a saturated hydrocarbon that contains any number of carbon atoms arranged one after the other in a chain
Ethane is the simplest of the straight-chain alkanes, which contain any number of carbon atoms, one after the other, in a chain. Propane (C3H8) has three carbon atoms bonded in a chain with eight electrons shared with eight hydrogen atoms. Butane (C4H10) has a chain of four carbons and ten hydrogens.
For the straight-chain alkanes with one to four carbon atoms, the official names and the common names are the same. They are methane, ethane, propane, and butane, respectively. A mixture of Latin and Greek prefixes are used to name the hydrocarbons having straight chains longer than four carbon atoms. The prefixes are pent- for 5, hex- for 6, hept- for 7, oct- for 8, and so on.
To draw a structural formula for a straight-chain alkane, write the symbol for carbon as many times as necessary to get the proper chain length. Then complete the formula with hydrogens and lines representing covalent bonds. Complete structural formulas show all the atoms and bonds in a molecule. Sometimes, however, shorthand or condensed structural formulas work just as well.
an alkane with one or more alkyl groups attached to the parent structure
An alkane with one or more alkyl groups is called a branched-chain alkane. The IUPAC rules for naming branched-chain alkanes are quite straightforward. The name of a branched-chain alkane is based on the name of the longest continuous carbon chain. Each alkyl substituent is named according to the length of its chain and numbered according to its position on the main chain. The compound with the following structural formula can be used as an example.
Find the longest chain of carbons in the molecule. This chain is considered the parent structure. In the example, the longest chain contains seven carbon atoms. Therefore, the parent hydrocarbon structure is heptane.
Number the carbons in the main chain in sequence. To do this, start at the end that will give the groups attached to the chain the smallest numbers. This has already been done in the example. As you can see, numbering the chain from right to left places the substituent groups at carbon atoms 2, 3, and 4. If the chain were numbered from left to right, the groups would be at positions 4, 5, and 6. These higher numbers would violate the rule.
Add numbers to the names of the substituent groups to identify their positions on the chain. These numbers become prefixes to the name of the alkyl group. In this example the substituents and positions are 2-methyl, 3-methyl, and 4-ethyl.
Use prefixes to indicate the appearance of the same group more than once in the structural formula. Common prefixes are di- (twice), tri- (three times), tetra- (four times), and penta- (five times). This example has two methyl substituents. Thus, dimethyl will be part of the complete name.
List the names of alkyl substituents in alphabetical order. For purposes of alphabetizing, ignore the prefixes di-, tri-, and so on. In this example, the 4-ethyl group is listed before the 2-methyl and 3-methyl groups (which are combined as 2,3-dimethyl in the name).
Use proper punctuation. This is very important in writing the names of organic compounds in the IUPAC system. Commas are used to separate numbers. Hyphens are used to separate numbers and words. The entire name is written without any spaces.
According to the IUPAC rules, the name of the compound in the example is 4-ethyl-2,3-dimethylheptane. Note that the name of the parent alkane, heptane, follows directly after the final prefix, dimethyl. (It would be incorrect to write the name 4-ethyl-2,3-dimethyl heptane.)
Much of the world's energy is supplied by burning fossil fuels. Fossil fuels are carbon-based because they are derived from the decay of organisms. Millions of years ago, marine organisms died, settled on the ocean floor, and were buried in ocean sediments. Heat, pressure, and bacteria changed the residue into petroleum and natural gas, which contain mostly aliphatic hydrocarbons.
Natural gas is an important source of alkanes of low molar mass. Typically, natural gas is composed of about 80% methane, 10% ethane, 4% propane, and 2% butane. The remaining 4% consists of nitrogen and hydrocarbons of higher molar mass. Natural gas also contains a small amount of the noble gas helium. In fact, natural gas is a major source of helium. Methane, the major constituent of natural gas, is especially prized for combustion because it burns with a hot, clean flame.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + heat
Geologists think that coal had its origin some 300 million years ago when huge tree ferns and mosses grew abundantly in swampy tropical regions. When the plants died, they formed thick layers of decaying vegetation. Layer after layer of soil and rock eventually covered the decaying vegetation, which caused a buildup of intense pressure. This pressure, together with heat from Earth's interior, slowly turned the plant remains into coal.
The first stage in the formation of coal is an intermediate material known as peat. Peat, shown in Figure 22.15, is a soft, brown, spongy, fibrous material. When first dug out of a bog, peat has a very high water content. After it has been allowed to dry, it produces a low-cost but smoky fuel. If peat is left in the ground, it continues to change. After a long period of time, peat loses most of its fibrous texture and becomes lignite, or brown coal. Lignite is much harder than peat and has a higher carbon content (about 50%). The water content, however, is still high. Continued pressure and heat slowly change lignite into bituminous, or soft coal. Bituminous coal has a lower water content and higher carbon content (70-80%) than lignite. In some regions of Earth's crust, even greater pressures have been exerted. In those places, such as eastern Pennsylvania, soft coal has been changed into anthracite, or hard coal. Anthracite has a carbon content that exceeds 80%, making it an excellent fuel source. Coal is classified by its hardness and carbon content.
