Organic Chemistry

PROBLEM 29 SOLVED

Most of the values given throughout this text are values determined in water. How

would the values of the following classes of compounds change if they were deter-

mined in a solvent less polar than water: carboxylic acids, alcohols, phenols, ammonium

ions and anilinium ions

SOLUTION A is the negative logarithm of an equilibrium constant,

(Section 1.17). Because we are determining how changing the polarity of a solvent affects

an equilibrium constant, we must look at how changing the polarity of the solvent affects

the stability of the reactants and products.

Carboxylic acids, alcohols, and phenols are neutral in their acidic forms (HB) and

charged in their basic forms A polar protic solvent will stabilize and more

than it will stabilize HB, thereby increasing Therefore, will be larger in water than

in a less polar solvent, which means that the value will be lower in water. So the

values of carboxylic acids, alcohols, and phenols determined in a less polar solvent than

water will be higher than those determined in water.

Ammonium ions and anilinium ions are charged in their acidic forms and neutral

in their basic forms (B). A polar solvent will stabilize and more than it will stabi-

lize B. Because is stabilized slightly more than will decrease, which means

that the value will be higher in water than in a less polar solvent. So the values of

ammonium ions and anilinium ions determined in a less polar solvent than water will be

slightly lower than those determined in water.

PROBLEM 30

Would you expect acetate ion to be a more reactive nucleophile in an re-

action carried out in methanol or in dimethyl sulfoxide?

PROBLEM 31

Under which of the following reaction conditions would (R)-2-chlorobutane form the most

(R)-2-butanol: in 50% water and 50% ethanol or in 100% ethanol?

10.11 Biological Methylating Reagents

If an organic chemist wanted to put a methyl group on a nucleophile, methyl iodide

would most likely be the methylating agent used. Of the methyl halides, methyl iodide

has the most easily displaced leaving group because is the weakest base of the

halide ions. In addition, methyl iodide is a liquid, so it is easier to handle than methyl

bromide or methyl chloride. The reaction would be a simple reaction.

In a living cell, however, methyl iodide is not available. It is only slightly solu-

ble in water, so is not found in the predominantly aqueous environments of biologi-

cal systems. Instead, biological systems use S-adenosylmethionine (SAM) and

as methylating agents, both of which are soluble in water.

Although they look much more complicated than methyl iodide, they perform the

same function—the transfer of a methyl group to a nucleophile. Notice that the methyl

N^5 -methyltetrahydrofolate

Nu ++CH 3 IICH 3 Nu −

−

SN 2

I-

HO- HO-

(CH 3 CO 2 - ) SN 2

pKa pKa

HB+ H+, Ka

HB+ H+

(HB+)

pKa pKa

Ka. Ka

(B-). B- H+

Ka [B

−] [H+]

[HB]

[B] [H+]

[HB+]

= Ka=

a neutral acid a positively charged acid

pKa Ka

(RNH 3 + ), (C 6 H 5 NH 3 + )?

pKa

pKa

Et 2 O

e.CH 3 Br + NH 3 CH 3 NH 3 + Br−

EtOH

CH 3 Br + NH 3 CH 3 NH 3 + Br−

+

+

394 CHAPTER 10 Substitution Reactions of Alkyl Halides