Acidez_e_basicidade_-_Parte_1

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Prof. Calçada 1 
 
Acidez e basicidade 
 
Questões de classe
 
1) Observe o exemplo a seguir: 
 
Usando setas curvas complete as reações a seguir de forma 
semelhante e classifique os reagentes como eletrófilos ou 
nucleófilos. 
 
2) Para cada reação abaixo indique se o equilíbrio está deslocado 
para a direita ou esquerda. 
 
 
3) Mostre a reação f acima usando setas curvas para mostrar o 
movimento dos elétrons. 
 
 
 
 
 
 
 
4) Coloque os álcoois a seguir em ordem crescente de acidez. 
 
 
 
 
 
 
 
 
 
 
 
5) Coloque os ácidos a seguir em ordem decrescente de acidez. 
 
 
6) Responda 
 
a) Por que o m-nitrofenol é menos ácido que seus isômeros 
de posição mas é mais ácido que o fenol? 
b) Coloque em ordem crescente de acidez: 
(A) fenol, (B)3,4-dimetilfenol, (C)ácido m-hidroxibenzoico, 
(D) 4-fluorometilfenol 
 
 
 
 
 
 
 
 
ORIENTAÇÃO DE ESTUDO 
 
1. Termine o capítulo 9 do livro 3. 
2. Leia as páginas 12 a 15 do livro do Mecanismos de 
Reações e resolva os exercícios 1 e 2. 
3. Resolva os extras a seguir. 
EXTRAS PARA CASA 
 
1) Em qual dos compostos abaixo o par isolado do nitrogênio não 
está deslocalizado sobre o anel aromático? 
 
2) O par de moléculas a seguir representa: 
 
