Elimination Reactions 1

Forty Q&A flashcards covering definitions, mechanisms, energetics, solvent effects, selectivity, and practical outcome predictions for E1 and E2 elimination reactions.

What general change occurs during an elimination reaction?

Elements such as HX or H₂O are removed from adjacent carbons of a substrate to create a new π bond.

In most elimination reactions, what class of compounds acts as the typical substrate and what class as the product?

Alkyl halides are the typical substrates, and alkenes are the products.

What is the elimination of HX from an alkyl halide called?

Dehydrohalogenation.

According to Zaitsev’s rule, which β-hydrogen is preferentially removed in an elimination?

The hydrogen on the more highly branched β-carbon, giving the more substituted alkene.

Under what two conditions is the Hofmann alkene favored over the Zaitsev alkene?

When the base is large and/or the leaving group is bulky.

Name the two main mechanistic types of elimination reactions.

E1 (unimolecular elimination) and E2 (bimolecular elimination).

How many steps are in an E1 mechanism, and what key intermediate is formed?

Two steps; a carbocation intermediate is formed after the leaving group departs.

Describe the basic sequence of events in an E2 reaction.

In one concerted step the base removes a β-hydrogen, the π bond forms, and the leaving group departs.

Which step is rate-determining in an E1 reaction?

The first step—departure of the leaving group to form the carbocation.

How many energy barriers appear on an E2 reaction coordinate diagram?

Only one, because the mechanism is concerted.

Why is an E1 reaction called “unimolecular”?

The rate-determining step involves only one molecule (the substrate) in the transition state.

Why is an E2 reaction called “bimolecular”?

The rate-determining step involves both the substrate and the base in the transition state.

How does increasing alkyl substitution at the carbon bearing the leaving group affect the rate of an E1 reaction?

It increases the rate because more substituted carbocations are more stable.

How does increasing substitution on the α-carbon affect E2 versus SN2 reactions?

E2 rates increase with more substitution, whereas SN2 rates decrease due to steric hindrance.

What relative base strength favors E2 rather than E1?

Strong, negatively charged bases (e.g., HO⁻, RO⁻) favor E2.

What solvent type best promotes E1 reactions, and why?

Polar protic solvents (water, ethanol, methanol, acetic acid) stabilize the carbocation and act as weak bases for proton removal.

Why do polar aprotic solvents accelerate E2 reactions?

They poorly solvate anions, leaving the base “naked” and more reactive.

Regarding regioselectivity, which alkene does an E2 reaction usually favor?

The more substituted (Zaitsev) alkene.

Regarding stereoselectivity, which geometric isomer is usually preferred in E2?

The trans (E) alkene is favored over the cis (Z) alkene.

What specific spatial arrangement of C–H and C–X bonds is required for a fast E2 reaction?

Antiperiplanar geometry (staggered conformation).

In antiperiplanar geometry, why are the bonds aligned that way for E2?

It keeps electron-rich bonds farther apart and allows optimal orbital overlap for forming the π bond.

In an SN1 versus E1 comparison, what happens to the carbocation in each pathway?

SN1: nucleophile attacks the carbocation; E1: base abstracts a β-proton to form the alkene.

Write the rate law for an E1 reaction.

Rate = k[substrate].

Write the rate law for an E2 reaction.

Rate = k[substrate][base].

Why is an E1 reaction considered entropically favorable?

It produces more molecules (carbocation + leaving group, then alkene + protonated base) than the reactant count.

Which elimination mechanism (E1 or E2) forms a discrete carbocation intermediate?

E1.

Which elimination mechanism proceeds in a single concerted step?

E2.

Why do polar protic solvents slow E2 reactions?

They solvate and “hide” the base, reducing its strength and reactivity.

Define the Zaitsev product.

The more substituted, more stable alkene formed in an elimination reaction.

Define the Hofmann product.

The less substituted alkene obtained under conditions favoring bulky bases or bulky leaving groups.

When is the Hofmann product most often observed in E2 reactions?

When bulky, non-nucleophilic bases (e.g., t-BuO⁻) or bulky leaving groups are involved.

During an E2 mechanism, what happens to the leaving group’s electron pair?

It departs with the leaving group, breaking the C–X bond as the π bond forms.

Give two examples of strong, negatively charged bases that promote E2 reactions.

Hydroxide ion (HO⁻) and alkoxide ions (RO⁻ such as EtO⁻).

Which mechanism(s) dominate for primary alkyl halides with strong nucleophiles?

SN2 predominates; E2 competes only with strong, bulky bases.

Why do primary alkyl halides rarely undergo E1 reactions?

They would form highly unstable primary carbocations.

Which mechanisms are available to tertiary alkyl halides, and which is excluded?

SN1, E1, and E2 are possible; SN2 is excluded due to steric hindrance.

Name three good nucleophiles that are weak bases and therefore favor substitution over elimination.

I⁻, Br⁻, and CN⁻ (also CH₃COO⁻).

Name two bulky, non-nucleophilic bases that drive elimination over substitution.

DBU and DBN (or tert-butoxide, KOC(CH₃)₃).

Which factor is pivotal in deciding between E1 and E2 pathways for a secondary substrate?

Base strength—strong bases give E2; weak bases allow E1 (usually mixed with SN1).

As the number of R groups on the α-carbon increases, what trend do you see for E2 and SN2 rates?

E2 rate increases, SN2 rate decreases.