Loudon Ch. 3 Review: Acids/Bases/Curved Arrows
Jacquie Richardson, CU Boulder – Last updated 9/16/2020
There are two different definitions of acids and bases that show up in this chapter:
1. Lewis acids (a.k.a. electrophiles, E or E+) accept an electron pair; Lewis bases (a.ka.
nucleophiles, Nu or Nu-) donate an electron pair
2. Brønsted-Lowry acids (a.k.a. acids) donate a proton (H+ ion); Brønsted-Lowry bases
(a.k.a. bases) accept a proton
Brønsted-Lowry acids/bases are a subset of Lewis acids/bases. If something is a Brønsted-
Lowry base it must also be a Lewis base but the reverse is not true. When an organic chemist
says “acid” or “base”, what we normally mean is Brønsted-Lowry acid or base. We use the
terms nucleophile for Lewis bases (things that love nuclei), and electrophile for Lewis acids
(things that love electrons). Confusingly, chemists will often use “nucleophile” to mean “only-
nucleophile-but-not-a-base”. Officially, though, bases are a subset of nucleophiles, and acids
are a subset of electrophiles. We’re going to look some more at the differences between them,
but first, here are some tools to help explain what’s going on during a reaction: curved arrows,
and frontier molecular orbitals.
Curved Arrows Show Electron Movement
We can show the flow of electrons to a Lewis acid by using curved arrows. These are used to
show the movement of electrons only, not anything else! Two electrons moving as a pair (like
in a reaction between a Lewis acid and base) are shown as a full-headed arrow. (One electron
moving by itself – a radical – is shown as a half-headed or fishhook arrow. We’ll cover these
later.) The Lewis base “attacks” the Lewis acid by pushing electrons towards it.
In other words, these arrows are used to take electrons from electron-rich areas (nucleophiles),
and give them to electron-poor areas (electrophiles). There are only three legal moves for
curved arrows. (This is also a good time to mention that organic chemists will usually neglect
to show lone pairs if they’re not involved in a particular step of a mechanism. But generally, if
you’re showing one LP on an atom, you should show all LPs on that atom.)
1. Bond to an atom → lone pair on that same atom (in this case, electrons from B-F
bond become a new lone pair on F.)
The electrons come out of the F-B bond and recreate the fourth lone pair on F.
Note that the total charge is conserved: there’s an overall -1 charge before the
reaction, so there has to be an overall -1 charge after the reaction. Since B gave up
electrons during the attack (it’s at the back end of the arrow), it loses its negative
charge. Since F gained electrons during the attack (it’s at the front end of the arrow),
it gains a negative charge.
2. Lone pair on an atom → bond to that same atom (in this case, lone pair on F becomes
a new B-F bond.)
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, Loudon Ch. 3 Review: Acids/Bases/Curved Arrows
Jacquie Richardson, CU Boulder – Last updated 9/16/2020
This is the reverse of the previous example: one of the lone pairs on F- is sent
towards B, creating a new F-B bond afterwards. Again, charge is conserved, and the
atom where the arrow ends (the B) gains negative charge while the bottom F loses its
negative charge. Note that before this reaction, B has an unfilled octet and needs to
accept electrons to fix that. Unfilled-octet species are a common example of Lewis
acids.
Here’s another example of this type. In this case, the lone pair on O becomes a new
O-C bond. Note that there’s also a second arrow happening here that converts the
C-Br bond into a LP on Br at the same time.
Not all Lewis acids have unfilled octets. Some have a full octet but also a δ+ charge
due to polar bonds, like the C atom shown above. It’s tempting to show the same
kind of reaction as in the first example, but if we only add new electrons to carbon
then we’ll violate the octet rule by having ten electrons around carbon.
Instead, something else needs to leave the carbon to free up some space. This makes
the reaction an electron pair displacement. In this case the lone pairs on O are
attacking the C and displacing the electrons out of the C-Br bond.
3. Bond to an atom → different bond to that same atom (in this case, B-H bond becomes
a new H-C bond.) This is another displacement reaction, but this time, we’re breaking
a π bond (the C=O) instead of a σ bond. To keep its octet, the C must lose a bond
when it gets attacked, and the easiest bond to break is the π bond. Even though B has
a negative charge, it does not have a lone pair to attack with. Instead, it attacks with
the electrons out of the B-H σ bond. Note that this is not an acid-base reaction,
because there is no proton being moved, but there is a hydride. The difference is that
a hydride (H-) brings electrons with it to do an attack, while a proton (H+) doesn’t.
Here’s another example of bond-to-bond electron transfer. Again, the B-H bond
electrons are being used to form a new H-H bond, which displaces the OH group.
This reaction has both a hydride (1) and a proton (2).
Here’s a more complicated example; this is part of a reaction we’ll see in Ch. 5. Here, the
alkene’s π electrons attack one of the Br atoms (bond to bond), which displaces the other Br
and sends it off with a negative charge (bond to LP). Meanwhile, the LP electrons on the
first Br are attacking one of the C atoms from the alkene (LP to bond).
