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Vollhardt Capítulo 7 (Haloalcanos 2)

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C H A P T E R

Further Reactions
of Haloalkanes

S E V E N

We have learned that the SN2 displacement process is an important reaction pathway for haloalkanes. But is it the only mechanism for
displacement available? Or are there other, fundamen-
tally different types of transformations that haloal-
kanes undergo? In this chapter, we shall see that
haloalkanes can indeed follow reaction pathways other
than SN2 displacement, especially if the haloalkanes
are tertiary or secondary. In fact, bimolecular substitu-
tion is only one of four possible modes of reaction.
The other three modes are unimolecular substitution
and two types of elimination processes. The elimina-
tion processes give rise to double bonds through loss
of HX and serve as our introduction into the prepara-
tion of multiply bonded organic compounds.

7-1 Solvolysis of Tertiary and Secondary Haloalkanes
We have learned that the rate of the SN2 reaction diminishes drastically when the reacting
center changes from primary to secondary to tertiary. These observations, however, pertain
only to bimolecular substitution. Secondary and tertiary halides do undergo substitution,
but by another mechanism. In fact, these substrates transform readily, even in the presence
of weak nucleophiles, to give substitution products.

For example, when 2-bromo-2-methylpropane (tert-butyl bromide) is mixed with water, it
is rapidly converted into 2-methyl-2-propanol (tert-butyl alcohol) and hydrogen bromide. Water
is the nucleophile here, even though it is poor in this capacity. Such a transformation, in which
a substrate undergoes substitution by solvent molecules, is called solvolysis, such as methano-
lysis, ethanolysis, and so on. When the solvent is water, the term hydrolysis is applied.

[ ]

š

CH3
Poor nucleophile
yet fast reaction!

CH3

H OH
Relatively fast

2-Bromo-2-methylpropane
(tert-Butyl bromide)

2-Methyl-2-propanol
(tert-Butyl alcohol)

An Example of Solvolysis: Hydrolysis

CH3CBr � HBr�
A

A

CH3

CH3

CH3COH
A

A
O

š
š š

š
š

š
š

š
š

Medicinal chemists use many
reactions to explore structure-
activity relationships in
physio logically active compounds.
Above, the bromocyclohexyl
substituent to a b-lactam is
converted to a cyclohexenyl
group by elimination of HBr.
b-Lactams are four-membered
ring amides featured in the
structure of many antibiotics, such
as penicillin and cephalosporin,
and their modifi cation is essential
in combating drug resistance.
The photo shows Petri dish
cultures of two strains of
Staphylococcus aureus bacteria
(opaque and grey), an organism
that causes boils, abscesses,
and urinary tract infections. At
left, one bacterial strain shows
sensitivity to penicillin (white
pellet) as indicated by the clear
zone of inhibited growth around
it. At right, a second strain of
bacteria shows resistance to
the drug and its growth is not
inhibited.

Unimolecular Substitution and
Pathways of Elimination

:Base
O

ON N

Br

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252 C h a p t e r 7 F u r t h e r R e a c t i o n s o f H a l o a l k a n e s

2-Bromopropane is hydrolyzed similarly, albeit much more slowly, whereas 1-bromopropane,
bromoethane, and bromomethane are unchanged under these conditions.

CH3

H

H OH
Relatively slow

2-Bromopropane
(Isopropyl bromide)

2-Propanol
(Isopropyl alcohol)

Hydrolysis of a Secondary Haloalkane

CH3CBr � HBr�
A

A

CH3

H

CH3COH
A

A
Oš

šš
š
š

š
š
ššš

Solvolysis also takes place in alcohol solvents.

