There is no home-treatment antidote for gasoline poisoning; call a poison control center. Liquid alkanes with approximately 5—16 carbon atoms per molecule wash away natural skin oils and cause drying and chapping of the skin, while heavier liquid alkanes those with approximately 17 or more carbon atoms per molecule act as emollients skin softeners.
Such alkane mixtures as mineral oil and petroleum jelly can be applied as a protective film. In this chapter we will investigate the alkanes, compounds containing only two elements, carbon and hydrogen, and having only single bonds. We will also investigate alkanes that have halogens incorporated into their structure. Recall that halogens are the elements in Family 7A on the periodic table and contain representative elements such as chlorine, fluorine, iodine, and bromine.
There are several other kinds of hydrocarbons, distinguished by the types of bonding between carbon atoms and by the properties that result from that bonding. In Chapter 8 we will examine hydrocarbons with double bonds, with triple bonds, and with a special kind of bonding called aromaticity. Then in Chapter 9, we will study some compounds considered to be derived from hydrocarbons by replacing one or more hydrogen atoms with an oxygen-containing group. Chapter 10 focuses on organic acids and bases.
Compounds isolated from nonliving systems, such as rocks and ores, the atmosphere, and the oceans, were labeled inorganic. For many years, scientists thought organic compounds could be made by only living organisms because they possessed a vital force found only in living systems.
What he expected is described by the following equation. This result led to a series of experiments in which a wide variety of organic compounds were made from inorganic starting materials. The vital force theory gradually went away as chemists learned that they could make many organic compounds in the laboratory. Today organic chemistry has been reclassified as the study of compounds that contain carbon, and inorganic chemistry is the study of the chemistry of all other elements.
It may seem strange that we divide chemistry into two branches—one that considers compounds of only one element and one that covers the plus remaining elements.
However, this division seems more reasonable when we consider that of tens of millions of compounds that have been characterized, the overwhelming majority are carbon compounds. The word organic has different meanings. Organic fertilizer, such as cow manure, is organic in the original sense; it is derived from living organisms. Organic foods generally are foods grown without synthetic pesticides or fertilizers.
Organic chemistry is the chemistry of compounds of carbon. Carbon is unique among the other elements in that its atoms can form stable covalent bonds with each other and with atoms of other elements in a multitude of variations.
The resulting molecules can contain from one to millions of carbon atoms. Organic compounds, like inorganic compounds, obey all the natural laws. Often there is no clear distinction in the chemical or physical properties among organic and inorganic molecules.
Nevertheless, it is useful to compare typical members of each class, as in Table 7. To further illustrate typical differences among organic and inorganic compounds, Table 7.
Many compounds can be classified as organic or inorganic by the presence or absence of certain typical properties, as illustrated in Table 7. Table 7. Classify each compound as organic or inorganic.
Which member of each pair has a higher melting point? Alkanes are organic compounds that consist entirely of single-bonded carbon and hydrogen atoms and lack any other functional groups.
Alkanes are the simplest and least reactive hydrocarbon species containing only carbons and hydrogens. They are commercially very important, being the principal constituent of gasoline and lubricating oils and are extensively employed in organic chemistry; though the role of pure alkanes such as hexanes is delegated mostly to solvents.
The distinguishing feature of an alkane, making it distinct from other compounds that also exclusively contain carbon and hydrogen, is its lack of unsaturation. That is to say, it contains no double or triple bonds, which are highly reactive in organic chemistry. Though not totally devoid of reactivity, their lack of reactivity under most laboratory conditions makes them a relatively uninteresting, though very important component of organic chemistry.
As you will learn about later, the energy confined within the carbon-carbon bond and the carbon-hydrogen bond is quite high and their rapid oxidation produces a large amount of heat, typically in the form of fire. The straight chain alkanes, methane CH 4 , ethane C 2 H 6 , and propane C 3 H 8 represent the beginning of a series of compounds in which any two members in a sequence differ by one carbon atom and two hydrogen atoms—namely, a CH 2 unit Fig.
The first 10 members of this series are given in Table 7. Note that as you increase the length of the carbon chain, the number of possible different structural isomers also increases. From propane C 3 H 8 o nward, you will notice that the only difference between longer chain hydrocarbons involves the addition of CH 2 units as you move up the series Fig. Any family of compounds in which adjacent members differ from each other by a definite factor here a CH 2 group is called a homologous series , and can be defined mathematically.
