Pentane constitutional isomers

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

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

There are three constitutional isomers of C5H12 -pentane... 

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. 

Structural (or constitutional) isomers are compounds with the same molecular formulas but different structural formulas (that is, different arrangements of the atoms in the molecule). Isomerism is especially important in organic chemistry because of the capacity of carbon atoms to be arranged in so many different ways continuous chains, branched chains, and rings. Structural formulas can be written so that every bond is shown, or in various abbreviated forms. For example, the formula for n-pentane (n stands for normal) can be written as ... 

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). 

We have just seen that the three C5H12 isomers all incorporate pentane in their names and are differentiated by the prefixes n- iso , and neo. Extending this approach to alkanes beyond C5H12 fails because we run out of descriptive prefixes before all the isomers have unique names. As difficult as it would be to invent different names for the 18 constitutional isomers of CgHjg, for example, it would be even harder to remember which structure corresponded to which name. For this and other reasons, organic chemists have developed systematic ways to name compounds based on their structure. The most widely used approach is called the lUPAC rules lUPAC stands for the International Union of Pure and Applied Chemistry. (See the boxed essay. What s in a Name Organic Nomenclature,)... 

Butane and isobutane have the same molecular formula C4H10- The two compounds have their atoms connected in a different order and are, therefore, constitutional isomers (Section 1.3). Pentane, isopentane, and neopentane are also constitutional isomers. They, too, have the same molecular formula (C5H12) but have different structures. 

Isomers are different compounds with the same molecular formula. Thus far, we have encountered three types of isomers. Constitutional isomers (Section 2.2) have the same molecular formula but a different connectivity of atoms in their molecules. Examples of pairs of constitutional isomers are pentane and 2-methylbutane, and... 

The number of constitutional isomers rapidly increases with the number of carbons in the series. For example, there are three constitutional isomers with the molecular formula C5H12 (pentanes see Figure 24.5), five of (hexanes), and 75 of... 

With alkanes having more than four carbons, the names of the straight-chain isomers are systematic and derive from Greek roots pentme for five C atoms, ejcane for six, and so on. There are three constitutional isomers for the five-carbon alkane, each having molecular formula C5H12 pentane, isopentane (or 2-methylbutane), and neopentane (or 2,2-dimethylpropane). 

Draw a constitutional isomer of pentane, and explain why branching does not change the ratio of C to H (the molecular formula remains C5H12). 

Next, we look for any constitutional isomers where the parent is pentane (five carbon atoms). There are only two such isomers. Specifically, we can either connect the extra CH3 group to positions C2 or C3 of the pentane chain ... 

To draw all of the constitutional isomers of C7H, we will follow the same methodical approach that we used in Problem 4.3. We start with a hnear chain of seven carbon atoms (heptane). Then, we draw aU possible substituted hexanes with molecular formula C7H16. There are only two possibUities - the methyl group can be placed at either C2 or C3. Then, we move on to the pentanes, and finally any possible butanes. This methodical analysis gives the following constitutional isomers ... 

Photochemical chlorination of pentane gave a mixture of three isomenc monochlorides The pnncipal monochlonde constituted 46% of the total and the remaining 54% was approximately a 1 1 mixture of the other two isomers Write structural formulas for the three monochlonde iso mers and specify which one was formed in greatest amount (Recall that a secondary hydrogen is abstracted three times faster by a chlonne atom than a pnmary hydrogen)... 

The above example illustrates the influence of the lattice on the constitution of the product. The photolysis of azobis-3-phenyl-3-pentane, 189, provides a case of influence on the product configuration (249). In this system, too, the crystal lattice favors disproportionation over combination. Two disproportionation products, the Z- and -pentenes 190 and 191, are formed in the solid, in a ratio of 1 3 the predicted equilibrium ratio is 1 1.6. While the nature of the processes determining this ratio is not clear, it was established that the Z isomer is formed under strong lattice control, whereas the E isomer is formed in a process with greater, but not complete, molecular freedom. 

Direct irradiation of an argon degassed 0.001 M pentane solution of cyclonona-1,2-diene (1) with wavelengths > 220 nm (Vycor filter) yielded bicyclo[6.1.0]non-l(9)-ene (6), cyclononyne 7 and tricyclo[4.3.0.0 ]nonane (3), ratio 94 3 3, as primary photoproducts. At low conversion of 1 (<20%) these isomers constituted more than 95% of the photoproducts higher conversions led to significant secondary reactions of bicyclo[6.1.0]non-l(9)-ene (6). 

Open-chain aliphatic hydrocarbons constitute alkanes, alkenes, alkynes, and their isomers. Alkanes have the general formula C H2 +2, where n is the number of carbon atoms in the molecnles, snch as methane, propane, n-pentane, and isooctane. Alkenes or olefins are nnsaturated compounds, characterized by one or more double bonds between the carbon atoms. Their general formula is C H2 . Examples are ethylene, 1-butene, and... 

Recall from Section 3.2 that two or more compounds with the same molecnlar formula but different properties are called isomers. Those 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 fonr C atoms two different compounds have the formula C4H10 (Table 15.3). The unbranched one is butane (common name, n-butane n- stands for normal, or having a straight chain), and the other is 2-methylpropane (common name, wobntane). Similarly, three compounds have the formula C5H12. The unbranched isomer is pentane (common name, n-pentane) the one with a methyl group at C-2 of a four-C chain is 2-methylbntane (common name, isopentane). The third isomer has two methyl branches on C-2 of a three-C chain, so its name is 2,2-dimethylpropane (common name, neopentane). 

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