In This Chapter
^ Checking out the nomenclature of organic compounds
^ Taking in testable trends in the properties of organic compounds
^ Discovering isomerism and chirality
^ Running through common organic reactions
M W Rganic chemistry Is a huge subfield, and the AP chemistry exam merely scratches the
Surface of it. Still, a good grasp of the concepts and naming rules covered in this chapter could get you those few extra points needed to push your score up one grade on exam day.
Don’t spend too much time here if you’re still struggling with the big-point-gaining chapters earlier in this book. Your time is better spent polishing your knowledge of the concepts in those earlier chapters, if you’re struggling, because so few questions on the AP exam address organic chemistry. But if you feel comfortable with the big-point-gaining chapters, then tackle this chapter with vigor and get ready to file lots of new information away in that chemistry-packed brain of yours — organic chemistry is heavy on memorization.
In addition to the few questions on each AP exam directly addressing organic chemistry, a basic knowledge of the information in this chapter, including basic structure and naming. can aid you in answering many other questions on the exam. Organic compounds are sometimes used in questions testing concepts such as kinetics, stoichiometry, and colligative properties.
Naming Names: Nomenclature and Properties of Hydrocarbons
Any study of organic chemistry begins with the study of Hydrocarbons. Hydrocarbons are some of the simplest and most important organic compounds and they contain only hydrogen and carbon atoms. Organic compounds Are based on carbon skeletons. Carbon skeletons can be modified — you can dress them up with chemically interesting atoms like oxygen, nitrogen, halogens, phosphorous, silicon, or sulfur. This cast of atomic characters may seem like a rather small subset of the more than 100 elements in the periodic table. It’s true: Organic compounds typically use only a very small number of the naturally occurring elements. Yet these molecules include the most biologically important compounds in existence. In this section you will learn how to recognize and name all of the types of hydrocarbons that appear on the AP exam, including alkenes, alkanes, alkynes, and cyclic hydrocarbons.
Starting with straight-chain alkanes
Starting with straigi linear hydrocarbons
The simplest of the hydrocarbons fall into the category of Alkanes. Alkanes are chains of carbon atoms connected by single covalent bonds. Chapter 5 describes how single covalent bonds result when atoms share pairs of valence electrons. Carbon molecules have four valence electrons. So, carbon atoms are eager to donate their four valence electrons to cova-lent bonds so that they can receive four donated electrons in turn, filling their valence shell. In other words, carbon really likes to form four bonds. In alkanes, each of these is a single bond with a different carbon or hydrogen partner.
The simplest of the alkanes, called Continuous – Or Straight-chain alkanes, Consist of one straight chain of carbon atoms linked with single bonds. Hydrogen atoms fill all the remaining bonds. Other types of alkanes include closed circles and branched chains.
We begin with straight-chain alkanes because they make clear the basic strategy for naming hydrocarbons. From the standpoint of naming, the hydrogen atoms in a hydrocarbon are more or less "filler atoms." Alkanes’ names are based on the largest number of consecutively bonded carbon atoms. So, the name of a hydrocarbon tells you about that molecule’s structure. To name a straight-chain alkane, simply match the appropriate chemical prefix with the suffix -ane. The prefixes relate to the number of carbons in the continuous chain, which we list in Table 25-1.
|
Table 25-1
|
The Carbon Prefixes
|
|
Number of Carbons
|
Prefix
|
Chemical Formula
|
Alkane
|
|
1
|
Meth-
|
CH4
|
Methane
|
|
2
|
Eth-
|
C2H6
|
Ethane
|
|
3
|
Prop-
|
C3H8
|
Propane
|
|
4
|
But-
|
C4H10
|
Butane
|
|
5
|
Pent-
|
C5H12
|
Pentane
|
|
6
|
Hex-
|
C6H14
|
Hexane
|
|
7
|
Hept-
|
C7H16
|
Heptane
|
|
8
|
Oct-
|
C8H18
|
Octane
|
|
9
|
Non-
|
C9H20
|
Nonane
|
|
10
|
Dec-
|
C10H22
|
Decane
|
The naming method in Table 25-1 tells you how many carbons are in the chain. Because you know that each carbon has four bonds and because you are fiendishly clever, you can deduce the number of hydrogen atoms in the molecule as well. Any carbon at the end of a chain must be bonded to three hydrogens and all interior carbons must be bonded to two. Consider the carbon structure of pentane, for example, shown in Figure 25-1.
