SN2 vs. E2
SN2 and E2 reactions share a number of similarities. Both require good leaving groups, and both mechanisms are concerted. SN2 reactions require a good nucleophile and E2 reactions require a strong base. However, a good nucleophile is often a strong base. Since the two reactions share many of the same conditions, they often compete with each other. The outcome of the competition is determined by three factors: the presence of antiperiplanar β -hydrogens, the degree of α and β branching, and the nucleophilicity vs. basicity of the reactant species.
In order for an E2 elimination to occur, there must be antiperiplanar β -hydrogens to eliminate. If there are none, the SN2 reaction will dominate. On the same token, the SN2 nucleophile needs a free path to the σ * C-LG antibond. α and β branching block this path and reduce the proportion of SN2 relative to E2 . E2 occurs even with extensive branching because it relies on the β -hydrogens, which are much more accessible than the σ * C-LG antibond.
The identity of the nucleophile or base also determines which mechanism is favored. E2 reactions require strong bases. SN2 reactions require good nucleophiles. Therefore a good nucleophile that is a weak base will favor SN2 while a weak nucleophile that is a strong base will favor E2. Bulky nucleophiles have a hard time getting to the α-carbon, and thus increase the proportion of E2 to SN2. Polar, aprotic solvents increase nucleophilicity, and thus increase the rate of SN2.
SN2
Requires an unhindered path to the back of the α carbon
Requires a good nucleophile
Polar, aprotic solvents increase nucleophilicity
Bulky groups on the nucleophile decrease nucleophilicity α and β branching block the path and hinder SN2

E2
Requires an antiperiplanar β –hydrogen (has staggered conformation with lower energy) antonym: synperiplanar
Enhanced by α and β -branching
Requires a strong base

(http://www.sparknotes.com/chemistry/organic4/sn2e2/section3.rhtml)

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(http://www.chem.sc.edu/faculty/shimizu/333/Chem_333/6a.ii.html)
SN2 and E2 Competition – One Step Concerted Reactions

SN2 and E2 reactions are one step reactions. The key bonds are broken and formed simultaneously, without any intermediate structures. These are referred to as concerted reactions. The SN2 and E2 mechanisms compete with one another in consuming the R-X compound. Approach of the nucleophile/base is always from the backside in SN2 reactions and mainly from the backside in E2 reactions (always from the backside in this chapter).

H Cβ R3 R2 Cα X R1 H B Nu E2 approach (usually from the backside in an "anti" conformation, "syn" is possible, but not common) SN2 approach (always from the backside, resulting in inversion of configuration = very specific stereochemistry) B ≈ Nu strong base ≈ strong nucleophile One step, concerted reactions, from the backside. Nu Cα Cβ H R3 R2 R1 H SN2 product (a nucleophile substitutes for a leaving group) E2 product (a pi bond forms with very specific stereochemistry) Cβ Cα R3 R2 H R1 X

Terms S = substitution E = elimination Nu: = strong nucleophile B: = strong base X = leaving group 2 = bimolecular kinetics (second order, rate in slow step depends on RX and Nu: /B: ) R-X = R-Cl, R-Br, R-I, R-OTs, ROH2 + stable leaving group

Relative rates of SN2 and SN1 reactions with different substitution patterns at Cα and Cβ of the R-X structure

Cβ CH2 H H H X Cβ CH2 H CH3 H X Cβ CH2 CH3 CH3 H X k 1 ethyl k 0.4 propyl k 0.03 2-methylpropyl k 0.00001 2,2-dimethylpropyl (neopentyl) All of these structures are primary R-X compounds at Cα, but substituted differently at Cβ. Reference compound Cα H H H X Cα H H CH3 X Cα CH3 H CH3 X Cα CH3 CH3 CH3 X k 0 t-butyl (tertiary) k 30 k 0.025 methyl (unique) k 1 ethyl (primary) Reference compound

Relative Rates of SN2 Reactions - Steric hindrance at the Cα carbon slows down the rate of SN2 reaction. 1 40 (very low) isopropyl (secondary)

Relative Rates of SN2 Reactions - Steric hindrance at the Cβ carbon also slows down the rate of SN2 reaction. H3C X CH3CH2 X CH X H3C H3C C X CH3 H3C CH3 10-5 10-4 1.0 106 these two rates are = 1,000,000 probably by SN2 reaction relative rates =

