Arrow Pushing: Mechanisms A few rules to follow: X H H 1) Arrows show the movement of ELECTRONS! HO HO They go from site of high electron density, usually a lone pair or alkene, to sites of low electron density, Correct OTs Incorrect OTs usually a cation, hydrogen, or unfilled orbital. H H H H 2) Balance the equation. Be sure to conserve mass and charge. If you start with an over charge of +1, +1 Incorrect 0 you need to end with an overall charge of +1. +1 Correct +1 Br Br CN Br CN Br H H 3) Draw out all intermediates. Include hydrogens, lone CN NC pairs, and charges. Be sure there are no carbons with X Incorrect H 5 bonds or other chemical errors in the intermediates. Correct 5 bonds to C OH Cl Cl HCl 4) The mechanism should give the observed product. + Do not forget about stereochemistry and regiochemistry. This mechanism HAS to go via a SN1 Let's try an example: HO cat. TsOH Given this reaction scheme we can determine that this will be an elimination type mechanism, specifically E1. CH HO CH3 2 The reaction scheme may not show all by products! CH CH3 cat. TsOH 3 + H2O Let's draw all hydrogens and balance the equation. H The lone pair on oxygen would be a site of high electron HO CH3 O CH3 O H O density and an acidic hydrogen (TsOH) is a site of low OH + OH CH3 H S CH3 S electron density. Note: TsOH is catalytic so it needs to O O O O be regenerated in our mechanism! H Now that we have made a charged species we want to CH H O CH3 3 find a way to stabalize or neutralize the charge. Water is a good leaving group since oxygen is more CH CH H O 3 3 + 2 electronegative than carbon the positive charge is more stable on a tertiary carbon. H C H OTs CH2 2 The TsO- from before can come back and deprotonate to CH CH3 3 + TsOH neutralize the positive charge by forming a C=C double bond thus regenerating the TsOH and giving the product. Let's put it all together: H OTs CH HO CH O CH H2C H 2 3 H 3 O O + OH OH CH CH3 + TsOH CH3 H S CH3 S 3 O O O O.