Each of the six compounds has the molecular formula C 6 H 14 O, the same molecular weight, , but different boiling points. Note that the boiling points of the alcohols are uniformly higher than the boiling points of the ethers even though they all have the same mass. The higher boiling point of the alcohols is due to their ability to hydrogen bond, as does water which has a very high boiling point for its mass.
A similar exercise could have been performed on hexanes 2 - 5 to produce a new series of alcohols and ethers. Stereoisomers are isomers that have the same atom connectivity but differ only in their orientation in space. Stereoisomers include geometrical isomers, diastereomers, and enantiomers. The most common definition of these three classes begins with enantiomers. Enantiomers are stereoisomers that are non-superimpoable mirror images of one another.
Diastereomers are defined traditionally as stereoisomers that are not mirror images of one another. Geometrical isomers cis-trans are stereoisomers about a double bond. Rather than discuss the more complex stereoisomers first -- for indeed we have been progressing from the more complex isomers to the less complex ones -- we will consider enantiomers in the next section first, and then work our way toward the other stereoisomers -- diastereomers and geometrical isomers.
Enantiomers are simply a pair of stereoisomers that are non-superimposable mirror images of one another. A substance must be chiral handed , i. Enantiomers come in pairs only and they are not superimposable upon one another. A hand is the most common chiral entity. Your left hand mirrors your right hand and they are not superimposable on one another. Many common objects are chiral: screws, spiral staircases, gloves, shoes, most knots, etc. For an object to be achiral, it must have a minimum of one plane of symmetry.
Some examples are: the human external body to a first approximation bilateral symmetry , a coffee mug, a pair of reading glasses, etc. Enantiomers are most commonly formed when a carbon atom sp3 hybridized contains four different substituents. There are two ways to attach the substituents to the quadrivalent carbon. The two arrangements are enantiomers of one another.
The carbon atom in such species is often said to be chiral but this is a misnomer. It is not the carbon atom that is chiral but rather the environment around the carbon atom. The property of chirality, as we have seen, is independent of chemistry and, for that matter, atoms. In the enantiomers 12 and 13 shown below How to manipulate JSmol structures , imagine that the black "ball" is totally invisible against the black background.
The four colored balls form a chiral environment independent of the black ball and the "sticks" that keep the four colored balls spaced apart. A ball achiral in a hand chiral is chiral. In summary, the most common source of chirality in organic chemistry is the "asymmetric carbon", or, better yet, the carbon at a center of asymmetry. We showed that the carbon only serves the capacity of keeping the four different atoms, or colored balls if you wish, where they belong. But an asymmetric carbon is not the only source of chirality.
Conformations of molecules, and molecules themselves, can be chiral without having an asymmetric carbon. Since the two higher priority groups 1 are on the same side of the double bond, this configuration is Z. Cycloalkane Stereoisomers. Stereoisomers are also observed in certain disubstituted and higher substituted cyclic compounds. Unlike the relatively flat molecules of alkenes, substituted cycloalkanes must be viewed as three-dimensional configurations in order to appreciate the spatial orientations of the substituents.
By agreement, chemists use heavy, wedge-shaped bonds to indicate a substituent located above the average plane of the ring note that cycloalkanes larger than three carbons are not planar , and a hatched line for bonds to atoms or groups located below the ring. As in the case of the 2-butene stereoisomers, disubstituted cycloalkane stereoisomers may be designated by nomenclature prefixes such as cis and trans. The stereoisomeric 1,2-dibromocyclopentanes shown to the right are an example.
In general, if any two sp 3 carbons in a ring have two different substituent groups not counting other ring atoms stereoisomerism is possible. This is similar to the substitution pattern that gives rise to stereoisomers in alkenes; indeed, one might view a double bond as a two-membered ring.
Four other examples of this kind of stereoisomerism in cyclic compounds are shown below. If more than two ring carbons have different substituents not counting other ring atoms the stereochemical notation distinguishing the various isomers becomes more complex.