Figure 25-1: I I I I I
Pentane’s I I I I I
Carbon — C-C-C-C-C—
Skeleton.
Only four carbon-carbon bonds are required to produce the five-carbon chain of pentane. This leaves many bonds open — two for each interior carbon and three for each of the terminal carbons. These open bonds are satisfied by carbon-hydrogen bonds, thereby forming a hydrocarbon, as shown in Figure 25-2.
Figure 25-2:
Pentane’s hydrocarbon
Structure.
H HHH H
CCC C H
HHHHH
H
C
If you add up the hydrogen atoms in Figure 25-2, you get 12. So, pentane contains 5 carbon atoms and 12 hydrogen atoms.
As the organic molecules you study get more and more complicated, it will become more and more important to draw the molecular structure to visualize the molecule. In the case of straight-chain alkanes, the simplest of all organic molecules, you can remember a convenient formula for calculating the number of hydrogen atoms in the alkane without actually drawing the chain:
Number of hydrogen atoms = (2 x number of carbon atoms) + 2
You can refer to the same molecule in a number of different ways. For example, you can refer to pentane by its name (ahem. . . Pentane), By its molecular formula, C5H12, or by the complete structure in Figure 25-2. Clearly, these different names include different levels of structural detail. A Condensed structural formula Is another naming method, one that straddles the divide between a molecular formula and a complete structure. For pentane, the condensed structural formula is CH3CH2CH2CH2CH3. This kind of formula assumes that you understand how straight-chain alkanes are put together. Carbons on the end of a chain, for example, are only bonded to one other carbon, so they have three additional bonds to be filled by hydrogen and are labeled as CH3 in a condensed formula. Interior carbons are bonded to two neighboring carbons and have only two hydrogen bonds, and so are labeled CH2.
Going out on a limb: Making branched alkanes by substitution
Not all alkanes are straight-chain alkanes. That would be too easy. Many alkanes are so-called Branched alkanes. Branched alkanes differ from continuous-chain alkanes in that carbon chains substitute for a few hydrogen atoms along the chain. Atoms or other groups (like carbon chains) that substitute for hydrogen in an alkane are called Substituents.
Naming branched alkanes is slightly more complicated, but you need only to follow these simple set of steps to arrive at a proper (and often lengthy) name:
1. Count the longest continuous chain of carbons. Tricky chemistry problems often show branched alkanes with the longest chain snaking through a few branches instead of obviously lined up in a row. Consider the two carbon structures shown in Figure 25-3. The two are actually the same structure, drawn differently! Yikes. In either case, the longest continuous chain in this structure has eight carbons. That wasn’t hard — and yippee, now you already know the basic name for the compound. It is an octane.
Cc
C1 C2 C3 C4 C5 C
C6
B
C7
7
C8
8
C
Figure 25-3′ Ci – c2-c3 - c4- c5- cb – c7 – c
One carbon i i i
Structure | | |
Drawn two c c c
Different I
Ways. I
2. Number the carbons in the chain Starting with the end which is closest to a branch.
You can always check to be sure you have done this step correctly by numbering the carbon chain from the opposite end as well. The correct numbering sequence is the one in which the substituent branches extend from the lowest-numbered carbons. For example, as it is drawn and numbered in Figure 25-3, the alkane has substituent groups branching off of its third, fourth, and fifth carbons. If the carbon chain had been numbered backward, these would be the fourth, fifth, and sixth carbons in the chain. Because the first set of numbers is lower, the chain is numbered properly. The longest chain in a branched alkane is called the Parent chain.
