Alkyl Halide: Definition, Classification, Examples and Properties

Consider for a moment working in a laboratory synthesizing a new drug that is poised to save thousands of human lives. The process will obviously be going through a series of reactions, one of which uses alkyl halides. These inconsequential compounds are truly behind a million chemical and related product syntheses that range from pharmaceuticals to agrochemicals, to plastics. Organic compounds where one or more hydrogen atoms in an alkane are replaced by one or more halogen atoms are called alkyl halides.

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This Story also Contains

1. Alkyl Halides
2. Major Reactions of Alkyl Halides
3. Importance and Applications
4. Some Solved Examples
5. Summary

The varied properties of alkyl halides makes them highly reactive with nucleophilic substitution and elimination, thus rendering them very useful in various industries and laboratories. Alkyl halides become important central intermediates besides merely being reagents, from which a great number of chemical products can be made. Some are so simple, such as PVC pipes, and others get very complicated, such as pharmaceuticals to aid man in fighting against illnesses. These applications in alkyl halide chemistry spread wide, conferring upon the chemist several advantages in the manipulation and easy building of molecules that lead to innovation and progress in most areas.

Alkyl Halides

Alkyl halides are those compounds in which the halogen atom is directly bonded to an sp3 hybridized carbon atom of an alkyl group. A general formula of alkyl halides is RX , where $\mathrm{R}=$ alkyl and $\mathrm{X}=$halogen. Such compounds can, depending on the number of halogen atoms, be classified as either primary,.

Alkyl halides are most commonly prepared by halogenation reactions of alkanes or substitution reactions of alcohols. The nature of physical properties, like boiling and melting points, depends on the type of halogen atom and the alkyl group. Their chemical properties are generally exemplified by their reactivity towards nucleophilic substitution and elimination reactions, which make the alkyl halides very important intermediates in organic synthesis.

Halogen atoms are more electronegative than carbon, therefore, the carbon-halogen bond of alkyl halide is polarised; the carbon atom bears a partial positive charge whereas the halogen atom bears a partial negative charge. As we go down the group in the periodic table, the size of the halogen atom increases. The fluorine atom is the smallest and the iodine atom is the largest. Consequently, the carbon-halogen bond length also increases from$\mathrm{C}=\mathrm{F}$ to $\mathrm{C}=\mathrm{I}$

Physical properties of Haloalkanes and Haloarenes:

(1) These are less soluble in $\mathrm{H}_2 \mathrm{O}$ but more soluble in Organic solvents

(2) Their density follows the order:

Iodide > Bromide > Fluoride > Chloride

(3) Their boiling point follows the order:

Iodide > Bromide > Fluoride > Chloride

(4) The boiling point of Isomeric haloalkanes decreases with the increase in branching

(5) The boiling point of isomeric dihalobenzene is nearly the same and follows the order

para > ortho > meta

Major Reactions of Alkyl Halides

Hunsdiecker Reaction:

This reaction is basically the decarboxylation of the silver salt of carboxylic acids to give alkyl halides. For example, if silver propanoate is treated with bromine, it gives 1 bromopropane which clearly shows the conversion of carboxylic acid to alkyl.

Reactions with NaCN and AgCN:

Alkyl halides react with sodium cyanide or silver cyanide to give nitriles, R-CN. By way of illustration, methyl bromide reacts with NaCN to give acetonitrile, a useful solvent as well as a precursor in organic synthesis.

Reactions with NaNO₂ and AgNO₂:

Alkyl halides react with NaNO₂ or AgNO₂ to give nitro compounds and alkyl nitrites respectively. These illustrate a method of converting alkyl halides to functional groups different from the normally resulting ones.

Finkelstein reaction:

The Finkelstein reaction is the replacement of one halogen by another. It involves mainly the substitution of alkyl chlorides or bromides with alkyl iodide using the medium of sodium iodide NaI in acetone. Swartz reaction involves the use of antimony trifluoride SbF3 in transforming alkyl halides into other halo forms.

Reactions with PCl₅, PCl₃, SOCl₂ and HX:

Alkyl halides react with phosphorus pentachloride, phosphorus trichloride, and thionyl chloride to form alkyl chlorides. Indeed, each of these reactions has been exploited commercially on a large scale in the conversion of alcohols to alkyl chlorides. In addition, a range of halides is formed by the reaction of alkyl halides with hydrogen halides, HX, with a typical example of their reactivity.

