Isomers Structural Formulas It is necessary to draw structural formulas for organic compounds because in most cases a molecular formula does not uniquely represent a single compound. Different compounds having the same molecular formula are called isomers, and the prevalence of organic isomers reflects the extraordinary versatility of carbon in forming strong bonds to itself and to other elements. When the group of atoms that make up the molecules of different isomers are bonded together in fundamentally different ways, we refer to such compounds as constitutional isomers. There are seven constitutional isomers of C4H10O, and structural formulas for these are drawn in the following table. These formulas represent all known and possible C4H10O compounds, and display a common structural feature. There are no double or triple bonds and no rings in any of these structures.. Note that each of the carbon atoms is bonded to four other atoms, and is saturated with bonding partners. Structural Formulas for C4H10O Isomers Kekulé Formula Condensed Formula Shorthand Formula Simplification of structural formulas may be achieved without any loss of the information they convey. In condensed structural formulas the bonds to each carbon are omitted, but each distinct structural unit (group) is written with subscript numbers designating multiple substituents, including the hydrogens. Shorthand (line) formulas omit the symbols for carbon and hydrogen entirely. Each straight line segment represents a bond, the ends and intersections of the lines are carbon atoms, and the correct number of hydrogens is calculated from the tetravalency of carbon. Non-bonding valence ****l electrons are omitted in these formulas. Developing the ability to visualize a three-dimensional structure from two-dimensional formulas requires practice, and in most cases the aid of molecular models. As noted earlier, many kinds of model kits are available to students and professional chemists, and the beginning student is encouraged to obtain one. Distinguishing Carbon Atoms When discussing structural formulas, it is often useful to distinguish different groups of carbon atoms by their structural characteristics. A primary carbon (1º) is one that is bonded to no more than one other carbon atom. A secondary carbon (2º) is bonded to two other carbon atoms, and tertiary (3º) and quaternary (4º) carbon atoms are bonded respectively to three and four other carbons. The three C5H12 isomers shown below illustrate these terms. Structural differences may occur within these four groups, depending on the molecular constitution. In the formula on the right all four 1º-carbons are structurally *****alent (remember the tetrahedral configuration of tetravalent carbon); however the central formula has two *****alent 1º-carbons (bonded to the 3º carbon on the left end) and a single, structurally different 1º-carbon (bonded to the 2º-carbon) at the right end. Similarly, the left-most formula has two structurally *****alent 2º-carbons (next to the ends of the chain), and a structurally different 2º-carbon in the middle of the chain. A consideration of molecular symmetry helps to distinguish structurally *****alent from non*****alent atoms and groups. The ability to distinguish structural differences of this kind is an essential part of mastering organic chemistry. It will come with practice and experience. Analysis of Molecular Formulas Although structural formulas are essential to the unique description of organic compounds, it is interesting and instructive to evaluate the information that may be obtained from a molecular formula alone. Three useful rules may be listed: The number of hydrogen atoms that can be bonded to a given number of carbon atoms is limited by the valence of carbon. For compounds of carbon and hydrogen (hydrocarbons) the maximum number of hydrogen atoms that can be bonded to n carbons is 2n + 2 (n is an integer). In the case of methane, CH4, n=1 & 2n + 2 = 4. The origin of this formula is evident by considering a hydrocarbon made up of a chain of carbon atoms. Here the middle carbons will each have two hydrogens and the two end carbons have three hydrogens each. Thus, a six-carbon chain (n = 6) may be written H-(CH2)6-H, and the total hydrogen count is (2 x 6) + 2 = 14. The presence of oxygen (valence = 2) does not change this relationship, so the previously described C4H10O isomers follow the rule, n=4 & 2n + 2 = 10. Halogen atoms (valence = 1) should be counted *****alent to hydrogen, as illustrated by C3H5Cl3, n = 3 & 2n + 2 = 8 = (5 + 3). If nitrogen is present, each nitrogen atom (valence = 3) increases the maximum number of hydrogens by one. Some Plausible Molecular Formulas C7H16O3, C9H18, C15H28O3, C6H16N2 Some Impossible Molecular Formulas C8H20O6, C23H50, C5H10Cl4, C4H12NO For stable organic compounds the total number of odd-valenced atoms is even. Thus, when even-valenced atoms such as carbon and oxygen are bonded together in any number and in any manner, the number of remaining unoccupied bonding sites must be even. If these sites are occupied by univalent atoms such as H, F, Cl, etc. their total number will necessarily be even. Nitrogen is also an odd-valenced atom (3), and if it occupies a bonding site on carbon it adds two additional bonding sites, thus maintaining the even/odd parity. Some Plausible Molecular Formulas C4H4Cl2, C5H9OBr, C5H11NO2, C12H18N2FCl Some Impossible Molecular Formulas C5H9O2, C4H5ClBr, C6H11N2O, C10H18NCl2 The number of hydrogen atoms in stable compounds of carbon, hydrogen & oxygen reflects the number of double bonds and rings in their structural formulas. Consider a hydrocarbon with a molecular structure consisting of a simple chain of four carbon atoms, CH3CH2CH2CH3. The molecular formula is C4H10 (the maximum number of bonded hydrogens by the 2n + 2 rule). If the four carbon atoms form a ring, two hydrogens must be lost. Similarly, the introduction of a double bond entails the loss of two hydrogens, and a triple bond the loss of four hydrogens. From the above discussion and examples it should be clear that the molecular formula of a hydrocarbon (CnHm) provides information about the number of rings and/or double bonds that must be present in its structural formula. By rule #2 m must be an even number, so if m < (2n + 2) the difference is also an even number that reflects any rings and double bonds. A triple bond is counted as two double bonds. The presence of one or more nitrogen atoms or halogen substituents requires a modified analysis. The above formula may be extended to such compounds by a few simple principles: [*]The presence of oxygen does not alter the relationship. [*]All halogens present in the molecular formula must be replaced by hydrogen. [*]Each nitrogen in the formula must be replaced by a CH moiety.
Stereoisomers As defined in an earlier introductory section, isomers are different compounds that have the same molecular formula. When the group of atoms that make up the molecules of different isomers are bonded together in fundamentally different ways, we refer to such compounds as constitutional isomers. For example, in the case of the C4H8 hydrocarbons, most of the isomers are constitutional. Shorthand structures for four of these isomers are shown below with their IUPAC names. Note that the twelve atoms that make up these isomers are connected or bonded in very different ways. As is true for all constitutional isomers, each different compound has a different IUPAC name. Furthermore, the molecular formula provides information about some of the structural features that must be present in the isomers. Since the formula C4H8 has two fewer hydrogens than the four-carbon alkane butane (C4H10), all the isomers having this composition must incorporate either a ring or a double bond. A fifth possible isomer of formula C4H8 is CH3CH=CHCH3. This would be named 2-butene according to the IUPAC rules; however, a close inspection of this molecule indicates it has two possible structures. These isomers may be isolated as distinct compounds, having characteristic and different properties. They are shown here with the designations cis and trans. The bonding patterns of the atoms in these two isomers are essentially *****alent, the only difference being the relative orientation or configuration of the two methyl groups (and the two associated hydrogen atoms) about the double bond. In the cis isomer the methyl groups are on the same side; whereas they are on opposite sides in the trans isomer. Isomers that differ only in the spatial orientation of their component atoms are called stereoisomers. Stereoisomers always require that an additional nomenclature prefix be added to the IUPAC name in order to indicate their spatial orientation, for example, cis (Latin, meaning on this side) and trans (Latin, meaning across) in the 2-butene case.
