Why alkyl iodides are purple in the presence of sunlight and Role of solvents in SN1 and SN2 reaction ?

Why alkyl iodides are purple in the presence of sunlight ?

The C –I bond in alkyl iodide compound is larger than the other carbon – halides bond in halo alkane. With increasing bond length,bond dissociation energy decreases . 

So C – I bond is weaker than other carbon – halides bond. Hence alkyl iodides are breaks easily in presence of sunlight and yield alkane along with I_2.

Since I₂ is purple in color, so due to presence of I₂ in product mixture alkyl iodides are seem to be purple in color.The reaction proceeds through radical mechanism.

Why  alkyl halides are insoluble in water  in spite of polar in nature ?

There are two type of force of attraction acts among the alkyl halides molecules ,  which one is Vander waals attraction force and another is dipole-dipole attraction force. 

On the other hand , water molecules  being polar, they can form inter-molecular hydrogen bond . 

Now, alkyl halides will be dissolve in water if breaks these dipole-dipole attraction force of alkyl halides and hydrogen bond of water molecules. 

Now, to break these dipole-dipole attraction force and hydrogen bond, a sufficient amount of energy is required which is not available through the formation of dipole-dipole attraction between alkyl halides and water molecules.

Consequently , alkyl halides are insoluble in water  in spite of polar in nature .

Role of solvents in SN¹ and SN² reaction .

The rate of SN¹ and SN² reaction depends on the nature of the solvents used.In case of SN¹ reaction, the nature of the solvents should be polar protic solvents, such as H₂O, CH₃OH and CH₃CH₂OH etc. 

This type of solvents are eligible for SN¹ reaction. Because, in such type of solvents, both carbo cationand halide ion gain stability through polar interaction. 

On the other hand, polar but aprotic solvents like dimethy sulphoxide ( DMSO), N,N-dimethyl formamide ( DMF), acetone ( CH₃COCH₃ ) etc are very much suitable solvents used in SN² reaction. 

Because, in such type of solvents, cation gets stability but anion is free. Consequently, the activity of anions increases as a nucleophile ( CH3O ^–, HO^– etc ).

Why backside attack occurs in SN²  reaction?

In SN²  reaction mechanism backside attackoccurs. This fact can be explained on the basis of orbital picture diagram of transition state.

The carbon atom which is attacked by the nucleophile ,are in sp2 hybridized . 

On the other hand , the p-orbital of carbon atom is perpendicular to the plane on which the three sp2hybridized  orbitals are present.

The one lobe of this p-orbital takes part in overlapping with the leaving group and another lobe overlaps with the nucleophile. For this reason, front side attack does not occur. 

If it is possible, then in the hypothetical transition, both the leaving group and the nucleophile will be overlapping in the same side of the p-orbital which is impossible in real case.Consequently, backside attack occurs in SN²  reaction. 

Summary 

• Why  alkyl halides are insoluble in water  in spite of polar in nature ?
• Why alkyl iodides are purple in the presence of sunlight ?
• Why backside attack occurs in SN²  reaction ?
• Role of solvents in SN¹ and SN² reaction ?

Calculate the no. of unpaired electron and LFSE of [ Fe ( H2O)6 ]+3 ion .Click here