| Front | Back | | --- | --- | | The rate equation for an $S_N2$ reaction is $\text{rate} = k[\text{halogenoalkane}][\text{nucleophile}]$, which makes it a second-order reaction overall because the rate-determining step is bimolecular. | The rate equation for an $S_N2$ reaction is $\text{rate} = k[\text{halogenoalkane}][\text{nucleophile}]$, which makes it a second-order reaction overall because the rate-determining step is bimolecular. | | Why must the nucleophile attack from the opposite side (180°) of the leaving group in an $S_N2$ mechanism? | To avoid steric hindrance and electrostatic repulsion from the electron-rich leaving group. | | What is the stereochemical outcome when a chiral halogenoalkane undergoes an $S_N2$ reaction? | Complete inversion of configuration (the stereochemistry flips like an umbrella turning inside out). | | The rate equation for an $S_N1$ reaction is $\text{rate} = k[\text{halogenoalkane}]$, which means the reaction is first-order overall and the rate-determining step is unimolecular. | The rate equation for an $S_N1$ reaction is $\text{rate} = k[\text{halogenoalkane}]$, which means the reaction is first-order overall and the rate-determining step is unimolecular. | | Why does the $S_N1$ reaction of a chiral halogenoalkane result in a racemic mixture of products? | The carbocation intermediate has a planar geometry, allowing the nucleophile to attack with equal probability from either side. | | Why do tertiary halogenoalkanes favor the $S_N1$ mechanism over the $S_N2$ mechanism? |

Steric hindrance blocks the backside attack required for $S_N2$.
Three alkyl groups provide inductive stabilization to the carbocation intermediate.

| | Why do primary halogenoalkanes favor the $S_N2$ mechanism over the $S_N1$ mechanism? |

Low steric hindrance allows easy backside attack by the nucleophile.
A primary carbocation intermediate is too unstable to form.

| | Why do polar protic solvents (e.g., water, ethanol) favor the $S_N1$ mechanism? | They form hydrogen bonds that solvate and stabilize the carbocation intermediate and the leaving group anion. | | Why do polar aprotic solvents (e.g., acetone, DMSO) favor the $S_N2$ mechanism? | They do not form hydrogen bonds to solvate the nucleophile, leaving the nucleophile highly reactive ("naked") for the attack. | |

In this $S_N1$ energy profile, why is the activation energy ($E_a$) for the first transition state much higher than for the second?

| The first step requires breaking a covalent bond to form a highly unstable carbocation intermediate, whereas the second step only involves bond formation. | |

What does the single peak in this $S_N2$ energy profile represent, and does it contain any intermediates?

| It represents a single transition state; there are no intermediates formed in this concerted reaction. | | During the transition state of an $S_N2$ reaction, the central carbon temporarily forms a highly unstable, pentavalent structure where both the nucleophile and leaving group are partially bonded. | During the transition state of an $S_N2$ reaction, the central carbon temporarily forms a highly unstable, pentavalent structure where both the nucleophile and leaving group are partially bonded. | | What is the key structural difference between a reaction intermediate (like in $S_N1$) and a transition state (like in $S_N2$)? | An intermediate is a stable species with fully formed bonds at a local energy minimum, whereas a transition state is an unstable maximum energy state with partial bonds. | | Why is a strong nucleophile required for an $S_N2$ mechanism, but a weak nucleophile is sufficient for an $S_N1$ mechanism? |

In $S_N2$, the nucleophile is involved in the slow step and must actively displace the leaving group.
In $S_N1$, the nucleophile only attacks after the highly reactive carbocation intermediate has already formed.

| | Which nucleophilic substitution mechanism ($S_N1$ or $S_N2$) do secondary halogenoalkanes undergo? | They can undergo both mechanisms depending on factors like solvent polarity and nucleophile strength. | | In the $S_N1$ mechanism, the rate-determining step is the slow departure of the leaving group to form a carbocation intermediate. | In the $S_N1$ mechanism, the rate-determining step is the slow departure of the leaving group to form a carbocation intermediate. | |

What is the term for the stereochemical change shown here, and why does it occur?

| Inversion of configuration. It occurs because the nucleophile must attack from the backside, exactly 180° opposite the leaving group. | |

Why does the geometry of the intermediate shown in step 1 lead to a mixture of enantiomers in the final product?

| The planar carbocation intermediate has no steric preference, allowing the nucleophile to attack from either side with equal probability. | | An $S_N1$ reaction is favored by polar protic solvents, whereas an $S_N2$ reaction is favored by polar aprotic solvents. | An $S_N1$ reaction is favored by polar protic solvents, whereas an $S_N2$ reaction is favored by polar aprotic solvents. | |

| H | |

| δ+ | |

| δ- | |

| C | |

| CH3 | |

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| STEP 2: |

Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

IB Chemistry R3.4.9 SN1 and SN2 Mechanisms Flashcards

Flashcards for R3.4.9 SN1 and SN2 mechanisms (HL only)