CLAYDEN. Organic Chemistry. 2ª edição. Oxford. 2012.
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CLAYDEN. Organic Chemistry. 2ª edição. Oxford. 2012.


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These two forms are not usually easy to inter-
convert\u2014in other words the C=C double bond is very rigid and cannot rotate.
If we look at the bonding in but-2-ene we can see why. The \u3c0 bond is made up of two parallel 
p orbitals. To rotate about the \u3c0 bond requires those orbitals to lose their interaction, pass 
through a state in which they lie perpendicular, and \ufb01 nally line up again. That transitional, 
perpendicular state is very unfavourable because all of the energy gained through \u3c0 bonding 
is lost. Alkenes are rigid and do not rotate.
H3C
CH3H
H H3C
HH
CH3H3C
H
CH3
H
overlap between p orbitals is lost
\u3c0 bond is broken
try to rotate one
end of \u3c0 bond
very 
unfavourable
process
trans alkene cis alkene
Alkenes are rigid...
Alkene isomers
Maleic and fumaric acids were known in the nineteenth century to have the same chemical composition and the same 
functional groups, and yet they were different compounds\u2014why remained a mystery. That is, until 1874 when van\u2019t Hoff 
proposed that free rotation about double bonds was restricted. This meant that, whenever each carbon atom of a double 
bond had two different substituents, isomers would be possible. He proposed the terms cis (Latin meaning \u2018on this side\u2019) 
and trans (Latin meaning \u2018across or on the other side\u2019) for the two isomers. The problem was: which isomer was which? 
On heating, maleic acid readily loses water to become maleic anhydride so this isomer must have both acid groups on 
the same side of the double bond.
HOOC
COOH
COOH
COOHH
H
H
H
H
H
O
O
O
heat
\u2013 H2O
heat
fumaric acid
trans-butenedioic acid
no change
maleic acid
cis-butenedioic acid
maleic anhydride
Compare that situation with butane. Rotating about the middle bond doesn\u2019t break any 
bonds because the \u3c3 bond is, by de\ufb01 nition, cylindrically symmetrical. Atoms connected only 
by a \u3c3 bond are therefore considered to be rotationally free, and the two ends of butane can 
spin relative to one another.
rotate one
end of \u3c3 bond
very 
easy
process
H
H3C H H
H CH3 H
H3C HH
H3C H H
H3C H CH3
H H
cylindrically symmetrical \u3c3 bond remains intact througout
very 
easy
process
Alkanes rotate freely...
The same comparison works for ethylene (ethene) and ethane: in ethylene all the atoms lie 
in a plane, enforced by the need for overlap between the p orbitals. But in ethane, the two 
ends of the molecule spin freely. This difference in rigidity has important consequences 
throughout chemistry, and we will come back to it in more detail in Chapter 16.
trans (E)
but-2-ene
cis (Z)
but-2-ene
×
do not
interconvert
It is in fact possible to intercon-
vert cis and trans alkenes, but it 
requires a considerable amount 
of energy\u2014around 260 kJ 
mol\u22121. One way to break the \u3c0 
bond is to promote an electron 
from the \u3c0 orbital to the \u3c0* 
orbital. If this were to happen, 
there would be one electron in 
the bonding \u3c0 orbital and one in 
the antibonding \u3c0* orbital, and 
hence no overall bonding. The 
energy required to do this corre-
sponds to light in the ultraviolet 
(UV) region of the spectrum. 
Shining UV light on an alkene 
can break the \u3c0 bond (but not 
the \u3c3 bond) and allows rotation 
to occur.
 \u25a0 In fact not all orientations 
about a \u3c3 bond are equally 
favourable. We come back to 
this aspect of structure, known 
as conformation, in Chapter 16.
H
H
H
H
H
H H H
H H
ethylene is flat and rigid
ethane spins freely
ROTAT ION AND R IG ID ITY 105
2069_Book.indb 105 12/12/2011 8:24:43 PM
Further reading
An excellent introduction to orbitals and bonding is Molecular 
Orbitals and Organic Chemical Reactions: Student Edition by Ian 
Fleming, Wiley, Chichester, 2009.
