We know that alkyl iodides are comparatively less stable than alkyl chlorides, fluorides and bromides. Alkyl iodides are often used as synthetic intermediates because of their advantages over the alkyl bromides. However, they are not used in the preparation of alkyl halides since they are costlier than the other halogens. We know that a compound with weaker bonds tends to hydrolyse faster.
Since it is observed that alkyl iodide hydrolyses faster, it is assumed that the strength of the C-X bond in the alkyl iodides has a lesser influence on the degree of the polarisation of the bond and more on the rate. Hence, if the difference between the energies of the starting and end product is higher, the faster would be the rate. This is because the activation energy is lower. In today’s lesson, we will learn about the preparation of nitroalkanes from alkyl halides and aluminium and iodine reaction with alkyl halides.
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Synthesis of Alkyl Iodide
You would know about the photochemical iodination of the alkanes with iodine, but it has almost no significance. However, the iodination of the carbonyl compounds along with their enol derivatives is more readily derived. For the activated methylene groups like malonates, the iodination process is derived under the phase transfer catalysis by using K₂CO₃ as the base and I₂ as a halogen source. Let us look at how alkyl iodide is synthesized.
Alkyl halides can be prepared by the addition of iodine-iodine to alkenes. When the elemental iodine is added across the double bonds, it yields vicinal di-iodo compounds. However, this method of preparation is not much in use since its reverse reaction is thermodynamically favourable.
Alkyl iodides are readily prepared by SN² halide exchange according to the conditions of Finklestein reaction. Though halide exchange is a reversible reaction of an alkyl chloride or bromide, a solution of sodium iodide immersed in acetone at reflux condition affects the conversion to alkyl iodide. This is because of the shift of the equilibrium positions that are caused due to the precipitation of sodium chloride, which is a by-product and is less soluble in acetone when compared to sodium iodide.
Due to the SN² nature of halide substitution, the secondary and tertiary halides tend to react slower with the iodide ion. They generally need various conditions like iron or zinc halide catalysis. Alkyl chlorides, fluorides and bromides can be converted to iodides by heating them with excessive HI\[_{aq}\], with or without the phase transfer catalysis.
To convert alkyl bromides to alkyl iodides, the poor solubility of potassium or sodium iodides is overcome in different methods, including using dipolar aprotic solvents like adding crown ether for solubilising the metal counterion and applying phase transfer catalysis.
The tertiary alkyl nitro compounds are converted to their corresponding iodides by reacting them with trimethylsilyl iodide. However, this reaction is restricted only to the tertiary systems since the primary and secondary nitroalkanes would yield nitriles and oximes.
Alkyl Iodide Aluminium and Iodine Reaction
Let us now look at the aluminium and iodine reaction of an alkyl iodide. Alkyl iodides tend to undergo elimination reactions with bases or nucleophiles. This results in loss of hydrogen iodide from the molecule and produces an alkene. There are two majorly occurring mechanisms,E₁ and E₂.
The most effective and preferred mechanism is E₂ for the synthesis of alkenes from alkyl iodide. The E₂ mechanism can be used for all forms of alkyl iodide, which are primary, secondary and tertiary. The E₁ reaction, on the other hand, is not synthetically useful since it occurs similar to SN¹ reactions. However, tertiary alkyl iodide and a few secondary alkyl iodides can react through this mechanism.
The E₂ mechanism process is one-stage and involves both the alkyl iodide and the nucleophile. This is a second-order reaction and depends on the concentration of both the reactants. The E₁ mechanism, on the other hand, involves a two-stage process. It includes loss of halide and forms a carbocation, followed by the loss of the susceptible proton for forming an alkene. The first stage is the rate-determining step which involves loss of the halide ion, which makes the reaction a first-order reaction.
The carbocation intermediate which is formed is stabilized by the substituent alkyl groups. In the mono-molecular substitution SN¹ reaction, first, the dissociation of the C-X bond in the alkyl halide takes place with the formation of a carbonium ion. Then a rapid reaction with the nucleophilic agent is followed.
