Eight carbons is typical for the size of the hydrocarbons present in gasoline. There are 13 structural isomers of C 8 H 18 , one of which is isooctane see below. Branched alkanes tend to burn more smoothly than straight-chain alkanes when they are used in internal combustion engines. Tetraethyllead, Pb CH 2 CH 3 4 , used to be widely used as an "anti-knocking" agent; this has been discontinued because of concerns about the release of lead into the atmosphere.
Other compounds can be used as antiknocking agents, such as MTBE methyl tert -butyl ether — see the section on ethers for more information on this compound. There are 35 structural isomers of C 9 H There are 75 structural isomers of C 10 H In diesel fuels, straight chains are preferable to branched chains because fuel is sprayed into the cylinder during the power stroke, and needs to ignite as it enters.
Hexadecane is assigned a cetane number of , and the highly branched heptamethylnonane has a cetane number of 0. Cycloalkanes are alkanes in which the ends of the carbon chain are joined together, making a ring of carbon atoms which requires the loss of two hydrogen atoms compared to the straight-chain alkanes.
They have the general formula C n H 2n for molecules containing one ring. One Ring, of course, rules them all. Cycloalkanes are named in a similar fashion to straight-chain alkanes; the prefix cyclo- is added to the stem indicating the number of carbon atoms in the ring.
Carbon groups that are attached to the ring are named as alkyl groups. Thus, the name "cyclobutane" indicates a ring of four carbon atoms, all linked by single bonds.
Free rotation is not possible around the carbon-carbon single bonds in rings, which leads to the introduction of stereoisomers — molecules having the same pattern of connectivity, but different arrangements of atoms in space. This deviation from the ideal tetrahedral bond angles of Because free rotation is not possible around the single bonds in a cycloalkane, when two methyl groups are added to cyclopropane, there are two possible arrangements: both methyl groups can be pointing to the same side of the ring cis , or to opposite sides of the ring trans : Download 3D Download 3D These molecules are not structural isomers of each other, because the order in which the atoms are connected is the same, but the two methyl groups are pointing in different directions in space, making them stereoisomers of each other.
These types of stereoisomers are referred to as geometric isomers. As a molecule, it's not completely "square. The ring is able to "pucker" slightly to relieve some of the eclipsing interactions, and so all of the carbon atoms do not lie in the same plane.
Cyclopentanes are a more stable than cyclopropanes and cyclobutanes, and are found in wide variety of naturally occurring molecules. Cyclohexane C 6 H 12 3D With cyclohexane, ring chemistry gets even more interesting and complicated.
However, cyclohexane can adopt one of two conformations which relieve this ring strain. In one of these conformations, the carbon atoms on either side of the ring bend slightly upwards; in this so-called "boat" conformation, the bond angles all Download 3D There are still a number of C—H eclipsing interactions, however, and this is not the lowest-energy conformation which is possible. In the other possible conformation, one of the carbon atoms bends upwards, and the one on the opposite side bends downwards, producing the "chair" conformation; in this conformation, not only are the bond angles Cyclohexane rings are very stable, and are present in large numbers of naturally occurring molecules.
Download 3D This is drastically oversimplified, of course; the chair and boat conformation represent the extremes of a large number of possible intermediate conformations, and the energetics of the ring system change a great deal when substituents are added to the ring. Cycloheptane C 7 H 14 3D Download 3D There is a slight amount of strain energy in cycloheptane and larger cycloalkanes, since it is difficult for them to adopt bond angles of This molecule is often found on traffic signs:.
There are two geometric isomers of the molecule, the cis and trans form. Norbornane rings show up in a lot of interesting molecules, such as camphor. Adamantane 3D Download 3D Adamantane is a tricyclic molecule; the fused cyclohexane rings in this molecule are extremely rigid.
Extending the structure of adamantane further into three dimensions results in the structure of diamonds. Cole at the University of Chicago in Prismane 3D Download 3D Prismane is so named because it looks like a molecular prism duh. Wiberg at Yale University. Dodecahedrane 3D Download 3D Dodecahedrane is a molecule of the molecular formula C 20 H 20 which is shaped like a dodecahedron a polyhedron having twelve faces.