 
a) enantiômeros 
b) diastereoisômeros 
672 - 3 A c i d s a n d B a s e s C H A P T E R 2
The reaction between boron trifl uoride and ethoxyethane (diethyl ether) to give the Lewis 
acid-base reaction product is shown on the previous page, and also in the form of electro-
static potential maps (above). As electron density is transferred, the oxygen becomes more 
positive (blue) and the boron more negative (red).
As we have seen in Section 2-2, the dissociation of a Brønsted acid HA is just the reverse 
of the combination of the Lewis acid H+ and the Lewis base A2:
Dissociation of a Brønsted Acid
H! "ðAAHO !
As a reminder (see Section 2-3), the use of the symbol for the free proton, H+, in the above 
equation is for convenience, and we will encounter it in future reaction schemes and mech-
anisms. Nonetheless, bear in mind that H+ in solution is always associated with a Lewis 
basic species such as a molecule of solvent.
Electrophiles and nucleophiles are similar to acids and bases
Many processes in organic chemistry exhibit characteristics of acid-base reactions. For 
example, heating an aqueous mixture of sodium hydroxide and chloromethane, CH3Cl, 
produces methanol and sodium chloride. As noted in Section 2-2, this process involves the 
same kind of movement of two pairs of electrons as does the acid-base reaction between 
sodium hydroxide and HCl:
Reaction of Sodium Hydroxide and Chloromethane
Na! Na!
Note: The Na!
ion is a
nonparticipating
“bystander”Methanol
HO "" CH3 ClO HO CH3O! ! ! !šð# šð# Clšðð#š#
H2O, ∆
Flow of Electrons Using Curved-Arrow Representation (Na+ Omitted)
ðð
ð
Clðš#ClðHO#š HO#šOOð
"" !! CH3 CH3
Compare to a
Brønsted acid-base 
reaction: 
šð#Clðš#ClðHO#š HO#šOOð
"" !! H H
Because the reaction between NaOH and CH3Cl results in substitution of a nucleophile 
(hydroxide) for another atom or group in the starting organic molecule, it is called a nucleo-
philic substitution.
The terms nucleophile and Lewis base are synonymous. All nucleophiles are Lewis bases. 
Nucleophiles, often denoted by the abbreviation Nu, may be negatively charged, such as 
!
68 S t r u c t u r e a n d R e a c t i v i t yC H A P T E R 2
hydroxide, or neutral, such as water, but every nucleophile contains at least one unshared pair 
of electrons. All Lewis acids are electrophiles, as shown earlier in the examples of Lewis acid-
base reactions. Species such as HCl and CH3Cl have closed outer shells and therefore are not 
Lewis acids. However, they may still behave as electrophiles, because they possess polar bonds 
that impart electrophilic character to the H in HCl and the C in CH3Cl, respectively.
Nucleophilic substitution is a general reaction of haloalkanes, organic compounds pos-
sessing carbon–halogen bonds. The two following equations are additional examples:
CH3 2CH3 !! CCH
H
CH3 2CH3CCH
Br
H
"I "
! ðð
Br! ðð!š šðð
I!ðð
CH3 2NH !! CH
H
H
CH3 32CH "
!
ðNHI!šð I!šðð
Although the haloalkanes in these examples contain different halogens and varied num-
bers and arrangements of carbon and hydrogen atoms, they behave very similarly toward 
nucleophiles. We conclude that it is the presence of the carbon–halogen bond that governs 
the chemical behavior of haloalkanes: The C–X bond is the structural feature that directs 
the chemical reactivity—structure determines function. The C–X bond constitutes the func-
tional group, or the center of chemical reactivity, of the haloalkanes. In the next section, 
we shall introduce the major classes of organic compounds, identify their functional groups, 
and briefl y preview their reactivity.
In Summary In Brønsted-Lowry terms, acids are proton donors and bases are proton accep-
tors. Acid-base interactions are governed by equilibria, which are quantitatively described 
Solved Exercise 2-14 Working with the Concepts: Using Curved Arrows
Using earlier examples in this section as models, add curved arrows to the fi rst of the two reactions 
immediately above.
Strategy
In the organic substrate, identify the likely reactive bond and its polarization. Classify the other 
reacting species, and look for reactions in the text between similar types of substances.