3. Count the number of carbons in each branch. These groups are called Alkyl groups And are named by adding the suffix -yl To the appropriate alkane prefix (Table 25-1 awaits your visit). The three most common alkyl groups are the methyl (one carbon), ethyl (two carbons), and propyl (three carbons) groups. Figure 25-3 has two methyl groups, one ethyl group, and no propyl groups.
Be careful when you find yourself dealing with alkyl groups made up of more than just a few carbons. A tricky drawing may cause you to misnumber the parent chain!
4. Attach the number of the carbon from which each substituent branches to the front of the alkyl group name. For example, if a group of two carbons is attached to the third carbon in a chain, like it is in Figure 25-3, the group is called 3-ethyl.
5. Check for repeated alkyl groups. If multiple groups with the same number of carbons branch off the parent chain, do not repeat the name. Rather, include multiple numbers, separated by commas, before the alkyl group name. Also, specify the number of instances of the alkyl group by using the prefixes di-, tri-, tetra-, and so on. For example, if one-carbon groups (in other words, methyl groups) branch off carbons four and five of the parent chain, the two methyl groups appear together as "4,5-dimethyl."
6. Place the names of the substituent groups in front of the name of the parent chain In alphabetical order. Prefixes like di-, tri-, and tetra – do not figure into the alphabetizing. So, the proper name of the organic molecule in Figure 25-3 is 3-ethyl-4,5-dimethyloctane.
Note that hyphens are used to connect all the naming elements except for the last connection to the parent chain, which includes neither a hyphen nor a space (. . . dimethyl-octane is wrong). We recommend practicing alkanes till you get them in your sleep before moving on to other organic compounds!
Getting unsaturated: Alkenes and alkynes
Carbons can do more than engage in four single bonds. There’s more to organic molecules than substituent-for-hydrogen swaps:
When carbons in an organic compound fill their valence shells entirely with single bonds, we say the compound is Saturated.
When hydrocarbons contain carbons that bond to each other more than once, creating double or triple covalent bonds, we say these hydrocarbons are Unsaturated Because they have fewer than the maximum possible number of hydrogens or substituents.
For every additional carbon-carbon bond formed in a hydrocarbon, two fewer covalent bonds to hydrogen are formed.
When neighboring carbons share four valence electrons to form a double bond, the resulting hydrocarbon is called an Alkene. Alkenes are characterized by these chemically interesting double bonds, which are more reactive than single carbon-carbon bonds (see Chapter 7 for a review of sigma and pi bonding). Double bonds also change the shape of a hydrocarbon, because the Sp2 Hybridized valence orbitals assume a trigonal planar geometry, as shown by the carbons of ethene in Figure 25-4. Saturated carbon is Sp3 Hybridized and has tetrahedral geometry (again, see Chapter 7 to review hybridization).
H
\
Figure 25-4′
Ethene carbons.
H
H
H
Naming alkenes is slightly more complicated than naming alkanes. In addition to the number of carbons in the main chain and any branching substituents, you must also note the location of the double bonds in an alkene and incorporate that information into the name. Nevertheless, the essential naming strategy for alkenes is quite similar to that for alkanes:
1. Locate the longest carbon chain that includes the double bond, and number it, Starting at the end closest to the double bond. In other words, double bonds trump sub-stituents when it comes to numbering the parent chain. Build the name of the parent chain by using the same prefixes as used for alkanes (refer to Table 25-1), but match the prefix with the suffix -ene. A three-carbon chain with a double bond, for example,
Is called "propene."
2. Number and name substituents that branch off the alkene In the same way as done for alkanes (see the section earlier in this chapter, "Starting with straight-chain alkanes — linear hydrocarbons" for more on naming alkanes). List the number of the substituted carbon, followed by the name of the substituent. Separate the sub-stituent number and name with a hyphen.
3. Identify the lowest-numbered carbon that participates in the double bond, and put that number Between the substituent names and the parent chain name (sandwiched by hyphens), but after all the substituent names. For example, if the second and third carbons of a five-carbon alkene engage in a double bond, then the molecule is called 2-pentene, not 3-pentene. If that same molecule has a methyl substituent at the fourth carbon, then the molecule is called 4-methyl-2-pentene.
