Vollhardt  Capítulo 6 (Haloalcanos)
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Vollhardt Capítulo 6 (Haloalcanos)

Disciplina:Química Orgânica II963 materiais31.941 seguidores
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carbon, they are
called a nucleophile (often
shown as Nu2 or Nu:).

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 C h a p t e r 6 235

table. What happens if we look at nucleophiles in a column of the periodic table? We shall
fi nd that the situation changes, because now solvent plays a role.

Solvation impedes nucleophilicity
If it is a general rule that nucleophilicity correlates with basicity, then the elements considered
from top to bottom of a column of the periodic table should show decreasing nucleophilic
power. Recall (Section 2-2) that basicity decreases in an analogous fashion. To test this pre-
diction, let us consider another series of experiments. In the equations below, we have explic-
itly added the solvent methanol to the reaction scheme, because, as we shall see, consideration
of the solvent will be important in understanding the outcome of these experiments.

Experiment 5

B

B
CH3CH2CH2OSCH3 �O3SCH3CH3CH2CH2Cl

CH3OH
(Solvent)

O

Cl �� �

O

Slow

B

B
CH3CH2CH2OSCH3 �O3SCH3CH3CH2CH2Br

O

O

Br �� � Faster
CH3OH

(Solvent)

B

B
CH3CH2CH2OSCH3 �O3SCH3CH3CH2CH2 I

O

O

� �I � Fastest
CH3OH

(Solvent)

Experiment 6

CH3CH2CH2Br CH3O � Br �CH3CH2CH2OCH3� � Not very
fast

CH3OH
(Solvent)

CH3S � Br �CH3CH2CH2SCH3 �CH3CH2CH2Br � Very fast
CH3OH

(Solvent)

Conclusion. Surprisingly, nucleophilicity increases in the progression down the periodic
table, a trend directly opposing that expected from the basicity of the nucleophiles tested.
For example, in the series of halides, iodide is the fastest, although it is the weakest base.

Increasing basicity

F2 , Cl2 , Br2 , I2

Increasing nucleophilicity in CH3OH

Moving one column to the left in the periodic table, sulfi de nucleophiles are more reactive
than the analogous oxide systems, and, as other experiments have shown, their selenium
counterparts are even more reactive. Thus, this column exhibits the same trend as that observed
for the halides. The phenomenon is general for other columns in the periodic table.

How can these trends be explained? An important consideration is the interaction of the
solvent methanol with the anionic nucleophile. We have largely ignored the solvent in our
discussion of organic reactions so far, in particular, radical halogenations (Chapter 3), in
which they play an insignifi cant role. Nucleophilic substitution features polar starting mate-
rials and a polar mechanism, and the nature of the solvent becomes more important. Let us
see how the solvent can become involved.

6 - 8 S t r u c t u r e a n d S N 2 R e a c t i v i t y : T h e N u c l e o p h i l e

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236 C h a p t e r 6 P r o p e r t i e s a n d R e a c t i o n s o f H a l o a l k a n e s

When a solid dissolves, the intermolecular forces that held it together (Section 2-6; Fig-
ure 2-6) are replaced by intermolecular forces between molecules and solvent. Such molecules,
especially the ions derived from the starting salts of many SN2 reactions, are said to be solvated.
Salts dissolve well in alcohols and water, because these solvents contain highly polarized
d1H–Od2 bonds that act by ion – dipole interactions. Thus, cations are solvated by the negatively
polarized oxygens (Figure 6-6A), anions by the positively polarized hydrogens (Figure 6-6B
and C). This solvation of anions is particularly strong, because the small size of the hydrogen
nucleus makes the d1 charge relatively dense. We shall study these interactions, called hydrogen
bonds, more closely in Chapter 8. Solvents capable of hydrogen bonding are also called protic,
in contrast to aprotic solvents, such as acetone, which will be discussed later.

Returning to the problem of our experimental results: What accounts for the increasing
nucleophilicity of negatively charged nucleophiles from the top to the bottom of a column
of the periodic table? The answer is that solvation weakens the nucleophile by forming a
shell of solvent molecules around the nucleophile and thus impeding its ability to attack an
electrophile. As we move down the periodic table, such as from F2 to I2, the solvated ion
becomes larger and its charge more diffuse. As a result, solvation is diminished along the
series and nucleophilicity increases. Figures 6-6B and C depict this effect for F2 and I2.
The smaller fl uoride ion is much more heavily solvated than the larger iodide. Is this true
in other solvents as well?

Decreasing solvation by protic solvent

F2 , Cl2 , Br2 , I2

Increasing nucleophilicity

Aprotic solvents: the effect of solvation is diminished
Other solvents that are useful in SN2 reactions are highly polar but aprotic. Several common
examples are shown in Table 6-5; all lack protons capable of hydrogen bonding but do
exhibit polarized bonds. Nitromethane even exists as a charge-separated species.

Polar, aprotic solvents also dissolve salts by ion–dipole interactions, albeit not as well as
protic solvents. Because they cannot form hydrogen bonds, they solvate anionic nucleophiles
relatively weakly. The consequences are twofold. First, compared to protic solvents, the reactiv-
ity of the nucleophile is raised, sometimes dramatically. For example, bromomethane reacts with

Table 6-5
Polar Aprotic
Solvents

CH3CCH3

O

Acetone

B
ðð

CH3C N
Ethanenitrile
(Acetonitrile)

q ð

HCN(CH3)2
N,N-Dimethylformamide

(DMF)

B
Oðð

š

CH3SCH3
Dimethyl sulfoxide

(DMSO)

B
Oðð

š

P

(CH3)2
(CH3)2N N(CH3)2N

Hexamethylphosphoric
triamide
(HMPA)

A

B

GD

O

ð

ðð

šš

CH3N

Nitromethane

G

J�

O �

Oð

ð

š

šš

ANIM
ATION ANIMATED MECHANISM:

Nucleophilic substitution (SN2)

Na++ F_ I
_

δ + δ + δ +

δ +

δ +

δ +
δ +

δ +

δ +δ −
δ −

δ −

δ −
δ −

δ −

O
O

O

O

O

O

O

O

O O

OO

O

O

O

Figure 6-6 (A) Solvation of Na1 by ion-dipole interactions with methanol. (B) Approximate repre-
sentation of the relatively dense solvation of the small F2 ion by hydrogen bonds to methanol.
(C) Approximate representation of the comparatively diminished solvation of the large I2 ion by
hydrogen bonds to methanol. The tighter solvent shell around F2 reduces its ability to participate
in nucleophilic substitution reactions.

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 C h a p t e r 6 237

potassium iodide 500 times faster in acetone than in methanol. Table 6-6 compares the rates of
SN2 reactions of iodomethane with chloride in three protic solvents—methanol, formamide, and
N-methylformamide—and one aprotic solvent, N,N-dimethylformamide (DMF). (Formamide
and N-methylformamide can form hydrogen bonds by virtue of their polarized N–H linkages.)
The rate of reaction in DMF is more than a million times greater than it is in methanol.

The second consequence of comparatively weaker solvation of anions by aprotic sol-
vents is that the nucleophilicity trend observed in protic solvents inverts. Thus, while the
reactivity of all anions increases, that of the smaller ones increases more than that of the
others. For many nucleophiles, including the halide series, base strength overrides solvation:
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