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Elimination reaction
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== Competition among mechanisms == The [[reaction rate]] is influenced by the reactivity of [[halogen]]s, [[iodide]] and [[bromide]] being favored. Fluoride is not a good leaving group, so eliminations with fluoride as the leaving group have slower rates than other halogens . There is a certain level of competition between the elimination reaction and [[nucleophilic substitution]]. More precisely, there are competitions between E2 and [[SN2 reaction|S<sub>N</sub>2]] and also between E1 and [[SN1 reaction|S<sub>N</sub>1]]. 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. For example, when a 3° haloalkane is reacts with an alkoxide, due to strong basic character of the alkoxide and unreactivity of 3° group towards S<sub>N</sub>2, only alkene formation by E2 elimination is observed. Thus, elimination by E2 limits the scope of the [[Williamson ether synthesis]] (an S<sub>N</sub>2 reaction) to essentially only 1° haloalkanes; 2° haloalkanes generally do not give synthetically useful yields, while 3° haloalkanes fail completely. With strong base, 3° haloalkanes give elimination by E2. With weak bases, mixtures of elimination and substitution products form by competing S<sub>N</sub>1 and E1 pathways. The case of 2° haloalkanes is relatively complex. For strongly basic nucleophiles (p''K''<sub>aH</sub> > 11, e.g., hydroxide, alkoxide, acetylide), the result is generally elimination by E2, while weaker bases that are still good nucleophiles (e.g., acetate, azide, cyanide, iodide) will give primarily S<sub>N</sub>2. Finally, weakly nucleophilic species (e.g., water, alcohols, carboxylic acids) will give a mixture of S<sub>N</sub>1 and E1. For 1° haloalkanes with β-branching, E2 elimination is still generally preferred over S<sub>N</sub>2 for strongly basic nucleophiles. Unhindered 1° haloalkanes favor S<sub>N</sub>2 when the nucleophile is also unhindered. However, strongly basic and hindered nucleophiles favor E2. In general, with the exception of reactions in which E2 is impossible because β hydrogens are unavailable (e.g. methyl, allyl, and benzyl halides),<ref>In rare cases in which β hydrogens are unavailable but substitution is disfavored, α-elimination to form a carbene can sometimes occur. In particular: (1) Trihalomethanes like chloroform can react with NaOH to form dihalocarbenes (substitution is electronically disfavored). (2) Allyl and benzyl chloride can react with lithium tetramethylpiperide (LiTMP) to form vinylcarbene and phenylcarbene, respectively (substitution is sterically disfavored). </ref> clean S<sub>N</sub>2 substitution is hard to achieve when strong bases are used, as alkene products arising from elimination are almost always observed to some degree. On the other hand, clean E2 can be achieved by simply selecting a sterically hindered base (e.g., potassium ''tert''-butoxide). Similarly, attempts to effect substitution by S<sub>N</sub>1 almost always result in a product mixture contaminated by some E1 product (again, with the exception of cases where the lack of β hydrogens makes elimination impossible).<ref>{{Cite book |last=Carey |first=Francis A. |title=Organic Chemistry |publisher=McGraw-Hill |year=2003 |isbn=0-07-242458-3 |edition=5th |location=New York |pages=350}}</ref> In one study<ref>{{cite journal | title = Deuterium Kinetic Isotope Effects in Gas-Phase SN2 and E2 Reactions: Comparison of Experiment and Theory |author1=Stephanie M. Villano |author2=Shuji Kato |author3=Veronica M. Bierbaum | journal = [[J. Am. Chem. Soc.]] | year = 2006 | volume = 128 | issue = 3 | pages = 736–737 | doi = 10.1021/ja057491d | pmid = 16417360}}</ref> 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 S<sub>N</sub>2 possible) on the other hand has a KIE of 0.85 consistent with a S<sub>N</sub>2 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.
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