Alternately, and especially when there are substituents present, the position of an unsaturation is indicated between the prefix and suffix of the parent chain name. So, 4-methyl-2-pentene may also be written 4-methylpent-2-ene.
Alkynes Are hydrocarbons in which neighboring carbons share six electrons to engage in triple covalent bonds. The naming strategy for alkynes is the same as that for alkenes, except that the alkyne parent chain is named by matching the prefix with the suffix -yne.
An important consequence of the presence of multiple bonds in alkenes and alkynes is their marked increase in reactivity over alkanes. Generally speaking, the more multiple bonds a compound has, the more reactive it tends to be. This is because of the propensity of multiple bonded carbons to undergo addition reactions, which are discussed at the end of this chapter.
Rounding ‘em Up: Circling Carbons with Cyclics and Aromatics
The compounds we discuss earlier in this chapter are linear or branched. However, hydrocarbons can be circular, or Cyclical, Hydrocarbons There are two important categories of cyclical hydrocarbons:
Cyclic aliphatic Hydrocarbons Aromatic Hydrocarbons
Chemists sometimes divide hydrocarbons into aliphatic and aromatic categories to highlight important differences in structure and reactivity. Without going into more technical detail than is useful here, aliphatic molecules and aromatic molecules have significantly different electronic configurations (which electrons go into which orbitals). As a result, the two types of hydrocarbons typically undergo different kinds of reactions. In particular, they tend to undergo different kinds of substitution reactions, ones in which some atom or group substitutes for hydrogen.
Shutting the circle with cyclic aliphatic hydrocarbons
Cyclic aliphatic hydrocarbons are like the hydrocarbons that we discuss earlier in this chapter, except that they form a closed ring. The rules for naming these compounds build on the rules for naming alkanes, alkenes, and alkynes. For example, a cyclical six-carbon alkane includes the name "hexane," but is preceded by the prefix Cyclo-, Making the final name "cyclohexane."
A single substituent or unsaturation on a cyclic aliphatic hydrocarbon does not require a number. So, a single methyl-for-hydrogen substitution on cyclohexane yields a compound with the name methylcyclohexane. Likewise, a lone double bond unsaturation on cyclo-hexane yields a compound with the name cyclohexene.
Multiple substitutions or unsaturations require numbering. In these cases, the same rules apply for deciding the rank of substituents as applied to alkenes and alkynes. Triple bonds
Outrank double bonds. Double bonds outrank other substitutions. So, number the carbons in the way that respects these rankings and produces the lowest overall numbers. A cyclo-hexane molecule with two methyl substituents on neighboring carbons, for example, is called 1,2-dimethylcyclohexane.
Analyzing aromatic hydrocarbons
Aromatic hydrocarbons have special properties because of their electronic structure. Aromatics are both cyclic and conjugated:
I Cyclic: The carbons form a ringlike structure.
Conjugated: Conjugation results from an alternation of double or triple bonds with single bonds.
Aromatic molecules have clouds of Delocalized pi electrons, Electrons that move freely through a set of overlapping P Orbitals. The model aromatic compound is benzene. Because of its cyclical, conjugated bonding, pi electrons delocalize evenly into rings above and below the plane of the flat benzene molecule. Aromatic compounds are very stable compared to their aliphatic counterparts.
Numbering substituents on aromatics follows the same basic pattern as followed for cyclic aliphatic compounds. A single substituent requires no numbering, as in methylbenzene. Multiple substituents are numbered by rank, with the highest-ranked substituent placed on carbon number one, and proceeding in a way that results in the lowest overall numbers. A benzene ring with ethyl and methyl substituents situated two carbons away from one another, for example, would be called 1-ethyl-3-methylbenzene.
