Associative and Dissociative Substitution

Associative substitution describes a pathway by which coordination and organometallic complexes interchange ligands. Associative mechanism resembles the SN2 mechanism in organic chemistry. The opposite pathway is dissociative substitution, being analogous to SN1 pathway. Intermediate pathways exist between the pure associative and pure dissociative pathways, these are called interchange mechanisms.


Associative mechanism

• Metal size should be large
• Ligand size should be small
• Incoming group should have pi bonding ability (CN- is a pi acid ligand)
• Rate depend on the nature of nucleophile
• Trigonal bipyramidal intermediate 

    L5MX + Y =   L5MXY        (SLOW)
    L5MXY =    L5MY + X        (FAST)

    Rate = k [L5MX] [Y]
 

Dissociative mechanism

• Metal size should be small
• Ligand size should be large
• Rate does not depend on nature of the nucleophile

    L5MX =  L5M + X   (SLOW)
    L5M + Y =  L5MY   (FAST)

    Rate = k [L5MX]

Zinc Oxide Nanomaterials

Zinc oxide is one of the most important II-VI semiconductor material with direct wide band gap (3.37 eV or 375 nm), good transparency, high electron mobility (>100 cm2/Vs) photoconductivity, strong room-temperature luminescence large exciton and biexciton energies of 60 meV and 15 meV respectively. It has attracted increasing attention due to its excellent optical and electrical properties, inexpensiveness, relative abundance, chemical stability towards air, ability to produce significant quantum confinement effect. 

Since ZnO is an important trace element for humans, it is environment friendly and suitable for in vivo applications. Zinc oxide has high refractive index, high thermal conductivity, antibacterial and UV-protection properties. Consequently, it is added in various materials and products including plastics, ceramics, glass, cement, rubber, lubricants, paints, ointments, adhesives, sealants, pigments, foods, batteries, ferrites, fire retardants etc. and non-toxicity, making it a suitable additive for textiles and surfaces that come in contact with humans.

Nanosized ZnO has immense importance due to its multifunctionality. Thermal stability, irradiation resistance and flexibility to form different nanostructures are the advantages that highlight its promising applications in solar cells and electronic devices, ultraviolet-light detectors and photo diodes and in catalysis.Structures like nanowires, nanobelts and nanorings are of great interest in photonics research, optoelectronics, nanotechnology, and biomedicine. Therefore, the controlled synthesis of various ZnO nanostructures such as nanocrystals, nanowires, nanobelts and other complex nano architectures has been extensively explored.

Zinc oxide nanostructure growth is heavily researched presently. The substance is likely to have the largest variety of nanostructures (and their associated properties) among all known materials. Its hexagonal lattice can easily match catalyst lattice structure and facilitate controlled growth patterns. Positive zinc surfaces and negative oxygen surfaces create electric dipoles that facilitate polarization growth along certain directions and planes under applied voltage and temperature. Different techniques such as sol-gel, spray pyrolysis, thermal evaporation, wet chemical processes etc are used for the synthesis of ZnO nanomaterials.

ZnO quantum dots exhibit emission bands in the ultraviolet and visible regions as shown by its photoluminescence spectra. The UV emission band at around 370 nm is usually attributed to the interband transition or the exciton combination in ZnO . Even though the emissions in the visible region are associated with the electronic defects due to surface states or trapping effects in the QDs, there are still
controversies related to the unambiguous electron transitions. In the visible region, the blue emission in ZnO QDs has considerable importance in biological fluorescence labeling. Several studies indicate that ZnO is one of the most efficient oxide-based phosphors in both photoluminescence (PL) and electroluminescence (EL).

Considerable studies have been done on the properties of metal incorporated fluorescent materials like Co, Ni and Fe doped ZnO aiming to develop efficient magnetic semiconductors. The properties of inner-transition metal doped ZnO nanoparticles have also been subjected to tremendous amount of studies. The combination of magnetic and optical properties provides the composites significantly important applications in biomedical fields including drug targeting, bioseparation and diagnostic analysis.

ENDOSULFAN

Endosulfan is an organochlorine insecticide and acaricide with a cyclodiene sub-group. It is highly toxic and can  bioaccumulate in organisms. It can also act as an endocrine disruptor i.e., it interferes with the synthesis, secretion, transport, binding, action, or elimination of natural hormones in the body that are responsible for the maintenance of homeostasis (normal cell metabolism), reproduction, development, and behavior.

Endosulfan has been used in agriculture around the world to control insect pests including whiteflys, aphids, leafhoppers, Colorado potato beetles and cabbage worms. Because of its non-specificity it impacts many beneficial insects also. It is also used as a wood preservative.

Comercial names : Beosit, Thiodan, Cyclodan, Malix, Thifor, Endocide etc.

Specifically, it is produced by the Diels-Alder reaction of hexachlorocyclopentadiene with cis-butene-1,4-diol and subsequent reaction of the adduct with thionyl chloride.Technical endosulfan is a 7:3 mixture of stereoisomers, designated α and β. α- and β-endosulfan are conformational isomers arising from the pyramidal stereochemistry of sulfur. α-Endosulfan is the more thermodynamically stable of the two, thus β-endosulfan irreversibly converts to the α form, although the conversion is slow.

Endosulfan breaks down into endosulfan sulfate and endosulfan diol, both of which, according to the EPA, have "structures similar to the parent compound and are also of toxicological concern. Since it neither  dissolves in water easily nor stick to soil particles readily, its transport to other regions is easier.

India the world's largest user of endosulfan, and a major producer with three companies—Excel Crop Care, H.I.L., and Coromandal Fertilizers—producing 4,500 tonnes annually for domestic use and another 4,000 tonnes for export.


In 2001, in Kerala, India, endosulfan spraying became suspect when linked to a series of abnormalities noted in local children. Initially endosulfan was banned, yet under pressure from the pesticide industry this ban was largely revoked.

Endosulfan is acutely neurotoxic to both insects and mammals, including humans.  Symptoms of acute poisoning include hyperactivity, tremors, convulsions, lack of coordination, staggering, difficulty breathing, nausea and vomiting, diarrhea, and in severe cases, unconsciousness. Doses as low as 35 mg/kg have been documented to cause death in humans, and many cases of sub-lethal poisoning have resulted in permanent brain damage. Farm workers with chronic endosulfan exposure are at risk of rashes and skin irritation.