The members of such a series, called homologs. In organic chemistry, homologs have properties that vary in a regular and predictable manner.
Thus, the principle of homology gives organization to organic chemistry in much the same way that the periodic table gives organization to inorganic chemistry. Instead of a bewildering array of individual carbon compounds, we can study a few members of a homologous series and from them deduce some of the properties of other compounds in the series. In Figure 7. Using this formula, we can write a molecular formula for any alkane with a given number of carbon atoms. In the homologous series of alkanes, what is the molecular formula for the member just above C 8 H 18?
Use the general formula for alkanes to write the molecular formula of the alkane with 12 carbon atoms. We can write the structure of butane C 4 H 10 by stringing four carbon atoms in a row,. The compound butane has this structure, but there is another way to put 4 carbon atoms and 10 hydrogen atoms together.
Place 3 of the carbon atoms in a row and then branch the fourth one off the middle carbon atom:. Now we add enough hydrogen atoms to give each carbon four bonds. There is a hydrocarbon that corresponds to this structure, which means that two different compounds have the same molecular formula C 4 H 10 , but a different arrangement of the atoms in space. Recall that compounds having the same molecular formula but a different arrangement in space are called structural isomers.
Structural isomers have different chemical and physical properties. The ball-and-stick models of these two compounds show them to be isomers; both have the molecular formula C 4 H Notice that C 4 H 10 is depicted with a bent chain in Figure 7. The four-carbon chain may be bent in various ways because the groups can rotate freely about the C—C bonds.
However, this rotation does not change the identity of the compound. It is important to realize that bending a chain does not change the identity of the compound; all of the following represent the same compound:. The formula of isobutane shows a continuous chain of three carbon atoms only, with the fourth attached as a branch off the middle carbon atom of the continuous chain.
Unlike C 4 H 10 , the compounds methane CH 4 , ethane C 2 H 6 , and propane C 3 H 8 do not exist in isomeric forms because there is only one way to arrange the atoms in each formula so that each carbon atom has four bonds. A continuous unbranched chain of carbon atoms is often called a straight chain even though the tetrahedral arrangement about each carbon gives it a zigzag shape.
Straight-chain alkanes are sometimes called normal alkanes , and their names are given the prefix n -. For example, butane is called n -butane. In alkanes, can there be a two-carbon branch off the second carbon atom of a four-carbon chain? A student is asked to write structural formulas for two different hydrocarbons having the molecular formula C 5 H She writes one formula with all five carbon atoms in a horizontal line and the other with four carbon atoms in a line, with a CH 3 group extending down from the first attached to the third carbon atom.
Do these structural formulas represent different molecular formulas? Explain why or why not. No; the branch would make the longest continuous chain of five carbon atoms. No; both are five-carbon continuous chains. Briefly identify the important distinctions between a straight-chain alkane and a branched-chain alkane. Draw the structural isomers for the following alkanes. Indicate whether the structures in each set represent the same compound or isomers.
Straight-chain alkanes and branched-chain alkanes have different properties as well as different structures. Write the condensed structural formula for each structural formula. Draw the line-angle formula for isohexane. Give the structural formula for the compound represented by this line-angle formula:. Cycloalkanes are very important in components of food, pharmaceutical drugs, and much more. However, to use cycloalkanes in such applications, we must know the effects, functions, properties, and structures of cycloalkanes.
Cycloalkanes are alkanes that are in the form of a ring; hence, the prefix cyclo- is used to name these alkanes. Stable cycloalkanes cannot be formed with carbon chains of just any length. Recall that in alkanes, carbon adopts the tetrahedral geometry in which the angles between bonds are Source: Wikipedia. For some cycloalkanes to form, the angle between bonds must deviate from this ideal angle, an effect known as angle strain.
Additionally, some hydrogen atoms may come into closer proximity with each other than is desirable become eclipsed , an effect called torsional strain. These destabilizing effects, angle strain and torsional strain are known together as ring strain. The smaller cycloalkanes, cyclopropane and cyclobutane, have particularly high ring strains because their bond angles deviate substantially from Thus, both of these ring conformations are highly unfavorable and unstable.