Decorating Skeletons: Organic Functional Groups
Surely you’ve noticed that in this chapter we have thus far limited our study of organic compounds to those made entirely of hydrogen and carbon. When studying the complex rules governing hydrocarbons, it is easy to forget that the periodic table contains more than 100 other elements. Although organic chemistry often concentrates on just a few of those elements, you’ll need to be familiar with many other compound types, and this time they’ll be made of more than just hydrogen and carbon. All of these exotic new compounds contain plain vanilla carbon chains, but each has a distinguishing feature composed of another, more exotic element. These embellishments are called Functional groups. These groups include noncarbon elements, and give organic compounds a dizzying array of different properties.
Because hydrocarbons are old news, we sometimes refer to them simply using the letter R, representing any hydrocarbon group: branched or unbranched, saturated or unsaturated.
Table 25-2 summarizes the other important organic compounds, their distinguishing features, and their endings. Some molecules contain more than one of these distinguishing features. Locations where one of these groups or a multiple bond appear are prime sites for reactions to occur. These sites are therefore called Functional groups. Carefully study the functional group column in Table 25-2 so you can recognize them quickly. These are the structures you should keep an eye out for later in this chapter.
|
Table 25-2
|
Functional Groups
|
|
Compound Name
|
Compound Formula
|
Functional Group
|
Prefix or Suffix
|
|
Alcohol
|
R-OH
|
-{-O H
|
-ol
|
|
Ether
|
R-O-R
|
— c — O — C —
|
Ether
|
|
Carboxylic acid
|
R-COOH
|
-oic acid
|
|
Ester
|
R-COOR
|
-oate
|
|
Aldehyde
|
R-CHO
|
‘O’
|
-al
|
|
Ketone
|
R-COR
|
‘O’
|
-one
|
|
Halocarbon
|
R-X
|
— c — X:
(X = halogen)
|
-oro, omo, or odo
|
|
Amine
|
R-NH2
|
-rr
|
-amine
|
Alcohols: Hosting a hydroxide
Alcohols Are hydrocarbons with a hydroxide group attached to them. Their general form is R-OH. To name an alcohol, you simply count the longest number of consecutive carbon atoms in the chain, find its prefix Table 25-2, and then attach the ending -ol To that prefix. For example, a one-carbon chain with an OH group attached is methanol, two is ethanol, three is propanol, and so on. Substituents (groups branching off the main chain) must, as always, be accounted for, and you must specify the number of the carbon atom in the chain to which the hydroxyl (OH) group is attached. Begin your numbering at the end of the chain closest to the OH, and attach the number before the prefix + -ol. For example, the compound shown in Figure 25-5 contains a six-carbon chain with a hydroxyl group. Its base name is therefore
Hexanol. It has methyl groups (with one carbon each) on the second and fourth carbons and its OH group lies on the third carbon. Its proper name is therefore 2,4-dimethyl-3-hexanol.
Some alcohols are Polyhydroxyl, Or contain multiple hydroxyl groups. In order to avoid confusion with multiples of any substituent groups, the prefixes indicating numbers of hydroxyl groups in an alcohol are attached to the suffix Ol, Making them diols, triols, etc. For example, a four carbon alcohol containing hydroxyl groups on the first and third carbons is 1,3-butane-diol. The numbers indicating the locations of the hydroxyl groups must come before the base name but after any substituent groups.
_ HH OH H H H
Figure 25-5:
An example H-C-C-C-C-C-C-H
Of an alcohol.
_ H CH, H CH, H H
Ethers: Invaded by oxygen
Perhaps most commonly known for their use as early anesthetics, Ethers Are carbon chains that have been infiltrated by an oxygen atom. These oxygen atoms lie conspicuously in the middle of a carbon chain like badly disguised spies in an enemy camp and have the general formula R-O-R. Ethers are named by naming the alkyl groups on either side of the lone oxygen as substituent groups individually (adding the ending -yl To their prefixes), and then attaching the word Ether To the end. For example, the compound shown in Figure 25-6 is an ether with a methyl group (one carbon) on one side of the oxygen and an ethyl (two carbons) on the other. Placing the substituents in alphabetical order, the compound’s proper name is ethyl methyl ether.
Figure 25-6:
An example of an ether.
