![]() Constitutional isomers have the same molecular formula but different spatial arrangements of the atoms in their molecules. For example, two alkanes have the formula C 4H 10: They are called n-butane and 2-methylpropane (or isobutane), and have the following Lewis structures:The compounds n-butane and 2-methylpropane are structural isomers (the term constitutional isomers is also commonly used). Properties of Some AlkanesHydrocarbons with the same formula, including alkanes, can have different structures. Pictured are the Lewis structures, ball-and-stick models, and space-filling models for molecules of methane, ethane, and pentane.A common method used by organic chemists to simplify the drawings of larger molecules is to use a skeletal structure (also called a line-angle structure).ĪlkaneMolecular FormulaMelting Point (☌)Boiling Point (☌)Phase at STPNumber of Structural IsomersmethaneCH 4–182.5–161.5gas1ethaneC 2H 6–183.3–88.6gas1propaneC 3H 8–187.7–42.1gas1butaneC 4H 10–138.3–0.5gas2pentaneC 5H 12–129.736.1liquid3hexaneC 6H 14–95.368.7liquid5heptaneC 7H 16–90.698.4liquid9octaneC 8H.7liquid18nonaneC 9H.8liquid35decaneC 10H.0liquid75tetradecaneC 14H 305.9253.5solid1858octadecaneC 18H 3828.2316.1solid60,523Table 1. Condensed structural formulas for ethane and pentane are shown at the bottom of, and several additional examples are provided in the exercises at the end of this chapter. ![]() These formulas have the appearance of a Lewis structure from which most or all of the bond symbols have been removed. Instead of the usual format for chemical formulas in which each element symbol appears just once, a condensed formula is written to suggest the bonding in the molecule. Because of the sp 3 hybridization, the bond angles in carbon chains are close to 109.5°, giving such chains in an alkane a zigzag shape.The structures of alkanes and other organic molecules may also be represented in a less detailed manner by condensed structural formulas (or simply, condensed formulas). Notice that the carbon atoms in the structural models (the ball-and-stick and space-filling models) of the pentane molecule do not lie in a straight line. The familiar plastics polyethylene, polypropylene, and polystyrene are also hydrocarbons. We use hydrocarbons every day, mainly as fuels, such as natural gas, acetylene, propane, butane, and the principal components of gasoline, diesel fuel, and heating oil. Many hydrocarbons are found in plants, animals, and their fossils other hydrocarbons have been prepared in the laboratory. In addition, hydrocarbons may differ in the types of carbon-carbon bonds present in their molecules. Even though they are composed of only two types of atoms, there is a wide variety of hydrocarbons because they may consist of varying lengths of chains, branched chains, and rings of carbon atoms, or combinations of these structures. The existence of so many organic molecules is a consequence of the ability of carbon atoms to form up to four strong bonds to other carbon atoms, resulting in chains and rings of many different sizes, shapes, and complexities.The simplest organic compounds contain only the elements carbon and hydrogen, and are called hydrocarbons. ![]()
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