Reaction with NaCN:

NaCN or KCN is ionic in nature i.e, Na⁺CN^-. Thus, the nucleophile in this case is CN^-. The reaction occurs as follows:
$\mathrm{R}-\mathrm{CH}_2-\mathrm{X} \xrightarrow{\mathrm{NaCN}} \mathrm{R}-\mathrm{CH}_2-\mathrm{CN}+\mathrm{NaX}$
For example:

$\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{Br}+\mathrm{NaCN} \rightarrow \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CN}+\mathrm{NaBr}$

Reaction with AgCN:

AgCN is covalent in nature and as a result, the C atom is covalently bonded to Ag. Thus, the attack of the nucleophile in this case will take place from the Lone pairs over N atom

$\mathrm{R}-\mathrm{CH}_2-\mathrm{X} \xrightarrow{\mathrm{AgC}} \mathrm{R}-\mathrm{CH}_2-\mathrm{NC}+\mathrm{AgX}$

For example

$\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{Br}+\mathrm{AgCN} \rightarrow \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{NC}+\mathrm{AgBr}$

The reaction occurs as follows:

$\mathrm{CH}_3 \mathrm{Br}+\mathrm{NaNO}_2 \rightarrow \mathrm{CH}_3-\mathrm{O}-\mathrm{NO}+\mathrm{NaB}$

Reaction with AgNO₂:

AgNO₂ is covalent and it does break into Ag⁺ and NO^-₂ ions. The O atom remains bonded to the Ag and hence the attack of the nucleophile takes place by the lone pairs over Nitrogen.

The reaction occurs as follows

$\mathrm{CH}_3 \mathrm{Br}+\mathrm{AgNO}_2 \rightarrow \mathrm{CH}_3-\mathrm{NO}_2+\mathrm{NaBr}$

The silver(I) salts of carboxylic acids react with halogens to give unstable intermediates which readily decarboxylate thermally to yield alkyl halides. The reaction is believed to involve homolysis of the C-C bond and a radical chain mechanism. In this reaction, the ester is formed as a by-product. The reaction occurs as follows:

$\mathrm{RCOOAg}+\mathrm{X}_2 \xrightarrow{\mathrm{CC}_4} \mathrm{R}-\mathrm{X}+\mathrm{CO}_2+\mathrm{AgX}$

For example:

$\mathrm{CH}_3 \mathrm{COOAg}+\mathrm{Br}_2 \xrightarrow{\mathrm{CCl}_4} \mathrm{CH}_3 \mathrm{COOBr}+\mathrm{AgBr}$

Mechanism

Chain Initiation

$\mathrm{CH}_3 \mathrm{COOBr}^{\mathrm{RDS}} \mathrm{CH}_3 \mathrm{COO}^*+\mathrm{Br}^*$

Chain Propagation

$\mathrm{CH}_3 \mathrm{COO}^{\bullet} \longrightarrow{ }^{\bullet} \mathrm{CH}_3+\mathrm{CO}_2$

${ }^{\bullet} \mathrm{CH}_3+\mathrm{CH}_3 \mathrm{COOBr} \longrightarrow \mathrm{CH}_3 \mathrm{Br}+\mathrm{CH}_3 \mathrm{COO} \cdot$

Chain Termination

$\begin{aligned} & { }^{\mathrm{CH}_3+\mathrm{Br}^*} \mathrm{CH}_3 \mathrm{Br}_{\text {(major) }} \\ & \mathrm{CH}_3 \mathrm{COO}^*+{ }^* \mathrm{CH}_3 \longrightarrow \mathrm{CH}_3 \mathrm{COOCH}_3 \text { (minor) } \\ & \mathrm{CH}_3 \mathrm{COO}^*+\mathrm{Br}^* \longrightarrow \mathrm{CH}_3 \mathrm{COOBr}_{\text {(minor) }}\end{aligned}$

For example:

The rate of reaction with changing the alkyl group (R) in the above reaction varies as

$1^{\circ}>2^{\circ}>3^{\circ}$

It is to be noted that with I₂, silver salt of carboxylic acid gives ester as the main product instead of alkyl iodide.

$2 \mathrm{RCOOAg}+\mathrm{I}_2 \xrightarrow{\mathrm{CCl}_4} \mathrm{RCOOR}+\mathrm{CO}_2+2 \mathrm{AgI}$

Importance and Applications

Alkyl halides find importance in peculiar types of reactivity and, therefore, comprise the backbone of various industrially important and academically distinctive processes. They can be a very important intermediate in the synthesis of active pharmaceutical ingredients used primarily in the pharmaceutical industry. Alkyl halides are used in synthesizing such topical anesthetics as lidocaine.

Alkyl halides have applications in the synthesis of herbicides, insecticides, and fungicides in agrochemicals. This area is almost a necessity in the development of new agricultural chemicals in crop protection practices and yield enhancement because the derived alkyl halides can be used to form a large variety of functional groups.

As such, alkyl halides are quite important in the making of polymers and plastics. One plastic compound that is much applied is polyvinyl chloride, whereby they are made from vinyl chloride using polymerization. To put it simply, the role of alkyl halides is projected in going into the formation of materials, which are integral to the modern-day way of living.

