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).