FAQs on Preparation of Nitroalkanes from Alkyl Halides
1. What is the primary method for preparing nitroalkanes from alkyl halides as per the CBSE syllabus?
The primary method involves the nucleophilic substitution reaction of an alkyl halide with silver nitrite (AgNO₂). When an alkyl halide (R-X) is heated with an ethanolic solution of silver nitrite, it forms a nitroalkane (R-NO₂) as the major product. The general reaction is: R-X + AgNO₂ → R-NO₂ + AgX.
2. Which specific reagent is used to ensure the formation of a nitroalkane instead of an alkyl nitrite?
To specifically prepare a nitroalkane, silver nitrite (AgNO₂) is the required reagent. This is because AgNO₂ is a predominantly covalent compound. In this case, the nitrogen atom, with its lone pair of electrons, acts as the nucleophilic centre, attacking the alkyl group to form a stable C-N bond, resulting in a nitroalkane.
3. Why does reacting an alkyl halide with silver nitrite (AgNO₂) yield a nitroalkane, while using potassium nitrite (KNO₂) gives an alkyl nitrite?
This difference in products is due to the chemical nature of the nitrite reagents and the ambidentate character of the nitrite ion (NO₂⁻).
- With Silver Nitrite (AgNO₂): This compound is mainly covalent. The bond between silver and oxygen is not easily broken. Therefore, the nitrogen atom's lone pair is more available for a nucleophilic attack on the alkyl halide, leading to the formation of a C-N bond and yielding a nitroalkane (R-NO₂).
- With Potassium Nitrite (KNO₂): This compound is ionic and readily dissociates in solution to give K⁺ and nitrite (O=N-O⁻) ions. In the nitrite ion, the oxygen atom carries a negative charge and has a higher electron density, making it a stronger nucleophilic site. The attack occurs through the oxygen atom, forming a C-O bond and yielding an alkyl nitrite (R-O-N=O) as the major product.
4. What is an ambidentate nucleophile, and how does this concept apply to the synthesis of nitroalkanes?
An ambidentate nucleophile is a chemical species that has two different nucleophilic centres, meaning it can attack an electrophile from two different sites. The nitrite ion (NO₂⁻) is a classic example.
- It can attack through the Nitrogen atom to form a nitro compound (R-NO₂).
- It can attack through one of the Oxygen atoms to form a nitrite ester (R-O-N=O).
5. How is the reaction of an alkyl halide with a nitrite ion similar to its reaction with a cyanide ion (CN⁻)?
The reactions are similar because, like the nitrite ion, the cyanide ion (CN⁻) is also an ambidentate nucleophile. The outcome depends on the reagent used:
- When an alkyl halide reacts with potassium cyanide (KCN), an ionic compound, the attack occurs through the carbon atom, forming an alkyl cyanide or nitrile (R-CN).
- When it reacts with silver cyanide (AgCN), a covalent compound, the attack occurs through the nitrogen's lone pair, forming an alkyl isocyanide (R-NC).
6. Can you provide a balanced chemical equation for the preparation of nitroethane from bromoethane?
Yes, the preparation of nitroethane from bromoethane using silver nitrite is represented by the following equation:
CH₃CH₂-Br + AgNO₂ → CH₃CH₂-NO₂ + AgBr
Here, bromoethane reacts with silver nitrite to yield the main product, nitroethane, and a precipitate of silver bromide.
7. Does the type of alkyl halide (primary, secondary, or tertiary) affect the synthesis of nitroalkanes?
Yes, the structure of the alkyl halide is crucial. This reaction generally follows an Sₙ2-like mechanism, which is sensitive to steric hindrance.
- Primary (1°) and Secondary (2°) alkyl halides are most suitable for this reaction, as they allow the nucleophile to attack effectively.
- Tertiary (3°) alkyl halides do not work well. Due to significant steric hindrance, they are highly prone to undergo elimination reactions when treated with a nucleophile/base like the nitrite ion, resulting in the formation of an alkene as the major product instead of the desired nitroalkane.
