Constitutional isomers number

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... 

Compounds like the two C4M [() molecules and the three C3I I 12 molecules, which have the same formula but different structures, are called isomers, from the Greek isos + meros, meaning "made of the same parts." Isomers are compounds that have the same numbers and kinds of atoms but differ in the way the atoms are arranged. Compounds like butane and isobutane, whose atoms are connected differently, are called constitutional isomers. We ll see shortly that other kinds of isomers are also possible, even among compounds whose atoms are connected in the same order. As Table 3.2 shows, the number of possible alkane isomers increases dramatically as the number of carbon atoms increases. 

Molecular Formula Possible Number of Constitutional Isomers Molecular Formula Possible Number of Constitutional Isomers... 

Constitutional isomerism becomes more complex as the size of the hydrocarbon molecule is increased. For example, there are three constitutional isomers of pentane, C5H12. The number of constimtional isomers increases quite rapidly with an increasing number of carbon atoms. Thus, there are five constimtional isomers of hexane, CeH, nine isomers of heptane, C7H16, 75 isomers of decane, C10H22, and 366,319 isomers of eicosane, C20H42. You can begin to understand why it is possible to make so many different molecules based on carbon. 

Figure 21. The equilibrium between the helical interlaced system precursor of the trefoil knot and its face-to-face analogous complex leading to the face-to-face complexes. Interconversion between the two isomeric cyclic products is, of course, not possible. For the cyclic compounds, the total number of atoms x connecting two phenolic oxygen atoms is 16 if n=4 (pentakis(ethyleneoxy) fragment) or 19 if n = 5 (hexakis(ethyleneoxy) linker). Each knot is represented by the letter k accompanied by the overall number of atoms included in the cycle. The face-to-face complexes contain two monocycles (letter m), the number of atoms in each ring also being indicated. It can be noted that each knot has a face-to-face counterpart. For instance [Cu2(k-90)]2+ and [Cu2(m-45)2]2+ are constitutional isomers. They are by no means topological stereoisomers [34, 35].

From a structural point of view a number of different isomers could be present. In principal the three different hydroxyl groups in the ligand are able to interact with the central metal ion thus giving rise to three different constitutional isomers. The stereochemistry of the different configurational isomers of gadobutrol is best described in view of the stereochemistry of the parent compound Gd-DOTA. In this complex chirality results from the restriction of free rotation around the bonds of the ligand caused by the inclusion of the central metal ion. Thus four stereoisomers are generated upon complexation of Gd(III) by DOTA. 

They have the same molecular mass, but differ in their manner of linkage, their branching, and in the order (sequence) of their atoms (constitutional isomers). The number of possible isomers increases rapidly in higher generations - by analogy with the classical case of the alkanes. 

Of course, it is the goal of a crossed aldol condensation to produce a single ,/i-u nsatu-rated carbonyl compound. One has to keep in mind that crossed aldol condensations may result in up to four constitutionally isomeric condensation products (starting from two aldehydes or an aldehyde and a symmetric ketone) or even in eight constitutional isomers (starting with an aldehyde and an unsymetrical ketone). These maximum numbers of structural isomers result if both starting materials... 

There are nine constitutional isomers with the empirical formula C3H60. However, since two of the compounds exist as pairs of stereoisomers, the total number of isomers equals eleven. The two methyloxiranes, A and B, are enantiomers, they are stable and very volatile. Acetone (C) is in equilibrium with its enol tautomer (propen-2-ol, D). Similarly, the two diastereomeric enols ( )- and (Z)-prop-l-en-l-ol (E and F) are in equilibrium with their tautomer propanal (or propionaldehyde, G). The other isomers are (last group of compounds from left to right) methoxyethene, prop-2-en-l-ol (commonly known as allyl alcohol), oxetane and cyclopropanol. 

Figure 1.9 illustrates the construction of a Lewis structure for a more complex example, nitromethane. In this case there are a number of reasonable (according to what we know now) constitutional isomers. Therefore, we must be told the connectivity, or which atoms are bonded to which. Once the connectivity shown in Figure 1.9 is given, the best structure that satisfies the octet rule has some formal charges. This still represents a quite stable molecule. 

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 ... 

Table 2.3 The Number of Constitutional Isomers for a Series of Hydrocarbons...

The DU is very useful when drawing isomers. Let s look at a simple example. Suppose we are asked to draw the constitutional isomers with the formula C5H 0- The DU for C5H o is [(2(5) + 2) — 10]/2 = 1. Therefore, this compound must have one double bond or one ring. Any compound that has five C s and one double bond or one ring will fit the formula. Although C5H 2 has only three isomers, having two fewer H s actually increases the number of isomers, because there is now a double bond or a ring to vary. 

There are three constitutional isomers of pentane (C5H12), whose common names are n-pentane, isopentane, and neopentane. The number of isomers increases rapidly as the number of carbon atoms increases. 

Constitutional isomers may differ in ways other than the branching of their carbon chain. They may differ in the position of a double bond or other group or by having a ring or some other feature. Notice how the following constitutional isomers all differ by the ways in which atoms are bonded to other atoms. (Check the number of hydrogens bonded to each carbon.) These compounds are not isomers of the pentanes just shown, however, because these have a different molecular formula (C5H10). 

Computed NMR chemical shifts have been used numerous times to help identify chemical structure. This might be to identify the correct structure from a number of possible constitutional isomers or the proper structure from a selection of possible diastereomers. How one judges the correct selection is the topic of this section. 

The maximum number of possible constitutional isomers increases dramatically as the number of carbon atoms in the alkane increases, as shown in Table 4.1. For example, there are 75 possible isomers for an alkane having 10 carbon atoms, but 366,319 possible isomers for one having 20 carbons. 

A limited number of chiral compounds having no stereogenic centers exist. For example, although A is achiral, constitutional isomer B is chiral. Make models and explain this observation. Compounds containing two double bonds that share a single carbon atom are called allenes. 

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