It was first synthesized in by the research group of Leo A. Paquette at Ohio State University. Alkyl halides , or haloalkanes , are alkanes in which one or more hydrogen atoms are replaced by halogen atoms fluorine, chlorine, bromine, or iodine. The carbon-halogen bond is more polar than the carbon-hydrogen bonds, but most alkyl halides are not very soluble in water.
Alkyl fluorides and chlorides having only one halogen atoms have densities that are higher than those of alkanes, but are slightly less dense than water, while alkyl bromides and iodides are generally more dense than water. Alkyl halides having more than one halogen atom are often more dense than water. Alkyl halides are named as alkanes, with the halogens named as halo- substituents. The halogens are named as fluoro- , bromo- , chloro- , and iodo- when attached to a carbon atom. For example, the name "1-chloropropane" indicated a three-carbon chain with a chlorine atom on carbon number 1.
A number of simple alkyl halides are usually known by their common names; for instance, trichloromethane is almost always referred to as "chloroform. It is a very common solvent, but it is mildly toxic; it is more dense than water, having a density of 1.
It is sometime used as a paint remover and degreaser. It is used to decaffeinate coffee beans; since it has such a low boiling point, the residual solvent can be removed from the beans at fairly low temperatures. Chloroform is a commonly used organic solvent, and like dichloromethane is more dense than water d 1. Chloroform vapor is a anesthetic: James Young Simpson was the first to use chloroform as an anesthetic during childbirth in presumably, not on himself!
However, since chloroform is carcinogenic, and toxic to the liver, it is not widely used for this purpose anymore. It's also useful for knocking out giant apes. Tetrachloromethane Carbon Tetrachloride 3D Download 3D Tetrachloromethane better known as carbon tetrachloride is a liquid at room temperature, with a density of 1. It used to be a common organic solvent, and was widely used for dry cleaning and spot removal.
However, it has been shown to be toxic and carcinogenic, so other solvents are used instead. Carbon tetrachloride is also a contributor to ozone depletion see Freon , and has been banned under the Montreal Protocols that set strict limits on the use of CFCs.
It was heavily used in dry cleaning, but it has been replaced by other solvents such as tetrachloroethylene under the Montreal Protocols see Freon Dichlorodifluoromethane Freon 3D Download 3D Dichlorodifluoromethane Freon is an example of the chlorofluorocarbons CFCs, or freons , which are organic compound containing fluorine and chlorine atoms. These compounds were developed in the s for use as refrigerants; prior to this, ammonia was used as a refrigerant, but the high toxicity of ammonia gas made this less than ideal.
The CFCs are relatively nontoxic, very unreactive, and boil at low temperatures, and were thus ideal for use in refrigeration equipment. In addition, CFCs were widely used as aerosol propellants in spray cans, and as foaming agents in the manufacture of plastic foams such as Styrofoam. Unfortunately, the low reactivity of the CFCs leads to a major environmental problem: they persist in the environment for a long time up to a century , and eventually make their way into the upper atmosphere, where they damage the ozone layer.
Ozone , O 3 , is an allotropic form of molecular oxygen, O 2 , which is found in the stratosphere, at an altitude of 10 to 50 km above the surface of the Earth. It is produced when photons h n of high-energy ultraviolet light wavelengths of nm or less from the Sun splits oxygen molecules into oxygen atoms O. The oxygen atoms combine with oxygen molecules to make ozone:. This ozone that is produced from this reaction absorbs ultraviolet light with wavelengths of nm or less, splitting into oxygen molecules and oxygen atoms,.
The ozone in the ozone layer thus protects the surface of the Earth and more importantly, the living organisms that make their home there from much of the damaging high-energy ultraviolet light from the Sun. When a molecule of Freon drifts into the upper atmosphere, photons of high-energy light can strike it, causing it to release a chlorine atom:. The chlorine atom has an unpaired electron, and is a highly reactive free radical , which reacts with ozone in the stratosphere, converting it to molecular oxygen:.
The chlorine atom is regenerated in this process, and behaves as a catalyst; one chlorine atom can destroy up to , ozone molecules. The mechanism for ozone destruction was first published in by F.