Solution
• The C–Br bond is the site of reactivity in the substrate and is polarized (!1)C–Br(!2). Iodide 
is a Lewis base and therefore a potential nucleophile (electron-pair donor). Thus the situation 
resembles that found in the reaction between hydroxide and CH3Cl just above and Example 3 at 
the beginning of the section.
• Follow the earlier patterns in order to add the appropriate arrows.
!!
CH2
CH3
3CH
"I!šðð šð!Brðš! ð
"CO BrO
!! !"
H CO HOI
CH2
CH3
3CH
Exercise 2-15 Try It Yourself
Add curved arrows to the second of the reactions immediately above Exercise 2-14 in the text.
92 S t r u c t u r e a n d R e a c t i v i t yC H A P T E R 2
 Average
 strength
Bond (kcal mol!1 )
COC 83
CPC 146
COH 99
BrOBr 46
HOBr 87
COBr 68
(a) Iodide ion, I2
(b) Hydrogen ion, H1
(c) Carbon in methyl cation, 1CH3
(d) Sulfur in hydrogen sulfi de, H2S
(e) Aluminum in aluminum trichloride, AlCl3
(f) Magnesium in magnesium oxide, MgO
 33. Circle and identify by name each functional group in the com-
pounds pictured.
(a) 
OH
 (b) 
(c) I (d) 
O
(e) H
O
 (f) 
(g) O
O
 (h) N
O
H
(i) 
O
OH
O
HO
 (j) 
O
O
O
 34. On the basis of electrostatics (Coulomb attraction), predict 
which atom in each of the follow ing organic molecules might 
react with the indicated reagent. Write “no reaction’’ if none 
seems likely. (See Table 2-3 for the structures of the organic 
molecules.) (a) Bromoethane, with the oxygen of HO2; 
(b)  propanal, with the nitrogen of NH3; (c) methoxyethane, 
with H1; (d) 3-hexanone, with the carbon of CH3
2; (e) eth-
anenitrile (acetonitrile), with the carbon of CH3
1; (f) butane, 
with HO2.
 35. Use curved arrows to show the electron movement in each 
 reaction in Problem 34.
 36. Name the following molecules according to the IUPAC system 
of nomenclature.
(a) 
CH3
CH3
H3C
CHCH3CH2
CH
(b) CH2 CH3
CH2
C
CH3
CH2CH2 CH2
CHCH3
CH
CH3
CH2CH3
CH3CH CH3
 26. You have just baked a pizza and turned off your oven. When you 
open the door to cool the hot oven, what happens to the total en-
thalpy of the system “oven 1 room”? The total entropy of the 
system? How about the free energy? Is the process thermody-
namically favorable? What can you say about the temperatures of 
the oven and of the kitchen after equilibrium has been reached?
 27. The hydrocarbon propene (CH3OCHPCH2) can react in two 
different ways with bromine (Chapters 12 and 14).
 (i) 
CH2P
Br
!CHCH2OCHCH3 OCH2 HBr
Br
A
CH2O
P
! CHCH3OCHCH3 OCH2
A
Br2
A
Br
P
! Br2 (ii)
(a) Using the bond strengths (kcal mol21) given in the table, cal-culate DH8 for each of these reactions. (b) DS8 ! 0 cal K21 mol21 
for one of these reactions and 235 cal K21 mol21 for the other. 
Which reaction has which DS8? Briefl y explain your answer. (c) 
Calculate DG8 for each reaction at 258C and at 6008C. Are both of 
these reactions thermodynamically favorable at 258C? At 6008C?
Problems
 28. (i) Determine whether each species in the following equations is 
acting as a Brønsted acid or base, and label it. (ii) Indicate whether 
the equilibrium lies to the left or to the right. (iii) Estimate K for 
each equation if possible. (Hint: Use the data in Table 2-2.)
(a) H2O 1 HCN 34 H3O
1 1 CN2 
(b) CH3O
2 1 NH3 34 CH3OH 1 NH2
2
(c) HF 1 CH3COO
2 34 F2 1 CH3COOH
(d) CH3
2 1 NH3 34 CH4 1 NH2
2
(e) H3O
1 1 Cl2 34 H2O 1 HCl
(f) CH3COOH 1 CH3S
2 34 CH3COO
2 1 CH3SH
 29. Use curved arrows to show electron movement in each acid-base 
reaction in Problem 28.
 30. Identify each of the following species as either a Lewis acid or a 
Lewis base, and write an equation illustrating a Lewis acid-base 
reaction for each one. Use curved arrows to depict electron-pair 
movement. Be sure that the product of each reaction is depicted 
by a complete, correct Lewis structure.
(a) CN2 (b) CH3OH (c) (CH3)2CH
1
(d) MgBr2 (e) CH3BH2 (f) CH3S
2
 31. For each example in Table 2-3, identify all polarized covalent 
bonds and label the appropriate atoms with partial positive or 
negative charges. (Do not consider carbon–hydrogen bonds.)
 32. Characterize each of the following atoms as being either nucleo-
philic or electrophilic.
286 H y d r o x y F u n c t i o n a l G r o u p : A l c o h o l s C H A P T E R 8
