1 (19)

Prévia do material em texto

Solutions for Introduction and Review
O HCH3CH2 CH3 N H
H
NCH3 CH3CH3CH2 C O H
O
CH3CH2 O
N CH3CH3
H
H
N HCH3
H
CH3CH2 C
O
O
CH3CH2 C
O
O
17 In Solved Problem 4, the structure of methylamine is shown to be similar to ammonia. It is 
reasonable to infer that their acid-base properties are also similar.
(a) This problem can be viewed in two ways. 1) Quantitatively, the pKa values determine the order of 
acidity. 2) Qualitatively, the stabilities of the conjugate bases determine the order of acidity (see 
Solved Problem 4 for structures): the conjugate base of acetic acid, acetate ion, is resonance-
stabilized, so acetic acid is the most acidic; the conjugate base of ethanol has a negative charge on a very 
electronegative oxygen atom; the conjugate base of methylamine has a negative charge on a mildly 
electronegative nitrogen atom and is therefore the least stabilized, so methylamine is the least acidic. 
(The first two pK values are from text Table 5.)a
 acetic acid > ethanol > methylamine
 pKa 4.74 pKa 15.9 pKa ≈ 40 (from text Appendix 4)
 strongest acid weakest acid
(b) Ethoxide ion is the conjugate base of ethanol, so it must be a stronger base than ethanol; Solved 
Problem 4 and text Table 5 indicate ethoxide is analogous to hydroxide in base strength. 
Methylamine has pKb 3.36. The basicity of methylamine is between the basicity of ethoxide ion and 
ethanol.
 ethoxide ion > methylamine > ethanol
strongest base weakest base
18 Curved arrows show electron movement, as described in text Section 14.
+
conjugate base
(a)
(b)
+ +
+
conjugate base
conjugate acid
conjugate acid
stronger acid stronger base
weaker base
weaker acidequilibrium favors PRODUCTS
stronger acid
stronger base
weaker acid
weaker base
equilibrium favors PRODUCTS
O HCH3
O S SO
O
O
O
O OS
O
O
O OO
O S
O
O
O
H H O HCH3
H
H HH
(c)
conjugate base, weaker base
conjugate acidstronger base
weaker acid
equilibrium favors 
PRODUCTS
stronger acid
+ +
more stable anion—
negative charge on the 
more electronegative 
atom
resonance stabilized
resonance stabilized
14