Nucleophilic Substitution 2

Fifty question-and-answer flashcards covering the mechanism, kinetics, energetics, stereochemistry, rearrangements, and factors affecting the SN1 nucleophilic substitution reaction.

What does the acronym SN1 stand for in organic chemistry?

Substitution Nucleophilic Unimolecular.

How many elementary steps are involved in an SN1 reaction?

Two steps.

Which step is rate-determining in the SN1 mechanism?

Formation of the carbocation (ionization of the C–X bond).

Does the nucleophile participate in the rate-determining step of an SN1 reaction?

No; only the substrate (alkyl halide) appears in the transition state of the slow step.

Write the rate law for the SN1 hydrolysis of tert-butyl bromide, (CH3)3CBr.

Rate = k[(CH3)3CBr].

What is the overall kinetic order of an SN1 reaction?

First order.

What reactive intermediate is characteristic of SN1 mechanisms?

A carbocation.

What is the hybridization and geometry of a simple carbocation?

sp2-hybridized and trigonal planar.

Why do SN1 reactions of chiral centers often give racemic mixtures?

Because the planar carbocation can be attacked by the nucleophile from either face.

What major product forms when (CH3)3CBr reacts with acetate ion under SN1 conditions?

tert-Butyl acetate, (CH3)3C–OCOCH3.

How many transition states appear on the potential‐energy diagram of an SN1 reaction?

Two transition states, one for each elementary step.

Is the first step of an SN1 reaction typically endothermic or exothermic?

Endothermic (bond breaking).

What factor primarily governs substrate reactivity in SN1 reactions?

Stability of the intermediate carbocation.

Arrange the following alkyl halides in decreasing SN1 reactivity: tertiary, secondary, primary, methyl.

Tertiary > Secondary > Primary > Methyl.

What electronic effect describes electron donation through sigma bonds that stabilizes carbocations?

The inductive effect.

Define hyperconjugation in the context of carbocation stability.

Delocalization of σ C–H (or C–C) bond electrons into the empty p orbital of the carbocation, dispersing positive charge.

Which typically has a stronger stabilizing influence on a carbocation—resonance or inductive effects?

Resonance effects.

Which is more stable: a benzyl carbocation or a simple tertiary alkyl carbocation?

They are comparably stable; benzyl is highly stabilized by resonance.

Name three possible fates of a carbocation intermediate.

Capture by a nucleophile (SN1), β-proton loss (elimination), or rearrangement to a more stable carbocation.

List the two common types of carbocation rearrangements observed in SN1 reactions.

Hydride shift and methyl shift.

What migrates during a hydride shift?

A hydride ion (H⁻) from an adjacent carbon moves to the carbocation center.

Under what condition does a methyl shift occur during an SN1 reaction?

When migration of a CH3⁻ group leads to a more stable (usually tertiary) carbocation and no hydride is available.

How does the stability of the leaving group influence SN1 rates?

More stable anions (weaker bases) are better leaving groups, lowering the activation energy.

Give an example of an excellent leaving group for SN1 reactions.

Tosylate (TsO⁻).

Why do polar solvents accelerate SN1 reactions?

They better solvate and stabilize the developing charges in the transition state and carbocation.

Name two typical polar protic solvents that favor SN1 mechanisms.

Water (H2O) and methanol (CH3OH).

Are SN1 reactions favored by weak or strong nucleophiles?

Weak nucleophiles are sufficient; nucleophile strength is not rate-limiting.

State the general rate equation for an SN1 reaction on substrate RX.

Rate = k[RX].

Does nucleophile concentration appear in the SN1 rate law?

No.

Which type of base (weak or strong) favors substitution (SN1) over β-elimination (E1)?

A weak base.

Explain why SN1 products often show partial, not complete, racemization.

Ion-pairing and steric shielding by the leaving group can slightly favor backside or frontside attack.

Describe the first elementary step in the SN1 mechanism.

Cleavage of the C–X bond to form a carbocation and a free leaving group (slow step).

Describe the second elementary step in the SN1 mechanism.

Fast attack of the nucleophile on the carbocation to form the substitution product.

Is step 1 (ionization) in SN1 generally slow or fast?

Slow.

Is step 2 (nucleophilic attack) generally slow or fast in SN1 reactions?

Fast.

How many energy maxima appear on an SN1 reaction coordinate diagram?

Two energy maxima corresponding to two transition states.

What property of polar solvents allows them to stabilize the transition state in SN1 reactions?

Ability to solvate and separate positive and negative charges.

Why does benzyl chloride undergo SN1 faster than simple primary alkyl chlorides?

Resonance stabilization of the benzyl carbocation.

Provide the approximate relative SN1 rate ratio for (CH3)3CCl versus CH3CH2Cl.

Tertiary is roughly 1.2 × 10^6 times faster than primary.

In SN1, what role does hyperconjugation play for a tertiary carbocation?

Multiple adjacent C–H or C–C σ bonds donate electron density, delocalizing the positive charge.

What is the biological significance of nitrosamines in the context of SN1 chemistry?

Nitrosamines can undergo SN1-like activation and are potential carcinogens found in smoked and cured foods.

Will a carbocation rearrange if doing so produces a less stable carbocation?

No; rearrangements occur only when a more stable carbocation can form.

Between nucleophilic capture and β-elimination, which pathway dominates when a weak nucleophile is present in SN1 conditions?

Nucleophilic capture (substitution).

Why is fluoride (F⁻) generally a poor leaving group in SN1 reactions?

Because F⁻ is a strong base and therefore an unstable anion.

Does the concentration of the nucleophile influence the rate of an SN1 reaction?

No; the rate is independent of nucleophile concentration.

Hyperconjugation involves overlap of which orbitals with the empty p orbital of a carbocation?

σ C–H or C–C bonding orbitals.

What type of solvent (protic or aprotic) generally favors SN1 reactions?

Polar protic solvents.

Which activation energy (Ea) is usually higher in an SN1 reaction, Ea[1] or Ea[2]?

Ea[1] (for carbocation formation) is higher.

Summarize the conditions that favor an SN1 reaction.

Tertiary (or stabilized) substrate, good leaving group, weak nucleophile, polar protic solvent, room or mild temperature.