Dr. Anand S. Burange

ZIEGLER –NATTA AND STEREOCHEMICAL CONTROL IN POLYMERIZATION Anand S. Burange, Wilson College, Mumbai asgburange@gmail.com Introduction The traditional route for the synthesis of LDPE (Low Density Polyethylene) requires very high pressure i.e. nearly 20,000 psi and high temperatur e (500 o C). These stringent reaction conditions have safety issues. But these severe conditions can be overcome by Ziegler-Natta catalyst where polymerization takes place at comparatively lower reaction temperature and pressure . The catalyst is also useful to synthesize HDPE which is composed of long linear chain that pack closely together to form rigid polymer. There are various Ziegler-Natta catalysts reported so far. The most commonly used, is the combination of TiCl 4 and triethylaluminium (Et 3 Al). The catalyst must be handled with care because it catches fire on contact with air. Before 1953, isotactic and syndiotactic addition polymers were unknown.  Karl Ziegler, Germany and...

BAEYER STRAIN THEORY AND STRAIN Anand S. Burange, Wilson College, Mumbai asgburange@gmail.com. Small rings like cyclopropane and cyclobutane have larger values of heat of combustion per –CH 2 - group compared to other larger rings. The answer to this observation lies in a theory proposed by Adolf Von Baeyer from University of Munich in 1885. His theory is also known as Baeyer’s Strain Theory but all the aspects discussed in theory are not completely accepted as some of them were proven wrong in the course of history. Now a days this theory, is modernized and then discussed for the better understanding of students/researchers. Baeyer’s Strain Theory Assumptions: Baeyer in his theory, he did few arguments where he failed to convince many aspects while some of them are well accepted. Few important arguments from his theory are as follows. # Argument 1 In cyclopropane and cyclobutane, all carbon atoms are SP 3 hybridized therefore expected bond angle (C-C-C bond angle) s...

CYCLOHEXANE STEREOCHEMISTRY Anand S. Burange, Wilson College, Mumbai asgburange@gmail.com Cyclohexane is an industrially important molecule mainly used for the production of epsilon caprolactum and adipic acid which are precursors to Nylon. Cyclohexane has a molecular formula C 6 H 12 and has a cyclic structure. At room temperature it exists as a liquid with boiling point 80.74 o C. Generally, representation of cyclohexane in a line structure form is shown as a hexagon but practically it is not the case. Cyclohexane has six C-C bonds and due to cyclic structure there is a restricted bond rotation. Rotations of bonds lead to the various conformations. The most important conformations of cyclohexane are enlisted below. Conformations of Cyclohexane 1. Boat 2. Chair 3. Twist boat 4. Twist Chair Boat Conformation In a boat conformation, four carbon atoms lie in a plane while remaining two carbons atoms (C1 and C4) are out of plane (above or below) but ...

Reactive intermediate: CARBOCATION Anand S. Burange, Wilson College, Mumbai asgburange@gmail.com The reaction intermediates formed during reaction and reactive intermediates are totally different from each other. Let us consider the example of oxidation of primary alcohols using permanganate or dichromate yields carboxylic acid where aldehyde is an reaction intermediates formed during the reaction and which is isolable and can be detected if we cease/quench the reaction earlier before the over oxidation. Whereas reactive intermediates are highly unstable, have very short life span, mostly non-isolable but they are the part of the reaction mechanism. Till date well known reactive intermediates mainly include carbocation, carbanion, carbene, nitrene, aryne, and radical. Stability of these reactive intermediates governs the rate of the reaction. Carbocation : Carbon is tetravalent (covalency= 04); the species where carbon is trivalent bearing one vacant p orbital is called as c...