substituents

Examples of substituents in the following topics:

• Naming Aromatic Compounds

  • Aromatic compounds are named based on the number and type of substituents on the ring.
  • When there is a single substituent on a benzene ring and the substituent contains six or fewer carbons, the substituent is included as a prefix to benzene.
  • If the substituent contains more than six carbons, the alkane portion is named first, and the aromatic ring portion is added as a suffix.
  • When there are multiple substituents, ring atoms are numbered to minimize the numbers assigned to the substituted positions.
  • Disubstituted benzene rings can be named based on the relative positions of the substituents: the prefix ortho- is used if the substituents occupy adjacent positions on the ring (1,2), meta- is used if the substituents are separated by one ring position (1,3), and para- if they are found on opposite sides of the ring (1,4).

• Substitution Reactions of Benzene Derivatives

  • In contrast, a nitro substituent decreases the ring's reactivity by roughly a million.
  • The first is the inductive effect of the substituent.
  • Clearly, the alkyl substituents activate the benzene ring in the nitration reaction, and the chlorine and ester substituents deactivate the ring.
  • Halogen ( X ), OR and NR2 substituents all exert a destabilizing inductive effect on an adjacent positive charge, due to the high electronegativity of the substituent atoms.
  • Consequently, all these substituents direct substitution to ortho and para sites.

• Electrophilic Substitution of Disubstituted Benzene Rings

  • When a benzene ring has two substituent groups, each exerts an influence on subsequent substitution reactions.
  • The activation or deactivation of the ring can be predicted more or less by the sum of the individual effects of these substituents.
  • The products from substitution reactions of compounds having a reinforcing orientation of substituents are easier to predict than those having antagonistic substituents.
  • The strongly activating hydroxyl (–OH) and amino (–NH2) substituents favor dihalogenation in examples 5 and six.
  • Substitution reactions of compounds having an antagonistic orientation of substituents require a more careful analysis.

• Configurational Stereoisomers of Cycloalkanes

  • Unlike the relatively flat molecules of alkenes, substituted cycloalkanes must be viewed as three-dimensional configurations in order to appreciate the spatial orientations of the substituents.
  • By agreement, chemists use heavy, wedge-shaped bonds to indicate a substituent located above the average plane of the ring (note that cycloalkanes larger than three carbons are not planar), and a hatched line for bonds to atoms or groups located below the ring.
  • In general, if any two sp3 carbons in a ring have two different substituent groups (not counting other ring atoms) stereoisomerism is possible.
  • If more than two ring carbons have different substituents (not counting other ring atoms) the stereochemical notation distinguishing the various isomers becomes more complex.

• Benzen Derivatives

  • Two commonly encountered substituent groups that incorporate a benzene ring are phenyl, abbreviated Ph-, and benzyl, abbreviated Bn-.
  • When more than one substituent is present on a benzene ring, the relative locations of the substituents must be designated by numbering the ring carbons or by some other notation.
  • Finally, if there are three or more substituent groups, the ring is numbered in such a way as to assign the substituents the lowest possible numbers, as illustrated by the last row of examples.
  • The substituents are listed alphabetically in the final name.

• Configurational Stereoisomers of Alkenes

  • The essential requirement for this stereoisomerism is that each carbon of the double bond must have two different substituent groups (one may be hydrogen).
  • In the first example, the left-hand double bond carbon has two identical substituents (A) so stereoisomerism about the double bond is not possible (reversing substituents on the right-hand carbon gives the same configuration).
  • In the next two examples, each double bond carbon atom has two different substituent groups and stereoisomerism exists, regardless of whether the two substituents on one carbon are the same as those on the other.

• Formulas Using Other Configurational Notations

  • Here, the zig-zag carbon chain lies in a plane and the absolute or relative configurations at the chiral centers are then designated by wedge or hatched bonds to substituent groups.
  • In the commonly used zig-zag drawings substituents may lie on the same side of the carbon chain, a syn orientation, or on opposite sides, an anti orientation.
  • For adjacent (vicinal) substituents this is opposite to their location in a Fischer formula.
  • Thus, the substituents in the erythro isomer have an anti orientation, but are syn in the threo isomer.
  • At sites having two substituents, such as carbon #5, the terms refer to the relative orientation of the highest order substituent, as determined by the C.I.P. sequence rules.

• Directed ortho-Metalation

  • Electrophilic substitution of aromatic rings generally gives a mixture of ortho and para substitution products when an existing substituent activates the ring or meta products when the substituent is deactivating.
  • In the case of anisole the methoxy substituent is a strongly activating group.
  • The strongly deactivating sulfonic acid substituent is easily converted to a DMG by amide formation.
  • A similar amide derivative of a carboxylic acid substituent may be used for DoM, as shown in the following diagram.

• Designating the Configuration of Chiral Centers

  • If we number the substituent groups from 1 to 4, with 1 being the highest and 4 the lowest in priority sequence, the two enantiomeric configurations are shown in the following diagram along with a viewers eye on the side opposite substituent #4.
  • Rule # 3 of the sequence rules allows us to order these substituents.
  • Here a stereogenic tetrahedral carbon has four different substituents, designated 1, 2, 3 & 4.
  • The viewing rule states that when the lowest priority substituent (4) is oriented behind the triangular face defined by the three higher priority substituents (shaded light gray here), a clockwise sequential arrangement of these substituents (1, 2 & 3) is defined as R, and a counter-clockwise sequence as S.
  • Below is the diagram for the 1:3:4-face, shaded light gray. oriented in front of substituent 2.

• Nomenclature

  • The ether functional group does not have a characteristic IUPAC nomenclature suffix, so it is necessary to designate it as a substituent.
  • To do so the common alkoxy substituents are given names derived from their alkyl component, as shown in the table on the right below.
  • Many simple ethers are symmetrical, in that the two alkyl substituents are the same.