Chemical Reactions of Haloalkanes: The replacement of one or more hydrogen atoms by halogens in hydrocarbons results in the formation of halogen derivatives of the hydrocarbons. In the case of aliphatic hydrocarbon, the product is haloalkane, in which the hydrogen atom is bonded to \(\text {sp}^{3}\) hybridized carbon atom. They are also known as alkyl halides.

Alkyl halides are identified by the alkyl group followed by the halide, for example, chloropropane containing propane as an alkane and chlorine as a halogen group. These compounds have been clinically tested and have a wide range of applications in industry and daily life. They are used as polar aprotic solvents and as starting materials for the synthesis of various organic compounds. In this article, we will study in detail the chemical properties/reactions of haloalkanes and some of their uses.

What are Haloalkanes?

Haloalkanes, also known as alkyl halides, are organic compounds composed of an alkane and one or more hydrogen atoms replaced by halogen. The general representation of haloalkanes is \(\mathrm{R}-\mathrm{X}\), where \(\mathrm{R}\) is an alkyl group and \(\mathrm{X}\) is a halogen such as \(\text {F}, \mathrm{Cl}, \mathrm{Br}, \text {I}\). Haloalkanes include methyl chloride or Chloromethane \(\left(\mathrm{CH}_{3} \mathrm{Cl}\right)\), Bromoethane or Ethyl bromide \(\text {C}_{2} \mathrm{H}_{5} \mathrm{Br}\), and others.

Know Everything About Haloalkanes Here

Types of Haloalkanes

There are three different kinds of haloalkanes- Primary, secondary and tertiary.

i. The carbon that carries the halogen atom in a primary \(\left(1^{\circ}\right)\) haloalkane is only attached to one other alkyl group. Primary haloalkanes include the following:

ii. The carbon with the halogen attached is joined directly to two other alkyl groups, which may be the same or different, in a secondary \(\left(2^{\circ}\right)\) haloalkane. Examples:

iii. The carbon atom holding the halogen is directly attached to three alkyl groups in a tertiary \(\left(3^{\circ}\right)\) haloalkane, which can be the same or different combination. Examples:

Reactivity of Haloalkanes

Haloalkanes are very reactive compounds. That is why these are used as the starting materials to prepare a large variety of organic compounds. The high reactivity of haloalkanes is due to the polarity in the \(\text {C}-\text {X}\) bond.

The polarity of the \(\text {C}-\text {X}\) bond is due to the difference in the electronegativities of \(\text {C}\) and \(\text {X}\) atoms. The halogen atom \(\text {X}\) is more electronegative than the \(\text {C}\) atom, and due to this, there is a partial positive charge on the \(\text {C}\) atom and a partial negative charge on the \(\text {X}\) atom.

This charge separation is called polarization, and the extent of polarization depends on the nature of halogen atom \(\text {X}\).

The reactivity of a haloalkane depends upon the bond energy of the \(\text {C}-\text {X}\) bond. The lower the bond energy, the greater is the reactivity of haloalkanes.

Chemical Reaction of Haloalkanes

The chemical reactions of haloalkanes can be divided into the following types:

1. Nucleophilic substitution reactions
2. Elimination reactions
3. Reactions with metals

The characteristic reactions are explained as follows:

Nucleophilic Substitution Reactions of Haloalkanes

Because of a partial positive charge on the carbon atom, it is vulnerable to attack by nucleophilic reagents (electron-rich species). When a strong nucleophile attacks this carbon atom, a new bond is formed with the incoming nucleophile, and the halide ion is removed.

Nucleophilic substitution reactions given by haloalkanes are given as follows:

Mechanism of Nucleophilic Substitution Reaction

The nucleophilic substitution in halogen derivatives containing \(\text {C}_{\text {sp}^{3}}-\text {X}\) bond can occur by any of the two mechanisms:

i. Unimolecular nucleophilic substitution reaction, \(\text {S}_{\text {N}^{1}}\)
ii. Bimolecular nucleophilic Substitution reaction, \(\text {S}_{\text {N}^{2}}\)

\(\text {S}_{\text {N}^{1}}\) Mechanism

In this mechanism, the reaction proceeds through the heterolytic fission of the \(\text {C}-\text {X}\) bond leading to the formation of a carbocation and a halide ion. The nucleophile then attacks the carbocation to form the substituted product. In this mechanism, the first step is slow, and the second step is fast.

