Elimination reaction

"Just as the chemistry of alkenes is dominated by addition reactions, the preparation of alkenes is dominated by elimination reactions. Additions and eliminations are, in many respects, two sides of the same coin."

"Unhindered primary alkyl substrates always react in a bimolecular way and almost always give predominantly substitution products, except when sterically hindered strong bases, such as potassium tert-butoxide, are employed. In these cases, the SN2 pathway is slowed down sufficiently for steric reasons to allow the E2 mechanism to take over. Another way of reducing substitution is to introduce branching. However, even in these cases, good nucleophiles still furnish predominantly substitution products. Only strong bases, such as alkoxides, RO-, or amides, R2N-, tend to react by elimination."

"Secondary alkyl systems undergo, depending on conditions, both eliminations and substitutions by either possible pathway: uni- or bimolecular. Good nucleophiles favor SN2, strong bases result in E2, and weakly nucleophilic polar media give mainly SN1 and E1."

"Tertiary systems eliminate (E2) with concentrated strong base and are substituted in nonbasic media (SN1). Bimolecular substitution is not observed, but elimination by E1 accompanies SN1."

"What is the mechanism of the chromium(VI) oxidation of alcohols? The first step is formation of an intermediate called a chromic ester; the oxidation state of chromium stays unchanged in this process. … The next step in alcohol oxidation is equivalent to an E2 reaction. Here water (or pyridine, in the case of PCC) acts as a mild base, removing the proton next to the alcohol oxygen. This proton is made unusually acidic by the electron-withdrawing power of the Cr(VI) (remember that it wants to become reduced!). HCrO3 is an exceptionally good leaving group, because the donation of an electron pair to chromium changes its oxidation state by two units, yielding Cr(IV)."

"Another mode of reactivity of carbocations, in addition to regular SN1 and E1 processes, is rearrangement by hydride or alkyl shifts. In such rearrangements, the migrating group delivers its bonding electron pair to a positively charged carbon neighbor, exchanging places with the charge. Rearrangement may lead to a more stable cation — as in the conversion of a secondary cation into a tertiary one. Primary alcohols also can undergo rearrangement, but they do so by concerted pathways and not through the intermediacy of primary cations."