polarized as a result of the high electronegativity of X (Sections 1-3 and 6-1). Electron 
withdrawal by the halogen also causes atoms farther away to be slightly positively charged. 
This phenomenon of transmission of charge, both negative and positive, through the s bonds 
in a chain of atoms is called an inductive effect. Here it stabilizes the negative charge on 
the alkoxide oxygen by electrostatic attraction. The inductive effect in alcohols increases 
with the number of electronegative groups but decreases with distance from the oxygen.
Figure 8-4 The smaller methoxide 
ion is better solvated than is the 
larger tertiary butoxide ion. 
(S 5 solvent molecules).
−−
S
S
S
S
S
S
S
S S
SO
O
Increasing 
inductive effect
Cl
Inductive Effect
of the Chlorine
in 2-Chloroethoxide
OCH2O CH2 !O O ðš#
Exercise 8-5
Rank the following alcohols in order of increasing acidity.
OH
Cl
Cl
Cl
OH OH OH
Exercise 8-6
Which side of the following equilibrium reaction is favored (assume equimolar concentrations of 
starting materials)?
(CH3)3CO
2 1 CH3OH ∆ (CH3)3COH 1 CH3O2
The lone electron pairs on oxygen make alcohols weakly basic
Alcohols may also be basic, although weakly so. Very strong acids are required to protonate 
the OH group, as indicated by the low pKa values (strong acidity) of their conjugate acids, 
the alkyloxonium ions (Table 8-3). Molecules that may be both acids and bases are called 
amphoteric (ampho, Greek, both).
The amphoteric nature of the hydroxy functional group characterizes the chemical reac-
tivity of alcohols. In strong acids, they exist as alkyloxonium ions, in neutral media as 
alcohols, and in strong bases as alkoxides.
RO !ROH
Alcohols Are Amphoteric
Strong acid
Mild base
Alkyloxonium
ion
Alcohol Alkoxide
ion
Strong base
Mild acid
š" š"ðR
H
H
O
"
O
i
f
ð
Table 8-3
pKa Values of 
Four Protonated 
Alcohols
Compound pKa
CH3O
!
H2 22.2
CH3CH2O
!
H2 22.4
(CH3)2CHO
!
H2 23.2
(CH3)3CO
!
H2 23.8
842 C a r b o x y l i c A c i d sC H A P T E R 1 9
Why do carboxylic acids dissociate to a greater extent than do alcohols? The difference is that 
the hydroxy substituent of a carboxylic acid is attached to a carbonyl group, whose positively 
polarized and sp2-hybridized carbonyl carbon exerts a powerful electron-withdrawing inductive 
effect. In addition, the carboxylate ion is signifi cantly stabilized by resonance, much as is the 
enolate ion formed by deprotonation of the !-carbon in aldehydes and ketones (Section 18-1).
Resonance in Carboxylate and Enolate Ions
(B ! base)Carboxylate ion
! BH
B
R
C
O
Oð ð
ð"
"E H "
! pKa 4–5
B
R
C
O
H
Oð ð
#½
! B"ðE EH
A
R
C
O
Oð ð
ð"
"
"E H "
"
A
R
C
O
Oð ð
½
½
"
E N
"
pKa 19–21
Enolate ion
! BH
B
C
CHR
Oð ð
"
E H " CHR CHR"
"
!
B
R# R# R# R#
C
C
H
H R
Oð ð
! B"ðE E
A A
H
A
C
Oð ð
"
E H
"
A
C
Oð ð
"
E N
"
In contrast with enolates, two of the three resonance forms in carboxylate ions are equivalent 
(Section 1-5). As a result, carboxylates are symmetric, with equal carbon–oxygen bond lengths 
(1.26 Å), in between the lengths typical of the carbon–oxygen double (1.20 Å) and single (1.34 Å) 
bonds in the corresponding acids (see Figure 19-1). The electrostatic potential map of acetate 
ion in the margin shows the equal distribution of negative charge (red) over the two oxygens.
Electron-withdrawing substituents increase the acidity 
of carboxylic acids
We saw previously (Section 8-3) that the inductive effect of electron-withdrawing groups 
close to the hydroxy function increases the acidity of alcohols. A similar phenomenon is 
observed in carboxylic acids. Table 19-3 shows the pKa values of selected acids. Note that 
two or three electron-withdrawing groups on the !-carbon can result in carboxylic acids 
whose strength approaches that of some inorganic (mineral) acids.
B
! H2O RCO
B
š"
O
"ð
ðððð
! HOH2š
!
Carboxylic Acids Dissociate Readily
Carboxylate ionKa 10"4
pKa 4–5
–10"5
š"RCOH š"
O
Acetate ion
Exercise 19-5
In the following sets of acids, rank the compounds in order of decreasing acidity.
(a) CH3CH2COOH
A
CH3CHCOOH
Br
CH3CBr2COOH
(b) CH3CHCH2COOH
A
Br
CH3CHCH2COOH
A
F
(c) 
COOH F COOH COOH
F
Turma	ITA	–	Frente	1	 
 