The \(\text {S}_{\text {N}^{1}}\) the mechanism is facilitated by polar solvents such as water, alcohol, or aqueous-organic solvents.

The \(\text {S}_{\text {N}^{1}}\) the reaction proceeds via the formation of a carbocation. So greater the stability of carbocation, the greater will be the ease of its formation from haloalkane, and the faster will be the rate of the reaction.

In the case of haloalkanes, the tertiary haloalkanes undergo \(\text {S}_{\text {N}^{1}}\) reaction very fast due to the high stability of tertiary carbocations.

\(\text {S}_{\text {N}^{2}}\) Mechanism

In this mechanism, the nucleophile, \(\text {Nu}^{-}\) attacks the partially positive carbon atom attached to the halogen atom from a direction opposite to the \(\text {C}-\text {X}\) bond, leading to the formation of a transition state. The carbon atom is simultaneously bonded to the incoming nucleophile and the leaving group in the transition state.

The transition state cannot be isolated and is highly unstable as the reaction proceeds. The carbon-nucleophile bond starts forming, and the carbon-leaving group bond starts weakening.

As this happens, the configuration of the carbon atom undergoes inversion.

During the formation of the transition state, the nucleophile approaches the carbon atom bearing the leaving group. Therefore, the presence of a bulky group/substituent on or near the carbon atom will inhibit/slow down the reaction. As a result, the reactivity in haloalkanes for \(\text {S}_{\text {N}^{2}}\) reactions follow the order-

The order of reactivity of haloalkanes towards \(\text {S}_{\text {N}^{1}}\) and \(\text {S}_{\text {N}^{2}}\) reactions can be summarized as–

Elimination Reactions

An elimination reaction involves the removal of one molecule of \(\text {HX}\) from the molecule of haloalkane. The elimination reactions, therefore, are also called dehydrohalogenation reactions.

In elimination reactions, the halogen atom of the haloalkanes and a hydrogen atom from the adjacent carbon (\(\beta\)-carbon) are involved.

The order of reactivity in elimination reactions is-

The reactivity of the elimination reaction also increases with an increasing number of substituent alkyl groups at the (\(\beta\)-carbon)

Haloalkanes, when boiled with alcoholic potassium hydroxide, form alkanes by the elimination of one molecule of \(\text {HX}\).

If a haloalkane can eliminate an \(\text {HX}\) molecule in more than one way, then the formation of the most substituted alkene predominates. This is called Saytzeff’s rule.

Saytzeff Rule

“In dehydrohalogenation reactions, the preferred product is the alkanes, which have more alkyl groups attached to the doubly bonded carbon atoms.”

For example, \(2\)-bromobutane can eliminate one molecule of \(\text {HBr}\) in the following two ways-

In this reaction, \(\text {but}-2-\text {ene}\) is a more substituted alkene than \(\text {but}-1-\text {ene}\). Therefore, \(\text {but}-2-\text {ene}\) is the major product.

Reaction with Metals

Haloalkanes react with active metals to form products that depend upon the nature of the metal used.

With Sodium metal

Haloalkanes, when heated with metallic sodium in ether solution, give alkanes. This reaction is known as the Wurtz reaction. This reaction is used for obtaining symmetrical alkanes. This reaction, however, fails with tertiary haloalkanes, and usually, a mixture of alkane and alkene is obtained.

With other Active Metals

Haloalkanes also react with other active metals such as lithium, aluminium, zinc, lead, etc., in dry ether to form the corresponding organometallic compounds.

With Magnesium

Haloalkanes on reacting with magnesium in dry ether form alkyl magnesium halides, commonly known as the Grignard reagent \((\text {R}-\text {Mg}-\text {X})\). Grignard reagent is an organometallic compound.

