Once that happens then the normal E1 mechanism takes place, viz., (1) the leaving group leaves, (2) the solvent grabs a beta-hydrogen, and (3) the bond collapses into a double bond. It will donate a proton to the $OH$ converting it from a really lousy leaving group ($OH$) to a good one ($HOH$). See Chad's Prep, 7.3 (~0.5 min).ĭehydration is the removal of water, i.e., $H$ and $OH$.
The resulting carbocation would be too unstable. (3) You will not get SN1 reaction with vinyl or aryl halides. That is essentially what happens with, for example, an iodide nucleophile (which is a large atom with electrons relatively far away from the nucleus). A “big fluffy pillow” is more easily squeezed into a small space than a cardboard box of the same size. Polarizability of the nucleophile is also related to this. But what if the nucleophile is very large? Then there will be some steric hindrance even though we generally assume that SN2 backside attack will occur with methyl and primary substrates under “normal conditions.” This just goes to show that the size of both the substrate and the nucleophile matter. Normally, a nucleophile can do backside attack against, for example, methylbromide. (2) The “strength” of a nucleophile is also related to its size. It is pretty safe to assume that any anion (atom or molecule with negative formal charge) is a strong nucleophile. Why? Because it ionizes leaving $Na^+$, a spectator ion, and $CN^-$ cyanide anion, a strong nucleophile. When dissolved in acetone it is a strong nucleophile. Example: $NaCN$ is a metal ($Na$) bonded to a non-metal $C$). (1) Metal bonded to a non-metal is an ionic bond therefore it will ionize in solution. Bulky substrates (like tertiary) tend towards E2 non-bulky (like primary) tend towards SN2.Ī bulky base is not a good nucleophile since it cannot squeeze close into the carbon, so it tends towards E2.Ī discussion of the above points: Chad's Prep, 7.7 (~4 min).Ī discussion of the “anti-periplanar” requirement for E2 reactions: Chad's Prep, 7.5 E2 Reactions (~4:50 min) These are $OH^-$ or substance with a negative charge on an oxygen ($RO^-$). If you have a negative charge on anything other than oxygen it is probably just a strong nucleophile/weak base → SN2.Ī strong nucleophile/strong base results in SN2-E2 reactions (that compete). These are nucleophiles that have a negative charge (except on oxygen) like the halides or contain sulfur, nitrogen or phosphorus. If you have a strong nucleophile that is a weak base, you are probably dealing with SN2 only.
If you have weak nucelophile/base you're probably dealing with SN1 & E1 and you'll get a mixture of the two. to leave creating a carbocation in 1st (slow) step then the solvent (weak base) strips off a β hydrogen forming a double bond any substrate except 1° (too unstable)
in one step any substrate (unhindered or not) Strong base deprotonates substrate forming pi bond and kicking off L.G. to leave creating a carbocation in 1st (slow) step then the weak nucleophile attacks the carbon relatively hindered substrate Strong nucleophile does backside attack on substrate in one step relatively unhindered substrate Here is a summary chart in a different format that you might find helpful. Usu only moves to adjacent carbon that is more substituted *aprotic is better but does not matter (DNM) SN1: solvent & nuc: usually the same (“solvolysis”) SN2, E2 - “stronger with 2 arms” SN1: carbocation stability controls (steric hindrance prevents backside attack) SN2: backside attack E1 primary OK (not forming a carbocation) SN1: step 3 $O$ has only one lone pair & has +1 formal charge, quite acidic (similar to $H_3O^+$), donates $H$ to another solvent molecule
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