Блог Анкара

In This Chapter

^ Fueling your knowledge of hydrocarbons

^ Naming alkanes, alkenes, and alkynes

^ Saturating and unsaturating hydrocarbons

I\ Ny study of organic chemistry begins with the study of Hydrocarbons. Hydrocarbons Ґ \ are some of the simplest and most important organic compounds. Organic compounds Are based on carbon skeletons. Hydrocarbon skeletons can be modified — you can dress them up with chemically interesting atoms like oxygen, nitrogen, halogens, phosphorus, 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. As an introductory chemistry student, you won’t be expected to know more than the basic structure of organic molecules and how to name them. So relax and get organic.

Single File Now: Linking Carbons into Continuous Alkanes

The simplest of the hydrocarbons fall into the category of Alkanes. Alkanes are chains of carbon molecules 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 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 partner.

As the name Hydrocarbon Suggests, these partners may be hydrogen or carbon. 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, but 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 and are listed in Table 21-1.

The naming method in Table 21-1 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. Consider the carbon structure of pentane, for example, shown in Figure 21-1.

Figure 21-1:

Pentane’s carbon skeleton.

-C-C-C-C-C—

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 21-2.

Figure 21-2:

Pentane’s hydrocarbon

Structure.

H HHH H

C CCC C H HHHHH

H

If you add up the hydrogen atoms in Figure 21-2, 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 21-2. 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. Here’s the lowdown:

Carbons on the end of a chain, for example, are only bonded to one other carbon, so they have three additional bonds that are 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, so they’re labeled CH2.

Your chemistry teacher will require you to draw structures of alkanes, given their names, and will require you to name alkanes, given their structure. If your teacher fails to make such requests, ask to see his credentials. You may be dealing with an impostor.

Q.

What is the name of the following structure, and what is its molecular formula?

HH HH

C – C – C – H

HH HH

Are four. Table 21-1 helpfully points out that four-carbon chains earn the prefix But-. What’s more, this molecule is an alkane (because it contains only single bonds), so it receives the suffix -ane. So, what you’ve got is butane. With four carbon atoms in a straight chain, ten hydrogen atoms are required to satisfy all the carbon bonds, so the molecular formula of butane is C4H10.

H

C

Butane; C4H10. First, count the number of carbons in the continuous chain. There

1. What is the name of the following structure, and what is its molecular formula?

HHH

HHH

Solve It

2. Draw the structure of straight-chain octane.

Solve It

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 a simple set of steps to arrive at a proper (and often lengthy) name.

1. Count the longest continuous chain of carbons.

Tricky chemistry teachers often draw 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 21-3. The two are actually the same structure, drawn differently! Yikes. In either case, the longest continuous chain in this structure has eight carbons.

H

C

Cc

C1 C2 C3 C4 Cb C

B

C7

7

C8

8

C

Figure 21-3: c

One carbon structure drawn two different ways.

2 3 4 b b

Cc

2. Number the carbons in the chain Starting with the end that’s closest to a branch.

You can always check to be sure you’ve 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’s drawn and numbered in Figure 21-3, the alkane has substituent groups branching off of its third, fourth, and fifth carbons. If the carbon chain had been numbered backwards, 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.

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 21-1 awaits your visit). The three most common alkyl groups are the methyl (one carbon), ethyl (two carbons), and propyl (three carbons) groups. Figure 21-3 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!

C

C

7

C

C

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 21-3, 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, don’t 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 – Don’t figure into the alphabetizing. So, the proper name of the organic molecule in Figure 21-3 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 (. . . dimethyl-octane is wrong).

Q.

Name the branched alkane shown in the A. Following structure:

/

CH.,

H, C-HC

\

CH

/

CH HC

/

H3C

3

\

CH

CH

CH,- C-CH,

CH

4-ethyl-2,6,6-trimethyloctane. First, notice how some of the bonds seem to zig-zag. This is a feature of many hydrocarbon structures. The longest continuous chain of carbon atoms in this alkane is eight carbons long. So, the parent chain is octane. Four alkyl groups branch off the parent chain: An ethyl group branches off the fourth carbon, two methyl groups branch off the sixth carbon, and another methyl group branches off the second carbon. Attaching appropriate prefixes and alphabetizing yields the name 4-ethyl-2,6,6-trimethyloctane.