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Jacquie Richardson, CU Boulder – Last updated 9/16/2020
There are two different definitions of acids and bases that show up in this chapter:
1. Lewis acids (a.k.a. electrophiles, E or E+) accept an electron pair; Lewis bases (a.ka.
nucleophiles, Nu or Nu-) donate an electron pair
2. Brønsted-Lowry acids (a.k.a. acids) donate a proton (H+ ion); Brønsted-Lowry bases
(a.k.a. bases) accept a proton
Brønsted-Lowry acids/bases are a subset of Lewis acids/bases. If something is a Brønsted-
Lowry base it must also be a Lewis base but the reverse is not true. When an organic chemist
says “acid” or “base”, what we normally mean is Brønsted-Lowry acid or base. We use the
terms nucleophile for Lewis bases (things that love nuclei), and electrophile for Lewis acids
(things that love electrons). Confusingly, chemists will often use “nucleophile” to mean “only-
nucleophile-but-not-a-base”. Officially, though, bases are a subset of nucleophiles, and acids
are a subset of electrophiles. We’re going to look some more at the differences between them,
but first, here are some tools to help explain what’s going on during a reaction: curved arrows,
and frontier molecular orbitals.
Curved Arrows Show Electron Movement
We can show the flow of electrons to a Lewis acid by using curved arrows. These are used to
show the movement of electrons only, not anything else! Two electrons moving as a pair (like
in a reaction between a Lewis acid and base) are shown as a full-headed arrow. (One electron
moving by itself – a radical – is shown as a half-headed or fishhook arrow. We’ll cover these
later.) The Lewis base “attacks” the Lewis acid by pushing electrons towards it.
In other words, these arrows are used to take electrons from electron-rich areas (nucleophiles),
and give them to electron-poor areas (electrophiles). There are only three legal moves for
curved arrows. (This is also a good time to mention that organic chemists will usually neglect
to show lone pairs if they’re not involved in a particular step of a mechanism. But generally, if
you’re showing one LP on an atom, you should show all LPs on that atom.)
1. Bond to an atom → lone pair on that same atom (in this case, electrons from B-F
bond become a new lone pair on F.)
The electrons come out of the F-B bond and recreate the fourth lone pair on F.
Note that the total charge is conserved: there’s an overall -1 charge before the
reaction, so there has to be an overall -1 charge after the reaction. Since B gave up
electrons during the attack (it’s at the back end of the arrow), it loses its negative
charge. Since F gained electrons during the attack (it’s at the front end of the arrow),
it gains a negative charge.
2. Lone pair on an atom → bond to that same atom (in this case, lone pair on F becomes
a new B-F bond.)
1
, Loudon Ch. 3 Review: Acids/Bases/Curved Arrows
Jacquie Richardson, CU Boulder – Last updated 9/16/2020
This is the reverse of the previous example: one of the lone pairs on F- is sent
towards B, creating a new F-B bond afterwards. Again, charge is conserved, and the
atom where the arrow ends (the B) gains negative charge while the bottom F loses its
negative charge. Note that before this reaction, B has an unfilled octet and needs to
accept electrons to fix that. Unfilled-octet species are a common example of Lewis
acids.
Here’s another example of this type. In this case, the lone pair on O becomes a new
O-C bond. Note that there’s also a second arrow happening here that converts the
C-Br bond into a LP on Br at the same time.
Not all Lewis acids have unfilled octets. Some have a full octet but also a δ+ charge
due to polar bonds, like the C atom shown above. It’s tempting to show the same
kind of reaction as in the first example, but if we only add new electrons to carbon
then we’ll violate the octet rule by having ten electrons around carbon.
Instead, something else needs to leave the carbon to free up some space. This makes
the reaction an electron pair displacement. In this case the lone pairs on O are
attacking the C and displacing the electrons out of the C-Br bond.
3. Bond to an atom → different bond to that same atom (in this case, B-H bond becomes
a new H-C bond.) This is another displacement reaction, but this time, we’re breaking
a π bond (the C=O) instead of a σ bond. To keep its octet, the C must lose a bond
when it gets attacked, and the easiest bond to break is the π bond. Even though B has
a negative charge, it does not have a lone pair to attack with. Instead, it attacks with
the electrons out of the B-H σ bond. Note that this is not an acid-base reaction,
because there is no proton being moved, but there is a hydride. The difference is that
a hydride (H-) brings electrons with it to do an attack, while a proton (H+) doesn’t.
Here’s another example of bond-to-bond electron transfer. Again, the B-H bond
electrons are being used to form a new H-H bond, which displaces the OH group.
This reaction has both a hydride (1) and a proton (2).
Here’s a more complicated example; this is part of a reaction we’ll see in Ch. 5. Here, the
alkene’s π electrons attack one of the Br atoms (bond to bond), which displaces the other Br
and sends it off with a negative charge (bond to LP). Meanwhile, the LP electrons on the
first Br are attacking one of the C atoms from the alkene (LP to bond).
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