E1 and E2 Reactions

Elimination reactions in organic chemistry

From smore on SN1 and SN2:

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What conditions favor elimination?

  • Primary alkyl haldies + weak nucleophile/strong base
  • secondary alkyl halides + strong Nuc/base AND/OR weak nucleophile/strong base
  • most tertiary alkyl halides (die to steric hindrance)

As you can see, there are two different mechanisms (E1 and E2). Just as in SN1 and SN2, the "1" and "2" represent the number of species present in the rate determining step. A slightly more confusing way to think about it is SN1 and E1 are two steps with an intermediate whereas SN2 and E2 are one-step mechanisms with a transition state.

Weak nucleophiles favor E1

The reason why is that the nucleophile is not involved in the slow step. Once the carbocation is formed, the nucleophile is really acting only as a Lewis base at this point. As can be seen from the flowchart above, in the cases of secondary or tertiary halides favoring the E1 mechanism (see below for mechanism), it is likely to be the solvent that actually causes the alkene to be formed.

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Strong Nucleophiles Favor E2

Primary alkyl halides combined with a weaker nucleophile that is a strong base that is too sterically hindered for substitution will undergo E2.

Secondary alkyl halides will do the same as primary given the same type of nucleophile. Secondary alkyl halides give the widest range of products due the possibility of almost every type of mechanism! See below for more complicated examples.

Tertiary alkyl halides favor elimination as can be seen in the flow chart with stronger nucleophiles favoring the E2 mechanism.

E2 Mechanism with transition state

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Look at whether the alkyl halide is symmetrical or asymmetrical

Two unsymmetrical examples: secondary and tertiary

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An assortment of elimination reactions

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Comments on reactions above

(e) there are actually two products formed: the cis (Z) and the trans (E) isomers

(h) 3-methyl-2-hexene and 3-methyl-3-hexene have geometric isomers as well.

Competition between substitution and elmination

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Mostly a secondary thing

As can be seen in the picture above AND in the flow chart, secondary, especially, and to much lesser extent, tertiary alkyl halides (steric hindrance being a problem for substitution), it is a weaker nucleophile that gives rise to the competition. In reality, both products are formed.
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What can you do to help your understanding?

Chemistry is a subject that must be practiced everyday if possible. Work through the lecture examples stopping the video clips and then restarting to check yourself. Take advantage of the practice in Mastering Chemistry to give you the practice you need to be successful. DO NOT PROCRASTINATE! Check announcements on Blackboard (Bishop State) or Sakai (USA) everyday. Keep a printed copy of the most recent course calendar (included in syllabus) next to your work area. Email me with questions!!