[h=3]Configurational Stereoisomers of Alkenes[/h] The carbon-carbon double bond is formed between two sp2 hybridized carbons, and consists of two occupied molecular orbitals, a sigma orbital and a pi orbital. Rotation of the end groups of a double bond relative to each other destroys the p-orbital overlap that creates the pi orbital or bond. Because the pi bond has a bond energy of roughly 60 kcal/mole, this resistance to rotation stabilizes the planar configuration of this functional group. As a result, certain disubstituted alkenes may exist as a pair of configurational stereoisomers, often designated cis and trans. The essential requirement for this stereoisomerism is that each carbon of the double bond must have two different substituent groups (one may be hydrogen). This is illustrated by the following general formulas. 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. Some examples of this configurational stereoisomerism (sometimes called geometric isomerism) are shown below. Note that cycloalkenes smaller than eight carbons cannot exist in a stable trans configuration due to ring strain. A similar restriction holds against cycloalkynes smaller than ten carbons. Since alkynes are linear, there is no stereoisomerism associated with the carbon-carbon triple bond. [h=3]Nomenclature of Alkene Stereoisomers[/h] Configurational stereoisomers of the kind shown above need an additional nomenclature prefix added to the IUPAC name, in order to specify the spatial orientations of the groups attached to the double bond. Thus far, the prefixes cis- and trans- have served to distinguish stereoisomers; however, it is not always clear which isomer should be called cis and which trans. For example, consider the two compounds on the right. Both compound A (1-bromo-1-chloropropene) and compound B ( 1-cyclobutyl-2-ethyl-3-methyl-1-butene) can exist as a pair of configurational stereoisomers (one is shown). How are we to name these stereoisomers so that the configuration of each is unambiguously specified? Assignment of a cis or trans prefix to any of these isomers can only be done in an arbitrary manner, so a more rigorous method is needed. A completely unambiguous system, based on a set of group priority rules, assigns a Z (German, zusammen for together) or E (German, entgegen for opposite) to designate the stereoisomers. In the isomers illustrated above, for which cis-trans notation was adequate, Z is equivalent to cis and E is equivalent to trans. [h=4]The Sequence Rule for Assignment of Alkene Configurations[/h] Assign priorities to double bond substituents by looking at the atoms attached directly to the double bond carbons. 1. The higher the atomic number of the immediate substituent atom, the higher the priority. For example, H– < C– < N– < O– < Cl–. (priority increases left to right) (Different isotopes of the same element are assigned a priority according to their atomic mass.) 2. If two substituents have the same immediate substituent atom, move to the next atom (away from the double bond) until a difference is found. For example, CH3– < C2H5– < ClCH2– < BrCH2– < CH3O–. Once the relative priorities of the two substituents on each of the double bond carbons has been determined, a cis orientation of the higher priority pair is designated Z, and a trans orientation is termed E. Applying these rules to the isomers of compounds A and B shown above, we assign the configuration of the 1-bromo-1-chloropropene isomer as E (Br has higher priority than Cl, and CH3 a higher priority than H). The configuration of the 1-cyclobutyl-2-ethyl-3-methyl-1-butene isomer is determined to be Z (C4H7 has higher priority than H, and the isopropyl group has higher priority than an ethyl group). The following example elaborates the priority determination for a more complex case. The line formula is expanded to give the structural formula in the center. The root name is heptene (the longest chain incorporating both carbons of the double bond), and the substituents (in red) are added to give the IUPAC name. In order to assign a configurational prefix the priority order of substituents at each double bond carbon must be determined. For carbon #3 the immediate substituent atoms are a chlorine and a carbon. The chlorine has a higher atomic number and therefore has higher priority (colored green and numbered 1). The more remote bromine atom does not figure in this choice. For carbon #4 the immediate substituent atoms are both carbons (colored orange). As a result, we must look at the next higher atomic number atoms in the substituent chain. These are also carbon, but the isopropyl group has two carbons (also orange) whereas the propyl group has only one. The priority order is therefore isopropyl (green) > propyl (magenta). Since the two higher priority groups (#1) are on the same side of the double bond, this configuration is (Z).
[h=3]Configurational Stereoisomers of Cycloalkanes[/h] Stereoisomers are also observed in certain disubstituted (and higher substituted) cyclic compounds. 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. As in the case of the 2-butene stereoisomers, disubstituted cycloalkane stereoisomers may be designated by nomenclature prefixes such as cis and trans. The stereoisomeric 1,2-dibromocyclopentanes shown to the right are an example. In general, if any two sp3 carbons in a ring have two different substituent groups (not counting other ring atoms) stereoisomerism is possible. This is similar to the substitution pattern that gives rise to stereoisomers in alkenes; indeed, one might view a double bond as a two-membered ring. Four other examples of this kind of stereoisomerism in cyclic compounds are shown below. If more than two ring carbons have different substituents (not counting other ring atoms) the stereochemical notation distinguishing the various isomers becomes more complex. For examples of how such compounds are named in the IUPAC system