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group
Protic and Lewis acids
So now we can draw a mechanism for the attack of a nucleophile on the carbonyl group.
The lone pair electrons on the nucleophile move into the π* orbital of the C=O double
bond and so break the πbond, though not, of course, the σ bond. Here is that process in
curly arrow terms.
The lone pair electrons on oxygen interact better with empty orbitals such as the 1s of
the proton and so carbonyl compounds are protonated on oxygen.
The resulting cation is even more electrophilic because of the positive charge but nucle-
ophiles still attack the carbon atom of the carbonyl group because the π* orbital still has
more contribution from carbon. The positive charge is neutralized even though the nucle-
ophile does not attack the positively charged atom.
Even σ bonds can be electrophilic if the atom at one end of them is sufficiently electronegative to
pull down the energy of the σ* orbital. Familiar examples are acids where the acidic hydrogen atom
is joined to strongly electronegative oxygen or a halogen thus providing a dipole moment and a rela-
tively low-energy σ* orbital.
These two diagrams suggest two different
ways of looking at the reaction between a base
and an acid, but usually both interactions are
important. Notice that an acid is just an elec-
trophile that has an electrophilic hydrogen atom
and a base is just a nucleophile that acts on a
hydrogen atom. This question is explored more
in Chapter 8. Bonds between carbon and halo-
gen are also polarized in some cases though the
electronegativity difference is sometimes very
small.
It is easy to exaggerate the importance of
single-bond polarization. The electronegativity
difference between H and Cl is 0.9 but that
Chemical reactions 121
O
O
Ononbondinglone pairs
bonding
orbitals
antibonding
orbitals
π
π∗
nonbonding
lone pair
small energy gap
Nu
O ONuNu
O O
H
H
O
H
OHNuNu
HCl B
charged 
base
δ+
H–Cl dipole
δ–
electrostatic
attraction
�
These are Pauling
electronegativities, calculated by
Linus Pauling (1901–94) who
won the chemistry Nobel prize in
1954 and the Nobel peace prize
in 1983 and from whose ideas
most modern concepts of the
chemical bond are derived. Born
in Portland, Oregon, he worked at
‘CalTech’ (the California Institute
of Technology at Pasadena) and
had exceptionally wide-ranging
interests in crystallography,
inorganic chemistry, protein
structure, quantum mechanics,
nuclear disarmament, politics,
and taking vitamin C to prevent
the common cold.
HCl B
σ* orbital 
of acid
non-bonding
lone pair 
of base
Quick guide to important electronegativities
H
2.1
Li B C N O F
1.0 2.0 2.5 3.0 3.5 4.0
Mg Al Si P S Cl
1.2 1.5 1.8 2.1 2.5 3.0
Br
2.8
I
2.5
between C and Br only 0.3 while the C–I bond is not polarized at all. When carbon–halogen σ bonds
act as electrophiles, polarity hardly matters but a relatively low-energy σ* orbital is vitally important.
The bond strength is also important in these reactions too as we shall see.
Some σ bonds are electrophilic even though they have no dipole at all. The halogens such as
bromine (Br2) are examples. Bromine is strongly electrophilic because it has a very weak Br–Br σ
bond. Symmetrical bonds have the energies of the σ orbital and the σ* orbital roughly evenly dis-
tributed about the nonbonding level. A weak symmetrical σ bond means a small energy gap while a
strong symmetrical σ bond means a large energy gap. Bromine is electrophilic but carbon–carbon σ
bonds are not. Reverting to the language of Chapter 4, we could say that the hydrocarbon framework
is made up of strong C–C bonds with low-energy populated and high-energy unpopulated orbitals,
while the functional groups react because they have low LUMOs or high HOMOs.
An example would be the rapid reaction between a sulfide and bromine. No reaction at all occurs
between a sulfide and ethane or any other simple C–C σ bond. Lone pair electrons are donated from
sulfur into the Br–Br σ* orbital, which makes a new bond between S and Br and breaks the old
Br–Br bond.
Summary: interaction between HOMO and LUMO leads to reaction
Organic reactions occur when the HOMO of a nucleophile overlaps with the LUMO of the elec-
trophile to form a new bond. The two electrons in the HOMO slot into the empty LUMO. The react-
ing species may be initially drawn together by electrostatic interaction of charges or dipoles but this is
not necessary. Thus at this simplest of levels molecular recognition is required for reaction. The two
components of a reaction must be matched in terms of both charge–charge attraction and the energy
and orientation of the orbitals involved.
Nucleophiles may donate electrons (in order of preference) from a lone pair, a πbond, or even a σ
bond and electrophiles may accept electrons (again in order of preference) into an empty orbital or
into the antibonding orbital of a πbond (π* orbital) or even a σ bond (σ* orbital). These antibond-
ing orbitals are of low enough energy to react if the bond is very polarized by a large electronegativity
difference between the atoms at its ends or, even for unpolarized bonds, if the bond is weak.
The hydrocarbon framework of organic molecules is unreactive. Functional groups such as NH2
and OH are nucleophilic because they have nonbonding lone pairs. Carbonyl compounds and alkyl
halides are electrophilic functional groups because they have low-energy LUMOs (π* for C=O and
σ* for C–X, respectively).
122 5 . Organic reactions
small
Br Br
Br Br
Br Br
H3C CH3
H3C CH3
σ
six degenerate
nonbonding
lone pairs
σ∗small
energy
gap
σ∗
large
energy
gap
strong 
bond
large
energy
gap
σ
nonbonding
electrons of
nucleophile
small gap
good overlap
large gap
poor overlap
energy
gap
weak
bond
�
Notice how putting charges in circles
(Chapter 2) helps here. There is no
problem in distinguishing the charge on
sulfur (in a ring) with the plus sign (not
in a ring) linking the two products of the
reaction.
Br Br
Me
S
Me
Me
S
Me
BrBr +
Organic chemists use curly arrows to represent reaction
mechanisms
You have seen several examples of curly arrows so far and you may already have a general idea of
what they mean. The representation of organic reaction mechanisms by this means is so important
that we must now make quite sure that you do indeed understand exactly what is meant by a curly
arrow, how to use it, and how to interpret mechanistic diagrams as well as structural diagrams.
A curly arrow represents the actual movement of a pair of electrons from a filled orbital into an
empty orbital. You can think of the curly arrow as representing a pair of electrons thrown, like a
climber’s grappling hook, across from where he is standing to where he wants to go. In the simplest
cases, the result of this movement is to form a bond between a nucleophile and an electrophile. Here
are two examples we have already seen in which lone pair electrons are transferred to empty atomic
orbitals.
Note the exact position of the curly arrow as the value of this representation lies in the precision
and uniformity of its use. The arrow always starts with its tail on the source of the moving electrons,
representing the filled orbital involved in the reaction. The head of the arrow indicates the final des-
tination of the pair of electrons—the new bond between oxygen and hydrogen or oxygen and alu-
minium in these examples. As we are forming a new bond, the head of the arrow should be drawn to
a point on the line between the two atoms.
When the nucleophile attacks an antibonding orbital, such as the weak Br–Br bond we have just
been discussing, we shall need two arrows, one to make the new bond and one to break the old.
The bond-making arrow is the same as before but the bond-breaking arrow is new. This arrow
shows that the two electrons in the bond move to one end (a bromine atom) and turn it into an anion.
This arrow should start in the centre of the bond and its head should rest on the atom (Br in this case)
at the end of the bond. Another example