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Pentane is an organic compound with the formula C 5 H 12 —that is, an alkane with five carbon atoms. The term may refer to any of three structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, pentane means exclusively the n-pentane isomer, in which case pentanes refers to a mixture of them; the other two are called isopentane (methylbutane) and neopentane ...
However, the chain of carbon atoms may also be branched at one or more points. The number of possible isomers increases rapidly with the number of carbon atoms. For example, for acyclic alkanes: [3] C 1: methane only; C 2: ethane only; C 3: propane only; C 4: 2 isomers: butane and isobutane; C 5: 3 isomers: pentane, isopentane, and neopentane
Constitution formula for isomers of C 5 H 8 with CAS numbers. The molecular formula C 5 H 8 may refer to any of the following hydrocarbons: Pentynes: 1-Pentyne; 2-Pentyne; 3-Methyl-1-butyne or isopentyne, CAS 598-23-2; Pentadienes: 1,2-Pentadiene, two cis-trans isomers, CAS 591-95-7; 1,3 Pentadiene, CAS 504-60-9 (racemic mixture)
Enumerating or counting structural isomers in general is a difficult problem, since one must take into account several bond types (including delocalized ones), cyclic structures, and structures that cannot possibly be realized due to valence or geometric constraints, and non-separable tautomers.
Unsaturated compounds generally carry out typical addition reactions that are not possible with saturated compounds such as alkanes. A saturated organic compound has only single bonds between carbon atoms. An important class of saturated compounds are the alkanes. Many saturated compounds have functional groups, e.g., alcohols.
When n is four or more, isomers are possible, distinguished by the position and conformation of the double bond. Alkenes are generally colorless non-polar compounds, somewhat similar to alkanes but more reactive. The first few members of the series are gases or liquids at room temperature.
In the Benson heat of formation group additivity dataset, cis isomers suffer a 1.10 kcal/mol stability penalty. Exceptions to this rule exist, such as 1,2-difluoroethylene, 1,2-difluorodiazene (FN=NF), and several other halogen- and oxygen-substituted ethylenes. In these cases, the cis isomer is more stable than the trans isomer. [10]
The gauche effect is very sensitive to solvent effects, due to the large difference in polarity between the two conformers.For example, 2,3-dinitro-2,3-dimethylbutane, which in the solid state exists only in the gauche conformation, prefers the gauche conformer in benzene solution by a ratio of 79:21, but in carbon tetrachloride, it prefers the anti conformer by a ratio of 58:42. [9]