Many areas of the country have recycling programs that focus on one or more of the commodity plastics that have been assigned a recycling code see Figure 8. These operations have been in effect since the s and have made the production of some plastics among the most efficient industrial operations today.
Figure 8. Each type of recyclable plastic is imprinted with a code for easy identification. The name of an alkene is derived from the name of the alkane with the same number of carbon atoms.
The presence of the double bond is signified by replacing the suffix -ane with the suffix -ene. The location of the double bond is identified by naming the smaller of the numbers of the carbon atoms participating in the double bond:. Molecules of 1-butene and 2-butene are structural isomers; the arrangement of the atoms in these two molecules differs.
As an example of arrangement differences, the first carbon atom in 1-butene is bonded to two hydrogen atoms; the first carbon atom in 2-butene is bonded to three hydrogen atoms. The compound 2-butene and some other alkenes also form a second type of isomer called a geometric isomer.
In a set of geometric isomers, the same types of atoms are attached to each other in the same order, but the geometries of the two molecules differ. Carbon atoms are free to rotate around a single bond but not around a double bond; a double bond is rigid. This makes it possible to have two isomers of 2-butene, one with both methyl groups on the same side of the double bond and one with the methyl groups on opposite sides. The 2-butene isomer in which the two methyl groups are on the same side is called a cis -isomer; the one in which the two methyl groups are on opposite sides is called a trans -isomer Figure 9.
The different geometries produce different physical properties, such as boiling point, that may make separation of the isomers possible:. Figure 9. These molecular models show the structural and geometric isomers of butene. This reaction is called an addition reaction. The hybridization of the carbon atoms in the double bond in an alkene changes from sp 2 to sp 3 during an addition reaction.
For example, halogens add to the double bond in an alkene instead of replacing hydrogen, as occurs in an alkane:. The reactant is a five-carbon chain that contains a carbon-carbon double bond, so the base name will be pentene. We begin counting at the end of the chain closest to the double bond—in this case, from the left—the double bond spans carbons 2 and 3, so the name becomes 2-pentene. Since there are two carbon-containing groups attached to the two carbon atoms in the double bond—and they are on the same side of the double bond—this molecule is the cis- isomer, making the name of the starting alkene cis pentene.
The product of the halogenation reaction will have two chlorine atoms attached to the carbon atoms that were a part of the carbon-carbon double bond:. This molecule is now a substituted alkane and will be named as such. The base of the name will be pentane. We will count from the end that numbers the carbon atoms where the chlorine atoms are attached as 2 and 3, making the name of the product 2,3-dichloropentane.
Hydrocarbon molecules with one or more triple bonds are called alkynes ; they make up another series of unsaturated hydrocarbons. The simplest member of the alkyne series is ethyne, C 2 H 2 , commonly called acetylene.
The Lewis structure for ethyne, a linear molecule, is:. The IUPAC nomenclature for alkynes is similar to that for alkenes except that the suffix -yne is used to indicate a triple bond in the chain. Chemically, the alkynes are similar to the alkenes.
The reaction of acetylene with bromine is a typical example:. Acetylene and the other alkynes also burn readily. An acetylene torch takes advantage of the high heat of combustion for acetylene. Benzene, C 6 H 6 , is the simplest member of a large family of hydrocarbons, called aromatic hydrocarbons. These compounds contain ring structures and exhibit bonding that must be described using the resonance hybrid concept of valence bond theory or the delocalization concept of molecular orbital theory.
To review these concepts, refer to the earlier chapters on chemical bonding. The resonance structures for benzene, C 6 H 6 , are:. Valence bond theory describes the benzene molecule and other planar aromatic hydrocarbon molecules as hexagonal rings of sp 2 -hybridized carbon atoms with the unhybridized p orbital of each carbon atom perpendicular to the plane of the ring.
Figure This condensed formula shows the unique bonding structure of benzene. Benzene does not, however, exhibit the characteristics typical of an alkene. To represent this unique bonding, structural formulas for benzene and its derivatives are typically drawn with single bonds between the carbon atoms and a circle within the ring as shown in Figure There are many derivatives of benzene. The hydrogen atoms can be replaced by many different substituents. Aromatic compounds more readily undergo substitution reactions than addition reactions; replacement of one of the hydrogen atoms with another substituent will leave the delocalized double bonds intact.
