Hydrocarbons constitutional isomers

As with hydrocarbons, constitutional isomers with branched chains have lower boiling points than their unbranched-chain isomers (Section 2.7C). Compare, for example, the boiling points of unbranched-chain 1-bromobutane (butyl bromide, bp 100°C) with the more branched and compact 2-bromo-2-methylpropane (ferf-butyl bromide, bp 72°C). Branched-chain constitutional isomers have lower boiling points because they have a more spherical shape and, therefore, decreased surface area, leading to smaller van der Waals forces between their molecules. 

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

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. 

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. 

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...

We have seen that there are two saturated C Hjq hydrocarbons. For the C5 hydrocarbons, there are three possible arrangements of the atoms. These three different C5H32 alkanes are examples of constitutional isomers. 

The number of constitutional isomers increases rapidly as the number of carbon atoms in saturated hydrocarbons increases. There are live isomeric alkanes (Table 27-3). 

Problem 2.9 Polymers of dienes (hydrocarbons containing two carbon-carbon double bonds), such as butadiene and isoprene, have the potential for head-to-tail and head-to-head isomerism and variations in double-bond position as well. How many constitutional isomers can form in the polymerization of (a) polybutadiene and (b) polyisoprene ... 

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... 

Calculated Number of Possible Constitutional Isomers for C ,Hn Polycyclic Hydrocarbons ... 

Saturated acyciic hydrocarbons have the general molecular formula C 2n 2- Each double bond or ring reduces the number of hydrogen atoms by two as compared with the formula for a saturated compound. Thus each ring or double bond provides one unit of hydrogen deficiency. For example, 1-hexene and cyclohexane have the same molecular formula (C6H12) and they are constitutional isomers. 

Acetaldehyde and propionaldehyde made up at least 90% of the C-C oxygenates for all experiments. The Cg oxygenates include 2,5-dimethyltetrahydrofuran, 2,4-dimethyltetrahydro-furan, 2,2,4-trimethyloxepane (not shown), and small amounts of other species (not shown) such as 3-ethoxy-l-butene and hexanal. The major product was 2,5-dimethyltetrahydrofuran, optimized at 21% selectivity for CgH,4/02 2.5, after which it did not change substantially. Its 2,4-constitutional isomer showed a similar trend but was minor in comparison (maximum -4% selectivity). Selectivity to Cg oxygenated hydrocarbons exceeded 30% for CgHj4/02 = 2.25 to... 

One important use of heats of combustion is to give us information about the relative stabilities of isomeric hydrocarbons. To illustrate, consider the heats of combustion of the four constitutional isomers given in Table 2.7. All four compounds undergo combustion according to this equation. 

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