3
CH
Oxygen atoms in ethers are often surrounded by two identical alkyl groups, in which case the prefix Di – Must be attached to the name of that substituent. For example, an oxygen surrounded by two propyl groups (with three carbons each) is called dipropyl ether.
Carboxylic acids: R-COOH brings up the rear
Carboxylic acids Appear to be ordinary hydrocarbons until you reach the very last carbon in their chain, whose three ordinary hydrogens have been usurped by a double-bonded oxygen and a hydroxyl group (an OH group). This gives a carboxylic acid the general form R-COOH. These compounds are named by attaching the suffix -oic acid To the end of the prefix. For example, the compound shown in Figure 25-7 has four carbons, the last in the chain attached to a double-bonded oxygen and a hydroxyl group. The compound is therefore called butanoic acid. The hydrogen of the COOH group can pop off as H+ leaving an R-COO – ion, which is why these compounds are called acids. Carboxylic acids are a favorite compound type for AP exam writers because they are both organics and acids. It is important to note, however, that all car-boxylic acids are weak acids, so learn to spot them among acid lists and keep their relative weakness in mind.
Figure 25-7:
An example of a carboxylic acid.
H3C’
3
CH,
CH,
OH
O
C
Esters: Creating two carbon chains
What if the same double-bonded oxygen/hydroxyl pair from a carboxylic acid has infiltrated deeper into the hydrocarbon chain and is not on an end, but rather in the middle? In order to accomplish this, the COOH group must lose the hydrogen of its hydroxyl group (which, as an acidic hydrogen, is no problem) to open up a bond to which a second hydrocarbon can attach. A compound of this nature, with the general formula R-COOR, is called an Ester. Lots of nice, smelly coumpounds are esters, including the compounds that give pears, apples, and bananas their pleasant aromas.
Because the carbon chain in an ester is also broken by an oxygen, you’ll need to choose one carbon chain as a lowly substituent group and the other to bear the proud suffix of -oate. This high-priority group is always the carbon chain that includes the carbon double-bonded to one oxygen and single bonded to the other. The group on the other side of the single-bonded oxygen is named as an ordinary substituent. For example, the compound shown in Figure 25-8 is phenyl ethanoate, because the high priority group has two carbons and the low priority group is a benzene ring.
H3C
3
C
C
/
Figure 25-8:
An example of an ester.
—O
\
C HC
HCCH
HCCH
\ _ /
CH CH
That’s right! The low-priority group on the other side of the oxygen doesn’t have to be a hydrocarbon chain! It can be a ring or a metal as well. In Figure 25-8, this group is a benzene ring.
Aldehydes: Holding tight to one oxygen
Aldehydes Are much like carboxylic acids except that they lack the second oxygen in the COOH group. Their final carbon shares a single bond with its neighboring carbon, a double bond with an oxygen, and a single bond with hydrogen. Aldehydes are named with the suffix -al (not to be confused with the -ol Of alcohols) and have the general formula R-CHO. For example, the compound in Figure 25-9 is pentanal, a five-carbon chain with a double-bonded oxygen taking the place of two of the hydrogen bonds on the final carbon.
Figure 25-9:
An example of an aldehyde.
. CH,
-CH,
3
*CH,
CH
O
Ketones: Lone oxygen sneaks up the chain
Much like esters are to carboxylic acids, Ketones Share the same basic structure as aldehydes, except that their double-bonded oxygen can be found hiding out in the midst of the carbon chain, giving them the general form R-COR. Ketones are named by adding the suffix -one To the prefix generated by counting up the longest carbon chain. Unlike esters, however, the carbon chain of a ketone is not broken by the double-bonded oxygen group, making naming them much simpler. The compound shown in Figure 25-10, for example, is called simply 2-butanone; the number before the name specifies the number of the carbon to which the oxygen is attached.
Figure 25-10:
An example of a ketone.
H3C
CH
CH
O
Halocarbons: Hello, halogen!