SN1 (and E1) relative reactivities of R-X compounds: R-X R SN1 > E1 Reference compound (relative SN1/E1 reactivity: methyl << primary << secondary < tertiary) (usually) Cβ CH2 CH3 CH3 CH3 X (B: = Nu: = strong) (B: = Nu: = strong)

(http://www.cpp.edu/~psbeauchamp/pdf/314_bare_bones_SN_E.pdf)
Pg 2

Nucleophilic Substitution Reactions - SN2 Reaction:

• Reaction is: o Stereospecific (Walden Inversion of configuration) o Concerted - all bonds form and break at same time o Bimolecular - rate depends on concentration of both nucleophile and substrate

• Substrate: o Best if primary (one substituent on carbon bearing leaving group) o works if secondary, fails if tertiary

• Nucleophile: o Best if more reactive (i.e. more anionic or more basic)

• Leaving Group: Best if more stable (i.e. can support negative charge well): o TsO- (very good) > I- > Br- > Cl- > F- (poor) o RF , ROH , ROR , RNH2 are NEVER Substrates for SN2 reactions o Leaving Groups on double-bonded carbons are never replaced by SN2 reactions

• Solvent: Polar Aprotic (i.e. no OH) is best. o For example dimethylsulfoxide ( CH3SOCH3 ), dimethylformamide ( HCON(CH3)2 ), acetonitrile ( CH3CN ). o Protic solvents (e.g. H2O or ROH) deactivate nucleophile by hydrogen bonding but can be used in some case

Elimination Reactions - E2 Reaction:

• Reaction is: o Stereospecific (Anti-periplanar geometry preferred, Syn-periplanar geometry possible) o Concerted - all bonds form and break at same time o Bimolecular - rate depends on concentration of both base and substrate o Favoured by strong bases

(http://www.chem.ualberta.ca/~vederas/Chem_164/handouts/pdf/sub_elim_rxn.pdf)

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http://www.chemgapedia.de/vsengine/vlu/vsc/en/ch/12/oc/vlu_organik/substitution/sn_e_konkurrenz/sn2_e2_konkurrenz.vlu/Page/vsc/en/ch/12/oc/substitution/sn_e_konkurrenz/a2_2_sn2_e2_base/sn2_e2_base.vscml.html
Substitution generally predominates and elimination occurs only during precise circumstances. Generally, elimination is favored over substitution when:

* steric hindrance around the α-carbon increases. * A stronger base is used. * temperature increases( increase entropy ) * the base is a poor nucleophile. Bases with steric bulk, (such as in Potassium tert-butoxide), are often poor nucleophiles.

In one study [4] the kinetic isotope effect (KIE) was determined for the gas phase reaction of several alkyl halides with the chlorate ion. In accordance with an E2 elimination the reaction with t-butyl chloride results in a KIE of 2.3. The methyl chloride reaction (only SN2 possible) on the other hand has a KIE of 0.85 consistent with a SN2 reaction because in this reaction type the C-H bonds tighten in the transition state. The KIE's for the ethyl (0.99) and isopropyl (1.72) analogues suggest competition between the two reaction modes.

https://en.wikipedia.org/wiki/Elimination_reaction
Stephanie M. Villano, Shuji Kato, and Veronica M. Bierbaum (2006). "Deuterium Kinetic Isotope Effects in Gas-Phase SN2 and E2 Reactions: Comparison of Experiment and Theory". J. Am. Chem. Soc. 128 (3): 736–737. doi:10.1021/ja057491d. PMID 16417360

FACTORS AFFECTING Sn2

Substrate[edit]
The substrate plays the most important part in determining the rate of the reaction. This is because the nucleophile attacks from the back of the substrate, thus breaking the carbon-leaving group bond and forming the carbon-nucleophile bond. Therefore, to maximise the rate of the SN2 reaction, the back of the substrate must be as unhindered as possible. Overall, this means that methyl and primary substrates react the fastest, followed by secondary substrates. Tertiary substrates do not participate in SN2 reactions, because of steric hindrance. Structures that can form highly stable cations by simple loss of the leaving group, for example, as a resonance-stabilized carbocation, are especially likely to react via an SN1 pathway in competition with SN2.

