The alkyl iodides being less stable than the fluorides, chlorides or bromides and also the fact that it’s little expensive has led to the use of other halogens for preparation of alkyl halides. These alkyl iodides work out as synthetic intermediate and hence offer specific advantages over alkyl bromides and hence are less frequently used for preparing alkyl halides.
It is observed that the alkyl iodide hydrolyses faster and hence it may be assumed that the strength of C-X bond has more influence on rate than does the degree of polarisation of the bond. We also know that compound with weaker bond hydrolysis faster. So larger the difference between the energies of the starting material and final product, the faster the rate. This is due to the fact that activation energy tends to be lower.
As alkyl iodide is less stable than alkyl chloride due to weaker C-X bond. Moreover, alkyl chloride C-X bond is more polarised than in alkyl iodide and C-Cl bond has more electronegative bond energy than the C-I bond. There are lots of interesting facts about these halides.
Alkyl Iodide Synthesis
The photochemical iodination of alkanes with elemental iodine is well known but has little or no significance. In comparison to this, the α iodination of carbonyl compounds and their enol derivatives is much more readily achieved. For activated methylene groups such as malonates, the iodination process is achieved straight forward under phase transfer catalysis using K2CO3 as base and I2 as one of the halogen sources.
- The alkyl halides can also be prepared by iodine – iodine addition to alkenes. This is possible by adding elemental iodine across double bonds to give vicinal di-iodo compounds but this is of little preparative use as the reverse reaction is thermodynamically favoured.
- Alkyl iodides are the least stable of alkyl halides but are readily prepared by SN2 halide exchange under the classical condition of Finkelstein. Although halide exchange is the reversible treatment of an alkyl bromide or chloride with a solution of sodium iodide in acetone at reflux effects conversion to alkyl iodide. This is due to the shift of equilibrium positions caused by the precipitation of the by-product sodium chloride which is less soluble in acetone than sodium iodide.
- Because of the SN2 nature of halide substitution, secondary and tertiary halides are slow to react with iodide ion and usually need different conditions such as zinc or iron halide catalysis. In fact, alkyl fluorides, alkyl bromides and alkyl chlorides can be converted to iodides by just simply heating with excess aqueous HI with or without phase transfer catalysis.
- For conversion of alkyl bromides to iodides, the generally poor solubility of sodium or potassium iodides has been overcome in a variety of methods, including the use of dipolar aprotic solvents such as the addition of crown ether to solubilise the metal counterion and application of phase transfer catalysis.
- Tertiary alkyl nitro compounds are converted to corresponding iodides by reaction with trimethyl silyl iodide. This particular reaction is restricted to tertiary systems because primary and secondary nitroalkanes give nitriles and oximes.
Alkyl Iodide Reactions
Alkyl iodides undergo elimination reactions with nucleophiles or bases and hydrogen iodide is lost from the molecule to produce an alkene. There are two commonly occurring mechanisms and they are E1 and E2.
- The E2 mechanism is the most preferred and effective for the synthesis of alkenes from alkyl iodide and can be used for primary, secondary and tertiary forms of alkyl iodide.
- The E1 reaction is not particularly useful from a synthetic point of view and occurs parallel to SN1 reaction. Tertiary alkyl iodide and some of the secondary alkyl iodides can react by this mechanism. The primary forms are not formed in such cases.
- The E2 mechanism is a one stage process involving both alkyl iodide and nucleophile and the reaction is categorised under second-order and also depends upon the concentration of both reactants.
- The E1 mechanism is a two-stage process which involves a loss of the halide to form a carbocation and is followed by loss of the susceptible proton to form an alkene. The rate-determining step is the first stage involving loss of halide ion and hence the reaction is considered as the first order.
- The carbocation intermediate is stabilised by substituent alkyl groups.
- In mono-molecular substitution SN1 the dissociation of C-X bond in alkyl halide takes place first along with the formation of a carbonium ion. This is followed by a rapid reaction with the nucleophilic agent.
