Methods and Reactions for the Preparation of Alkyl Halides
Alkyl halides (or haloalkanes) are the compounds in which one or more hydrogen atoms in an alkane are replaced by halogen atoms (fluorine, chlorine, bromine, or iodine). These are organic compounds with the general formula RX, where R denotes the alkyl group and X denotes the halogen (group 17 elements). Alkyl halides and aryl halides (also known as haloarenes) are the two different types of substituted hydrocarbons.
The major difference between both alkyl halides and aryl halides is that haloalkanes are derived from alkanes (open chain hydrocarbons) and haloarenes are derived from aromatic hydrocarbons. Now we will discuss the preparation of alkyl halides. Both haloalkanes and haloarenes can be prepared from other organic compounds. Some of the methods are given below.
Preparation of Alkyl Halides
1. Preparation of Alkyl Halides from Alkenes
The addition of hydrogen halides to alkenes either follows Markovnikov’s rule or the Kharash effect. All the electrophilic addition reactions of alkenes following the Markovnikov rule are known as Markovnikov addition reactions. (In a simple definition, it states that “Hydrogen is added to the carbon with the most hydrogens and the halide is added to the carbon with least hydrogens”.)
General Reaction
\[\underset{\text{Alkene}}{R - CH = CH_2} + \underset{\text{Hydrogen halide}}{H - X \to R - CH_2 - CH_2X}\] OR \[\underset{\text{Alkyl halide}}{R - CH_2X - CH_2}\]
Conversion of \[- C = C - (\text{Alkenes}) \text{into} - X (\text{Alkyl halides})\]
\[\underset{\text{Symmetric alkene}}{R - CH = CH - R } + \underset{\text{Hydrogen halide}}{H - X} \to \underset{\text{Alkyl halide}}{R - CH_2 - CHX - R}\]
Preparation of Alkyl Chloride / Alkyl Bromides / Alkyl Iodides:
\[\underset{\text{Symmetric alkene}}{R - CH = CH - R} + \underset{\text{Hydrogen chloride}}{H - Cl} \to \underset{\text{Alkyl chloride}}{R - CH_2 - CHCl - R} \]
\[ \underset{\text{Unsymmetric alkene}}{R - CH = CH - R' \,\,\,\, + \,\,\,\,H - X} \to\underset{\text{Hydrogen chloride}}{R - CH_2 - CHX - R'} \to \underset{\text{Alkyl halide}}{R - CHX - CH_2 - R'} \]
2. Preparation of Alkyl Halides by Free Radical Halogenation
In free radical halogenation, we get a mixture of mono-substituted, di-substituted, tri-substituted, and even tetra-substituted halo-alkanes (alkyl halides). Since we require only one type of alkyl halide and not all in the form of a mixture, So this method is not used. Free radical halogenation of alkane is a process where single hydrogen from the alkane group is substituted by a single halogen so as to form an alkyl halide which is alternatively known as haloalkanes. The radical formation is generally initiated by the presence of light which is a good example of a photochemical reaction. One of the simplest examples is the reaction of methane with chlorine gas in the presence of sunlight to result in the formation of chloromethane and hydrogen chloride gas.
\[CH_{3}CH_{2}CH_2CH_{3} \xrightarrow[]{Cl_{2} / UVlight} CH_{3}CH_{2}CHClCH_{3} + CH_{3}CH_{2}CH_{2}CH_{2}Cl\]
The structure of the alkane has been evaluated in order to choose between the high reactivity of chlorine and the high selectivity of bromine which is as follows:
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All the carbons in the above reaction are equivalent and therefore, the high reactivity of chlorine (Cl_{2}) is followed.
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Here, as the above carbons are not equivalent to one another, therefore, there is a greater chance of the high selectivity of the bromine (\[Br_{2}\]) which will result in the formation of the tertiary halide.