Uses

Alkyl halides have a variety of uses and applications in our daily lives, which are listed below:

1. In laboratories, alkyl halides are used as synthetic intermediate compounds.
2. They are used as cleaning agents.
3. Haloalkanes have commercial applications such as fire extinguishers.
4. Neutrinos are detected using carbon tetrachloride.
5. In tropical regions, ethyl chloride can be used as a cooling agent for the skin.
6. In chemical processes, water-insoluble alkyl halides are used as hydrophobic solvents.
7.  Alkyl halides are commonly used to remove varnish or paint.
8. Haloalkanes are used as propellants.
9. Natural haloalkanes found in the oceans can sometimes act as deterrents to ocean attackers.
10. Refrigerators use CFCs or chlorofluorocarbons as refrigerants. They do, however, deplete the ozone layer and are thus harmful. They can also be used to make expanded polystyrene.
11. Dichloromethane is a blowing agent for foamed plastics and is used in plastic welding.

Alkyl halides have numerous applications in chemistry. However, the fact that these compounds have serious health and environmental consequences cannot be overlooked. Carbon tetrachloride, which is commonly used as a fabric cleaner, has been linked to liver damage. Similarly, chloroform, a common anaesthetic, has been shown to be carcinogenic. As a result, the use and circulation of alkyl halides have been limited to some extent.

Summary

Haloalkanes, also known as alkyl halides, are organic compounds composed of an alkane and one or more hydrogen atoms replaced by halogen. \(\text {R}–\text {X}\) is the general representation of haloalkanes. They undergo distinct reactions such as nucleophilic reactions, elimination reactions, wurtz reactions, and so on. Grignard’s reagent is a well-known organometallic halide that is used in a variety of chemical reactions.

We now know that haloalkanes are used in our daily lives, such as they are used as flame retardants, refrigerants, fire extinguishers, propellants, solvents, and in the pharmaceutical industry.

Frequently Asked Questions (FAQs) on Chemical Reactions of Haloalkanes

Q.1. What type of haloalkanes show \(\text {S}_{\text {N}^{1}}\) reaction?
Ans: Tertiary haloalkanes undergo \(\text {S}_{\text {N}^{1}}\) reactions, whereas secondary and primary haloalkanes undergo \(\text {S}_{\text {N}^{2}}\) reactions. Organic chemists use these reactions to change the functional group of molecules.

Q.2. What are the chemical properties of haloalkanes?
Ans: The chemical properties of haloalkanes are as follows:
i. Because the carbon attached to the halogen in haloalkanes generally has a partially positive charge, haloalkanes are more reactive to nucleophiles.
ii. radical reactions are observed in haloalkanes. Haloalkanes and Mg react to form Grignard reagents via a radical reaction mechanism.
iii. Organolithium compounds are formed when haloalkanes react with \(\text {Li}\).
iv. They show oxidative addition reactions, yielding organometallic compounds.
v. Haloalkanes undergo coupling in the Wurtz reaction, yielding symmetrical alkanes.
vi. They go through elimination reactions.
vii. Substitution reactions occur in haloalkanes. A nucleophile can substitute the halogen of a haloalkane.

Q.3. How many types of the reaction occurred in haloalkanes?
Ans: The chemical reactions of haloalkanes can be divided into the following types:
1. Nucleophilic substitution reactions
2. Elimination reactions
3. Reactions with metals.

Q.4. Which is the best reaction for the preparation of haloalkanes?
Ans: The reaction of alcohols with thionyl chloride as a suitable reagent is the best method of preparation of haloalkanes. This is because this method contributes to the production of pure alkyl halide.
Alkyl chlorides are formed when alcohol reacts with thionyl chloride \(\left(\text {SOCl}_{2}\right)\).
The byproducts of this reaction are gaseous in nature. As a result, the byproducts are easily released into the atmosphere, leaving only the pure alkyl halide.

Q.5. The most important chemical reactions of the haloalkanes are substitution reactions. What are \(\left(\text {SOCl}_{2}\right)\) reactions?
Ans: The substitution reactions of haloalkanes are the most important chemical reactions. \(\text {S}_{\text {N}^{1}}\) reactions are nucleophilic substitution reactions in which only one component is involved in the rate-determining step. The \(\text {S}_{\text {N}^{1}}\) the reaction mechanism is two-step in nature. To begin, the carbocation is formed by removing the leaving group (bond-breaking step). The nucleophile then attacks the carbocation (bond-forming step). The protonated nucleophile is then deprotonated to yield the desired product. And the first step is to determine the rate.

Nomenclature Of Haloalkanes And Haloarenes