Butane structural isomers

Isomers are substances having the same molecular formula and molecular weight, but differing in physical and chemical properties. Since branched and straight-chain alkanes with the same molecular formula can exist as distinct structures having different geometrical arrangement of the atoms, they are termed structural isomers. One example is C H,j (butane) which has two isomers ... 

Compounds having the same molecular formula but different molecular structures are called structural isomers. Butane and 2-methylpropane are referred to as structural isomers of C4H10. They are two distinct compounds with their own characteristic physical and chemical properties. 

Isomers are distinctly different compounds, with different properties, that have the same molecular formula. In Section 22.1, we considered structural isomers of alkanes. You will recall that butane and 2-methylpropane have the same molecular formula, C4H10, but different structural formulas. In these, as in all structural isomers, the order in which the atoms are bonded to each other differs. 

Identify each of the following pairs as structural isomers, geometrical isomers, or not isomers (a) butane and cyclobutane (b) cyclopentane and pentene ... 

The molecules butane and 2-methylpropane are structural isomers. Which of the following characteristics would be the same for both isomers, assuming constant temperature where necessary ... 

Figure 11.19 Two structural isomers having the molecular formula C4Hi0. (a) butane, (b) 2-methylpropane.

For a given molecular formula there is often more than one way of joining the atoms together, whilst still satisfying the rules of valency. Such variants are called structural isomers or constitutional isomers - compounds with the same molecular formula but with a different arrangement of atoms. A simple example is provided by C4H10, which can be accommodated either by the straight-chained butane, or by the branched-chain isobutane (2-methylpropane). 

This second branched molecule is called isobutane. Compounds sharing the same molecular formula but having different structures are called structural isomers. Normal butane and isobutane have different physical properties. The number of structural isomers for the alkanes is included in Table 15.1. It can be seen in this table that as the number of carbon atoms increases that the number of possible isomers also increases. The fact that numerous isomers exist for most organic compounds is another reason why there are so many organic compounds. 

The possibility of having branched-chain hydrocarbons that are structural isomers of the continuous-chain hydrocarbons begins with butane (n = 4). The HJPAC rules for the systematic naming of these hydrocarbons follow. 

Constitutional isomers (or structural isomers) are isomers that differ in their bonding sequence that is, their atoms are connected differently. Let s use butane as an example. If you were asked to draw a structural formula for C4Hi0, either of the following structures would be correct ... 

If all alkanes had unbranched (straight-chain) structures, their nomenclature would be simple. Most alkanes have structural isomers, however, and we need a way of naming all the different isomers. For example, there are two isomers of formula C4H10. The unbranched isomer is simply called butane (or n-butane, meaning normal butane), and the branched isomer is called isobutane, meaning an isomer of butane. ... 

Isomers with at least some atoms connected to different other atoms in the two (or more) compounds, such as butane and methylpropane, are said to be structural isomers of each other. (The fourth carbon atom is added to the end of a three-carbon chain to get butane, but in the middle of a three-carbon chain to get methylpropane.) Only structural isomers will be considered in this text. (Other types of isomers, in which all the atoms are bonded to the same other atoms in each of the isomers but in different spatial arrangements, will be taken up in more advanced texts.)... 

Butane and isobutane are isomers, two different compounds with the same molecular formula (Section 1.4A). They belong to one of the two major classes of isomers called constitutional or structural isomers. The two isomers discussed in Section 1.4A, CH3OCH3 and CH3CH2OH, are also constitutional isomers. We will learn about the second major class of isomers, called stereoisomers, in Section 4.13B. 

As we shall see, there are several different types of structural isomer. Isomers have to be named so that you can tell the difference between them. Consider the first member of the alkane series that shows structural isomerism, butane. 

In the example given above, there were eight possible arrangements of the constituent atoms that corresponded to the same molecular formula. Each one of these possible arrangements is called an isomer. Isomers may be divided into two fundamental classes. The first type consists of those isomers which differ in the exact number of each type of bond that connects the atoms in the various molecules. These are called structural isomers, because the structure is different in each one. In the above example, the ethers and the alcohols are structural isomers of each other. In the second type, the isomers all have exactly the same connectivity, i.e. the manner and order in which the atoms are connected to each other are the same, but differ only in the precise position that each group occupies relative to all the others in the molecule. These are called stereoisomers. In the above example, the two isomers of butan-2-ol are stereoisomers. 

Example Butane (CH3(CH2)2CH3) and methylpropane (CH3CH(CH3)CH3) are structural isomers with the same molecular formula (C4H10). The molecules have a different connectivity. 