Academic research used alkyl halides for the mechanism study and development of new methods relevant to their synthesis. It is chemistry ten alkyl halide reactions with nucleophiles or electrophiles that gave an insight into the behavior of organic molecules, hence contributing to progress in the field of Organic Chemistry.

Finkelstein Reaction

Finkelstein's reaction is a method of preparation of alkyl iodides from alkyl chlorides or alkyl bromides. In this reaction, alkyl chlorides or bromides are treated with NaI in the presence of acetone to form alkyl iodides. The reaction occurs as follows:

$\mathrm{R}-\mathrm{X}+\mathrm{NaI} \rightarrow \mathrm{R}-\mathrm{I}+\mathrm{NaX}$

We use NaI because it is soluble in acetone as it is covalent. All other sodium halides are ionic and thus not soluble.

For example:

$\mathrm{CH}_2=\mathrm{CH}-\mathrm{CH}_2-\mathrm{Cl} \xrightarrow{\mathrm{Nal} / \text { Acetone }} \mathrm{CH}_2=\mathrm{CH}-\mathrm{CH}_2-\mathrm{I}$

Swarts Reaction

Halide exchange is also used for the preparation of alkyl fluorides by Swarts Reaction. Alkyl chloride/bromide is heated in the presence of AgF, Hg₂F₂, CoF₂ or SbF₃ to give alkyl fluoride.

For example:

$\mathrm{CH}_3-\mathrm{Br}+\mathrm{AgF} \longrightarrow \mathrm{CH}_3-\mathrm{F}+\mathrm{AgBr}$

The reaction of alcohols ROH with PCl₅ and PCl₃ yields an alkyl halide RCl. The reactions of alcohols with $\mathrm{PCl}_5, \mathrm{PCl}_3$ and $\mathrm{SOCl}_2$occurs as follows:

$\begin{aligned} & \mathrm{PCl}_5 \rightarrow \mathrm{POCl}_3+\mathrm{HCl}+\mathrm{RCl} \\ & \mathrm{PCl}_3 \rightarrow \mathrm{H}_3 \mathrm{PO}_3+\mathrm{HCl}+\mathrm{RCl} \\ & \mathrm{SOCl}_2 \rightarrow \mathrm{SO}_2+\mathrm{HCl}+\mathrm{RCl}\end{aligned}$

$\mathrm{POCl}_3$ and $\mathrm{H}_3 \mathrm{PO}_3$are generated in the liquid phase and hence they are very hard to separate while$\mathrm{SO}_2$ and HCl are gases and thus they are easy to remove. Hence, for chlorination, we always use $\mathrm{SOCl}_2$ as the best option among the given reagents.

Mechanism

The reactions occur as follows:

$\begin{aligned} & \mathrm{RCOOH}+\mathrm{PCl}_3 / \mathrm{PCl}_5 / \mathrm{SOCl}_2 \rightarrow \mathrm{RCOCl} \\ & \mathrm{R}-\mathrm{OH}+\mathrm{PCl}_3 / \mathrm{PCl}_5 / \mathrm{SOCl}_2 \rightarrow \mathrm{R}-\mathrm{Cl}\end{aligned}$

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Some Solved Examples

Example 1
Question:

Which of the following are monohalides?

1)a,b and c only

2)a, b only

3)c, d only

4 a,b,c and d

Solution:
All of the given compounds are monohalides containing only one halogen atom. Therefore, option (4) is correct.

Example 2
Question:

Which ones are dihalogen derivatives of alkanes?

1)a, b only

2)a, b and d only

3)b only

4)a,b and c only

Solution:

Dihalogen derivatives of hydrocarbons contain two halogen atoms apart from carbon and hydrogen

Here, in

It is Dihalogen derivatives of alkene not alkane.

In (c) Hydrogen is replaced from the alkane group, not from the Benzene group, so it is a Dihalogen derivative of alkane.

a,b, and c are Dihalogen derivatives of alkene.

Therefore, option (4) is correct.

Example 3
Question:

Which of the following are geminal dihalides?

a) $\mathrm{Et}-\mathrm{CHBr}_2$
b) $\mathrm{Et}-\mathrm{CHI}-\mathrm{CH}_2 \mathrm{Cl}$

c)

1) a, c only

2)an only

3)b only

4)a,b and c only

Solution:
Compounds where 2 halogen atoms are attached to the same carbon atom are geminal dihalides. Hence, the correct answer is Option (1) which includes a and c.

Summary

The importance of alkyl halides in both industrial and academic uses cannot be overemphasized. Their chemical reactivity in a wide spectrum of reactions is what makes them so important in organic synthesis. The formation of pharmaceuticals and agrochemicals, principal materials like PVC-the list is by no means complete--to even the most basic uses in specialized fields of application, their study enriches our knowledge about organic chemistry and opens up new applications in very divergent sectors.