PHA284
Organic Chemistry II
Ankara University
Faculty of Pharmacy
Department of Pharmaceutical Chemistry
Ring-Activating and
Ring-Deactivating Substituents
Substituents Can Affect Both the Reactivity of the
Ring and the Orientation of the Incoming Group
A substituent group present on a benzene ring can affect both the reactivity
of the ring toward electrophilic substitution and the orientation that the
incoming group takes on the ring.
• A substituent can make the ring more reactive than benzene. Such a group
is called an activating group.
• A substituent can make the ring less reactive than benzene. Such groups
are called deactivating groups.
How Do Substituents Affect Reactivity?
Ring-Activating and
Ring-Deactivating Substituents
Consider the relative nitration rates of the following compounds, all under the
same reaction conditions:
• Taking benzene as the standard, we see that some substituents (for
example, OH and CH3) speed up the reaction, and other substituents (Cl
and NO2) retard the reaction.
• We know from other evidence that hydroxyl and methyl groups are more
electron donating than hydrogen, whereas chloro and nitro groups are
more electron withdrawing than hydrogen.
• The electrostatic potential maps illustrate the effects of electron-donating
and electron-withdrawing groups on the electron density in the benzene
ring.
Ortho, Para-Directing and Meta-Directing
Groups
Ortho, Para-Directing and Meta-Directing
Groups
Nitration of Toluene: The Effect of AIkyl Substitution
1. Toluene reacts about 25 times faster than benzene under the same
conditions.
2. Nitration of toluene gives a mixture of products.
In ortho or para substitution of toluene, the positive charge is
spread over two secondary carbons and one tertiary (30) carbon
(bearing the CH3 group).
Meta substitution of toluene does not show the large rate enhancement
seen with ortho and para substitution.
Ortho, Para-Directing Groups
Consider now the other ortho, para-directing groups listed in Table.
In each of them, the atom attached to the aromatic ring has an
unshared electron pair.
Meta-Directing Groups
In the Table, notice that each meta-directing group is connected to the
aromatic ring by an atom that is part of a double or triple bond, at the other
end of which is an atom more electronegative than carbon (for example, an
oxygen or nitrogen atom).
In nitrobenzene, the nitrogen has a formal charge of +1, as
shown on the structures. The equations for forming the
intermediate benzenonium ion are:
Reactions of the Side Chain of
Alkylbenzenes
Hydrocarbons that consist of both aliphatic and aromatic groups are also
known as arenes. Toluene, ethylbenzene, and isopropylbenzene are
alkylbenzenes. Phenylethene, usually called styrene, is an example of an
alkenylbenzene. :
A- Benzylic Halogenation of the Side Chain
B- Additions to the Double Bond of Alkenylbenzenes
C- Oxidation of the Side Chain
An important characteristic of side-chain oxidations is that oxidation takes
place initially at the benzylic carbon.
References
• Organic Chemistry 11e, T.W. Graham Solomons, Craig B. Fryhle, Scott A.
Snyder, John Wiley & Sons, Inc., 2014, ISBN 978-1-118-13357-6 (cloth) Binder-
ready version ISBN 978-1-118-14739-9
• Organic Chemistry: A Short Course, 13th Ed., D.J. Hart, C.M. Hadad, L.E.
Craine, H. Hart, Brooks/Cole, Cengage Learning, 2012, ISBN-13: 978-1-111-
42556-2
• Organic Chemistry, 6th Ed., L. G. Wade, Pearson Education, Inc., 2006, ISBN 0-
13-147871-0
• Organic Chemistry, 2nd Ed., Jonathan Clayden, Nick Greeves, and Stuart
Warren,, Oxford University Press, 2012, ISBN: 9780199270293
• Organic Chemistry, Mukherjee, S.M., et al., New Age International Ltd, 2008.
ProQuest Ebook Central,
http://ebookcentral.proquest.com/lib/ankara/detail.action?docID=3017383.