Example Unbranched butane has a higher boiling point (-0.5°C) than its branched-chain structural isomer, methylpropane (-11.6°C). 

Finally, there are algorithms available for simply recognizing when two structures are tautomers. This is sufficient to locate all isomers in a database. In general, two structures are considered to be structural isomers if they share the same molecular formula. Tautomers are a special type of structural isomer in which the connectivity of the atoms, as well as the molecular formula, is the same. For example, butane (smiles CCCC) and isobutane (smiles CC(C)C) are strucural isomers but not tautomers. Butyraldehyde (smiles CCCC=0) and but-l-en-l-ol (smiles CCC=CO) are structural isomers as well as tautomers. A direct comparison of the molecular formulae readily shows the structural isomerism. There is a text graph representation that can allow easy detection of tautomers. 

Many hydrocarbons exist as structural isomers, which are compounds that have the same molecular formula but different structures. For example, both butane and isobutane have the same molecular formula of C4H10 (see Problem 11.19 on p. 457). Calculate the mole percent of these molecules in an equilibrium mixture at 25°C, given that the standard free energy of formation of butane is —15.9 kJ/mol and that of isobutane is —18.0 kJ/mol. Does your result support the notion that straight-chain hydrocarbons (that is, hy-... 

FIGURE 24.2 Structures of the first four olkanes. Note thot butane can exist in two structurolly different forms, called structural isomers. 

In the alkane series, as the number of carbon atoms increases, the number of structural isomers increases rapidly. For example, butane, C4H10, has two isomers decane, C10H22, has 75 isomers and the alkane C30H62 has over 400 million, or 4 X 10, possible isomers Obviously, most of these isomers do not exist in nature nor have they been synthesized. Nevertheless, the numbers help to explain why carbon is found in so many more compounds than any other element. 

As we saw earlier, there are two arrangements of the atoms represented by the molecular formula C4HJ0 butane and 2-methylpropane. Molecules having the same molecular formula but a different arrangement of atoms are called constitutional, or structural, isomers. These isomers are unique compounds because of their structural differences, and they have different physical and chemical properties. Butane... 

Two or more compounds with the same molecular formula but different properties are called isomers. Isomers with different arrangements of bonded atoms are constitutional (or structural) isomers alkanes with the same number of C atoms but different skeletons are examples. The smallest alkane to exhibit constitutional isomerism has four C atoms two different compounds have the formula C4H10, as shown in Table 15.3 on the next page. The unbranched one is butane (common name, /t-butane n- stands for normal, or having a straight chain), and the other is 2-methylpropane (common name, i obutane). Similarly, three compounds... 

Butane and all larger alkanes exhibit stmctural isomerism. Structural isomerism occurs when two molecules have the same atoms but different bonds. That is, the molecules have the same formulas but different arrangements of the atoms. For example, butane can exist as a straight-chain molecule (normal butane, or u-butane) or with a branched-chain stmcture (called isobutane), as shown in Figure 20.4. Because of their different stmctures, these structural isomers have different properties. 

Further examples are the structural isomers of CjHj and 03 . The linear and cyclic structures of both the C3H2 and 03 ions are formed in electron impact ionization from methylacetylen (CH3CCH) (Smith and Adams, 1987), n-butane, or propane (Hansel et al, 1989). The linear and cyclic isomers of C3H and C3H are distinguished by either their different association rates with CO or then-different reactivity with C2H2. In all cases, the linear (or open-chain) structural isomers 1-C3H, 1-C3H ) are more reactive then the respective cyclic forms (C-C3H+, C-C3H3+). 

Thus there are only five distinct structural isomers of C6H14 hexane, 2-methylpentane, 3-methylpentane, 2,3-dimethyl butane, and 2,2-dimethylbutane. 

Draw all the structural isomers for CgHig that have the following root name (longest carbon chain). Name the structural isomers, a. heptane b. butane... 

Have the same molecular formulae but have different atom connectivities, i.e., they have the same types and number of atoms but are structural isomers (e.g., -butane and isobutane) or positional isomers (e.g., 1-butanol and 2-butanol). 

Notice that both butanes have the same molecular formula, C4H10. These two possible butanes are structural isomers because they have the same molecular formulas, but they have different atom-to-atom bonding sequences. The straight-chain isomer is called a normal alkane and the other is a branched alkane (> Figure 1.12). 

Isooctane is 2,2,4-trimethylpentane. Draw structural formulas for and name a branched heptane, hexane, pentane, and butane that are structural isomers of isooctane. 

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