Practice Problems. Conformational Isomerism. Structural formulas show the manner in which the atoms of a molecule are bonded together its constitution , but do not generally describe the three-dimensional shape of a molecule, unless special bonding notations e. The importance of such three-dimensional descriptive formulas became clear in discussing configurational stereoisomerism, where the relative orientation of atoms in space is fixed by a molecule's bonding constitution e.
Here too it was noted that nomenclature prefixes must be used when naming specific stereoisomers. In this section we shall extend our three-dimensional view of molecular structure to include compounds that normally assume an array of equilibrating three-dimensional spatial orientations, which together characterize the same isolable compound.
We call these different spatial orientations of the atoms of a molecule that result from rotations or twisting about single bonds conformations. We know this is not strictly true, since the carbon atoms all have a tetrahedral configuration. The actual shape of the extended chain is therefore zig-zag in nature.
However, there is facile rotation about the carbon-carbon bonds, and the six-carbon chain easily coils up to assume a rather different shape. Many conformations of hexane are possible and two are illustrated below. Extended Chain Coiled Chain. For an animation of conformational motion in hexane. Ethane Conformers. The simple alkane ethane provides a good introduction to conformational analysis. Here there is only one carbon-carbon bond, and the rotational structures rotamers that it may assume fall between two extremes, staggered and eclipsed.
In the following description of these conformers, several structural notations are used. The first views the ethane molecule from the side, with the carbon-carbon bond being horizontal to the viewer.
The hydrogens are then located in the surrounding space by wedge in front of the plane and hatched behind the plane bonds. If this structure is rotated so that carbon 1 is canted down and brought closer to the viewer, the "sawhorse" projection is presented. Finally, if the viewer looks down the carbon-carbon bond with carbon 1 in front of 2, the Newman projection is seen.
To see an eclipsed conformer of ethane orient itself as a Newman projection, and then interconvert with the staggered conformer and intermediate conformers.
The most severe repulsions in the eclipsed conformation are depicted by the red arrows. There are six other less strong repulsions that are not shown. The low-molecular mass alkanes, from 1 to 4 carbons, are frequently used as cooking fuels; molecules of intermediate length carbons are found in gasoline and diesel fuels; longer carbon chains are found in motor oils, lubricants, emollients, greases, waxes, paraffin, and other applications.
Many of these compounds are isolated from petroleum, and may serve as the starting points for the synthesis of more complex molecules. Methane is an odorless, nontoxic, flammable gas, which has a boiling point of It is produced by the bacterial decomposition of organisms in the absence of oxygen, and is found in natural gas, marsh gas, bovine flatulence, etc.
It burns cleanly, and so is useful in cooking. Ethanethiol is added to natural gas to give it an unpleasant odor, thereby making gas leaks detectable. Both propane and butane are used as LPG fuels liquefied petroleum gas for outdoor cooking, either in camping stoves or outdoor gas barbecue grills.
Propane liquefies at Butane is a gas above In this form, it is used in cigarette lighters and lighter sticks. It is also possible to obtain a molecule with the formula C 4 H 10 by having a chain of three carbons with a one-carbon branch a methyl group attached to the middle carbon. This molecule is given the common "trivial", i. Butane and isobutane are structural isomers of each other, having the same molecular formula, but with the atoms connected in a different order.
This slight difference is enough to give butane and 2-methylpropane different physical properties: for instance while butane boils at The lower boiling point is a result of the more compact structure of the 2-methylpropane isomer, which has less surface area than the straight-chain isomer, and thus fewer London forces between its molecules.
Notice that as the number of carbons increases, the boiling point also increases; this is a result of the longer carbon chains having more London forces acting between them. There are three structural isomers of C 5 H 12 :. Mixtures of structural isomers of hexane are commonly used as organic solvents. There are five structural isomers of C 6 H 14 :.
Using common or non-systematic names is not practical for molecules with a large number of carbon atoms, since it would be necessary to memorize unique names for each possible compound. And as you can see in Table 1 below , the number of possible structural isomers increases drastically as more carbons are added! Systematic names, in which there is a one-to-one correspondence between the structure and the name, are a huge benefit to organic chemists.
There are 9 structural isomers of C 7 H 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.
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