Halocarbons Are simply hydrocarbons with a halogen or more tacked on to a carbon in place of a hydrogen atom. (A Halogen Is a group VIIA element.) Halogens in a halocarbon are always named as substituent groups — Fluoro, chloro, bromo, Or Iodo, One for each of the four halogens that form halocarbons (F, Cl, Br or I). The shorthand for a halocarbon is R-X, where X is the halogen. The compound shown in Figure 25-11 has two bromo groups attached, one to the second carbon and one to the third in a five-carbon alkane. Its official name is, therefore, 2,3-dibromopentane.
Br
CH
CH
Figure 25-11:
An example of a
Halocarbon.
H3C
3
CH
CH
Br
Amines: Hobnobbing with nitrogen
Amines Are as conspicuous as Waldo on the very first page of a Where’s Waldo Book. No tricksters wearing red-striped shirts are waiting to fool you among these organic molecules. Amines and their derivatives are the only compounds that you’ll encounter in basic organic chemistry that contain nitrogen atoms. Amines have the basic form R-NH2, and they’re named by naming the carbon chain as a substituent (with the ending -yl), And then adding the suffix -amine. The structure in Figure 25-12, for example, is called ethylamine because it’s a two-carbon ethyl chain with an amine group.
Figure 25-12:
An example of an amine.
-CH,
H, C
3
‘ NH,
Rearranging Carbons: Isomerism
How about this: Two organic molecules have identical chemical formulas. Each atom in one molecule is bonded to the same groups as in the other. They’re identical molecules, right? Wrong! (Mischievous chemistry gods point and snicker.) Many organic molecules are Iso-mers, Compounds with the same formula and types of bonds, but with different structural or spatial arrangements. Who cares about such subtle differences? Well, you might. Consider thalidomide, a small organic molecule widely prescribed to pregnant women in the late 1950s and early 1960s as a treatment for morning sickness. Thalidomide exists in two isomeric forms that rapidly switch from one to the other in the body. One isomer is very effective at combating morning sickness. The other isomer causes serious birth defects. Isomers matter.
Isomers can be confusing. They fall into different categories and subcategories. So, before committing your brain to a game of isomeric Twister, peruse the following breakdown: Keep in mind, however that all isomers, no matter what their specific classification, have both the same number and type of atoms.
Structural isomers Have identical molecular formulas but differ in the arrangement of bonds.
Stereoisomers Have identical connectivities — all atoms are bonded to the same types of other atoms — but differ in the arrangement of atoms in space.
• Diastereomers Are stereoisomers that are Not Non-superimposable mirror images of each other. Two types of diastereomers exist: Geometric isomers (or Cis-trans isomers) Are diastereomers that differ in the arrangement of groups around a double bond, or the plane of a ring. Conformers And Rotamers Are diastereomers that differ because of rotations about individual bonds (we don’t cover them in this book because they are beyond the scope of general chemistry).
• Enantiomers Are stereoisomers that Are Non-superimposable mirror images of each other.
You already know how to recognize and name stereoisomers appropriately by carefully studying the structure of the main chain and branching substituents. This section focuses on the trickier category: stereoisomers.
Picking sides with geometric isomers
Geometric isomers Or Cis-trans isomers Are a good place to start in the world of stereoisomers because they’re the easiest of the stereoisomers to understand. In the following sections, we explain how isomers relate to alkenes, alkanes that aren’t straight-chain, and alkynes.
Straight-chain alkanes are immune from geometric isomerism because their carbon-carbon single bonds can rotate freely. Unsaturate (or add another bond to) one of those bonds, however, and you’ve got a different story. Alkenes have double bonds that resist rotation. Furthermore, the Sp2 Hybridization of double-bonded carbons gives them trigonal planar bonding geometry (see Chapter 7 for an introduction to hybridization). The result is that groups attached to these carbons are locked on one side or the other of the double bond. Convince yourself of this by examining Figure 25-13.
Figure 25-13:
Cis And Trans Isomers ofan alkene.
