How To Use Curved Arrows in Illustrating Reactions
How To Use Curved Arrows in Illustrating Reactions
** Up to this
point we have not indicated how bonding changes occur in the reactions we have presented,
but this can easily be done using curved-arrow notation.
** Curved
arrows show the direction of electron flow in a reaction mechanism.
(1) Draw
the curved arrow so that it points from the source of an electron pair to the
atom receiving the pair. (Curved arrows can also show the movement of single
electrons. We shall discuss reactions of this type in a later chapter.)
(2)
Always show the flow of electrons from a site of higher electron density to a
site of lower electron density.
(3) Never
use curved arrows to show the movement of atoms. Atoms are assumed to follow the
flow of the electrons.
(4)
Make sure that the movement of electrons shown by the curved arrow does not
violate the octet rule for elements in the second row of the periodic table.
Examples
Example (1): Reaction of Water with Hydrogen Chloride
** The reaction
of hydrogen chloride with water provides a simple example of how to use curved arrow
notation.
** Here we
invoke the first of many “A Mechanism for the Reaction” boxes, in which we show
every key step in a mechanism using color-coded formulas accompanied by explanatory
captions
** The curved
arrow begins with a covalent bond or unshared electron pair (a site of higher
electron density) and points toward a site of electron deficiency.
** We see here that
as the water molecule collides with a hydrogen chloride molecule, it uses one
of its unshared electron pairs (shown in blue) to form a bond to the proton of
HCl.
** This bond
forms because the negatively charged electrons of the oxygen atom are attracted
to the positively charged proton.
** As the bond
between the oxygen and the proton forms, the hydrogen–chlorine bond of HCl
breaks, and the chlorine of HCl departs with the electron pair that formerly
bonded it to the proton. (If this did not happen, the proton would end up
forming two covalent bonds, which, of course, a proton cannot do.)
** We,
therefore, use a curved arrow to show the bond cleavage as well. By pointing
from the bond to the chlorine, the arrow indicates that the bond breaks and the
electron pair leaves with the chloride ion.
Example (2): acid–base reactions
The following
acid–base reactions give other examples of the use of the curved-arrow notation:
Solved problem
Add curved
arrows to the following reactions to indicate the flow of electrons for all of
the bond-forming and bond- breaking steps.
Strategy:
** Recall the
rules for use of curved arrows presented at the beginning of this subject.
** Curved
arrows point from the source of an electron pair to the atom receiving the
pair, and always point from a site of higher electron density to a site of
lower electron density.
** We must also
not exceed two electrons for a hydrogen atom, or an octet of electrons for any
elements in the second row of the periodic table.
** We must also
account for the formal charges on atoms and write equations whose charges are
balanced.
Answer:
(1) In (a):
** The hydrogen
atom of HCl is partially positive (electrophilic) due to the electronegativity
of the chlorine atom.
** The alcohol
oxygen is a source of electrons (a Lewis base) that can be given to this
partially positive proton.
** The proton must
lose a pair of electrons as it gains a pair, however, and thus the chloride ion
accepts a pair of electrons from the bond it had with the hydrogen atom as the
hydrogen becomes bonded to the alcohol oxygen.
(2) In (b):
The carboxylic acid hydrogen is partially
positive and therefore electrophilic, and the amine provides an unshared pair
of electrons that forms a bond with the carboxylic acid hydrogen, causing
departure of a carboxylate anion.
Reference: Organic chemistry / T.W. Graham Solomons , Craig B.Fryhle , Scott A.snyder , / ( eleventh edition) / 2014.
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