- The two reactions differ by the fact that SN2 reaction is of first-order both in terms of alkyl halide and the nucleophilic agent. The SN1 reaction is of first-order only with respect to an alkyl halide.
So we can also say that in the first case the reaction rate is proportional to the concentration of both reacting species and in the second case the reaction rate depends only upon the concentration of alkyl halide or in this case alkyl iodide.
In less polar solvents the reaction rate of thiourea with alkyl iodide drops sharply in the following order.
Primary > Secondary > Tertiary
The increased polarity of the medium facilitates the SN1 reaction mechanism.
RX → R+ + X–
The reactivity is further influenced by the branching of the carbon chain and by the possible presence of a double bond and position of a second halogen atom. Among the tested monovalent and divalent halo compounds, methyl iodide and methylene iodide along with benzyl bromide and allyl bromide are found most reactive.
Why Alkyl Iodide Cannot be Prepared Directly?
Alkyl lithium compounds are generally prepared by reaction between lithium metal and an alkyl halide using a hydrocarbon or ethereal solvent along with an atmosphere of dry nitrogen or argon.
Alkyl chlorides are preferable and not alkyl iodide as these compounds react very quickly with resultant alkyl lithium. The concentrations of solutions of n-butyl lithium can be determined by double titration method. Lithium alkyl attack diethyl ether and are less stable in tetrahydrofuran.
2RCO2Ag + I2 → RCO2R + CO2 + 2AgI
Acids can also form alkyl iodides if iodine and red mercuric oxide are used. The aliphatic carboxylic acids may be converted into esters and alkyl halides:
RCOOH + R(COO)4Pb → X2 → RCOOR + 2RX
Alkyl chloride may be prepared without simultaneous ester formation by the action of lead tetraacetate and lithium chloride on carboxylic acid in benzene. Again the iodide forms of alkyls cannot be considered as these forms react very fast and are not preferred.
Alkyl Iodide Free Radical Halogenation
With the help of activating light of wavelength 184.9 nm, the Iodine can be used in the free radical mechanism. But we need to note here is that iodination using I2 alone are hardly used and if so then maybe because the Hydrogen iodide which is formed goes ahead and reduces the alkyl iodide.
The direct free radical halogenation method of aliphatic hydrocarbons with the help of iodine is found to be significantly endothermic in nature as compared to other forms of halogens. Under such conditions, the necessary chain reaction does not take place.
But again when the Iodine from aluminium iodide (AlI3) reacts with an alkane in dibromo methane at 20oC, it gives out a good amount of iodo alkane. The preparation of alkyl halides can be carried out by nucleophilic substitution reaction of alcohols from alkanes. This is done by the free radical halogenation as well as by the addition of hydrogen halide from alkenes. These molecules are considered as precursor molecules for the preparation of other functionally substituted organic compounds by replacement of halide leaving the group with a nucleophile.
Alkanes are unreactive compounds because they are non-polar and lack functional groups at which reactions can take place. Free radical halogenation, therefore, provides a method by which alkanes can be functionalized.
Limitations of Radical Halogenation
The limitations of radical halogenation are the number of similar C-H bonds that are present in all but the simplest alkanes and hence reactions are difficult to achieve. These reactions are carried out in three stages.
- Initiation steps, propagation steps and finally the termination steps.
- In the propagation step, a halogen radical abstracts a hydrogen from methane to produce a methyl radical.
- This is followed by the regeneration of the halogen atom.
- Finally, the halogen atom reacts with another chlorine atom regenerate the halogen molecule. The two methyl radicals combine to produce ethane.
- The halogenation of methane is plagued by the lack of selectivity which becomes apparent as the reaction proceeds.
- As the halogen (iodo) methane is formed the concentration increases. As the methyl halogen has C-H bonds it can also undergo iodination forming di-iodo methane or most preferred dichloromethane.