Which compound in each of the following pairs will react...

Which compound in each of the following pairs will react faster in an $\mathrm{S}_{\mathrm{N}}^{2}$ reaction with $\mathrm{OH}^{-} ?$ (a) $\mathrm{CH}_{3} \mathrm{Br}$ or $\mathrm{CH}_{3} \mathrm{I}$ (b) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{I}$ in ethanol or in dimethyl sulfoxide (c) $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}$ or $\mathrm{CH}_{3} \mathrm{Cl}$ (d) $\mathrm{H}_{2} \mathrm{C}=\mathrm{CHBr}$ or $\mathrm{H}_{2} \mathrm{C}=\mathrm{CHCH}_{2} \mathrm{Br}$

Which compound in each of the following pairs will react faster in an $\mathrm{S}_{\mathrm{N}}^{2}$ reaction with $\mathrm{OH}^{-} ?$
(a) $\mathrm{CH}_{3} \mathrm{Br}$ or $\mathrm{CH}_{3} \mathrm{I}$
(b) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{I}$ in ethanol or in dimethyl sulfoxide
(c) $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}$ or $\mathrm{CH}_{3} \mathrm{Cl}$
(d) $\mathrm{H}_{2} \mathrm{C}=\mathrm{CHBr}$ or $\mathrm{H}_{2} \mathrm{C}=\mathrm{CHCH}_{2} \mathrm{Br}$

Key Concepts

Solvent Effects

The nature of the solvent plays a significant role in SN2 reactions. Polar aprotic solvents are preferred for SN2 mechanisms because they do not strongly solvate anions, enhancing nucleophilicity and thereby increasing the reaction rate. In contrast, polar protic solvents can form hydrogen bonds with the nucleophile, reducing its reactivity and slowing down the reaction.

Steric Hindrance

Steric hindrance refers to the spatial congestion around the reactive center that can impede the approach of the nucleophile. In SN2 reactions, less substituted (such as primary or methyl) carbons are favored because they offer minimal steric resistance to the backside attack, whereas tertiary centers, which are heavily blocked by bulky groups, react much more slowly or not at all under SN2 conditions.

Substrate Structure and Hybridization

The reactivity of substrates in SN2 reactions is also influenced by the hybridization of the carbon bearing the leaving group. Electrophilic centers that are sp3 hybridized are generally reactive in SN2 processes, whereas substrates with sp2 hybridization, such as vinyl halides, are much less reactive due to the increased electron density and partial double bond character, which hinders the backside attack necessary for the reaction.

SN2 Reaction Mechanism

The SN2 reaction mechanism is a bimolecular nucleophilic substitution process where the rate depends on both the nucleophile and the electrophile. It occurs in a single, concerted step with a backside attack that leads to inversion of configuration at the reaction center. The requirement for the nucleophile to approach from the opposite side of the leaving group makes the reaction particularly sensitive to steric hindrance around the electrophilic carbon.

Leaving Group Ability

Leaving group ability is crucial in SN2 reactions because a good leaving group can stabilize the negative charge once it departs, thereby facilitating the reaction. Generally, larger, softer anions are better leaving groups due to their ability to disperse the negative charge. This factor significantly influences reaction rate, as substrates with more effective leaving groups will typically undergo substitution faster.

Transcript

00:01 So we have the following pairs of compounds, and we want to know which will react faster in an sn2 reaction with hydroxide.

00:07 So in a here, we have bromomethane versus iodomethane.

00:13 Well, in this case, we're going to see that iodomethane will react faster.

00:21 And the reason for that is iodine actually has more electrons than bromine, and is therefore more willing to give up its electrons, because we have to remember the different electron shells.

00:35 And because iodine has more electron shells, once we get to the valence electrons, which are the electrons that are actually reacting, they are actually less attracted to the nucleus.

00:47 And therefore, iodine is more willing to give up one of its or a pair of its valence electrons, whereas bromine is more, the valence electrons are more attracted to its nucleus because it's smaller.

01:02 And then so for b, we are given the pair of iotoethane in the presence of ethanol or dms.

01:13 And in this case here, we're going to see that it's actually going to be more reactive.

01:22 An s &2 reaction will be more reactive in dms solvent.

01:26 And for s &2 reactions, it's key to know that we are looking for a polar a -protic solvent.

01:33 And the reason for that is, so for example, ethanol, it's a protic solvent, and it's actually going to solvate our hydroxide nucleophile, and that will reduce the nucleophilicity of hydroxide, which we do not want.

01:46 And the reason we want a polar solvent and polar apodic solvent, like dms, is that, say, for example, our nucleophile is sodium iodide.

02:01 Then here we're going to say that dms, which is polar, is actually really good at solvating our metal cat -ion...

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