Prof. Calçada 2018 2 
c) isômeros constitucionais 
d) duas moléculas do mesmo composto 
3) Sobre aminoácido glicina (ácido aminoetanoico) são feitas as 
seguintes afirmações: 
(1) Existe em forma cristalina 
(2) É opticamente ativo 
(3) É solúvel em água 
(4) Forma zwitterion 
São corretas: 
a) 1, 2 e 3 b) 1, 2 e 4 
c) 1, 3 e 4 d) 2, 3 e 4 
 
4) A ordem correta de basicidade dos compostos a seguir é: 
 
 
5) A ordem decrescente de acidez dos compostos a seguir é: 
 
 
6) Qual dos compostos a seguir é o mais ácido? Dica: retire o próton 
e veja qual espécie terá mais formas ressonantes. 
 
7) Qual a ordem correta de força das seguintes aminas: 
 
 
 
 
 
 
8) Qual dos compostos a seguir é o ácido mais fraco? 
 
 
9) Em pH próximo de 6,0 um amino ácido está na forma de 
zwitterion. A diminuição e o aumento de pH fazem o aminoácido: R = 
B 
 
a) ficar ácido e básico, respectivamente. 
b) ficar básico e ácido, respectivamente. 
c) perder a carga e ficar uma molécula neutra. 
d) perder sua atividade óptica (exceto a glicina). 
 
10 ) DESAFIO: 
 
 
RESPOSTAS 
 
1) C 2) C 3) C 
4) B 
 
O ácido conjugado da protonação de I é estabilizado por 
ressonância, logo I é mais básico. 
Aminas 2a são mais básicas que 1a e a carbonila deslocaliza o par 
de elétrons do N diminuindo a basicidade. 
 
5) B 
Fenóis são ácidos por natureza. A presença de grupos que removem 
elétrons (-NO2, - X, -NR3+, -CHO, -COX, -COOR, -CN) no anel, 
estabiliza o íon fenóxido e aumenta a acidez. Por outro lado, a 
presença de grupos que cedem elétrons ( grupos alquila e –OR) 
desestabiliza o íon e diminui a acidez do fenol. Além disso, o 
isômero meta é estabilizado mais pelo efeito indutivo que pela 
ressonância, assim os isômeros orto e para são mais ácidos. De 
forma semelhante os isômeros meta (do metoxi) são menos 
desestabilizados que o para. 
 
6) B – fica estabilizado por ressonância. 
 
 
 
 
 
7) C 
Aminas aromática são menos básicas que as aminas cíclicas. Além 
disso o grupo nitro retira elétrons e diminui a basicidade. 
 
