CHAPTER 12B: Unit 2. Structures and Nomenclature: Alkanes & Cycloalkanes
| Learning Objective: Identify and name simple (straight-chain) alkanes given formulas and write formulas for straight-chain alkanes given their names.
The simplest organic compounds are composed of carbon and hydrogen atoms only. As we know, there are several different kinds of hydrocarbons. They are distinguished by the types of bonding between carbon atoms and the properties that result from that bonding. Hydrocarbons with only carbon-to-carbon single bonds (C–C) and existing as a continuous chain of carbon atoms also bonded to hydrogen atoms are called alkanes (or saturated hydrocarbons). Saturated, in this case, means that each carbon atom is bonded to four other atoms (hydrogen or carbon)—the most possible; there are no double or triple bonds in the molecules.Classification of Carbon atoms:Each of the carbon atoms within a hydrocarbon structure can be classified as a primary, (10), secondary (20) and tertiary (30) or quaternary ( 40) carbon atoms.A primary carbon ( 10C) is bonded to one other C.A secondary carbon ( 20C) is bonded to two other C’s.A tertiary carbon ( 30C) is bonded to three other C’s.A quaternary carbon ( 40C) is bonded to four other C’s.
|
 |
Alkane: The IUPAC name for an alkane is based on the longest continuous chain of carbon atoms in the molecule. A group of carbon atoms attached to the chain is an alkyl group. Both the position and identity of alkyl group are prefixed to the name of the longest carbon chain. A suffix -ane is used at the end to indicate the alkane family compounds.Here are three simplest alkanes. All other alkanes condensed structural formula are given below.  |
 Consider the series in above figure. The sequence starts with C3H8, and a CH2 unit is added in each step moving up the series. Any family of compounds in which adjacent members differ from each other by a definite factor (here a CH2 group) is called a homologous series. The members of such a series, called homologs, have properties that vary in a regular and predictable manner. 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.Figure 1.3 Members of a Homologous Series Each succeeding formula incorporates one carbon atom and two hydrogen atoms more than the previous formula.The principle of homology allows us to write a general formula for alkanes: CnH2n + 2. Using this formula, we can write a molecular formula for any alkane with a given number of carbon atoms. For example, an alkane with eight carbon atoms has the molecular formula C8H(2 × 8) + 2 = C8H18. |
| Branched Chain Alkanes:Learning ObjectiveLearn how alkane molecules can have branched chains and recognize compounds that are isomers. We can write the structure of butane (C4H10) by stringing four carbon atoms in a row, –C–C–C–C–and then adding enough hydrogen atoms to give each carbon atom four bonds:  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: C4H10. The two compounds have different properties—for example, one boils at −0.5°C; the other at −11.7°C. Different compounds having the same molecular formula are called isomers. The compound with this branched chain is called isobutane ( “Butane and Isobutane”).Butane and Isobutane The ball-and-stick models of these two compounds show them to be isomers; both have the molecular formula C4H10.Notice that C4H10 is depicted with a bent chain in Figure 1.4. 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 C4H10, the compounds methane (CH4), ethane (C2H6), and propane (C3H8) 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.Next beyond C4H10 in the homologous series is pentane. Each compound has the same molecular formula: C5H12. (Table 1.2 “The First 10 Straight-Chain Alkanes” has a column identifying the number of possible isomers for the first 10 straight-chain alkanes.) 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 CH3 branch off the second carbon atom of the continuous chain. The compound at the far right, discovered after the other two, was named neopentane (from the Greek neos, meaning “new”). Although all three have the same molecular formula, they have different properties, including boiling points: pentane, 36.1°C; isopentane, 27.7°C; and neopentane, 9.5°C. Condensed Structural and Line-Angle FormulasLearning ObjectivesWrite condensed structural formulas for alkanes given complete structural formulas.Draw line-angle formulas given structural formulas.We use several kinds of formulas to describe organic compounds. A molecular formula shows only the kinds and numbers of atoms in a molecule. For example, the molecular formula C4H10 tells us there are 4 carbon atoms and 10 hydrogen atoms in a molecule, but it doesn’t distinguish between butane and isobutane. A structural formula shows all the carbon and hydrogen atoms and the bonds attaching them. Thus, structural formulas identify the specific isomers by showing the order of attachment of the various atoms.Unfortunately, structural formulas are difficult to type/write and take up a lot of space. Chemists often use condensed structural formulas to alleviate these problems. The condensed formulas show hydrogen atoms right next to the carbon atoms to which they are attached, as illustrated for butane: The ultimate condensed formula is a line-angle formula, in which carbon atoms are implied at the corners and ends of lines, and each carbon atom is understood to be attached to enough hydrogen atoms to give each carbon atom four bonds. For example, we can represent pentane (CH3CH2CH2CH2CH3) and isopentane [(CH3)2CHCH2CH3] as follows: Alkyl group:In previous chapter we briefly mentioned about alkyl groups. A substituent group derived from an alkane by removal of a hydrogen atom is called alkyl group and it is denoted by R- in general. Both IUPAC and common names are used to indicate alkyl groups. The prefix sec- is an abbreviation for secondary meaning a carbon bonded to two other carbons. The prefix tert- is an abbreviation for tertiary meaning a carbon bonded to three other carbons.Below are the names of most common alkyl groups: |