Cyclopentane is a more stable molecule with a small amount of ring strain, while cyclohexane is able to adopt the perfect geometry of a cycloalkane in which all angles are the ideal Cycloalkanes larger than cyclohexane have ring strain and are not as commonly encountered in organic chemistry.
Representative Cycloalkane Structures. Average bond angles and strain energy are indicated. It is also a potent, quick-acting anesthetic with few undesirable side effects in the body. It is no longer used in surgery, however, because it forms explosive mixtures with air at nearly all concentrations. Carbon atoms participating in chemical bonds within a molecule can be classified based on the number of carbon-carbon bonds that are formed.
The number of carbon neighbors that a carbon atom has can help determine the reactivity of that carbon position. Thus, it is important to be able to recognize whether a carbon atom is primary, secondary, tertiary, or quaternary in its structure Fig. Classification of carbon atoms as primary, secondary, tertiary, or quaternary. In the molecules above, the center carbon is evaluated for the number of carbon atoms that are bonded directly with the center carbon.
A primary carbon is bonded to one carbon, a secondary carbon is bonded to two carbons, a tertiary carbon is bonded to three carbons, and a quaternary carbon is bonded to four carbons.
Within any given molecule, each carbon atom can be classified Fig. Classification of Carbon Atoms Within a Molecule. Alkanes are the simplest family of hydrocarbons — compounds containing carbon and hydrogen only with only carbon-hydrogen bonds and carbon-carbon single bonds. Alkanes are not very reactive and have little biological activity; all alkanes are colorless and odorless. Because alkanes have relatively predictable physical properties and undergo relatively few chemical reactions other than combustion, they serve as a basis of comparison for the properties of many other organic compound families.
Nearly all alkanes have densities less than 1. Natural gas is composed chiefly of methane, which has a density of about 0. The density of air is about 1. Because natural gas is less dense than air, it rises. When a natural-gas leak is detected and shut off in a room, the gas can be removed by opening an upper window. On the other hand, bottled gas can be either propane density 1. Both are much heavier than air density 1. If bottled gas escapes into a building, it collects near the floor.
This presents a much more serious fire hazard than a natural-gas leak because it is more difficult to rid the room of the heavier gas. Both the melting points and boiling points of alkanes are characteristic of the intermolecular forces found between the molecules. The electronegativity difference between carbon and hydrogen 2.
These forces will be very small for a molecule like methane but will increase as the size of the molecules increase. Therefore, the melting and boiling points of the alkanes increases with the molecular size, due to the increase in London dispersion forces. Notice that the first four alkanes are gases at room temperature, and solids do not start to appear until about C 17 H Adapted from: Techstepp. Regarding isomers, the more branched the chain, the lower the boiling point tends to be.
London dispersion forces are smaller for shorter molecules and only operate over very short distances between one molecule and its neighbors. It is more difficult for short, bulky molecules with substantial amounts of branching to lie close together compact compared with long, thin molecules. Cycloalkanes are similar to alkanes in their general physical properties, but they have higher boiling points , melting points , and densities than alkanes.
This is due to stronger London forces because the ring shape allows for a larger area of contact. Alkanes both normal and cycloalkanes are virtually insoluble in water but dissolve in organic solvents. The liquid alkanes are good solvents for many other covalent compounds.
When a molecular substance dissolves in water, the following must occur:. Breaking either of these attractions requires energy, although the amount of energy required to break the London dispersion forces in a compound, such as methane, is relatively negligible; this is not true of the hydrogen bonds in water.
Recall that hydrogen bonds are much stronger. To simplify, a substance will dissolve if sufficient energy is released when the new bonds are formed between the substance and the water to make up for the energy required to break the original attractions. The only new attractions between the alkane and the water molecules are the London dispersion forces.
These forces to do not release a sufficient amount of energy to compensate for the energy required to break the hydrogen bonds in water. Therefore, the alkane does not dissolve as shown in Figure 7. Long Chain Hydrocarbons are Insoluble in Water. Neither the saturated and unsaturated hydrocarbons in this sunflower oil do not have strong enough intermolecular forces to disrupt the hydrogen bonds between the water molecules. In this case the oil is less dense than the water and will float on top of the water layer.