The following are typical examples of substituted benzene derivatives:. Toluene and xylene are important solvents and raw materials in the chemical industry. Styrene is used to produce the polymer polystyrene. One possible isomer created by a substitution reaction that replaces a hydrogen atom attached to the aromatic ring of toluene with a chlorine atom is shown here. Draw two other possible isomers in which the chlorine atom replaces a different hydrogen atom attached to the aromatic ring:.
Since the six-carbon ring with alternating double bonds is necessary for the molecule to be classified as aromatic, appropriate isomers can be produced only by changing the positions of the chloro-substituent relative to the methyl-substituent:.
Strong, stable bonds between carbon atoms produce complex molecules containing chains, branches, and rings. The chemistry of these compounds is called organic chemistry. Hydrocarbons are organic compounds composed of only carbon and hydrogen. The alkanes are saturated hydrocarbons—that is, hydrocarbons that contain only single bonds.
Alkenes contain one or more carbon-carbon double bonds. Alkynes contain one or more carbon-carbon triple bonds. Both reactions result in bromine being incorporated into the structure of the product. The difference is the way in which that incorporation takes place. In the saturated hydrocarbon, an existing C—H bond is broken, and a bond between the C and the Br can then be formed.
Unbranched alkanes have free rotation about the C—C bonds, yielding all orientations of the substituents about these bonds equivalent, interchangeable by rotation. Since these concepts pertain to phenomena at the molecular level, this explanation involves the microscopic domain.
They are the same compound because each is a saturated hydrocarbon containing an unbranched chain of six carbon atoms. In the following, the carbon backbone and the appropriate number of hydrogen atoms are shown in condensed form:. In acetylene, the bonding uses sp hybrids on carbon atoms and s orbitals on hydrogen atoms. In benzene, the carbon atoms are sp 2 hybridized. Skip to main content.
Organic Chemistry. Search for:. Hydrocarbons Learning Objectives By the end of this section, you will be able to: Explain the importance of hydrocarbons and the reason for their diversity Name saturated and unsaturated hydrocarbons, and molecules derived from them Describe the reactions characteristic of saturated and unsaturated hydrocarbons Identify structural and geometric isomers of hydrocarbons. Example 1: Drawing Skeletal Structures Draw the skeletal structures for these two molecules:.
Show Answer Each carbon atom is converted into the end of a line or the place where lines intersect. Show Answer. Example 2: Interpreting Skeletal Structures Identify the chemical formula of the molecule represented here:. Show Answer There are eight places where lines intersect or end, meaning that there are eight carbon atoms in the molecule.
Location of the hydrogen atoms:. Show Answer C 9 H Example 3: Naming Halogen-substituted Alkanes Name the molecule whose structure is shown here:. Show Answer 3,3-dibromoiodopentane. Example 4: Naming Substituted Alkanes Name the molecule whose structure is shown here:. Show Answer The longest carbon chain runs horizontally across the page and contains six carbon atoms this makes the base of the name hexane, but we will also need to incorporate the name of the branch.
Show Answer 4-propyloctane. Show Answer The reactant is a five-carbon chain that contains a carbon-carbon double bond, so the base name will be pentene.
Show Answer reactant: transhexene, product: 3,4-dichlorohexane. Three-dimensional representations of both structures are as follows:. The systematic names for branched hydrocarbons use the lowest possible number to indicate the position of the branch along the longest straight carbon chain in the structure. Thus the systematic name for isobutane is 2-methylpropane, which indicates that a methyl group a branch consisting of —CH 3 is attached to the second carbon of a propane molecule.
Similarly, Section 2. The compound has a chain of five carbon atoms, so it is a derivative of pentane. There are two methyl group branches at one carbon atom and one methyl group at another. Using the lowest possible numbers for the branches gives 2,2,4-trimethylpentane for the systematic name of this compound. As with alkanes, more than one structure is possible for alkenes with four or more carbon atoms. For example, an alkene with four carbon atoms has three possible structures.
Just as a number indicates the positions of branches in an alkane, the number in the name of an alkene specifies the position of the first carbon atom of the double bond. The name of a compound does not depend on its orientation.
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