Don't you mean the Li+ ion is unipositive, not uninegative?
In essence, since they have different proton numbers, even if they have isoelectronic configurations, their behaviour will be different.
Behaviour here refers to its chemical reactivity - the nature of the reactions that it has any propensity to undergo.
If have a question too! (sorry for invading your tread)
If protic solvents form hydrogen bonds with nucleophiles and slow down Sn2 reaction tremendously then why is it that most of the nucleophilic substitution (of halogenoalkanes in h2 chem) reagents and conditions involve ethanol as solvent?
Originally posted by TenSaru:If have a question too! (sorry for invading your tread)
If protic solvents form hydrogen bonds with nucleophiles and slow down Sn2 reaction tremendously then why is it that most of the nucleophilic substitution (of halogenoalkanes in h2 chem) reagents and conditions involve ethanol as solvent?
First, some additional info for H2 Chem students, to serve as a background for this discussion.
Solvents may be classified as protic polar, aprotic polar and aprotic non-polar (for obvious reasons, 'protic non-polar' solvents do not exist).
If a reactant in the rate-determining step is charged (eg. OH- ion in SN2), increasing the polarity of the solvent will decrease the rate of the reaction. If none of the reactants in the rate-determining step are charged (eg. alkyl halide in SN1), increasing the polarity of the solvent will increase the rate of reaction.
This occurs because :
For charged reactants (in the rate determining step, eg. SN2), a polar solvent will stabilize the (charged) reactants more extensively than it will stabilize the transition state (which is relatively less charged; as charge is spread out over more atoms), increasing the activation energy and decreasing the rate of reaction.
For non-charged reactants (in the rate determining step, eg. SN1), a polar solvent will stabilize the (charged) transition state more extensively than it will stabilize the (uncharged) reactants, decreasing the activation energy and increasing the rate of reaction.
For SN2 reactions, the nucleophile is presumably charged (otherwise, SN1 would more likely predominate). Consequently, a protic polar solvent stabilizes the nucleophile more extensively (by way of strong hydrogen bonds) and hence decreases the rate of reaction for reasons discussed above.
Since using protic polar solvents slow down SN2 reactions, shall we use (aprotic) non-polar solvents (eg. CCl4) instead? The problem with using non-polar solvents, is that the negatively charged nucleophile (eg. OH- ion) will not dissolve in non-polar solvents, nor be miscible with the (relatively) non-polar alkyl halide substrate, as they would rather form ionic (electrostatic) interactions with each other instead (eg. Na+ and OH-), hence the reaction is unable to proceed.
So do we have no better solution (pun intended) to this dilemma? Actually, we do. The solvent to dissolve (pun intended again) this difficulty, lies in a middle-man so to speak. Something polar enough to dissolve the charged nucleophile, but not solvate it so extensively (via hydrogen bonds) that it slows down the rate of the SN2 reaction.
In short, we need an aprotic polar solvent, such as dimethyl sulfoxide (DMSO). (See Wikipedia for a list of aprotic polar solvents). Thus the rate of an SN2 reaction with a negatively charged nucleophile will be greater in an aprotic polar solvent than in a protic polar solvent.
Consequently, an aprotic polar solvent is the solvent of choice for an SN2 reaction in which the nucleophile is negatively charged, while a protic polar solvent is used if the nucleophile is a neutral molecule. For SN1 reactions in which the uncharged alkyl halide is the only reactant in the rate determining step, a protic polar solvent is most effective in increasing the rate of the SN1 reaction.
Now, to address TenSaru's question on why despite slowing down the SN2 reaction, ethanol is often used as a solvent in the H2 Chem syllabus.
