What are Alkyl halides and Aryl halides?

In organic chemistry, an alkyl halide is formed when an atom of hydrogen is switched by a halogen in a hydrocarbon or aliphatic compound. An aryl halide is formed when an atom of hydrogen is substituted by a halogen atom in an aromatic compound. Metals react with aryl halides and alkyl halides and they also go through nucleophilic substitution reactions and elimination reactions.

Reactions of Alkyl halides

The following reactions are undergone by alkyl halides:

• Reactions with metals.
• Nucleophilic substitution reaction.
• Elimination reactions.

The reaction of Alkyl halides with metals

Several organic bromides, iodides, and chlorides would react with specific metallic compounds to yield a compound, which could contain a carbon-metal bond. Such compounds are called organometallic compounds. Grignard reagents are significant organometallic compounds. The Grignard reagent is an alkyl magnesium halide that is represented as RMgX. When an alkyl halide is reacted with magnesium (Mg) metal, Grignard reagents are achieved. The reaction of an alkyl halide with Mg happens in dry ether.

${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{Br}+\mathrm{Mg}\stackrel{\mathrm{dry} \mathrm{ether}}{\to }{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{MgBr}$

Wurtz reaction

A coupling reaction in which two alkyl halides reacted with sodium metal in dry ether to form higher alkanes is the Wurtz reaction. The general equation is written as,

$$\begin{gathered} 2\mathrm{RX}+2\mathrm{Na}\to \mathrm{RR}+2\mathrm{NaX} \\ \mathrm{Here}, \mathrm{R}=\mathrm{Alkyl} \mathrm{group}, \mathrm{X}=\mathrm{Halogen} \end{gathered}$$

Elimination reactions of alkyl halides

Alkene is obtained in the elimination reaction of alkyl halides. When an alkyl halide containing a beta-hydrogen is heated with a solution containing alcoholic potassium hydroxide, a hydrogen atom is eliminated from beta-carbon, an atom of halogen is eliminated from alpha-carbon and an alkene is obtained. As a beta-hydrogen is given out, the reaction is named beta-elimination.

When beyond one beta atom of hydrogen is found, extra alkenes are formed. One alkene is viewed as a major product and the major product is decided on the Saytzeff rule. Based on this rule, “In dehydrohalogenation reaction, the major alkene product could have the maximum number of alkyl groups bonded to the carbon atoms that are double bonded”.

For example, when 2-bromopentane undergoes an elimination reaction, 2-pentene is the major product and the minor product is 1-pentene.

Nucleophilic substitution reaction

The alkyl halide containing a partial +ve charge on the C atom linked to the halogen interacts with a nucleophile, a substitution reaction occurs and a halide ion is produced as the atom of halogen departs as the leaving group. Because the nucleophile initiates the substitution reaction, the reaction is a nucleophilic substitution reaction.

Alkyl halides go through two distinct nucleophilic substitution reactions mechanisms:

• Bimolecular substitution reaction (S_N2).
• Unimolecular substitution reaction (S_N1).

S_N2 reactions

In the S_N2 reaction, the carbon-halide bond is fragmented as the entering nucleophile interacts with an alkyl halide, and a new carbon-nucleophile bond is made. The reaction occurs in a single phase and no formation of intermediates happens. As the reaction goes by, the generation of a carbon-nucleophile bond happens and the carbon-leaving group bond gets weakened. In the transition state, the nucleophile and the leaving group are simultaneously attached to the atom of carbon. The transition state structure is unstable. The unstable structure is because of the existence of five atoms linked to a carbon atom.

For example: When methyl chloride is attacked by the hydroxide ion (nucleophile), the product is methanol and the chloride ion. The reaction’s rate is second-order kinetics.

In S_N2 reactions, the reactivity of primary alkyl halides is more when compared to secondary alkyl halides along with tertiary alkyl halides. The most reactive nature of primary alkyl halides is because they contain only a small quantity of atoms of hydrogen. The least reactive nature of tertiary alkyl halides is due to the bulky groups, which hinder the incoming nucleophiles. The sequence of reactivity is,

$\mathrm{Primary} \mathrm{alkyl} \mathrm{halides}>\mathrm{Secondary} \mathrm{alkyl} \mathrm{halides}>\mathrm{Tertiary} \mathrm{alkyl} \mathrm{halides}$

S_N1 reactions

S_N1 reaction occurs in polar protic solvents. This type of substitution reaction generally happens in two stages. In the 1^st stage, a carbocation & a halide ion are produced, when the polarized carbon-halogen bond goes through a slow cleavage. In the 2^nd stage, the carbocation is attacked by the nucleophile & the product is produced.

For example, tertiary butyl alcohol and bromide ion are formed when tert-butyl bromide reacts with hydroxide ions (OH^-). First-order kinetics is the reaction’s rate. The reaction could be written as,

In the first stage, a carbocation & a bromide ion are produced when the polarized carbon-bromine bond goes through a slow cleavage. This stage is reversible but the step is slow.

In the next stage, tertiary butyl alcohol is formed when the carbocation is attacked by OH^-.

