¹ mechanism via formation of carbocation as intermediate. The reactivity order for S_N¹ reaction is

Benzyl > Allyl > 3° > 2° > 1° > CH₃X.

A mechanism for the reaction of tert-butyl chloride with water apparently involves two steps:

Stereochemistry of S_N¹:

Key features:

More stable will be the carbocation intermediate; faster will be the SN¹ mechanism.

Polar solvents lead to polar transition state which in turn accelerates the SN¹ reaction.

If the initial compound is chiral then SN¹ reaction ends up with racemization of the product.

Weaker bases being leaving group favor SN¹ reaction.

S_N² mechanism

In case of S_N² reactions the halide ion leaves from the front side whereas the nucleophiles attacks from the back side; due to this reason S_N² reactions are always accompanied by the inversion of configuration. Thus formation of another enantiomer is lead by S_N² reaction of an optically active halide i.e. optical activity is retained but with opposite configuration.

Stereochemistry of SN²:

Key features:

In SN² reaction the stereochemistry around carbon atom of the substrate undergoes inversion and is known as walden inversion.

The rate of reaction depends on the steric bulk of the alkyl group.

Increase in the length of alkyl group decreases the rate of reaction. Alkyl branching next to the leaving group decreases the rate drastically.

Under the following conditions S_N¹ and S_N² reactions take place:

• The alkyl is secondary and tertiary.
• The solvent is Protic or Aprotic.
• To stabilize the intermediate stage..

E1 reaction

It is a unimolecular reaction. Rate determining step consist of formation of carbocation intermediate. Stability of carbocation intermediate determines the reactivity of E1 reaction.

Order of reactivity for E1 reaction is 30° >  20° > 10°. Both elimination and substitution reaction involves the use of (same reactive intermediate) carbocation. Therefore both the products are formed in comparable amount. This reaction is favored by entropy of reaction therefore increase in temperature favors the E1 reaction.

Stereochemistry of E1 reaction:

E1 eliminations generally lead to the more stable stereochemistry.

The rate of the E1 reaction depends only on the substrate, therefore more stable the carbocation is, faster will be the reaction. Slowest step is the formation of the carbocation. Alkenes formation doesn’t require strong base, since there is no leaving group that needs to be displaced. So there is no requirement for the stereochemistry of the starting material;

E2 reaction

It’s a biomolecular reaction. It is a single step reaction whose rate depends on the concentration of base and substrate. Reactivity depends on both strength of base and nature of alkyl halide. Order of reactivity for E1 reaction is 30° > 20° > 10°. This reaction proceeds at room temperature.

Example:

General mechanism:

Stereochemistry of E2 reaction:

E2 eliminations may or may not lead to the more stable stereochemistry. Initial material for this reaction has two sp³ hybridized carbons which on rehybridization forms two sp² hybridized carbons. The C-X bond and the C-H bond lines up in the same plane and faces in anti directions to each other.