First of all, in regards to the H2 Chem syllabus, I'll like to point out that for the hydrolysis (ie. reaction with water) of alkyl halides (a.k.a. halogenoalkanes) to form alcohols, which is a nucleophilic substitution reaction (it could be SN2 or SN1, depending on whether the akyl halide is primary, secondary or tertiary; ie. tertiary alkyl halide presents greatest steric hinderance for nucleophile in SN2 *and* greatest stability of carbocation intermediate of SN1; conversely primary alkyl halide presents least steric hinderance for nucleophile in SN2 *and* least stability of carbocation intermediate of SN1), the H2 Chem student is expected to use aqueous solvent.
(The reason why it is called 'hydrolysis' is because water may function as the nucleophile here; after the nucleophilic attack, the loss of a proton to rid the positive formal charge on the O atom consequently yields an alcohol. However, to speed up the rate of reaction, hydroxide ions are introduced in the form of an alkali. Notice that water is the protonated conjugate acid of the hydroxide ion, and hence the initial hydrolysis product is the protonated conjugate acid of the alcohol generated; subsequent loss of a proton generates the same alcohol product that the hydroxide ion nucleophile would directly generate.)
In contrast, for elimination or dehydrohalogenation of alkyl halides to form alkenes, the H2 Chem student is expected to use alcoholic solvent (eg. ethanol).
The reason for this has to do with the behaviour of the hydroxide ion. Although both water and alcohols are protic polar solvents, but water affords more extensive ion-dipole and hydrogen bond interactions for the hydroxide ion, compared to an alcohol solvent (due to the one less partially positively charged H atom available, and the presence of the non-polar alkyl group of the alkanol/alcohol).
Consequently, the hydroxide ion is (relatively) more unstable in alcoholic solvent than in aqueous solvent. The more unstable a negatively charged species is, the greater its propensity to behave as a base. (Since a base accepts protons in a bid to stabilize itself).
In summary, this is the reason why alcoholic solvent is used for elimination reactions of alkyl halides, while aqueous solvent is used for hydrolysis reaction of alkyl halides (as far as the H2 Chem syllabus is concerned).
(Unfortunately, JCs do not teach H2 Chem students what I've just explained, which means JC students resort to blind memorization without understanding. I make it a point to share with my tuition students these and other relevant explanations, so that they can understand more clearly and consequently can appreciate Chemistry more deeply, than they would in their JC environment where blind memorization is the unfortunate norm.)
But back to TenSaru's question, why use aqueous or alcoholic solvents for SN2 reactions, since water and alcohols are protic polar solvents and will slow down SN2 reactions? Wouldn't an aprotic polar solvent work better?
Several points.
1. The H2 Chem syllabus does not explore the role of protic polar, aprotic polar, and aprotic non-polar solvents, and how they affect SN2 and SN1 reactions. Hence, the H2 Chem student has only two choices - aqueous or alcoholic.
2. Hydrolysis (nucleophilic substitution reactions with water or hydroxide ions as the nucleophile) of alkyl halides, is not differentiated into SN2 vs SN1 in the old (ie. before 2010) H2 Chem syllabus, and therefore generally employ water (a protic polar solvent) as the solvent of choice, ie. aqueous conditions. Even though this works better for SN1 than for SN2.
Even though the new H2 Chem syllabus starting 2010 does require the H2 Chem student to draw both the SN2 and SN1 nucleophilic substitution mechanisms, but the only factor that influences SN2 vs SN1 that they need to know would be tertiary vs secondary vs primary alkyl halide (ie. tertiary alkyl halide presents greatest steric hinderance for nucleophile in SN2 *and* greatest stability of carbocation intermediate of SN1; conversely primary alkyl halide presents least steric hinderance for nucleophile in SN2 *and* least stability of carbocation intermediate of SN1), and not the role of the protic polar, aprotic polar and aprotic non-polar solvent. Hence water is still employed as the solvent of choice for hydrolysis reactions, while alcohols are employed as the solvent of choice for elimination reactions and nucleophilic substitutions involving non-polar reactants (my next point).