8) A 9) B 10 ) Ver no HD 
319C H A P T E R 8P r o b l em s
24. Name the following alcohols according to the IUPAC nomen-
clature system. Indicate stereochemistry (if any) and label the 
hydroxy groups as primary, secondary, or tertiary.
 (a) CH3CH2CHCH3
OH
A
 (b) CH3CHCH2CHCH2CH3
Br
A
OH
A
 (c) HOCH2CH(CH2CH2CH3)2 (d) 
C
H3C
CH2Cl
H
OH
A
; G^
 (e) 
CH2CH3
OH/∑
 (f) 
OH
Br
[
~
 (g) C(CH2OH)4 (h) 
H OH
CH2OH
CH2OH
H OH
 (i) 
]
-
OH
CH2CH2OH
 ( j) ClH3C
CH2OH
CH2CH3
25. Draw the structures of the following alcohols. (a) 2-(Trimethyl-
silyl)ethanol; (b) 1-methylcyclopropanol; (c) 3-(1-methylethyl)-
2-hexanol; (d) (R)-2-pentanol; (e) 3,3-dibromocyclohexanol.
26. Rank each group of compounds in order of increasing boiling 
point. (a) Cyclohexane, cyclohexanol, chlorocyclohexane; 
(b) 2,3-dimethyl-2-pentanol, 2-methyl-2-hexanol, 2-heptanol.
27. Explain the order of water solubilities for the compounds in each 
of the following groups. (a) Ethanol . chloroethane . ethane; 
(b) methanol . ethanol . 1-propanol.
28. 1,2-Ethanediol exists to a much greater extent in the gauche 
conformation than does 1,2-dichloroethane. Explain. Would 
you expect the gauche: anti conformational ratio of 2-chloro-
ethanol to be similar to that of 1,2-dichloroethane or more like 
that of 1,2-ethanediol?
29. The most stable conformation of trans-1,2-cyclohexanediol is 
the chair in which both hydroxy groups are equatorial. (a) Draw 
the structure or, better yet, make a model of the compound in 
this conformation. (b) Reaction of this diol with the chlorosilane 
R3SiCl, R 5 (CH3)2CH (isopropyl), gives the corresponding 
disilyl ether shown below. Remarkably, this transformation 
causes the chair to fl ip, giving a conformation where both silyl 
ether groups are in axial positions. Explain this observation by 
means of either structural drawings or models.
OSiR3]
~OSiR3
30. Rank the compounds in each group in order of decreasing acidity.
(a) CH3CHClCH2OH, CH3CHBrCH2OH, BrCH2CH2CH2OH
(b) CH3CCl2CH2OH, CCl3CH2OH, (CH3)2CClCH2OH
(c) (CH3)2CHOH, (CF3)2CHOH, (CCl3)2CHOH
Problems
31. Write an appropriate equation to show how each of the follow-
ing alcohols acts as, fi rst, a base, and, second, an acid in solu-
tion. How do the base and acid strengths of each compare with 
those of methanol? (a) (CH3)2CHOH; (b) CH3CHFCH2OH; 
(c) CCl3CH2OH.
32. Given the pKa values of 22.2 for CH3O
!
H2 and 15.5 for CH3OH, 
calculate the pH at which (a) methanol will contain exactly 
equal amounts of CH3O
1
H2 and CH3O
2; (b) 50% CH3OH and 
50% CH3O
1
H2 will be present; (c) 50% CH3OH and 50% 
CH3O
2 will be present.
33. Do you expect hyperconjugation to be important in the stabili-
zation of alkyloxonium ions (e.g., R
1
OH2, R2
1
OH)? Explain your 
answer.
34. Evaluate each of the following possible alcohol syntheses as 
being good (the desired alcohol is the major or only product), 
not so good (the desired alcohol is a minor product), or worth-
less. (Hint: Refer to Section 7-9 if necessary.)
(a) CH3CH2CH2CH2Cl CH3CH2CH2CH2OH
H2O, CH3CCH3
O
B
(b) CH3OSO2 CH3 CH3OH
HO!, H2O, ∆
(c) 
I
HO!, H2O, ∆
OH
(d) CH3CHCH2CH2CH3
I
CH3CHCH2CH2CH3
H2O, ∆A
OH
A
(e) CH3CHCH3 CH3CHCH3
CN
A
OH
AHO!, H2O, ∆
(f ) CH3OCH3 CH3OH
HO!, H2O, ∆
(g) 
H3C Br
H2O
H3C OH
(h) CH3CHCH2Cl
CH3
A
CH3CHCH2OH
CH3
AHO!, H2O, ∆
35. For every process in Problem 34 that gives the designated prod-
uct in poor yield, suggest a superior method if possible.
36. Give the major product(s) of each of the following reactions. 
Aqueous work-up steps (when necessary) have been omitted.
(a) CH3CH CHCH3
H3PO4, H2O, ∆
P
(Hint: See Section 8-4.)
(b) CH3CCH2CH2CCH3
O
B
O
B
1. LiAlH4, (CH3CH2)2O
2. H", H2O 
(c) 
C
O
NaBH4, CH3CH2OH
B
H