H3C
3 ‘
H
\ / 3
H
H3C
3
H
\ /
H CH
A)
B)
In the left-hand structure of Figure 25-13, the carbon chain continues along the same side of the carbon-carbon double bond. Both methyl (-CH3) groups lie on the same side of the unsat-uration. This is called Cis Configuration. In the right-hand structure, the carbon chain swaps sides as it proceeds across the double bond. The methyl groups lie on opposite sides of the unsaturation. This is called a Trans Configuration.
Naming Cis-trans Isomers is simple. Attach the appropriate Cis – Or Trans – Prefix before the number referring to the carbon of the double bond. For example, the left-hand structure in Figure 22-1 is Cis-2-butene, while the structure on the right is Trans-2-butene.
Although straight-chain alkanes happily avoid isomerism by rotating merrily about their single bonds, the four bonds of sp3-hybridized carbons assume tetrahedral geometry. Detailed representations like the one shown for methane in Figure 25-14 reveal this geometry. In the structure of methane, the bonds depicted as straight lines run in the plane of the page. The bond drawn as a filled wedge projects outward from the page. The bond drawn as a dashed wedge projects behind the page. These filled and dashed wedge symbols are known as Stereo bonds Because they are helpful in identifying stereoisomers.
Figure 25-14:
Stereo bonds in methane.
C
HH
H
When alkanes close into rings, they can no longer freely rotate about their single bonds, and the tetrahedral geometry of sp3-hybridized carbons creates Cis-trans Isomers. Groups bonded to ring carbons are locked above or below the plane of the ring, as shown in Figure 25-15.
The figure depicts two different versions of trans-1,2-dimethylcyclohexane. In both versions, the adjacent methyl substituents are locked in Trans Positions, on opposite sides of the ring.
The upper set of structures shows the plane of the ring as seen from above, and highlights the Trans – Configuration of the methyl groups with stereo bonds.
The lower set of structures shows the same rings rotated 90 degrees downward and toward you.
H, C,
CH,
____■ ,
CH CH,
H3CV CH2
,
CH CH,
Figure 25-15:
Two isomers of Trans-1, ,-dimethyl-cyclo-hexane.
H3C
3
CH CH,
H3C
3
CH CH,
CH
The Trans Configuration of the methyl groups is most clear in the lower structures. The front-most methyl group is highlighted with explicit hydrogen atoms to emphasize its position above or below the plane of the ring.
Alkynes also contain carbon-carbon bonds that cannot rotate freely. However, the Sp Hybridization of the carbons in these bonds leads to linear bonding geometry. The two-carbon alkyne, ethyne, is shown in Figure 25-16. Each carbon locks three of its valence electrons into the axis of the triple bond. Each has only one valence electron remaining with which to bond to hydrogen. No Cis-trans Isomerism is possible in this scenario.
Figure 25-16:
No iso-merism is possible at the triple bonds of alkynes, such as ethyne.
■ CC-H
H
Changing Names: Some Common Organic Reactions
The carbon-based organic molecules presented in this chapter can morph into one another through chemical reactions. Such reactions are rather common, and the two most common types are substitution and addition reactions.
Alcohols are particularly prone to a chemical process called Substitution In which their OH group is replaced by a different atom. For example, the OH group on 2-pentanol can be replaced by the halogen fluorine, turning the alcohol into a halocarbon called 2-fluoro-pentane (see Figure 25-17).
OH
CH3— CH2— CH2— CH — CH3 + F2
3 2 2 3 2
Figure 25-17:
The
Process of F substitution.
CH3 CH2 CH2 CH CH3
3 2 2 3
The double bonds of alkenes also make them particularly likely to react with other compounds through a process called Addition. If the double bond between two carbon molecules is broken, it allows each of those two carbon atoms to form a bond with another atom or molecule. The reaction shown in Figure 25-18, for example, shows water being added across the double bond of 1-hexene. The water molecule itself is split into two pieces — simple hydrogen and a hydroxide — and each of them is added to one of the two carbons that used to share a double bond, forming either 1-hexanol or 2-hexanol.
Figure 25-18: HOH
The process
Of addition. H2C CH — CH2— CH2— CH2— CH3
2 2 2 2 3
H3C
3
OH
CH CH
CH
CH
CH