Nucleophile[edit]
Like the substrate, steric hindrance affects the nucleophile's strength. The methoxide anion, for example, is both a strong base and nucleophile because it is a methyl nucleophile, and is thus very much unhindered. tert-Butoxide, on the other hand, is a strong base, but a poor nucleophile, because of its three methyl groups hindering its approach to the carbon. Nucleophile strength is also affected by charge and electronegativity: nucleophilicity increases with increasing negative charge and decreasing electronegativity. For example, OH− is a better nucleophile than water, and I− is a better nucleophile than Br− (in polar protic solvents). In a polar aprotic solvent, nucleophilicity increases up a column of the periodic table as there is no hydrogen bonding between the solvent and nucleophile; in this case nucleophilicity mirrors basicity. I− would therefore be a weaker nucleophile than Br− because it is a weaker base. Verdict - A strong/anionic nucleophile always favours SN2 manner of nucleophillic substitution.

Solvent[edit]
The solvent affects the rate of reaction because solvents may or may not surround a nucleophile, thus hindering or not hindering its approach to the carbon atom. Polar aprotic solvents, like tetrahydrofuran, are better solvents for this reaction than polar protic solvents because polar protic solvents will hydrogen bond to the nucleophile, hindering it from attacking the carbon with the leaving group. A polar aprotic solvent with low dielectric constant or a hindered dipole end will favour SN2 manner of nucleophillic substitution reaction. Examples: DMSO, DMF, acetone etc. In polar aprotic solvent, nucleophilicity parallels basicity.

Leaving group[edit]
The stability of the leaving group as an anion and the strength of its bind to the carbon atom both affect the rate of reaction. The more stable the conjugate base of the leaving group is, the more likely that it will take the two electrons of its bond to carbon during the reaction. Therefore, the weaker the leaving group is as a conjugate base, and thus the stronger its corresponding acid, the better the leaving group. Examples of good leaving groups are therefore the halides (except fluoride, due to its strong bond to the carbon atom) and tosylate, whereas HO− and H2N− are not.

A common side reaction taking place with SN2 reactions is E2 elimination: the incoming anion can act as a base rather than as a nucleophile, abstracting a proton and leading to formation of the alkene. This is more common when the incoming ion is sterically hindered in which case abstracting a proton is much easier. Elimination reactions are usually favoured at elevated temperatures[3] because of increased entropy. This effect can be demonstrated in the gas-phase reaction between a sulfonate and a simple alkyl bromidetaking place inside a mass spectrometer:[4][5]

With ethyl bromide, the reaction product is predominantly the substitution product. As steric hindrance around the electrophilic center increases, as with isobutyl bromide, substitution is disfavored and elimination is the predominant reaction. Other factors favoring elimination are the strength of the base. With the less basic benzoate substrate, isopropyl bromide reacts with 55% substitution. In general, gas phase reactions and solution phase reactions of this type follow the same trends, even though in the first, solvent effects are eliminated.

Wiki:Sn2 reaction http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch08/ch8-0.html https://www.ncbi.nlm.nih.gov/gquery/?term=substitution+vs+elimination

https://books.google.com.my/books?id=FVXC2Ky792wC&pg=PA56&dq=elimination+vs+substitution&hl=en&sa=X&redir_esc=y#v=onepage&q=elimination%20vs%20substitution&f=false

Elimination vs Substitution Reaction. A Dichotomy between Brønsted–Lowry and Lewis Basicity
Francisco Méndez, Arlette Richaud, and Julio A. Alonso
Org. Lett., 2015, 17 (4), pp 767–769
Publication Date (Web): January 29, 2015 (Letter)
DOI: 10.1021/ol5034628

Ortho-Substitution Rearrangement vs. Elimination Reaction of Certain Benzyl-Type Quaternary Ammonium Ions with Sodium Amide1
Frank N. Jones, Charles R. Hauser
J. Org. Chem., 1962, 27 (5), pp 1542–1547
Publication Date: May 1, 1962 (Article)
DOI: 10.1021/jo01052a012

http://pubs.acs.org/action/doSearch?AllField=substitution+vs+elimination&type=within&publication=

http://www.masterorganicchemistry.com/2012/05/31/walkthrough-of-substitution-reactions-1-introduction/