Isomer constitutional

With the experience we have gained so far. it should be fairly easy to draw a structure for any formula. It is also possible to crudely estimate the stability of the compound represented by this structure. As an example, let s show the structure for the compound with the formula C2HeO. We quickly discover that there are two ways to assemble these atoms, depending on whether we start with a C—C—O or a C—O—C arrangement of the nonhydrogen atoms. 

Which of these two structures is correct Both of them satisfy the octet rule and neither has formal charges, so both are predicted to be of comparable stability. On the basis of what we have discussed so far, we cannot predict which is more stable. In fact, both of these compounds are quite stable and can be put in a bottle. But they are different compounds. Ethyl alcohol is the alcohol found in beverages. It is a liquid at room temperature. In contrast, dimethyl ether is a gas at room temperature and is quite poisonous. As was mentioned in Section 1.7, compounds such as these, with the same molecular formula but different arrangements of bonded atoms (different structures or different connectivities), are called constitutional isomers (or structural isomers). Constitutional isomerism is very common in organic compounds. This is another reason why it is necessary to show the structure of the compound under discussion rather than just the molecular formula. 

As the number of atoms in a formula increases, the number of possible constitutional isomers increases dramatically. As an illustration, consider the series of hydrocarbons (compounds made up of only carbon and hydrogen) shown in Table 2.3. Although there is only one compound with the formula CH4, there are 75 constitutional isomers with the formula C10H22 (all 75 have been prepared in the lab and identified) and more than 4 billion with the formula C30H62  

A general method for calculating the number of isomers of a given molecular formula has not yet been developed. The problem is just too complex. However, several 

How many of the virtually limitless number of organic compounds are known The organization Chemical Abstracts Service (CAS) has the task of reviewing every article in every chemical journal that is published and tabulating all of the compounds that have been characterized and reported in these articles. Most of these compounds are organic. The CAS registry listed more than 22 million compounds as of 2003. 

Compounds with the same formula, but which give different ions in solution, exhibit ionization isomerization. The difference is in which ion is included as a ligand and 

Many other examples, and even more possibilities, exist. Enthusiasm for preparing and characterizing such compounds is not great at this time, and new examples are more likely to be discovered only as part of other studies. 

Examples of a complete series of coordination isomers require at least two metal ions and sometimes more. The total ratio of ligand to metal remains the same, but the ligands attached to a specific metal ion change. This is best described by example. 

For the empirical formula Pt(NH3)2Cl2, there are three possibilities  

Other examples are possible with different metal ions and with different oxidation states  

In the introduction we noted that both Berzelius and Wohler were fascinated by the fact that two different compounds with different properties, ammonium cyanate and urea, possessed exactly the same molecular formula, CH4N2O. Berzelius had studied examples of similar phenomena earlier and invented the word isomer to describe different compounds that have the same molecular formula. 

We can illustrate isomerism by referring to two different compounds, nitromethane and methyl nitrite, both of which have the molecular formula CH3NO2. Nitromethane, 

PROBLEM 1.14 There are many more isomers of CH3NO2 other than nitromethane and methyl nitrite. Some, such as carbamic acid, an intermediate in the commercial preparation of urea for use as a fertilizer, are too unstable to isolate. Given the information that the nitrogen and both oxygens of carbamic acid are bonded to carbon and that one of the carbon-oxygen bonds is a double bond, write a Lewis structure for carbamic acid. 

PROBLEM 1.15 Write structural formulas for all the constitutionally isomeric compounds having the given molecular formula. 

SAMPLE SOLUTION (a) Begin by considering the ways in which two carbons and one oxygen may be bonded. There are two possibilities C—C—O and C—O—C. Add the six hydrogens so that each carbon has four bonds and each oxygen two. There are two constitutional isomers ethyl alcohol and dimethyl ether. 

Hydrate isomerism requires water to play two roles, as (1) a ligand and as (2) an additional occupant (or solvate) within the crystal structure. Solvent isomerism broadens the definition to allow for the possibility of ammonia or other ligands participating as solvates. 

CrCls 6 H2O is a classic example. Three different crystalline compounds that each feature 6-coordinate Cr(III) have this empirical formula [Cr(H20)6]Cl3 (violet), [CrCl(H20)5]Cl2 H2O (blue-green), and [CrCl2(H20)4]Cl 2 H2O (dark green). These three hydrate isomers can be separated from commercial CrCls 6 H2O, with trans-[CrCl2(H20)4]Cl 2 H2O the major component. Other examples of hydrate isomers are  

FIGURE 9.20 The cations [c/s-Ni(phen)2CI(H20)]+ and [c/s-Ni(phen)2(H20)2]2+ co-crystallize with three PFg counterions and HjO solvate molecules (not shown). If the counterions were Cl , these would be hydrate isomers. (Molecular structure drawing created from CIF data, with hydrogen atoms omitted for clarity.) 

Carbanions, comprising a negatively charged C atom, C in state (3,1, -1,0)  

Isonitriles (very important in Ugi s multicomponent reactions, see Chapter 5) are characterized by the functional group R - NC, whose carbon atom may formally be described as carbanion-like or carbene-like  

The molecule generator MOLGEN, up to version 3.5 [19], generates compounds on the level of RC or CSC (if we enter particular valences, e.g. 5 for P, or 4 or 6 for S), while from version 4.0 onwards [148] it is possible to go to level 1C. Molecules (compounds) generated are unlabeled molecular graphs, i.e. equivalence classes of labeled molecular graphs that we introduce as follows  

15 Definition (Labeled molecular graphs of n atoms in ) Let n denote the number of atoms in the molecular graphs to be defined. Consider a set of chemical elements, and denote by 

Since molecules are often considered in a reduced, H-suppressedform [107 ], we extend the data introduced in Definition 1.1 as follows. 

