A Notation for Substituents

Before we discuss the effects of substituents on the energies and coefficients of conjugated systems, it will be convenient to have at our disposal a notation for the various types of substituents which we shall come across. There are three common types, which we shall designate with the letters C-, Z- and X- (Fig. 2.1), each of 

Molecular Orbitals and Organic Chemical Reactions Reference Edition Ian Fleming 2010 John Wiley Sons, Ltd 

C-Substituents are simple conjugated systems of carbon atoms, like vinyl or phenyl. They may be n donors or 7i acceptors, depending upon what they are conjugated with, responding to and stabilising electron demand or electron excess, as appropriate. Their effect on the a framework is small, because the point of attachment is a carbon atom, and C—C single bonds are not strongly polarised. 

1 C-Substituents. We saw in Chapter 1 with butadiene (Fig. 1.39) that a simple double bond, the most simple of the C-substituents, lowers the total n energy when it is conjugated to another double bond to 

This exercise shows that the effect on the energy of the % molecular orbitals of adding simple conjugation in the form of a n bond or of a benzene ring is very similar—a C-substituent lowers the 

There are three common types of substituents, each of which modifies the orbitals of conjugated systems in a different way (Fig. 2.1). They are (a) simple conjugated systems, like vinyl or phenyl, which we shall designate with the letter C (b) re-bonded systems which are also electron withdrawing, like formyl, acetyl, 

1 C-Substituents. A double bond, lowers the % energy when it is conjugated to another double bond (see pages 28-30). The phenyl group is similar— the filled n molecular orbitals in styrene come at 2.14/3, 1.41/3, 1.00/3, 0.66/3 below the a level, and the LUMO at 0.66/3 above the a level. The total n stabilisation is 2 x 5.21/3, whereas the total n stabilisation for the separate components benzene and ethylene is 2 x 5.0/3. A C-substituent raises the energy 

2 Z-Substituents. The simplest Z-substituent is the formyl group in acrolein 2.1. A simple Hiickel calculation gives Fig. 2.2 in which we are looking at the p orbitals from above. This figure gives us what we want, but no insight. 

The energies for the molecular orbitals for these two extremes are shown in Fig. 2.3. The true orbital energy for the orbitals of acrolein must be in between those of the corresponding orbitals of the allyl cation and butadiene. We can perhaps expect the true structure to be more like the butadiene system than the allyl cation system (for the same reason that we prefer to draw it as 2.1 rather than 2.2). We can see that the effect of having a Z-substituent conjugated with the double bond of 

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Consider the domain of a six-membered ring with single unsaturation. Table 3 expresses the synthetic route with one substituent. Again, the symbol V represents an arbitrary substituent. Square brackets surrounding a set of symbols indicates optionality of those symbols (as in regular expression notation). For example, the string may reduce... 

These parameters were intended to apply to XGY systems in which the skeletal group is phenylene. Hammett found it necessary to define an additional set of parameters, cTp , in order to account for substituent effects in 4-substituted benzene systems with an active site that has a lone pair on the atom adjacent to the benzene ring. The reference set was the ionization constants of 4-substituted phenols in water at 25 °C. Brown and his coworkers later defined another set of constants, o p, to account for substituent effects in benzene derivatives with electronically deficient active sites. The reference set was the rate constants for the solvolysis of 4-substituted cumyl chlorides in 90% aqueous acetone at 25 °C. Finally, Wepster and coworkersand Taft both independently proposed constants intended to represent substituent effects in benzene derivatives with minimal delocalized effect. Using the Taft notation these constants are written as cr . The reference systems had a methylene group inserted between the benzene ring and the active... 

In modern chemistry texts the cisHrans nomenclatnre has been replaced by a notational system known as E-Z where corresponds to cis and Z to trans. This system is less ambiguous as it is based on a more precise atomic number criterion for ranking substituents. In this system, when the higher atomic number atoms are on opposite sides of the double bond the configuration is (Carey 2000). 

The lUPAC nomenclature is based on the longest continuous chain (alkane, alkene, or alkyne based) that contains the carboxyl carbon (the -COOH unit). As with aldehydes, the carbon of the COOH group is always at one terminus of the chain and must receive the lowest possible number (1), which is omitted from the name. The suffix for carboxylic acids is oic acid, with the word acid separated from the first part of the name. As with aldehydes and ketones, the carboxyl unit is higher in priority than an alkene or alkyne. Compound 38 is an eight-carbon acid with an alkane backbone, so it is octanoic acid (note the short notation for the carboxyl group). Substituents are numbered relative to the carbonyl carbon of the COOH unit, so 39 is 2,2-dimethyl-5-phenyloctanoic acid. In a similar manner, 40 is named 16-chloro-4-ethylheptadecanoic acid. 

The Wiswesser Line Notation (WLN) was introduced in 1946, in order to organize and to systematically describe the cornucopia of compounds in a more concise manner. A line notation represents a chemical structure by an alphanumeric sequence, which significantly simplifies the processing by the computer [9-11], (n many cases the WLN uses the standard symbols for the chemical elements. Additionally, functional groups, ring systems, positions of ring substituents, and posi-... 

It IS convenient m equations such as this to represent generic alcohols and alkyl halides as ROH and RX respectively where R stands for an alkyl group In addition to con venience this notation lets us focus more clearly on the functional group transformation that occurs the OH functional group of an alcohol is replaced as a substituent on car bon by a halogen usually chlorine (X = Cl) or bromine (X = Br)... 

Isomeric alkenes may be either constitutional isomers or stereoisomers There is a sizable barrier to rotation about a carbon-carbon double bond which corresponds to the energy required to break the rr component of the double bond Stereoisomeric alkenes are configurationally stable under normal conditions The configurations of stereoisomeric alkenes are described according to two notational systems One system adds the prefix CIS to the name of the alkene when similar substituents are on the same side of the double bond and the prefix trans when they are on opposite sides The other ranks substituents according to a system of rules based on atomic number The prefix Z is used for alkenes that have higher ranked substituents on the same side of the double bond the prefix E is used when higher ranked substituents are on opposite sides... 

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