Preparation of Nitroalkanes from Alkyl Halides

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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.

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

FAQ (Frequently Asked Questions)

Q1. By Which Name Reaction are Alkyl Iodides Prepared?

Alkyl iodides can be prepared when an alkyl chloride or bromide is treated with the solution of sodium iodide in acetone. This reaction is called Finklestein reaction in which the SN² halide exchange takes place. It is also termed as halogen exchange reaction. Since sodium iodide is soluble in acetone, whereas sodium chloride and sodium bromide are not. The equilibrium tends to shift by the precipitation of the insoluble salt. The reaction’s equilibrium position is dependant on the nucleophilicity of the anion, irrespective of the anion being present and if one anion is stabilized better than the other in the solvent.

Q2. Can Alkyl Iodides be Prepared Directly?

No, alkyl iodides cannot be prepared directly. The alkyl lithium compounds are usually prepared when lithium metal reacts with an alkyl halide using a hydrocarbon or an ethereal solvent in the atmosphere of argon or dry nitrogen. Alkyl chlorides are preferred over alkyl iodides since these compounds tend to react faster with the resultant alkyl lithium. Alkyl chloride can be prepared without the simultaneous formation of ester when lead tetraacetate and lithium chloride react on the carboxylic acid in benzene. The forms of alkyl iodides are not considered since they react faster and are not preferred. Hence, alkyl iodides cannot be prepared directly.

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