When the stability of the carbocation is greater, the formation from alkyl halide could be greater, and the reaction rate could be faster. The reactivity towards S_N1 reaction is given as,

$\mathrm{Tertiary} \mathrm{alkyl} \mathrm{halides}>\mathrm{Secondary} \mathrm{alkyl} \mathrm{halides}>\mathrm{Primary} \mathrm{alkyl} \mathrm{halides}$

Because of the higher stability of tertiary carbocations, tertiary alkyl halides react rapidly towards S_N1 reaction.

Reactions of Aryl halides

Aryl halide undergoes the given reactions:

• Reaction with metals.
• Nucleophilic substitution reaction.
• Electrophilic substitution reaction.

Reactions of Aryl halides with metals

Wurtz-Fittig reaction

When a combination of an aryl halide and an alkyl halide interacts with Na in dry ether, the product is an alkylarene. This reaction is the Wurtz-Fittig reaction.

Fittig reaction

When an aryl halide interacts with Na in dry ether, analogous compounds are formed. This reaction is known as the Fittig reaction.

Nucleophilic substitution reactions of Aryl halides

The following reasons state the less reactivity of aryl halides towards nucleophilic substitution reactions:

Effect of resonance: In aryl halides, the pairs of electrons on halogen are linked together with pi-electrons found in the ring. The possible resonating structures of chlorobenzene are drawn as,

Due to the resonance, the carbon-chlorine bond gets a partial double bond nature. So, the cleavage of bonds in aryl halides is tough when compared to alkyl halides. So, the reactivities of aryl halides are less towards these reactions.

Dissimilarity in the hybridization of an atom of carbon in carbon-hydrogen bond: In an alkyl halide, the hybridization of a C atom linked to a halogen atom is sp³ and in an aryl halide, the hybridization of a carbon atom bonded to a halogen is sp². The sp² hybridized carbon with more s-nature is more electronegative and could have the pair of electrons of carbon-halogen bond more firmly when compared to sp³ hybridized carbon, which has less s-nature. Longer bonds are easy to break when compared to short bonds, so aryl halides’ reactivity is less towards nucleophilic substitution reactions.

Electrophilic substitution reactions of Aryl halides

Aryl halides undergo electrophilic reactions like halogenation (chlorination, fluorination, bromination, etc.), sulfonation, Friedel-Crafts reaction, and nitration. The nature of aryl halide towards the electrophilic attack is due to the –I effect of halogen; the ring of benzene is slightly deactivated to electrophilic substitution reaction. Because of several resonating structures, there is a more electron or –ve charge over ortho- (o-) and para- (p-) directions of the ring when compared to meta- directions. So, aryl halides are para directing and ortho directing.

Halogenation

When an aryl halide-like chlorobenzene reacts with chlorine in ferric chloride solvent, the products would be para and ortho compounds. The electrophile would attack the o-position and p-position of the aryl halide. The para isomer could be the main product and the minor product is the ortho isomer.

Nitration

In the nitration, in the presence of sulfuric acid, nitric acid would form NO₂. Due to two electronegative atoms of oxygen, an electrophilic center is found above N in NO₂. The o-position and p-position of the aryl halide like chlorobenzene are attacked by NO₂. The major product is the p- isomer and the minor product is the o- isomer.

Friedel-Crafts reaction

The electrophile is either an alkyl group or a carbonyl group in this reaction. The ortho position and para position of the aryl halide is attacked by the electrophile. The major product is the p-isomer and the minor product is the o-isomer.

Context and Applications

This topic is vital for,

• Bachelors in Chemistry
• Masters in Chemistry
• Masters in Organic Chemistry
• Doctor of Philosophy in Organic Chemistry

Practice Problems

Question 1: Alkyl halides & aryl halides are otherwise known as ___ respectively.

1. Haloalkanes & haloarenes
2. Alkanes & benzenes
3. Alkanes & arenes
4. None of the above

Answer: Option 1 is correct.

Explanation: Alkyl halides & aryl halides are otherwise known as haloalkanes & haloarenes respectively. These compounds comprise one or more halogens as their substituent.

Question 2: Alcohol could be formed from alkyl halide by _____.

1. Dehydrohalogenation
2. Elimination
3. Substitution
4. Addition

Answer: Option 3 is correct.

Explanation: Alkyl halide could be converted into alcohol by a nucleophilic substitution reaction. The hydroxide ion could play the role of the nucleophile.

Question 3: Alkyl fluorides could be readily synthesized by _____.

1. Free radical fluorination
2. Sandmeyers reaction
3. Swartz reaction
4. None of the above

Answer: Option 3 is correct.

Explanation: Alkyl fluorides can be readily produced by the Swartz reaction. Alkyl bromides or alkyl chlorides on heating with silver fluoride would result in alkyl fluorides.

Question 4: The examples of ambident nucleophiles are_____.

1. Cyanides
2. Nitrites
3. Only cyanides
4. Both 1 and 2

Answer: Option 4 is correct.

Explanation: Ambident nucleophiles have two nucleophilic centers. Cyanides and nitrites are examples of ambident nucleophiles.

Question 5: The product formed between the reaction of haloalkanes and potassium cyanide (KCN) is _____.

1. Alkyl cyanides
2. Isocyanides
3. Alcohol
4. None of the above

Answer: Option 1 is correct.

Explanation: When alkyl halide interacts with KCN, the halogen in alkyl halide is substituted by cyanide functional group & a nitrile (alkyl cyanide) is the product.