SN1 SN2 E1 E2 Reaction Mechanism - Test Review

This paragraph introduces the topic of SN1, SN2, E1, and E2 reactions, emphasizing the importance of understanding these mechanisms for students preparing for tests on the subject. The speaker recommends watching another video for an overview and then delves into the specifics of the SN2 reaction, explaining why methyl and primary alkyl halides are more reactive in SN2 reactions than tertiary alkyl halides. The paragraph also discusses the difficulty a nucleophile faces when approaching a tertiary carbon due to steric hindrance from methyl groups.

The second paragraph focuses on solvolysis, a process typically associated with SN1 reactions, where the solvent acts as a nucleophile. The speaker explains the preference for tertiary alkyl halides in SN1 reactions due to the stability of tertiary carbocations. The paragraph also covers the identification of the correct substrate for an SN1 reaction and the concept of resonance stabilization in carbocations, using a benzylic tertiary alkyl halide as an example. The speaker then discusses the major substitution product of a reaction between a secondary alkyl halide and a protic solvent, highlighting the competition between SN2, SN1, and E1 reactions.

In this paragraph, the speaker discusses the trend of nucleophile strength in polar protic versus polar aprotic solvents. It explains that in a polar protic solvent, the strength of the nucleophile increases towards iodide, while in a polar aprotic solvent, fluoride is the strongest nucleophile. The paragraph uses the example of fluoride in water being solvated and stabilized, which makes it less effective as a nucleophile compared to iodide. The speaker also explains the concept of a racemic mixture of stereoisomers and how the mechanism of the reaction (SN1 or E1) can affect the product formed.

The fourth paragraph focuses on the rate law expressions for E1 reactions. The speaker explains that the overall order of an E1 reaction is second order, being first order with respect to the substrate and first order with respect to the base. The paragraph clarifies that E1 reactions are elimination reactions that use bases, unlike SN1 and SN2 reactions which use nucleophiles. It also discusses the difference between nucleophiles and bases, and how ethanol acts as both in the solvolysis reaction of a secondary carbocation.

This paragraph delves into the stereochemistry changes that occur during SN2 reactions, noting that these reactions proceed with inversion of stereochemistry. The speaker uses the example of a reaction between R2 bromobutane and potassium iodide in acetone to illustrate this point. The paragraph also addresses the characteristics of the essential SN2 reaction as a concerted reaction mechanism, where bond formation and bond breaking occur simultaneously. It corrects the misconception that SN2 reactions involve a carbocation intermediate, which is not the case, and explains that rearrangements do not occur during SN2 reactions.

The sixth paragraph discusses the role of polar protic and aprotic solvents in influencing reaction mechanisms. The speaker identifies various solvents as polar aprotic or protic and explains how these solvents affect SN1 and SN2 reactions. The paragraph also provides examples of crown ethers and their ability to solvate cations, thereby freeing up nucleophiles to react. The speaker concludes by identifying acetic acid as a polar protic solvent that favors SN1 reactions due to its high polarity and the presence of an OH group.

In this paragraph, the speaker ranks protic solvents based on their polarity and effectiveness in SN1 reactions. Water is identified as the most polar and reactive solvent for SN1 reactions, with the reactivity increasing as the number of carbon-hydrogen bonds decreases. The speaker provides a ranking of the solvents from most to least reactive for SN1 reactions: water, methanol, ethanol, and hexane. The paragraph emphasizes the importance of understanding the polarity of solvents and their impact on reaction mechanisms.

The speaker discusses the reagents needed to synthesize 2-methoxybutane from 2-chlorobutane. The paragraph explains why sodium methoxide and potassium ethoxide are not suitable reagents, as they would lead to an E2 reaction rather than the desired substitution reaction. The speaker then identifies methanol as the correct reagent, which can act as both a solvent and a nucleophile in a solvolysis reaction, leading to the formation of 2-methoxybutane through an SN1 mechanism.

This paragraph focuses on predicting the major product of an E2 reaction between a secondary alkyl halide and potassium terpetoxide in tert-butanol. The speaker explains that the bulky nature of the terpetoxide base will lead to the formation of the less stable alkene, known as the Hofmann product, due to the steric hindrance it causes. The paragraph contrasts this with the use of sodium hydroxide, a smaller and less hindered base, which would lead to the formation of the more stable Zaitsev product. The speaker then discusses the stability of alkenes and how the number of substituent groups on the double-bonded carbon atoms affects this stability.

The speaker discusses the factors that determine the stability of alkenes formed in E1 reactions. The paragraph explains that the more substituent groups (R groups) attached to the double-bonded carbon atoms, the more stable the alkene. The speaker identifies the most stable alkene as the one with four R groups (tetra-substituted alkene) and uses this principle to predict the major elimination product of the given reaction. The paragraph concludes with the identification of the correct answer based on the stability of the potential alkene products.

In this paragraph, the speaker outlines the mechanism of an acid-catalyzed E1 alcohol dehydration reaction. The speaker identifies the steps involved in the mechanism, starting with the protonation of the alcohol by the acid, followed by the formation of a carbocation intermediate. The paragraph then discusses the role of water as a weak base, which abstracts a proton leading to the formation of the alkene product. The speaker clarifies which steps occur in the actual E1 mechanism and correctly identifies the step that does not occur, which involves water acting as a nucleophile and reacting with the carbocation.

The speaker ranks different carbocations by their stability, starting with the most stable. The paragraph explains that tertiary allylic carbocations are more stable than regular tertiary carbocations due to resonance stabilization. It then places tertiary carbocations above secondary carbocations, which in turn are more stable than primary carbocations. The speaker concludes by providing the correct order of carbocations from most stable to least stable, which corresponds to answer choice C.