Butane constitutional isomers

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... 

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... 

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. 

Constitutional isomerism is not limited to alkanes—it occurs widely throughout organic chemistry. Constitutional isomers may have different carbon skeletons (as in isobutane and butane), different functional groups (as in ethanol and dimethyl ether), or different locations of a functional group along the chain (as in isopropylamine and propylamine). Regardless of the reason for the isomerism, constitutional isomers are always different compounds with different properties, but with the same formula. 

Alkanes are a class of saturated hydrocarbons with the general formula C H2n. -2- They contain no functional groups, are relatively inert, and can be either straight-chain (normal) or branched. Alkanes are named by a series of IUPAC rules of nomenclature. Compounds that have the same chemical formula but different structures are called isomers. More specifically, compounds such as butane and isobutane, which differ in their connections between atoms, are called constitutional isomers. 

Unlike the constitutional isomers butane and isobutane (Section 3.2), which have their atoms connected in a different order, the two 1,2-dimethyl-cyclopropanes have the same order of connections but differ in the spatial orientation of the atoms. Such compounds, which have their atoms connected in the same order but differ in three-dimensional orientation, are called stereochemical isomers, or stereoisomers. 

I One possibility is that the protons are chemically unrelated and thus nonequivalent. If so, the products formed on replacement of Ii by X would be different constitutional isomers. In butane, for instance, the -CH3 protons are different from the -CH2- protons, would give different products on replacement by X, and would likely show different NMR absorptions. 

Constitutional isomers (Sections 3.2, 9.9) Isomers that have their atoms connected in a different order. For example, butane and 2-methylpropane are constitutional isomers. 

In contrast, the constitutional isomers of butane have different physical properties. Thus, n-butaneboils at-0.5°C but Mobutane boils at-12 C. Although the composition of the two butanes is the same, their three-dimensional geometries are quite different as a result of different patterns of chemical bonding. Note, for example, that one of the carbon atoms of wobutane makes bonds to three other carbon atoms. No carbon atom in n-butane does. Thus, the chemistry of the two butanes differs in many ways, but not as a consequence of handedness. They are simply examples of different ways to put together the same set of structural elements. Think about a Lego set. 

Two butanes, n—butane and lYobutane, provide a simple example of constitutional isomers. 

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). 

When two different compounds have the same molecular formula but differ in the nature or sequence of bonding, they are called constitutional isomers. For example, ethanol and dimethylether have same molecular formula, C2HgO, but they differ in the sequence of bonding. Similarly, butane and isobutane are two constitutional isomers. Constitutional isomers generally have different physical and chemical properties. 

Alkanes having a particular molecular formula can exist as different constitutional isomers. For example, the alkane having the molecular formula C4H10can exist as two constitutional isomers-the straight chain alkane (butane) or the branched alkane (2-methylpropane Following fig.). These are different compounds with different physical and chemical properties. 

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 ... 

These two compounds are isomers because they are different compounds with different properties, yet they have the same molecular formula. They are constitutional isomers because their atoms are connected differently. The first compound (n-butane for normal butane) has its carbon atoms in a straight chain four carbons long. The second compound ( isobutane for an isomer of butane ) has a branched structure with a longest chain of three carbon atoms and a methyl side chain. 

Isobutane (4-1) and n-butane (4-2) are familiar examples of constitutional isomers. Each has the molecular formula C4H10 but the C and H atoms are joined differently in these two molecules. In polymers the major types of constitutional differences involve positional isomerism and branching. 

Constitutional isomers like butane and isobutane belong to the same family of compounds they are both alkanes. In contrast, constitutional isomers like CH3CH2OH and CH3OCH3 have different functional groups and belong to different families CH3CH2OH is an alcohol and CH3OCH3 is an ether. 

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. 

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