UC Berkeley CHEM 3A - Organic Chemistry: Mechanisms & Reactions Flashcards

Question 1

What is the rate law for an $E2$ reaction?

Accepted Answer

The rate law for an $E2$ reaction is $Rate = k[R-X][Base^-]$, indicating that the reaction rate depends on the concentration of both the substrate (alkyl halide) and the base.

Question 2

What do curved arrows represent in organic reaction mechanisms?

Accepted Answer

Curved arrows depict the movement of electron pairs. They originate from an electron source (lone pair or bond) and point to an electron sink (atom forming a new bond or breaking an old one).

Question 3

Define an electrophile.

Accepted Answer

An electrophile (electron-lover) is an electron-deficient species that accepts an electron pair to form a new bond. It typically has a partial positive charge, a full positive charge, or an incomplete octet.

Question 4

Define a nucleophile.

Accepted Answer

A nucleophile (nucleus-lover) is an electron-rich species that donates an electron pair to form a new bond. It typically has a lone pair of electrons or a $\pi$ bond, and can be negatively charged or neutral.

Question 5

What is a leaving group in organic reactions?

Accepted Answer

A leaving group is an atom or group of atoms that departs with an electron pair during a reaction. Good leaving groups are typically weak bases (e.g., halides like $Cl^-$, $Br^-$, $I^-$, or tosylates $OTs^-$).

Question 6

Order the stability of carbocations from most to least stable.

Accepted Answer

Allylic/Benzylic > Tertiary ($3^\circ$) > Secondary ($2^\circ$) > Primary ($1^\circ$) > Methyl. Stability is primarily due to hyperconjugation and resonance.

Question 7

How does hyperconjugation stabilize carbocations?

Accepted Answer

Hyperconjugation is the stabilizing interaction between the empty p-orbital of the carbocation and the adjacent $\sigma$ bonds (C-H or C-C). The electrons in these $\sigma$ bonds are delocalized into the empty p-orbital, spreading out the positive charge.

Question 8

What are carbocation rearrangements, and what types are common?

Accepted Answer

Carbocation rearrangements occur when a less stable carbocation can rearrange to a more stable one by shifting an adjacent group. Common types include:
*   **Hydride shift:** A hydrogen atom with its bonding electrons moves (e.g., $1,2$-hydride shift).
*   **Alkyl shift:** An alkyl group with its bonding electrons moves (e.g., $1,2$-alkyl shift).

Question 9

Describe the $S_N2$ reaction mechanism.

Accepted Answer

$S_N2$ (Substitution, Nucleophilic, Bimolecular) is a concerted, one-step mechanism where the nucleophile attacks the electrophilic carbon from the backside, simultaneously displacing the leaving group. There is a single transition state.

Question 10

What is the rate law for an $S_N2$ reaction?

Accepted Answer

The rate law for an $S_N2$ reaction is $Rate = k[R-X][Nu^-]$, indicating that the reaction rate depends on the concentration of both the substrate (alkyl halide) and the nucleophile.

Question 11

What is the stereochemical outcome of an $S_N2$ reaction?

Accepted Answer

$S_N2$ reactions proceed with **inversion of configuration** at the electrophilic carbon. If the carbon is chiral, an R enantiomer will become S, and vice-versa (Walden inversion).

Question 12

List the factors that favor an $S_N2$ reaction.

Accepted Answer

*   **Substrate:** Methyl > Primary ($1^\circ$) > Secondary ($2^\circ$). Tertiary ($3^\circ$) substrates are too sterically hindered.
*   **Nucleophile:** Strong, unhindered nucleophiles are preferred.
*   **Solvent:** Aprotic polar solvents (e.g., DMSO, acetone, acetonitrile) enhance nucleophilicity.
*   **Leaving Group:** Good leaving groups (weak bases) are essential.

Question 13

Describe the $S_N1$ reaction mechanism.

Accepted Answer

$S_N1$ (Substitution, Nucleophilic, Unimolecular) is a two-step mechanism. The leaving group departs first to form a carbocation intermediate, followed by nucleophilic attack on the carbocation.

Question 14

What is the rate law for an $S_N1$ reaction?

Accepted Answer

The rate law for an $S_N1$ reaction is $Rate = k[R-X]$, indicating that the reaction rate depends only on the concentration of the substrate (alkyl halide), as the rate-determining step is the formation of the carbocation.

Question 15

What is the stereochemical outcome of an $S_N1$ reaction?

Accepted Answer

$S_N1$ reactions typically lead to **racemization** if the reacting carbon is chiral. The planar carbocation intermediate can be attacked from either face, resulting in a mixture of enantiomers (often slightly more inversion than retention).

