The term isomer is derived from Greek, meaning "equal parts". Constitutional isomers differ only in the connectivity of their atoms. As the number of carbon atoms increases, the number of possible constitutional isomers increases rapidly. The five constitutional isomers of the hexanes are illustrated in structures 1 - 5. Structure 1 can be rearranged to form the other four constitutional isomers.
If you use an imaginary pair of "chemical scissors" to cut off a methyl group from the chain you will be left with a C 1 and C 5 fragment. You can reattach the C 1 residue at C 2 the same as C 4 of the C 5 residue to form isomer 2 , or add the C 1 residue to C 3 of the C 5 fragment to obtain isomer 3. The remaining isomers 4 and 5 , can be formed from two C 1 units and a straight chain C 4 fragment.
All five hexane constitutional isomers have the same molecular formula, C 6 H 14 , and the same molecular weight, However, each one of the hexanes has a unique boiling point. If once again we employ our "chemical scissors" to cut a unique C-H bond in n-hexane, structure 1, and insert a divalent oxygen at the point of scission, we will have formed three constitutionally isomeric hexanols: 1-hexanol 6 , 2-hexanol 7 and 3-hexanol 8.
Using the same technique to cut unique C-C bonds in n-hexane 1 followed by oxygen insertion, three ethers are formed: methyl n-pentyl ether 9 , n-butyl ethyl ether 10 and di-n-propyl ether 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. Branch isomers are a kind of structural isomer found in organic molecules i. Carbon can bind to other carbon atoms in addition to bonding to hydrogen atoms, so once a carbon "chain" bordered by a hydrogen atom chain grows long enough for the atoms to move more freely in space, secondary carbon chains may appear at one or more points from one end.
As you might expect, this significantly affects the chemical behavior of these molecules. Alkanes are compounds containing only carbon and hydrogen atoms joined in single bonds. Since each carbon atom can form four bonds, the door is open to a wealth of isomers within this class of organic compounds, found in abundance in fossil fuels. You have already seen that butane C 4 H 10 has an isomer, 2-methylpentane.
These are the only two isomers of this molecule. C 5 H 10 , on the other hand, has three isomers, while C 6 H 14 has nine. There is no "number of chain isomers formula" for alkanes, and the number quickly grows cumbersome for example, decane, or C 10 H 22 , has a whopping 75 isomers. Instead, you should be able to construct a few of them given a particular alkane formula.
For an example of a program that acts as a combination generator for isomers so that you can see their respective physical structure in space, see the Resources. Note that if you try to input a formula for which there is no isomer, the program quickly returns a null result. You may want to experiment with drawing some of these prospective compounds to see why they are impossible to generate, given basic chemical bonding principles. All of the alkanes containing 4 or more carbon atoms show structural isomerism, meaning that there are two or more different structural formulas that you can draw for each molecular formula.
Alkanes with carbons, 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. However, C 4 H 10 , has more than possible structure. The four carbons can be drawn in a row to form butane or the can branch to form isobutane. Likewise the molecular formula: C 5 H 12 has three possible isomer. The compound at the far left is pentane because it has all five carbon atoms in a continuous chain.
The compound in the middle is isopentane; like isobutane, it has a one CH 3 branch off the second carbon atom of the continuous chain. Although all three have the same molecular formula, they have different properties, including boiling points: pentane, Of the structures show above, butane and pentane are called normal alkanes or straight-chain alkanes , indicating that all contain a single continuous chain of carbon atoms and can be represented by a projection formula whose carbon atoms are in a straight line.
The other structures, isobutane, isopentane, and neopentane are called called branched-chain alkanes. As the number of carbons in an akane increases the number of possible isomers also increases as shown in the table below.
Akanes can be represented in many different ways. The figure below shows some of the different ways straight-chain butane can be represented. Note that many of these structures only imply bonding connections and do not indicate any particular geometry. The bottom two structures, referred to as "ball and stick" and "space filling" do show 3D geometry for butane.
Because the four-carbon chain in butane may be bent in various ways 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, butane:. The nomenclature of straight alkanes is based on the number of carbon atoms they contain.
The number of carbons are indicated by a prefix and the suffix -ane is added to indicate the molecules is an alkane. Likewise, the prefix for six is hex so the name for the straight chain isomer of C 6 H 14 is called hexane.
The first ten prefixes should be memorized, because these alkane names from the basis for naming many other organic compounds. Pentane, C 5 H 12 , has three chain isomers. If you think you can find any others, they are simply twisted versions of the ones below.
If in doubt make some models. Draw all of the isomers for C 6 H 14 O that contain a 6 carbon chain and an alcohol -OH functional group.
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