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Western University
Chemistry 1027A/B
Mel Usselman

October 22, 2012 Organic Chemistry Lecture Notes - No tutorial this week - Tutorial starts again next week E. Beta Elimination Reactions - C-LG can undergo elimination reaction in addition to the substitution reactions already discussed - Strong nucleophile base running around looking for a C+ - Now looking at a molecule that has both a C+ and a hydrogen that runs into either one of them or both of them that’s what give s you a beta elimination reaction - In elimination reactions, the Nu: Acts as a base (then denoted by B) and attacks a H instead of attacking C as a nucleophile o Because of the bad aim of the species with the lone pair – sometimes get substitution/elimination - Leaving groups are always delta minus and the carbon is a delta plus - Because the H atom and the LG group come off C atoms that are adjacent (often labeled alpha and Beta) this reaction is called a B elimination) - In many cases elimination reactions and substitution reactions occur together - All species considered earlier as nucleophiles can also act as bases- the only difference is whether the species attacks a carbon or a base o The only difference is where it comes into the molecule - We classify a group as a nucleophile or base depending on how it reacts, not on what its structure is - A large anion which can attack a more exposed H atom better than the sheltered C will normally give more elimination that substitution - A stable alkene as a product will also proceed through an elimination reaction - The more alkyl groups on the double bonds- the more stable that double bond - The reactions occur together o Will ask on the exam which mixture will favor elimination or substitution wont ask for exact ratio - Zaitsev’s rule: the alkene with the most alkyl groups will form in the greater amount o E.g. This is an alkyl halide  The LG is bromine – which is good  The anion in this example is large(CH3)3CO-K+)  Is it a skinny Nu base or a big one base?  It is a big one  Therefore the product will most likely be elimination - Elimination always occurs with substitution- always (might only be like 1% but it always happens) - There are two different types of elimination reactions o They are almost identical to the 2 substitution reactions that we already know - E2 mechanism: o Similar to SN2 o One step o Double collision o Elimination is on the Beta carbon to where the hydrogen is o Whenever you have SN2 you have E2 o The transition state requires the concentration of both the LG and the base- this is why it is considered E2 o Predominates when you have a large bulky base or when you have a super stable double bond - E1 Mechanism: o The nucleophile base hits the hydrogen next door to the carbon o Similar to SN1 o Whenever you have SN1 you also have E1 o But when told which type of reaction is occurring you should be able
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