In most organic solvents, the primary forces of attraction between the solvent molecules are the London dispersion forces. Therefore, when an alkane dissolves in an organic solvent, the London dispersion forces are broken and are replaced by new London dispersion forces between the mixture. The two processes more or less cancel each other out energetically; thus, there is no barrier to solubility. Due to the solubility and density of alkanes, oil spills into the ocean or other bodies of water can have devastating environmental consequences.
The oil cannot dissolve or mix with the water, and because it is less dense than water, it floats on top of the surface of the water creating an oil slick, as depicted in Figure 7. Since the oil slick remains at the surface of the water, the organisms most affected by oil slicks are those found on the surface of the ocean or near the shorelines, including sea otters and seabirds.
The chemical constituents of the oil are toxic though ingestion, inhalation, and through skin and eye irritation. The leak was a mile below the surface, making it difficult to estimate the size of the spill. One liter of oil can create a slick 2.
Without referring to a table, predict which has a higher boiling point—hexane or dodecane. If 25 mL of hexane were added to mL of water in a beaker, which of the following would you expect to happen?
Without referring to a table or other reference, predict which member of each pair has the higher boiling point. For which member of each pair is hexane a good solvent?
Unlike the complex transformations of combustion, the halogenation of an alkane appears to be a simple substitution reaction in which a C-H bond is broken and a new C-X bond is formed. The chlorination of methane, shown below, provides a simple example of this reaction. However, one complication is that all the hydrogen atoms of an alkane may undergo substitution, resulting in a mixture of products, as shown in the following unbalanced equation.
The relative amounts of the various products depend on the proportion of the two reactants used. In the case of methane, a large excess of the hydrocarbon favors formation of methyl chloride as the chief product; whereas, an excess of chlorine favors formation of chloroform and carbon tetrachloride.
The following facts must be accomodated by any reasonable mechanism for the halogenation reaction. We shall confine our attention to chlorine and bromine, since fluorine is so explosively reactive it is difficult to control, and iodine is generally unreactive. Chlorinations and brominations are normally exothermic.
Energy input in the form of heat or light is necessary to initiate these halogenations. If light is used to initiate halogenation, thousands of molecules react for each photon of light absorbed.
Halogenation reactions may be conducted in either the gaseous or liquid phase. In gas phase chlorinations the presence of oxygen a radical trap inhibits the reaction. In liquid phase halogenations radical initiators such as peroxides facilitate the reaction.
The most plausible mechanism for halogenation is a chain reaction involving neutral intermediates such as free radicals or atoms. A chain reaction mechanism for the chlorination of methane has been described. Bromination of alkanes occurs by a similar mechanism, but is slower and more selective because a bromine atom is a less reactive hydrogen abstraction agent than a chlorine atom, as reflected by the higher bond energy of H-Cl than H-Br.
To see an animated model of the bromination free radical chain reaction. When alkanes larger than ethane are halogenated, isomeric products are formed. Thus chlorination of propane gives both 1-chloropropane and 2-chloropropane as mono-chlorinated products. Four constitutionally isomeric dichlorinated products are possible, and five constitutional isomers exist for the trichlorinated propanes. Can you write structural formulas for the four dichlorinated isomers?
The halogenation of propane discloses an interesting feature of these reactions. All the hydrogens in a complex alkane do not exhibit equal reactivity. For example, propane has eight hydrogens, six of them being structurally equivalent primary , and the other two being secondary. Solutions of bromine in CCl 4 have an intense red-orange color. When Br 2 in CCl 4 is mixed with a sample of an alkane, no change is initially observed.
When it is mixed with an alkene or alkyne, the color of Br 2 rapidly disappears. The reaction between 2-butene and bromine to form 2,3-dibromobutane is just one example of the addition reactions of alkenes and alkynes. Addition of HBr to 2-butene, for example, gives 2-bromobutane. H 2 adds across double or triple bonds in the presence of a suitable catalyst to convert an alkene or alkyne to the corresponding alkane.
Addition reactions provide a way to add new substituents to a hydrocarbon chain and thereby produce new derivatives of the parent alkanes.
0コメント