Check your understanding
 To check that you have mastered the concepts presented in this chapter, attempt the problems that are 
available in the book\u2019s Online Resource Centre at http://www.oxfordtextbooks.co.uk/orc/clayden2e/
Conclusion
We have barely touched the enormous variety of molecules, but it is important that you real-
ize at this point that these simple ideas of structural assembly can be applied to the most 
complicated molecules known. We can use AOs and combine them into MOs to solve the 
structure of very small molecules and to deduce the structures of small parts of much larger 
molecules. With the additional concept of conjugation in Chapter 7 you will be able to grasp 
the structure of any organic compound. From now on we shall use terms like AO and MO, 2p 
orbital, sp2 hybridization, \u3c3 bond, energy level, and populated orbital without further expla-
nation. If you are unsure about any of them, refer back to this chapter.
Looking forward
We started the chapter with atomic orbitals, which we combined into molecular orbitals. But 
what happens when the orbitals of two molecules interact? This is what happens during chemi-
cal reactions, and it\u2019s where we are heading in the next chapter.
CHAPTER 4   STRUCTURE OF MOLECULES106
2069_Book.indb 106 12/12/2011 8:24:43 PM
Online support. The icon in the margin indicates that accompanying interactive resources are provided online to help 
your understanding: just type www.chemtube3d.com/clayden/123 into your browser, replacing 123 with the number of 
the page where you see the icon. For pages linking to more than one resource, type 123-1, 123-2 etc. (replacing 123 
with the page number) for access to successive links.
Connections
 Building on
\u2022 Drawing molecules realistically ch2
\u2022 Ascertaining molecular structure 
spectroscopically ch3
\u2022 What determines molecular shape and 
structure ch4
Arriving at
\u2022 Why molecules generally don\u2019t react 
with each other
\u2022 Why sometimes molecules do react with 
each other
\u2022 How molecular shape and structure 
determine reactivity
\u2022 In chemical reactions electrons move 
from full to empty orbitals
\u2022 Identifying nucleophiles and electrophiles
\u2022 Representing the movement of electrons 
in reactions by curly arrows
 Looking forward to
\u2022 Reactions of the carbonyl group ch6
\u2022 The rest of the chapters in this book
5Organic reactions
Chemical reactions
Most molecules are at peace with themselves. Bottles of sulfuric acid, sodium hydroxide, 
water, or acetone can be safely stored in a laboratory cupboard for years without any change 
in the chemical composition of the molecules inside. Yet if these compounds are mixed, 
chemical reactions, in some cases vigorous ones, will occur. This chapter is an introduction to 
the behaviour of organic molecules: why some react together and some don\u2019t, and how to 
understand reactivity in terms of charges, orbitals, and the movement of electrons. We shall 
also be introducing a device for representing the detailed movement of electrons\u2014the mech-
anism of the reaction\u2014called the curly arrow.
To understand organic chemistry you need to be \ufb02 uent in two languages. The \ufb01 rst is the 
language of structure: of atoms, bonds, and orbitals. This language was the concern of the 
last three chapters: in Chapter 2 we looked at how to draw structures, in Chapter 3 how to 
\ufb01 nd out what those structures are, and in Chapter 4 how to explain structure using electrons 
in orbitals.
But now we need to take up a second language: that of reactivity. Chemistry is \ufb01 rst and 
foremost about the dynamic features of molecules\u2014how to create new molecules from old 
ones, for example. To understand this we need new terminology and tools for explaining, 
predicting, and talking about reactions.
Molecules react because they move. Atoms have (limited) movement within molecules\u2014
you saw in Chapter 3 how the stretching and bending of bonds can be detected by infrared 
spectroscopy, and we explained in Chapter 4 how the \u3c3 bonds of alkanes (but not the \u3c0 bonds 
of alkenes) rotate freely. On