3. Name the branched alkane shown in the following structure:

CH

H, C

3 \

H, C

\

CH

H3C

3

CH

CH

CH

CH

CH

H3C

3

CH CH

CH

CH

Solve It

4. Name the branched alkane shown in the following structure:

CH

H3C

3

CH

CH

Solve It

5. Draw the alkane 3,4-diethyl-5-propyldecane.

Solve It

6. Draw the alkane 3-propyl-2,2,4,4-tetra-methylheptane.

Solve It

Getting Unsaturated: Alkenes and Alkynes

JjjtJW^ Carbons can do more than enagage in four single bonds. There’s more to organic molecules 41/ M \, than substituent-for-hydrogen swaps. When carbons in an organic compound fill their

MM ) valence shells entirely with single bonds, we say the compound is Saturated. But many 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 molecule, 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 5 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 21-4. Saturated carbon is sp3 hybridized and has tetrahedral geometry (again, see Chapter 5 to review hybridization).

Figure 21-4:

Ethene, a

C

Alkene.

Two-carbon

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 in the previous section:

1. Locate the longest carbon chain, and number it, Starting at the end closest to the double bond.

In other words, double bonds trump substituents 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 21-1), 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 that you do for alkanes.

List the number of the substituted carbon, followed by the name of the substituent. Separate the substituent 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 third carbon, then the molecule is called 3-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, 3-methyl-2-pentene may also be written 3-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.

The trick to drawing hydrocarbons is to start at the end of the name and work backwards. The prefix preceding the -ane, – ene, Or -yne Ending always tells you how many carbons are in the longest chain, so begin by drawing that parent chain. From there, work through the substituent groups, tacking them on as you go. Finally, add hydrogens into the structure wherever there are empty bonds, and voila! A portrait of a hydrocarbon.

KPLf

Q. Name the following alkene and the following alkyne:

H. C

3 *

.CH,

CH

HC

CH

CH

CH

C

The alkene is 2-methyl-2-butene (or 2-methylbut-2-ene); the alkyne is 1-butyne. The figure on the left shows a four-carbon alkene with the double bond between the second and third carbon atoms. Numbering the chain from either side yields the same numbers with respect to the double bond. However, numbering from right to left gives a lower number to the methyl substituent, so the compound is 2-methyl-2-butene (or 2-methylbut-2-ene). The figure on the right shows a four-carbon alkyne, with the triple bond located between the first and second carbon atoms. There are no substituents. The compound is therefore 1-butyne.

7. Name the following unsaturated hydrocarbon:

3 -

3

CH CH

H3C

3

CH2 CH3

CH C CH

H2C

2

CH

Solve It

8. Draw the compound 5-ethyl-5-methyl-3-octyne (or 5-ethyl-5-methyloct-3-yne).

Solve It

9. Draw the compound 4,4-dimethyl-2-pen-tene (or 4,4-dimethylpent-2-ene).

Solve It

Rounding ‘em Up: Circular Carbon Chains

The compounds we cover earlier in this chapter are linear or branched. However, hydrocarbons can be circular, or Cyclical. Among the cyclical carbons, there are two important categories, the Cyclic aliphatic Hydrocarbons and the 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.

Wrapping your head around cyclic aliphatic hydrocarbons

Cyclic aliphatic hydrocarbons are like the hydrocarbons that we explain earlier in this chapter, except that they form a closed ring. The rules for naming these compounds build on the rules we provide earlier in this chapter. 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 doesn’t require a number. So, a single bromine-for-hydrogen substitution on cyclohexane yields a compound with the name bromocyclohexane. Likewise, a lone double bond unsaturation on cyclohexane 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. Triple bonds outrank double bonds. Double bonds outrank other substituents. So, number the carbons in the way that respects these rankings and produces the lowest overall numbers. A cyclohexane molecule with two methyl substituents on neighboring carbons, for example, is called 1,2-dimethylcyclohexane.

Sniffing out aromatic hydrocarbons

Aromatic hydrocarbons have special properties because of their electronic structure. Aromatics are Both cyclic and conjugated. Conjugation results from an alternation of double or triple bonds with single bonds. Noncyclic hydrocarbons can be conjugated, too, but they can’t be aromatic. 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, the resonance structures for which are depicted in Chapter 5. 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 bromobenzene. 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 chlorine and methyl substituents situated two carbons away from one another, for example, would be called 1-chloro-3-methylbenzene.

Name the following cyclic hydrocarbon:

3-methylcyclohexene. The structure is a six-carbon cyclic alkene with a methyl substituent. Numbering starts with the highest priority group, which is the double bond. Number so that the carbons of the double bond receive numbers 1 and 2, and the carbon to which the methyl group is attached gets the lowest possible number. This means numbering counterclockwise as shown in the figure. The name of the compound is 3-methylcyclohexene. You don’t need to specify the number of the carbon where the double bond appears; because it’s the highest-ranking feature, the double bond is assumed to start at carbon number one.

A.

10. Name the following cyclic hydrocarbon:

H, C.