CH3

CH3OH
Solvent

CH3
2-Chloro-

2-methylpropane
2-Methoxy-

2-methylpropane

Solvolysis of 2-Chloro-2-methylpropane in Methanol

CH3CCl � HCl�
A

A

CH3

CH3

CH3COCH3
A

A
š

š
šš

š
š

š
š

š
š

Solvolysis—the
solvent is also
the nucleophile

The relative rates of reaction of 2-bromopropane and 2-bromo-2-methylpropane with
water to give the corresponding alcohols are shown in Table 7-1 and are compared with
the corresponding rates of hydrolysis of their unbranched counterparts. Although the process
gives the products expected from an SN2 reaction, the order of reactivity is reversed from
that found under typical SN2 conditions. Thus, primary halides are very slow in their reac-
tions with water, secondary halides are more reactive, and tertiary halides are about 1 mil-
lion times as fast as primary ones.

These observations suggest that the mechanism of solvolysis of secondary and, espe-
cially, tertiary haloalkanes must be different from that of bimolecular substitution. To under-
stand the details of this transformation, we shall use the same methods that we used to study
the SN2 process: kinetics, stereochemistry, and the effect of substrate structure and solvent
on reaction rates.

Reminder
Nucleophile: red
Electrophile: blue
Leaving group: green

REACTION

Methyl and Primary
Haloalkanes:

Unreactive in Solvolysis

CH3Br
CH3CH2Br

CH3CH2CH2Br
Essentially no reaction with H2O

at room temperature

7-2 Unimolecular Nucleophilic Substitution
In this section, we shall learn about a new pathway for nucleophilic substitution. Recall that
the SN2 reaction

• Has second-order kinetics

• Generates products stereospecifi cally with inversion of confi guration

• Is fastest with halomethanes and successively slower with primary and secondary
halides

• Does not take place with tertiary substrates at all

Exercise 7-1

Whereas compound A (shown in the margin) is completely stable in ethanol, B is rapidly converted
into another compound. Explain.

Br

B

H3C A AA A

A

H3C CH2Br

Table 7-1

Relative Reactivities of
Various Bromoalkanes
with Water

 Relative
Bromoalkane rate

CH3Br 1

CH3CH2Br 1

(CH3)2CHBr 12

(CH3)3CBr 1.2 3 10
6

In
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in
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Podemos inferir que o carbocátion não seria estabilizado, e que portanto a sobstituição ou não ocorre, ou ocorre em Sn2 (eliminação E1 ou E2 ainda não foi tratada).

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Callout
Mesmo que a água não seja um bom nucleófilo, ela ainda pode reagir com tais compostos pois o carbocárion formado é melhor estabilizado, fazendo com que, portanto, possa ser realizada uma Sn1

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A possúi:nullnull1. impedimento estérico.nullnull2. carbono primário ligado ao grupo de partidanullnullnullnull...enquanto B:nullnull1. é um carbono terceario, e, portanto, forma um carbocárion mais estabilizado no caso da saída do bromo.nullnull2. não sofre tamanho impedimento estérico.

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 C h a p t e r 7 253

In contrast, solvolyses

• Follow a fi rst-order rate law

• Are not stereospecifi c

• Are characterized by the opposite order of reactivity

Let us see how these fi ndings can be accommodated mechanistically.

Solvolysis follows fi rst-order kinetics
In Chapter 6, the kinetics of reaction between halomethanes and nucleophiles revealed a
bimolecular transition state: The rate of the SN2 reaction is proportional to the concentration
of both ingredients. Similar studies have been carried out by varying the concentrations of
2-bromo-2-methylpropane and water in formic acid (a polar solvent of very low nucleophi-
licity) and measuring the rates of solvolysis. The results of these experiments show that the
rate of hydrolysis of the bromide is proportional to the concentration of only the starting
halide, not the water.

Rate 5 k[(CH3)3CBr] mol L
21 s21

What does this observation mean? First, it is clear that the haloalkane has to undergo
some transformation on its own before anything else takes place. Second, because the fi nal
product contains a hydroxy group, water (or, in general, any nucleophile) must enter the
reaction, but at a later stage and not in a way that will affect the rate law. The only way
to explain this behavior is to postulate that any steps that follow the initial reaction of the
halide are relatively fast. In other words, the observed rate is that of the slowest step in
the sequence: the rate-determining step. It follows that only those species