 *Parenthesis is used to indicate the branching from the parent compound. IUPAC Nomenclature Learning Objective: Name alkanes by the IUPAC system and write formulas for alkanes given IUPAC names. As noted in Table 1.2 “The First 10 Straight-Chain Alkanes”, the number of isomers increases rapidly as the number of carbon atoms increases. There are 3 pentanes, 5 hexanes, 9 heptanes, and 18 octanes. It would be difficult to assign unique individual names that we could remember. A systematic way of naming hydrocarbons and other organic compounds ha s been devised by the International Union of Pure and Applied Chemistry (IUPAC). These rules, used worldwide, are known as the |
 |
methyl
CH3–ethane
ethyl
CH3CH2–propane
propyl
CH3CH2CH2– isopropyl
CH3–butane
butyl*
CH3CH2CH2CH2–*There are four butyl groups, two derived from butane and two from isobutane. We will introduce the other three where appropriate.Simplified IUPAC rules for naming alkanes are as follows (demonstrated in Example 12.1).1. Name alkanes according to the LCC of carbon atoms in the molecule (rather than the total number of carbon atoms). This LCC, considered the parent chain, determines the base name, to which we add the suffix -ane to indicate that the molecule is an alkane.2. If the hydrocarbon is branched, number the carbon atoms of the LCC. Numbers are assigned in the direction that gives the lowest numbers to the carbon atoms with attached substituents. Hyphens are used to separate numbers from the names of substituents; commas separate numbers from each other. (The LCC need not be written in a straight line; for example, the LCC in the following has five carbon atoms.)
3. Place the names of the substituent groups in alphabetical order before the name of the parent compound. If the same alkyl group appears more than once, the numbers of all the carbon atoms to which it is attached are expressed. If the same group appears more than once on the same carbon atom, the number of that carbon atom is repeated as many times as the group appears. Moreover, the number of identical groups is indicated by the Greek prefixes di-, tri-, tetra-, and so on. These prefixes are not considered in determining the alphabetical order of the substituents. For example, ethyl is listed before dimethyl; the di- is simply ignored. The last alkyl group named is prefixed to the name of the parent alkane to form one word.When these rules are followed, every unique compound receives its own exclusive name. The rules enable us to not only name a compound from a given structure but also draw a structure from a given name. The best way to learn how to use the IUPAC system is to put it to work, not just memorize the rules. It’s easier than it looks. Example 1Draw the structure for each compound.2,3-dimethylbutane4-ethyl-2-methylheptaneSolutionIn drawing structures, always start with the parent chain.The parent chain is butane, indicating four carbon atoms in the LCC.
Then add the groups at their proper positions. You can number the parent chain from either direction as long as you are consistent; just don’t change directions before the structure is done. The name indicates two methyl (CH3) groups, one on the second carbon atom and one on the third.
Finally, fill in all the hydrogen atoms, keeping in mind that each carbon atom must have four bonds.
The parent chain is heptane in this case, indicating seven carbon atoms in the LCC.–C–C–C–C–C–C–C–Adding the groups at their proper positions gives
Filling in all the hydrogen atoms gives the following condensed structural formulas:
Note that the bonds (dashes) can be shown or not; sometimes they are needed for spacing.Examples of different molecules:| Homework Exercises:
Briefly identify the important distinctions between an alkane and an alkyl group.How many carbon atoms are present in each molecule?a. 2-methylbutaneb. 3-ethylpentaneHow many carbon atoms are present in each molecule?a. 2,3-dimethylbutaneb. 3-ethyl-2-methylheptaneDraw the structure for each compound.a. 3-methylpentaneb. 2,2,5-trimethylhexanec. 4-ethyl-3-methyloctaneDraw the structure for each compound.a. 2-methylpentaneb. 4-ethyl-2-methylhexanec. 2,2,3,3-tetramethylbutaneName each compound according to the IUPAC system.a. |
Homework Exercises: What is the molecular formula of cyclooctane?What is the IUPAC name for this compound? AnswersC8H16ethylcyclopropaneAdditional ExercisesDraw the structure for each compound.a. ethylcyclobutaneb. propylcyclopropaneDraw the structure for each compound.a. methylcyclohexaneb. butylcyclobutaneCycloalkyl groups can be derived from cycloalkanes in the same way that alkyl groups are derived from alkanes. These groups are named as cyclopropyl, cyclobutyl, and so on. Name each cycloalkyl halide.a. b. Halogenated cycloalkanes can be named by the IUPAC system. As with alkyl derivatives, monosubstituted derivatives need no number to indicate the position of the halogen. To name disubstituted derivatives, the carbon atoms are numbered starting at the position of one substituent (C1) and proceeding to the second substituted atom by the shortest route. Name each compound.a. b. Answers 1.a. b. 3.a. cyclopentyl bromideb. cyclohexyl chloride |
| IN CLASS PRACTICE PROBLEMS: |
| Draw the proper structure for 3-methylpentane
Draw the proper structure for 3-ethyl-2-methylpentane If the molecule C6H12 does not contain a double bond, and there are no branches in it, what will its structure look like?Write the name of the structure |
4. Write the name of the structure |
 |
b.