As we learned in Chapter 23, the chemical and physical properties of a substance depend on the arrangements, as well as the identities, of its atoms. 

Isomers are substances that have the same number and kind of atoms— the same imkcular formula—but with different structural formulas. Because their structures are d erent, isomers are dijferent substances and have different properties. 

Isomers can be broadly divided into two major classes constitutional isomers and stereoisomers. In Chapter 2 3 we learned about some isomeric organic compounds. In this chapter, we will take a more systematic look at some three-dimensional aspects of organic structures—2l subject known as stereochemistiy ( spatial chemistry ). 

Consdtudanal (or structural) isomers differ in the order in which their atoms are bonded together. 

In our studies of hydrocarbons in Sections 23-1 through 23-7, we saw some examples of constitutional isomerism in hydrocarbons. Recall, for instance, that there are three isomers of C5H12. 

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The earlier sections have only considered the way atoms are bonded to each other in a molecule (topology) and how this is translated into a computer-readable form. Chemists define this arrangement of the bonds as the constitution of a molecule. The example in Figure 2-39, Section 2.5.2.1, shows that molecules with a given empirical formula, e.g., C H O, can have several different structures, which are called isomers [lOOj. Isomeric structures can be divided into constitutional isomers and stereoisomers (see Figure 2-67). 

Constitutional isomers are molecules with different connectivities between the atoms. Either the structures have different functional groups (these are called structural isomers), or the same functional group is placed in different positions (positional isomers). 

Different compounds that have the same molecular formula are called isomers If they are different because their atoms are connected m a dif ferent order they are called constitutional isomers... 

Formamide (/eft) and formaldoxime (right) are constitutional isomers both have the same molecular formula (CH3NO) but the atoms are con nected m a different order... 

Wnte structural formulas for all the constitutional isomers of molecular formula C3HgO that contain... 

Methane is the only alkane of molecular formula CH4 ethane the only one that is C2H6 and propane the only one that is C3Hj Beginning with C4H10 however constitutional isomers (Section 1 8) are possible two alkanes have this particular molecular formula In one called n butane, four carbons are joined m a continuous chain The nmn butane stands for normal and means that the carbon chain is unbranched The second isomer has a branched carbon chain and is called isobutane... 

There are three constitutional isomers of C5H12 n pentane (CH3CH2CH2CH2CH3) isopentane [(CH3)2CHCH2CH3] and neopen tane [(CH3)4C]... 

From among the 18 constitutional isomers of CgHig wnte structural formulas and give the lUPAC names for those that are named as derivatives of... 

Exclusive of compounds with double bonds four hydrocarbons are constitutional isomers of cis and trans 1 2 dimethylcyclopropane Identify these compounds... 

The CIS and trans forms of 1 2 dimethylcyclopropane are stereoisomers Stereoisomers are isomers that have their atoms bonded m the same order—that is they have the same constitution but they differ m the arrangement of atoms m space Stereoiso mers of the cis-trans type are sometimes referred to as geometric isomers You learned m Section 2 18 that constitutional isomers could differ m stability What about stereoisomers We can measure the energy difference between as and trans 1 2 dimethylcyclo propane by comparing their heats of combustion As illustrated m Figure 3 20 the two compounds are isomers and so the difference m their heats of combustion is a direct measure of the difference m their energies Because the heat of combustion of trans 1 2 dimethylcyclopropane is 5 kJ/mol (12 kcal/mol) less than that of its cis stereoisomer it follows that trans 1 2 dimethylcyclopropane is 5 kJ/mol (12 kcal/mol) more stable than as 1 2 dimethylcyclopropane... 

Determine whether the two structures in each of the following pairs represent constitutional isomers different conformations of the same compound or stereoisomers that cannot be inter converted by rotation about single bonds... 

The constitutional isomer of ethanol dimethyl ether (CH3OCH3) IS a gas at room temperature Suggest an explanation for this observation I... 

You have seen that measurements of heats of reaction such as heats of combustion can pro vide quantitative information concerning the relative stability of constitutional isomers (Section 2 18) and stereoisomers (Section 3 11) The box in Section 2 18 described how heats of reaction can be manipulated arithmetically to generate heats of formation (AH ) for many molecules The following material shows how two different sources of thermo chemical information heats of formation and bond dissociation energies (see Table 4 3) can reveal whether a particular reaction is exothermic or en dothermic and by how much... 

Butene has an unbranched carbon chain with a double bond between C 1 and C 2 It IS a constitutional isomer of the other three Similarly 2 methylpropene with a branched carbon chain is a constitutional isomer of the other three... 

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... 

Two constitutional isomers of molecular formula CgHi20 are formed in the following reac tion Ignoring stereochemistry suggest reasonable structures for these Diels-Alder adducts... 

The keto and enol forms are constitutional isomers Using older terminology they are referred to as tautomers of each other... 

It IS important to recognize that an enol is a real substance capable of mdepen dent existence An enol is not a resonance form of a carbonyl compound the two are constitutional isomers of each other... 

When compounds of the type represented by A are allowed to stand in pentane they are converted to a constitutional isomer... 

Excluding enantiomers there are three isomeric cyclopropanedicarboxyhc acids Two of them A and B are constitutional isomers of each other and each forms a cyclic anhydnde on being heated The third diacid C does not form a cyclic anhydride C is a constitutional isomer of A and a stereoisomer of B Identify A B and C Construct molecular models of the cyclic anhy dndes formed on heating A and B Why doesn t C form a cyclic anhydride" ... 

Constitutional isomers (Section 1 8) Isomers that differ in re spect to the order in which the atoms are connected Butane (CH3CH2CH2CH3) and isobutane [(CH3)3CH] are constitu tional isomers... 

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