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Clayden_-_Organic_chemistry

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The most common type of nucleo-
phile has a nonbonding lone pair of electrons. Usually these are on a heteroatom such as O, N, S,
or P.
These four neutral molecules, ammonia, water, trimethylphosphine, and dimethylsulfide, all have
lone pairs of electrons in sp3 orbitals and in each case this is the donor or nucleophilic orbital. The
group VI atoms (O and S) have two lone pairs of equal energy. These are all nonbonding electrons
and therefore higher in energy than any of the bonding electrons.
118 5 . Organic reactions
orbitals have the same energy small difference in energy 
of filled and empty orbitals
large difference in energy 
of filled and empty orbitals
orbital
energy
new
MOs
new
MOs
new
MOs
Nu E Nu
E
Nu
E
�
We saw exactly the same response
when we combined AOs of different
energies to make MOs in Chapter 4.
•Molecules repel each other because of their outer coatings of electrons.
Molecules attract each other because of:
• attraction of opposite charges
• overlap of high-energy filled orbitals with low-energy empty orbitals
For reaction, molecules must approach each other so that they have:
• enough energy to overcome the repulsion
• the right orientation to use any attraction
H
N
H
H
ammonia
H
O
H
water
Me
S
Me
dimethylsulfide
Me
P
Me
Me
trimethylphosphine
Anions are often nucleophiles too and these are
also usually on heteroatoms such as O, S, or halogen
which may have several lone pairs of equal energy.
The first diagram for each of our examples shows the
basic structure and the second diagram shows all the
lone pairs. It is not possible to allocate the negative
charge to a particular lone pair as they are the same.
There are a few examples of carbon nucleophiles with lone pairs of electrons, the most famous
being the cyanide ion. Though linear cyanide has a lone pair on nitrogen and one on carbon, the
nucleophilic atom is usually anionic carbon rather than neutral nitrogen as the sp orbital on carbon
has a higher energy than that on the more electronegative nitrogen. Most anionic nucleophiles con-
taining carbon have a heteroatom as the nucleophilic atom such as the anion methane thiolate
shown above.
Neutral carbon electrophiles usually have a π
bond as the nucleophilic portion of the molecule.
When there are no lone pair electrons to supply
high-energy nonbonding orbitals, the next best is
the lower-energy filled π orbitals rather than the
even lower-energy σ bonds. Simple alkenes are
weakly nucleophilic and react with strong electrophiles such as bromine. In Chapter 20 we shall see
that the reaction starts by donation of the π electrons from the alkene into the σ* orbital of the
bromine molecule (which breaks the Br–Br bond) shown here with a curly arrow. After more steps
the dibromoalkane is formed but the molecules are attracted by overlap between the full π orbital
and the empty σ* orbital.
It is possible for σ bonds to act as nucleophiles
and we shall see later in this chapter that the boro-
hydride anion, BH4
–, has a nucleophilic B–H bond
and can donate those electrons into the π∗ orbital of
a carbonyl compound breaking that bond and even-
tually giving an alcohol as product. The first stage of the reaction has electrons from the B–H single
bond of nucleophilic anion BH4
–, which lacks lone pair electrons or πbonds, as the nucleophile.
In this section you have seen lone pairs on anions and neutral molecules acting as nucleophiles
and, more rarely, πbonds and even σ bonds able to do the same job. In each case the nucleophilic
electrons came from the HOMO—the highest occupied molecular orbital—of the molecule. Don’t
worry if you find the curly arrows strange at the moment. They will soon be familiar. Now we need to
look at the other side of the coin—the variety of electrophiles.
Electrophiles have a low-energy vacant orbital
Electrophiles are neutral or positively charged species with an empty atomic orbital (the opposite of
a lone pair) or a low-energy antibonding orbital. The simplest electrophile is the proton, H+, a
species without any electrons at all and a vacant 1s orbital. It is so reactive that it is hardly ever found
and almost any nucleophile will react with it.
Each of the nucleophiles we saw in the previous section will react with the proton and we shall
look at two of them together. Hydroxide ion combines with a proton to give water. This reaction is
Chemical reactions 119
H O
H O
hydroxide
Me S
Me S
methane thiolate bromide
Br
Br
�
This point will be important in Chapter 6
as well.
C N
C N
cyanide ion
C N
sp lone pairsp lone pair
H H
HH
BrBr
Br
Br
H
H
H
H
H
B
H
H H O
H
B
H H H O H OH
+
H NuH H
empty 1s orbitalproton
H
reaction with anionic nucleophile
Nu
governed by charge control. Then water itself reacts with the proton to give H3O
+, the true acidic
species in all aqueous strong acids.
We normally think of protons as acidic rather than electrophilic but an acid is just a special kind
of electrophile. In the same way, Lewis acids such as BF3 or AlCl3 are electrophiles too. They have
empty orbitals that are usually metallic p orbitals. We saw above how BF3 reacted with Me3N. In that
reaction BF3 was the electrophile and Me3N the nucleophile. Lewis acids such as AlCl3 react violent-
ly with water and the first step in this process is nucleophilic attack by water on the empty p orbital of
the aluminium atom. Eventually alumina (Al2O3) is formed.
Few organic compounds have vacant atomic orbitals and most organic electrophiles have low-
energy antibonding orbitals. The most important are π* orbitals as they are lower in energy than σ*
orbitals and the carbonyl group (C=O) is the most important of these—indeed it is the most impor-
tant functional group of all. It has a low-energy π* orbital ready to accept electrons and also a partial
positive charge on the carbon atom. Previously we said that charge attraction helped nucleophiles to
find the carbon atom of the carbonyl group.
Charge attraction is important in carbonyl reac-
tions but so are the orbitals involved. Carbonyl com-
pounds have a low-energy bonding π orbital.
Carbonyl compounds have a dipole because in this
filled orbital the electrons are more on electronegative
oxygen than on carbon. The same reason (electro-
negative oxygen) makes this an exceptionally low-
energy orbital and the carbonyl group a very stable
structural unit. This orbital is rarely involved in reac-
tions. Going up the energy scale we next have two
degenerate (equal in energy) lone pairs in nonbond-
ing orbitals. These are the highest-energy electrons in
the molecule (HOMO) and are the ones that react
with electrophiles.
When we consider the carbonyl group as an electrophile, we must look at antibonding orbitals
too. The only one that concerns us is the relatively low-energy π* orbital of the C=O double bond
(the LUMO). This orbital is biased towards the carbon to compensate for the opposite bias in the
filled π orbital. How do we know this if there are no electrons in it? Simply because nucleophiles,
whether charged or not, attack carbonyl groups at the carbon atom. They get the best overlap with
the larger orbital component of the π* orbital.
120 5 . Organic reactions
H
O
H
H
O
H
HH O HH
hydroxide as 
nucleophile
water as 
nucleophile
H
O
H
Cl
Al
Cl Cl
H
O
H
Al Cl
Cl
Cl
water as 
nucleophile
empty
p orbital
Al2O3
new
σ bond
Protic acids (also known as Brønsted acids) are
electrophiles (like HCl) that can donate protons (H+) to
nucleophiles. They will be discussed in detail in Chapter 8.
Lewis acids are also electrophiles but they donate more
complicated cations to nucleophiles. They are usually metal
halides such as LiCl, BF3, AlCl3, SnCl4, and TiCl4. We shall
meet them in many later chapters, particularly in Chapters
22–8 when we discuss carbon–carbon bond formation.
O Nu
C=O dipole
δ–
electrostatic
attraction
charged 
nucleophile
δ+
O
Ononbondinglone pairs
bonding
orbitals
π
high-energy filled orbitals of the carbonyl