Allylic bromination: When there is a presence of halogen around the unsaturated carbon such as alkenes, the expected reaction that occurs is the addition reaction to the double bond carbons which results in the formation of the vicinal dihalide (halogens on adjacent carbons). So as to avoid the reaction of the halogen to the carbons that are in the double bond, that is, alkene carbons, the concentration of the halogen is kept low so that the substitution reaction takes place at the allylic position rather than addition at the double bond. So, the reaction ends up in the formation of the halogen with the carbon that is placed next to the double-bonded carbons which is known as allylic halides. It is obtained by a radical chain mechanism.
\[CH_{2}=CHCH_{3} + X_{2}\] (in low conc.) \[\leftrightarrows CH_{2} = CHCH_{2} X + HX \]
Electrophilic substitution reaction: By this method, the preparation of aryl bromides and aryl chlorides becomes very easy. In the presence of the lewis acid, the electrophilic substitution reaction results in the formation of the aryl chlorides or aryl bromides by using halogens such as bromines and chlorines. In order to generate the proper electrophiles, certain specific conditions need to be maintained for the reaction to occur. The conditions for the reaction to occur is that the reaction should be carried out in the dark and there should be the presence of Lewis acid. Thus, the reactions to obtain the electrophiles are:
\[Cl_{2} + Fe \rightarrow FeCl_{3}\]
\[FeCl_{3} + Cl_{2} \rightarrow FeCl_{4} + Cl^{+} (to obtain Cl^{+} as an electrophile) \]
\[ Br_{2} + Fe \rightarrow FeBr_{3}\]
\[ FeBr_{3} + Br_{2} \rightarrow FeBr_{4} + Br^{+} (to obtain Br^{+} as an electrophile) \]
While HCl and HBr are the byproducts of the reaction, \[ Cl^{+} and Br^{+}\] are the electrophiles of the above-mentioned reactions. Therefore, the electrophilic substitution reaction for the preparation of aryl bromide and aryl chloride is
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3. Preparation of Alkyl Halides from Alcohols
In this reaction of synthesis of alkyl halides, the hydroxyl group of alcohol is replaced with the halogen atom attached to the other involved compound. The reaction requires a catalyst for primary and secondary alcohol whereas tertiary alcohol doesn’t require any catalyst.
\[CH_{3}CH_{2}OH + SOCl_{2} \overset{\Delta}{\rightarrow} CH_{3}CH_{2}Cl + SO_{2} + HCl\]
\[CH_{3}CH_{2}OH + PCl_{2} \overset{\Delta}{\rightarrow} CH_{3}CH_{2}Cl + P(OH)_{3} + HCl\]
\[CH_{3}CH_{2}OH + PCl_{5} \overset{\Delta}{\rightarrow} CH_{3}CH_{2}Cl + P(OH)_{3} + HCl\]
\[CH_{3}CH_{2}OH + PBr_{3} \overset{\Delta}{\rightarrow} CH_{3}CH_{2}Br + P(OH)_{3} + HBr\]
Preparation of Aryl Halides
1. Preparation of Aryl Halides by Electrophilic Substitution Reactions
Aryl halides can be prepared by an electrophilic aromatic substitution reaction of arenes with halogens in the presence of Lewis acid.
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2. Preparation of Aryl Halides through Sandmeyer’s Reaction
Aryl halides can be prepared by mixing the solution of freshly prepared diazonium salt from the primary aromatic amine with cuprous chloride or cuprous bromide. In a Sandmeyer reaction, a diazonium salt is reacted with copper (I) bromide, copper (I) chloride, or potassium iodide (KI) to form the respective aryl halide. The diazonium salt can be prepared from aniline by reacting nitrous acid at cold temperatures.
Did You Know?
The order of reactivity of halogen acids towards alcohol is:
Order of Reactivity for Halogen acids: HI>HBr>HCl
In the case of halogen acids, bond length increases from HCl to HI. The longer the bond length, the lesser will be the dissociation energy, and hence, more will be reactivity.
FAQs on Preparation of Alkyl Halides in Organic Chemistry
1. What are alkyl halides and how are they prepared?
Alkyl halides (haloalkanes) are organic compounds in which one or more hydrogen atoms of an alkane are replaced by a halogen atom (F, Cl, Br, or I), and they are prepared mainly from alcohols, alkanes, and alkenes.
