Page 23 - Color Atlas of Biochemistry

14 Basics
Chemical reactions
Chemical reactions are processes in which
electrons or groups of atoms are taken up
into molecules, exchanged between mole-
cules, or shifted within molecules. Illustrated
here are the most important types of reaction
in organic chemistry, using simple examples.
Electron shifts are indicated by red arrows.
A. Redox reactions
In redox reactions (see also p. 32), electrons
are transferred from one molecule (the reduc-
ing agent) to another (the oxidizing agent).
One or two protons are often also transferred
in the process, but the decisive criterion for
the presence of a redox reaction is the elec-
tron transfer. The reducing agent is oxidized
during the reaction, and the oxidizing agent is example of the S N 2mechanism.Both reac-
reduced.
Fig. A shows the oxidation of an alcohol
into an aldehyde (1) and the reduction of
the aldehyde to alcohol (2). In the process,
one hydride ion is transferred (two electrons
and one proton; see p. 32), which moves to
the oxidizing agent A in reaction 1.The super-
fluous proton is bound by the catalytic effect
of a base B. In the reduction of the aldehyde
(2), A-H serves as the reducing agent and the
acid H-Bisinvolvedasthe catalyst.
B. Acid–base reactions
In contrast to redox reactions, only proton
transfer takes place in acid–base reactions
(see also p. 30). When an acid dissociates (1), In
water serves as a proton acceptor (i. e., as a
base). Conversely, water has the function of
an acid in the protonation of a carboxylate
anion (2).
C. Additions/eliminations
A reaction in which atoms or molecules are
taken up by a multiple bond is described as
addition. The converse of addition—i. e., the
removal of groups with the formation of a
double bond, is termed elimination.When
water is added to an alkene (1a), a proton is
first transferred to the alkene. The unstable
carbenium cation that occurs as an intermedi-
ate initially takes up water (not shown), be-
fore the separation of a proton produces alco-
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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hol (1b). The elimination of water from the
alcohol (2, dehydration) is also catalyzed by
an acid and passes via the same intermediate
as the addition reaction.
D. Nucleophilic substitutions
Areactioninwhich onefunctionalgroup (see
p.10) is replaced by another is termed substi-
tution. Depending on the process involved, a
distinction is made between nucleophilic and
electrophilic substitution reactions (see
chemistry textbooks). Nucleophilic substitu-
tions start with the addition of one molecule
to another, followed by elimination of the so-
called leaving group.
The hydrolysis of an ester to alcohol and
acid (1) and the esterification of a carboxylic
acid withanalcohol (2)are shownhereas an
tions aremadeeasierbythe marked polarity
of the C=O double bond. In the form of ester
hydrolysis shown here, a proton is removed
from a water molecule by the catalytic effect
of thebaseB.The resulting stronglynucleo-
–
philic OH ion attacks the positively charged
carbonyl C of the ester (1a), and an unstable
3
sp -hybridized transition state is produced.
From this, either water is eliminated (2b)
and the ester re-forms, or the alcohol ROH is
eliminated (1b) and thefreeacid results. In
esterification (2), thesamesteps take placein
reverse.
Further information
rearrangements (isomerizations, not
shown), groups are shifted within one and
the same molecule. Examples of this in bio-
chemistry include the isomerization of sugar
phosphates (see p. 36) and of methylmalonyl-
CoA to succinyl CoA (see p.166).