3. In the H2 Chem syllabus, for the formation of nitriles, the H2 Chem student is required to heat or reflux alcoholic KCN with alkyl halides, to generate nitriles in a nucleophilic substitution reaction (whether by SN2 or by SN1). There is yet another reason for alcohol as the choice of solvent, rather than water (even though both are protic polar solvents; as stated earlier, the H2 Chem syllabus is not concerned over this aspect).
The reason being :
The alkyl halide electrophile (eg. bromoethane) is largely non-polar. The nucleophile is ionic (eg. K+ and CN-). If water is used, yes the nucleophile reactant is miscible with water, as ion-dipole interactions are established. But the electrophile (the alkyl halide) reactant, being (largely) non-polar (and hence incapable of either ion-dipole, hydrogen bonding or even the weaker permanent dipole-dipole interactions to any significant extent), will not dissolve well in water. Hence, the reaction is unable to proceed.
But if alcohol solvent is utilized (in spite of it being a protic polar solvent; as discussed earlier, whether by SN2 (unfavourable) or SN1 (favourable), the role of protic polar vs aprotic polar vs aprotic non-polar solvents are ignored by the H2 Chem syllabus), in place of aqueous solvent, then because the alcohol molecule consists of both a polar (hydroxy) group and a non-polar (alkyl) group, it is able to effective function as a middle-man, someone that is able to form favourable interactions with both reactants.
As an analogy, the ionic nucleophile (eg. CN- and its counter ion K+) are the ruling elite snobbish aristocratic class, while the non-polar alkyl halide (eg. bromoethane) are the lower working class peasants or serfdom. The elite snobs woud rather interact with each other than with the lower class (ionic bonding is incomparably stronger than ion - induced dipole interactions). So the reaction (which requires both reactants to meet and react with each other) is unable to proceed to any appreciable extent. However, if you've a bourgeoisie middle class that could get along with both the elite snobs and the lower class, you might actually have a chance to get them to meet and the reaction to proceed.
Enter the bourgeoisie alcohol. Part elite polar (hydroxy group), part working class non-polar (alkyl chain). The hydroxy group is capable of ion-dipole and/or hydrogen bonding with the nucleophile (eg. CN- ion), and the alkyl chain is capable of instantaneous dipole - induced dipole van der Waals interaction with the electrophile (eg. bromoethane).
Hence, utilizing an alcoholic solvent is a cunning manipulative trick, to get the two otherwise immiscible reactants (ionic CN- ion, and non-polar alkyl halide) to interact, and the reaction to proceed.
(Again, tragically, JCs do not explain these underlying rationales to the H2 Chem students. Which only serves to perpetuate the myth that Organic Chem is all tough memory work. Nonsense. My tuition students have the privilege of understanding these underlying rationales, and hence have the opportunity to enjoy Organic Chem without having to suffer torturous blind memorizations.)
So finally again, to summarily address TenSaru's question :
>>> If protic solvents form hydrogen bonds with nucleophiles and slow down Sn2 reaction tremendously then why is it that most of the nucleophilic substitution (of halogenoalkanes in h2 chem) reagents and conditions involve ethanol as solvent? <<<
Because :
1) Ethanol is useful in allowing both reactants (usually one non-polar and one ionic) to be more miscible, and hence for the reaction to proceed. Between the two protic polar solvents of water and alcohol, the alcohol is the superior choice in this regard; even though (see next point)
2) Indeed, an aprotic polar solvent (eg. DMSO) would be better for SN2 reactions, but the H2 Chem syllabus does not concern itself with the role of protic vs aprotic polar solvents. Nor as much with SN1 vs SN2 pathways. So ethanol is fine for the H2 syllabus (even though DMSO would be better for SN2).
3) Just a reminder that in the H2 syllabus, water or aqueous solvent is used for hydrolysis (to generate alcohols), while alcoholic solvents are used for other nucleophilic substitution reactions (eg. to generate nitriles), and for elimination reactions (to generate alkenes).