- General formula: R–X (where R = alkyl group, X = halogen).
- From alcohols using HX, PCl5, PCl3, or SOCl2.
- From alkanes by free radical halogenation.
- From alkenes by addition of HX or X2.
2. How do you prepare alkyl halides from alcohols?
Alkyl halides are prepared from alcohols by replacing the –OH group with a halogen using reagents like HX, PCl5, PCl3, or SOCl2.
- With hydrogen halide: R–OH + HCl → R–Cl + H2O (often in presence of ZnCl2).
- With thionyl chloride: R–OH + SOCl2 → R–Cl + SO2(g) + HCl(g).
- With PCl5: R–OH + PCl5 → R–Cl + POCl3 + HCl.
3. How are alkyl halides prepared from alkanes?
Alkyl halides are prepared from alkanes by free radical halogenation in the presence of light or heat.
- Example (chlorination of methane): CH4(g) + Cl2(g) → CH3Cl(g) + HCl(g) (in UV light).
- The reaction proceeds via initiation, propagation, and termination steps.
- It may produce a mixture of mono- and poly-halogenated products.
4. How do you prepare alkyl halides from alkenes?
Alkyl halides are prepared from alkenes by electrophilic addition of hydrogen halides (HX) or halogens (X2) across the double bond.
- Addition of HBr to ethene: CH2=CH2 + HBr → CH3–CH2Br.
- Addition of Br2: CH2=CH2 + Br2 → BrCH2–CH2Br.
- Follows Markovnikov’s rule in unsymmetrical alkenes (except in presence of peroxide with HBr).
5. What is Markovnikov’s rule in the preparation of alkyl halides?
Markovnikov’s rule states that in the addition of HX to an unsymmetrical alkene, the hydrogen attaches to the carbon with more hydrogen atoms and the halogen attaches to the more substituted carbon.
- Example: CH3–CH=CH2 + HBr → CH3–CHBr–CH3.
- The major product is 2-bromopropane.
- This occurs due to formation of the more stable carbocation intermediate.
6. What is the Finkelstein reaction in preparation of alkyl halides?
The Finkelstein reaction is a halogen exchange reaction where an alkyl chloride or bromide reacts with sodium iodide in acetone to form an alkyl iodide.
- General reaction: R–Cl + NaI → R–I + NaCl.
- Sodium chloride precipitates in acetone, driving the reaction forward.
- It is useful for preparing alkyl iodides.
7. What is the Swarts reaction in the preparation of alkyl halides?
The Swarts reaction is used to prepare alkyl fluorides by heating alkyl chlorides or bromides with metallic fluorides like AgF, Hg2F2, or SbF3.
- Example: R–Cl + AgF → R–F + AgCl.
- It is specifically used for introducing fluorine into alkyl halides.
- This method is important because direct fluorination is difficult to control.
8. How are alkyl halides prepared from carboxylic acids?
Alkyl halides can be prepared from carboxylic acids by the Hunsdiecker reaction, which involves decarboxylation of silver salts of acids with halogen.
- General reaction: RCOOAg + Br2 → R–Br + CO2 + AgBr.
- The product has one carbon less than the original acid.
- This method is useful for preparing alkyl bromides.
9. What is the difference between primary, secondary, and tertiary alkyl halides?
Primary, secondary, and tertiary alkyl halides are classified based on the number of carbon atoms attached to the carbon bearing the halogen.
- Primary (1°): Halogen-bearing carbon attached to one carbon (e.g., CH3CH2Cl).
- Secondary (2°): Attached to two carbons (e.g., CH3CHClCH3).
- Tertiary (3°): Attached to three carbons (e.g., (CH3)3CCl).
10. Why is thionyl chloride (SOCl2) preferred for preparing alkyl chlorides?
Thionyl chloride (SOCl2) is preferred because it converts alcohols to alkyl chlorides with gaseous by-products that are easily removed.
- Reaction: R–OH + SOCl2 → R–Cl + SO2(g) + HCl(g).
- The gases escape, shifting equilibrium toward product formation.
- It gives a pure alkyl chloride without contamination by inorganic salts.