Name each compound according to the IUPAC system.a.
b.
What is a substituent? How is the location of a substituent indicated in the IUPAC system?Briefly identify the important distinctions between a common name and an IUPAC name.Answers1. An alkane is a molecule; an alkyl group is not an independent molecule but rather a part of a molecule that we consider as a unit. 3.a. 6b. 10 5.a.
b.
c.
7.a. 2,2,4,4-tetramethylpentaneb. 3-ethylhexane9. Common names are widely used but not very systematic; IUPAC names identify a parent compound and name other groups as substituents Additional ExerciseName each compound.
SolutionThe LCC has five carbon atoms, and so the parent compound is pentane (rule 1). There is a methyl group (rule 2) attached to the second carbon atom of the pentane chain. The name is therefore 2-methylpentane.The LCC has six carbon atoms, so the parent compound is hexane (rule 1). Methyl groups (rule 2) are attached to the second and fifth carbon atoms. The name is 2,5-dimethylhexane.The LCC has eight carbon atoms, so the parent compound is octane (rule 1). There are methyl and ethyl groups (rule 2), both attached to the fourth carbon atom (counting from the right gives this carbon atom a lower number; rule 3). The correct name is thus 4-ethyl-4-methyloctane. Additional ExerciseDraw the structure for each compound.4-ethyloctane3-ethyl-2-methylpentane3,3,5-trimethylheptaneAdditional ExercisesWhat is a CH3 group called when it is attached to a chain of carbon atoms—a substituent or a functional group?Which type of name uses numbers to locate substituents—common names or IUPAC names?AnswerssubstituentIUPAC namesAdditional ExerciseName each compound.
Common names: In older times, before IUPAC system was established, all naturally occurring and new compounds and common names. Common names are still used and very popular among chemists in modern days along with IUPAC names. Some common names compounds are: Isobutane, neopentane etc.CycloalkanesLearning ObjectiveName cycloalkanes given their formulas and write formulas for these compounds given their names.Cycloalkane nomenclature: the IUPAC name for the cycloalkane is obtained by placing the prefix cyclo- before the alkane name that corresponds to the number of carbon atoms in the ring. Alkyl groups attached to the ring are located by using a ring numbering system.The hydrocarbons we have encountered so far have been composed of molecules with open-ended chains of carbon atoms. When a chain contains three or more carbon atoms, the atoms can join to form ring or cyclic structures. The simplest of these cyclic hydrocarbons has the formula C3H6. Each carbon atom has two hydrogen atoms attached (Figure 1.6 “Ball-and-Spring Model of Cyclopropane”) and is called cyclopropane.Figure: Ball-and-Spring Model of Cyclopropane
The springs are bent to join the carbon atoms.he cycloalkanes—cyclic hydrocarbons with only single bonds—are named by adding the prefix cyclo- to the name of the open-chain compound having the same number of carbon atoms as there are in the ring. Thus the name for the cyclic compound C4H8 is cyclobutane. The carbon atoms in cyclic compounds can be represented by line-angle formulas that result in regular geometric figures. Keep in mind, however, that each corner of the geometric figure represents a carbon atom plus as many hydrogen atoms as needed to give each carbon atom four bonds.
Some cyclic compounds have substituent groups attached. Example 5 interprets the name of a cycloalkane with a single substituent group.ExampleDraw the structure for each compound.cyclopentanemethylcyclobutaneSolutionThe name cyclopentane indicates a cyclic (cyclo) alkane with five (pent-) carbon atoms. It can be represented as a pentagon.
The name methylcyclobutane indicates a cyclic alkane with four (but-) carbon atoms in the cyclic part. It can be represented as a square with a CH3 group attached.