Alkane molecules in which one or more hydrogen atoms are replaced by halogens like fluorine, chlorine, bromine or iodine are termed as alkyl halide. The functional group is halogen atom. If the same arrangement is seen in an alkene molecule and the halogen atom is attached to a double bonded carbon then it is better known as vinyl halide.
The generic structure for all alkyl halides is RX and for vinyl halides is CH2 = CHX. A molecule with two halogens bonded to same carbon is geminal di halide while a molecule with two halogen atoms attached to neighbouring carbons is vicinal di halide.
The vinyl compound with the generic formula of –CH = CH2 is meant for any organic compound that contains a vinyl group or as per IUPAC ethenyl, considered as a derivative of ethene, CH2 = CH2. Here one hydrogen atom is replaced with one of the halogens (X-CH = CH2). In case of chloride functional group, we get to see vinyl chloride which is a precursor to PVC or commonly known as vinyl. So whenever any alkene with a substituent halide bonded directly on one of the unsaturated carbons then it is termed as vinyl halide.
Vinyl halides can be synthesised in various methods. The common method is to elimination reactions involving the di halogen compounds. These elimination reactions give vinyl halides. The elimination reactions can be from either the geminal or vicinal di halide.
Under these given conditions we usually find the hydrogen halide getting eliminated and the elimination of decarboxylative and elimination of a silyl halide are also common. In some cases either a molecule of a halogen and a mixed halogen or rather a di-halogen from a poly-halide is also seen to get vinyl halides.
The overall addition of hydrogen halide to a triple bond is a common method to produce vinyl halide and this can be either a direct addition of hydrogen halide or can be applied for adding another species which further helps in converting into a halide.
The formal addition of alkyl halides to triple bonds to produce synthesise vinyl halide is also seen. The halogenation of alkyne might also lead to the production of di halo alkane.
The compounds of vinyl chloride, bromide and iodides can be prepared directly from ketones and then finally reduced by metallation or protonation method. The traditional methods to converting ketones into vinyl halides can be classifed into two major categories.
- Reaction of ketones with hydrazine followed by adding the halogen in the presence of a base. This method is used to synthesise an intermediate in total synthesis nakiterpiosin. The method is a variant for wolf kishner reduction process.
- Second method is to transform ketones into vinyl halides involving reaction of ketones with phosphorus halide like PCl5, phosphorus tri-bromide or thionyl chloride.
Vinyl chloride is considered to be a hybrid structure where chlorine is joined to carbon by a double bond, while chlorine attains a positive charge and the other carbon bears a negative charge.
In our daily life we come across several of these compounds but in their derivative forms. We use them every moment of our lives. PVC or poly vinyl chloride are nothing derivatives of vinyl chloride.
- PVC pipes
- PVC construction materials
- PVC daily use utensils
- PVC castings
Compared to SN2 substitution the SN1 is even more disfavoured for compounds like vinyl chloride as it leads to the formation of positive charge on sp hybridised carbon. This occurs after the stronger bond is broken and is not easily polarised than compared to vinyl chloride.
The possible SN2 reaction mechanism in unsaturated molecules with pie orbitals of the double bond. This leads to rearrangement of electrons in the given space and begin the interactions with incoming nucleophile.
For any alkene molecule there is a presence of huge electron cloud close to the line of the incoming nucleophile. This creates an unfavourable coulombic interaction and hence prevent the nucleophile approach. Finally the possibility of inversion at such an unsaturated carbon atom.
The transition state of a saturated carbon is a trigonal bipyramid, where the three groups that are not involved in substitution reaction orient themselves on this trigonal plane and then approaching along with leaving group interact along the axis. This interaction is found to be orthogonal to the plane.
The hypothetical transtion state at an unsaturated carbon atom would be based upon an octahedral framework constituting approaching as well as the leaving group interacting along the axis of orthogonal to a plane need to have two lobes of the ‘p’ orbital of carbon and perpendicular to the other two sigma bonds which are not involved in bond cleavage or bond formation. However sp2 hybridised carbon inversion of configuration does not take place in general.
An alkynyl halide cannot undergo an SN2 reaction mainly due to the fact that this would require the incoming nucleophile to approach through carbon at the other end of triple bond and moreover, there is no specific mechanism by which a carbon can get its configuration inverted.
Vinyl halides also cannot go for SN1 as well because the nucleophile approaches from behind the sp2 hybridised carbon atom and is the repelled by pie electrons cloud present in double bond.
R – CH = CHCl –> X (not possible) --> R – CH = CH+ + Cl-
The vinyl halides exhibit extreme low reactivity towards nucleophilic substitution and it’s only because of this reason the vinyl halide cannot be used in place of alkyl halides in Friedel Crafts reaction.
Similarly, the aryl halides also cannot undergo these type of reactions due to low reactivity toward nucleophilic substitution reactions. Aryl and vinyl halides cannot get converted into phenols or alcohols, or into ethers, or into amines, or into nitriles as they are unresponsive to these reactions.
The low reactivity of both vinyl and aryl halides towards displacement has been given out to two reasons:
- Differences existing in the sigma bond energies due to differences in hybridisation of carbon
- Delocalisation of electrons due to presence of resonance structure
The resonance structure for aryl and vinyl haldies is considered to be a major concern as far low reactivity is concerned. For aryl halides the kekule structure comprising chlorine joined to carbon by a double bond would bear a positive charge while the ortho and para positions of the benzene ring bear a negative charge. Similarly, vinyl halides also faces the same dilemma. The halogen is joined to carbon by a double bond and bears a positive charge, whereas the other carbon automatically bears a negative charge.
Now the other aryl and vinyl halides would comprise structures exactly analogous to these. Contribution of these resonance structures help stabilise the halo benzenes and vinyl halide molecules and thus give double bond character to carbon chlorine bond. The low reactivity of these halides twoards nucleophilic substitution is partly due to the resonance stabilisation of the halides. In alkyl halides the carbon holding halogen is sp3 hybridised but in both aryl and vinyl halides the carbon is sp2 hybridised. The bond to halogen from carbon is shorter and quite strong as well, which makes the molecule more stable.
The carbon – halogen bonds of aryl and vinyl halides are short which is unusual but that itself makes the bond strength comparatively high. If the cabon – halogen bond in both aryl and vinyl halides has double bond character then it should be shorter than carbon – halogen bonds of alkyl halides.
A bond formed by sp2 orbital overlap should be shorter than a corresponding sp3 orbital. The dipole moments of aryl and vinyl halides are found to be very less. Organic halogen compounds are usually polar in character and displacement of electrons toward the more electronegative element makes these halogens comparatively negative and the corresponding carbon relatively positive. The mobile pie electrons of benzene ring and carbon – carbon double bond are easy to displace and hence aryl and vinyl halides have larger dipole moments than the normal alkyl halides.
In rate determining step of nucleophilic aromatic substitution a nucleophile attaches itself to carbon having the halogen attached to it. This carbon atom becomes tetrahedral and the ring acquires a negative charge. These type of reactions destroys the aromaticity of ring and disrupts resonance between the various forms.