Question 16

List the factors that favor an $S_N1$ reaction.

Accepted Answer

*   **Substrate:** Tertiary ($3^\circ$) > Secondary ($2^\circ$). Primary ($1^\circ$) and methyl substrates do not form stable carbocations.
*   **Nucleophile:** Weak nucleophiles are sufficient, as they are not involved in the rate-determining step.
*   **Solvent:** Protic polar solvents (e.g., water, alcohols) stabilize the carbocation intermediate and solvate the leaving group.
*   **Leaving Group:** Good leaving groups are essential.

Question 17

Compare and contrast $S_N1$ and $S_N2$ reactions based on mechanism, rate law, and stereochemistry.

Accepted Answer

*   **Mechanism:** $S_N2$ is one-step (concerted); $S_N1$ is two-step (carbocation intermediate).
*   **Rate Law:** $S_N2$: $Rate = k[R-X][Nu^-]$; $S_N1$: $Rate = k[R-X]$.
*   **Stereochemistry:** $S_N2$: Inversion of configuration; $S_N1$: Racemization.
*   **Substrate Preference:** $S_N2$: Methyl > $1^\circ$ > $2^\circ$; $S_N1$: $3^\circ$ > $2^\circ$.

Question 18

Describe the $E2$ reaction mechanism.

Accepted Answer

$E2$ (Elimination, Bimolecular) is a concerted, one-step mechanism where a strong base removes a $\beta$-hydrogen, while the leaving group departs and a $\pi$ bond forms between the $\alpha$ and $\beta$ carbons.

Question 19

What is the stereochemical requirement for an $E2$ reaction?

Accepted Answer

For an $E2$ reaction to occur, the $\beta$-hydrogen and the leaving group must be in an **anti-periplanar** conformation. This allows for optimal orbital overlap during the concerted bond breaking and forming.

Question 20

List the factors that favor an $E2$ reaction.

Accepted Answer

*   **Substrate:** Tertiary ($3^\circ$) > Secondary ($2^\circ$) > Primary ($1^\circ$). More substituted alkenes are more stable (Zaitsev's rule).
*   **Base:** Strong, often sterically hindered bases (e.g., $t-BuOK$, $DBN$, $DBU$) are preferred.
*   **Solvent:** Aprotic polar solvents are often used, but protic solvents can also work.
*   **Leaving Group:** Good leaving groups are essential.
*   **Temperature:** High temperatures favor elimination over substitution.

Question 21

Describe the $E1$ reaction mechanism.

Accepted Answer

$E1$ (Elimination, Unimolecular) is a two-step mechanism. The leaving group departs first to form a carbocation intermediate (rate-determining step), followed by a weak base removing a $\beta$-hydrogen to form an alkene.

Question 22

What is the rate law for an $E1$ reaction?

Accepted Answer

The rate law for an $E1$ reaction is $Rate = k[R-X]$, indicating that the reaction rate depends only on the concentration of the substrate (alkyl halide), as the rate-determining step is the formation of the carbocation.

Question 23

List the factors that favor an $E1$ reaction.

Accepted Answer

*   **Substrate:** Tertiary ($3^\circ$) > Secondary ($2^\circ$). Primary ($1^\circ$) substrates do not form stable carbocations.
*   **Base:** Weak bases (often the solvent itself, e.g., $H_2O$, $CH_3OH$) are sufficient.
*   **Solvent:** Protic polar solvents stabilize the carbocation intermediate.
*   **Leaving Group:** Good leaving groups are essential.
*   **Temperature:** High temperatures favor elimination over substitution.

Question 24

Define stereospecificity in the context of organic reactions.

Accepted Answer

A reaction is stereospecific if the stereochemistry of the reactant dictates the stereochemistry of the product. For example, $S_N2$ reactions are stereospecific because a specific enantiomer of the reactant yields a specific enantiomer of the product (inversion). $E2$ reactions are also stereospecific due to the anti-periplanar requirement.

Question 25

Explain Zaitsev's Rule and its relevance to elimination reactions.

Accepted Answer

Zaitsev's Rule states that in an elimination reaction, the major product will be the most substituted (most stable) alkene. This typically means the hydrogen is removed from the $\beta$-carbon that has the fewest hydrogens (i.e., the most substituted $\beta$-carbon). This rule applies to both $E1$ and $E2$ reactions, though bulky bases can lead to Hofmann products (least substituted alkene).

UC Berkeley CHEM 3A - Organic Chemistry: Mechanisms & Reactions

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