Coal, which is usually found in seams from 1 to 3 meters thick, is obtained from both underground and surface mines. In North America, coal mines are usually less than 100 meters underground. Much of the coal is so close to the surface that it is strip-mined, as shown in Figure 22.16. By contrast, many coal mines in Europe and other parts of the world extend 1000 to 1500 meters below Earth's surface.
Composition of Coal
Coal consists largely of condensed aromatic compounds of extremely high molar mass. These compounds have a high proportion of carbon compared with hydrogen. Due to the high proportion of aromatic compounds, coal leaves more soot upon burning than do the more aliphatic fuels obtained from petroleum. The majority of the coal that was once burned in North America contained about 7% sulfur, which burns to form the major air pollutants SO2 and SO3.
Coal may be distilled to obtain a variety of products: coke, coal tar, coal gas, and ammonia. Coke is the solid material left after coal distillation. It is used as a fuel in many industrial processes and is the crucial reducing agent in the smelting of iron ore. Because it is almost pure carbon, coke produces intense heat and little or no smoke when it burns. Coal gas consists mainly of hydrogen, methane, and carbon monoxide, all of which are flammable. Coal tar can be distilled further into benzene, toluene, naphthalene, phenol, and pitch. The ammonia from distilled coal is converted to ammonium sulfate for use as a fertilizer
any member of a class of organic compounds containing covalently bonded fluorine, chlorine, bromine, or iodine
Halocarbons are a class of organic compounds containing covalently bonded fluorine, chlorine, bromine, or iodine. A halocarbon is a carbon-containing compound with a halogen substituent. The IUPAC rules for naming halocarbons are based on the name of the parent hydrocarbon. The halogen groups are named as substituents.
Common names of halocarbons consist of two parts. The first part names the hydrocarbon portion of the molecule as an alkyl group, such as methyl- or ethyl-. The second part gives the halogen with an -ide ending.
Very few halocarbons are found in nature, but they can be readily prepared and used for many purposes. For example, halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) is used as an anesthetic. Hydrofluorocarbons are used as refrigerants in automobile air-conditioning systems.
The attractions between halocarbon molecules are primarily the result of the weak van der Waals interactions called dispersion forces. These attractions increase with the degree of halogen substitution. Thus, more highly halogenated organic compounds have higher boiling points
a common type of organic reaction; involves the replacement of an atom or group of atoms by another atom or group of atoms
Organic reactions often proceed more slowly than inorganic reactions. This is because organic reactions commonly involve the breaking of relatively strong covalent bonds. Catalysts are often needed. Many organic reactions are complex, often producing a mixture of products. The desired product must then be separated by distillation, crystallization, or other means. A common type of organic reaction is a substitution reaction, in which an atom, or a group of atoms, replaces another atom or group of atoms.
A halogen can replace a hydrogen atom on an alkane to produce a halocarbon. The symbol X stands for a halogen in this generalized equation.
R - H + X2 --> R - X + HX
alkane halogen halocarbon hydrogen halide
Sunlight or another source of ultraviolet radiation usually serves as a catalyst. From the generalized equation, you can write a specific one.
CH4 + CL2 --------> CH3Cl + HCl
methane chlorine uv light chloromethane hydrogen-chlorine
Even under controlled conditions, this simple halogenation reaction produces a mixture of mono-, di-, tri-, and tetrachloromethanes
Treating benzene with a halogen in the presence of a catalyst causes the substitution of a hydrogen atom in the ring. Iron compounds are often used as catalysts for aromatic substitution reactions. For example, a rusty nail dropped in the reaction flask can act as a catalyst.
Halogens on carbon chains are readily displaced by hydroxide ions to produce an alcohol and a salt.
the —OH functional group present in alcohols
The —OH functional group in alcohols is called a hydroxyl group or hydroxy function. Aliphatic alcohols can be classified into structural categories according to the number of R groups attached to the carbon with the hydroxyl group. If one R group is attached, the alcohol is a primary alcohol; if two R groups, a secondary alcohol; if three R groups, a tertiary alcohol.
Both IUPAC and common names are used for alcohols. When using the IUPAC system to name continuous-chain and substituted alcohols, drop the -e ending of the parent alkane name and add the ending -ol. The parent alkane is the longest continuous chain of carbons that includes the carbon attached to the hydroxyl group. In numbering the longest continuous chain, the position of the hydroxyl group is given the lowest possible number. Alcohols containing two, three, and four — OH substituents are named diols, triols, and tetrols, respectively.
Common names of aliphatic alcohols are written in the same way as those of the halocarbons. The alkyl group ethyl, for example, is named and followed by the word alcohol, as in ethyl alcohol.