Question

The correct order of \(\mathrm{S}_{\mathrm{N}} 2\) reactivity among the following is

Short answer

Question: Arrange the following alkyl halides in the correct order of SN2 reactivity: Ethyl chloride (A), Isopropyl chloride (B), Ethyl bromide (C), and Ethyl iodide (D). Answer: The correct order of SN2 reactivity among the given alkyl halides is Ethyl iodide (D) > Ethyl bromide (C) > Ethyl chloride (A).

Step-by-step solution

Analyze the steric effect of the substrate

The rate of SN2 reactions is highly influenced by the steric effect of the substrates. Smaller and less sterically hindered substrates tend to be more reactive in SN2 reactions. In our case, we can see all options (A, B, C, D) are primary alkyl halides, which means the steric effect will not play a significant role in determining the reactivity in this case.

Analyze the leaving group ability

The leaving group's ability is another essential factor influencing SN2 reactivity. A good leaving group is required for an SN2 reaction, as it "leaves" the compound as a solitary anion. The leaving group ability typically increases going down the halogen group on the periodic table (i.e., from F to I). Therefore, iodide (I-) is a better leaving group than bromide (Br-), which is a better leaving group than chloride (Cl-). Based on the leaving group ability, we can order the given alkyl halides in terms of their reactivity as follows: D (Ethyl iodide) > C (Ethyl bromide) > A (Ethyl chloride)

Analyze the nucleophilicity of the nucleophile

The nucleophilicity of a nucleophile has a significant effect on the rate of an SN2 reaction. As there are no specific nucleophiles given in the exercise, we cannot accurately determine the order of reactivity based on this factor. However, we can still compare the reactivity based on the leaving group ability as concluded in Step 2.

Determine the correct order of SN2 reactivity

Based on the leaving group ability, the correct order of SN2 reactivity among the given alkyl halides is: D (Ethyl iodide) > C (Ethyl bromide) > A (Ethyl chloride) In conclusion, the correct order of SN2 reactivity among the given compounds is Ethyl iodide > Ethyl bromide > Ethyl chloride.

Key concepts

Steric Effects in SN2 Reactions

In understanding the SN2 mechanism, steric hindrance plays a critical role. Steric effects refer to the resistance to the attack of a nucleophile caused by the bulkiness of substituents around the carbon atom bearing the leaving group.

During an SN2 reaction, the nucleophile approaches this carbon atom and, if the path is cluttered with large groups, the nucleophile may be unable to reach the reactive center. Hence, the more sterically hindered a substrate is, the slower the SN2 reaction will proceed. Primary alkyl halides, being the least hindered, tend to react faster than secondary and tertiary alkyl halides.

For students to grasp this concept, imagine a crowd around a door: the more people there are, the harder it is to get through. In the case of primary alkyl halides, the crowd isn't dense, so the nucleophile—the person trying to get through—can pass relatively easily.

Leaving Group Ability

In the realm of SN2 reactions, the ability of the leaving group to depart from the substrate significantly impacts the reaction rate. A good leaving group stabilizes the negative charge it acquires after breaking away from the carbon atom. The ease with which a leaving group detaches is usually ascribed to both its size and electronegativity; halogens, for example, are decent leaving groups, with iodide being the best among them.

Enhancing Leaving Group Ability

Common strategies to improve leaving group ability include the use of resonance or inductive effects to distribute the negative charge. With iodide having a larger atomic size and better charge distribution than bromide or chloride, molecules with iodide as a leaving group are generally more reactive in SN2 mechanisms.

Students can liken the leaving group to a slipperiness factor; the easier it is for something to slip away (leave), the faster and smoother the reaction will proceed.

Nucleophilicity

Nucleophilicity, a term describing the strength or ability of a nucleophile to donate a pair of electrons, is key in driving an SN2 reaction. Not all nucleophiles are created equal; they vary based on charge, electronegativity, steric bulk, and the solvent being used.

Factors Influencing Nucleophilicity

Nucleophiles with negative charges and less electronegative atoms tend to be stronger, as they have electrons they are more willing to share. Solvent also influences nucleophilicity; in polar aprotic solvents, nucleophilicity increases down the periodic table, while in polar protic solvents, the trend can reverse.

For a student-faced comparison, think of nucleophiles as people offering gifts. A person with a bigger, less cumbersome gift (a stronger nucleophile) can hand it over more easily than someone with a small, complex package to unwrap (a weaker nucleophile).

Primary Alkyl Halides

Primary alkyl halides are organic compounds where a halogen atom is bound to a primary carbon, which is a carbon attached to only one other carbon. They are pivotal in SN2 reactions because of their minimal steric hindrance and favorable leaving group characteristics.

Since primary carbons are bonded to only one other carbon, there is less 'crowding' around the reactive site. This simplicity not only makes it easier for the nucleophile to attack but also means that the reaction barrier is lower and the reaction occurs more quickly as compared to secondary or tertiary carbons.

Visualizing this concept can be straightforward: picture a small tree with an apple (the halogen) hanging from an outer branch (the carbon). It's much easier to pluck this apple than if the tree were densely packed with branches.