,ChL

HC =

= CH

Solve It

11. Draw the compound 4-methylcyclopentene.

Solve It

C

12. Draw the compound 4-butyl-3-ethylcyclopentyne.

Solve It

Answers to Questions on Carbon Chains

Do you feel like you’re on the straight and narrow path to understanding carbon chains, or do you feel like you’ve been running around in circles? Take a deep breath, relax, and check your answers to the practice problems presented in this chapter.

D The structure is propane; its molecular formula is C3H8. The figure shows a three-carbon chain with only single bonds. Therefore, it is propane. Its molecular formula is C3H8.

CM The Oct – Prefix here tells you that this alkane is eight carbons long. Draw eight linked carbons and fill in the empty bonds with hydrogen. Your structure should look like this one:

H

H

H

H

H

H

H

H

HC

H3C

CH

CH

CH

CH

CH

CH

CH

CM 4-ethyl-3-methyl-5-propylnonane. The structure shown is a nine-carbon alkane, so its parent chain name is nonane. Begin numbering from the end that gives the substituent groups the lowest numbers. A one-carbon group extends from the third carbon (3-methyl), a two-carbon group extends from the fourth carbon (4-ethyl), and a three-carbon group extends from the fifth carbon (5-propyl). Alphabetize these substituents and tack on the nonane ending, and you have 4-ethyl-3-methyl-5-propylnonane.

MM 2,2-dimethylpropane. Note that this is the proper Systematic Name, and that other Common Names may also be used. The common name for this structure is neopentane.

CM The name 3,4-diethyl-5-propyldecane tells you first and foremost that this compound is a ten-carbon alkane. The substituent names indicate two ethyl groups extending from the third and fourth carbons and a propyl group extending from the fifth carbon in the chain. So your artwork should look like this figure:

H, CX

CH

H3C

/CH2^ /CH\ /CH2^ YCH2K /CH3

CH CH

CH2 CH2

22

CH2 CH2

22

H3C H2C

, 2 .

C

C

C

C

C

C

C

H

H

H

H

H

H

H

H

H

Or

CH,

MM The Heptane Ending tells you that you’re dealing with a seven-carbon alkane here, so begin by drawing a seven-carbon parent chain. Next, attach the substituents to the main chain by decoding their names. 3-propyl Tells you that a three-carbon substituent group extends from the third carbon in the chain, and 2,2,4,4-tetramethyl Tells you that four single-carbon substituent groups are attached to the chain, two on the second carbon and two on the fourth. Your drawing should look like this:

H3C

3

CH3 H3C

3 3 .

H3C

3

CH,

• CH3

CH

CH

CH

CH

H, C

CH3

3

EM 5,5-diethyl-2-methyl-3-heptene (or 5,5-diethyl-2-methylhept-3-ene). Note first that this structure contains a double bond, meaning it’s an alkene and will end with the suffix -ene. It has seven carbons in the parent chain, so its base name is Heptene. In general, you begin numbering the chain at the end closest to the double bond because it has the highest priority. However, in this case, the double bond is between carbons three and four (so it’s 3-heptene) no matter which end you begin numbering from. The substituents serve as the tiebreaker here to help you decide whether to number from the left or the right. Choose the end that yields the lowest substituent numbers and then name the substituents. You should get 5,5-diethyl-2-methyl-3-heptene (or 5,5-diethyl-2-methylhept-3-ene).

MM The ending 3-octyne Tells you that this compound is an eight-carbon alkyne with the triple bond between the third and fourth carbons. The substituent names 5-ethyl And 5-methyl Tell you that the chain includes both a two-carbon (ethyl) and a one-carbon (methyl) branch off the fifth carbon. If your drawing looks like the following figure, you’ve done well.

CH

CH

CH

H3C

3

CH, CH3

CH

H3C

3

C

C

C

MM The ending 3-pentene Tells you that this compound is a five-carbon alkene with the double bond

Between the second and third carbons. The substituent name 4,4-dimethyl Tells you that two single-carbon substituent groups are present, both extending from the fourth carbon in the chain. Your artwork should look like the following figure:

ChL

H, C -

3

CH -

CH

C

CH3

3

Jj 3,3-dimethylcyclopropene. This is a five-carbon cyclic alkene with two methyl substituents. Number the ring so the two carbons sharing the double bond receive the numbers 1 and 2, and the methyl substituents receive the lowest number. Doing so causes the two methyl groups to extend from the carbon numbered 3. The name is therefore 3,3-dimethylcyclopropene.

|f| The ending Cyclopentene Indicates that this is a five-carbon cyclic alkene. Draw a five-carbon ring containing a single double bond and number around the ring so the two carbons sharing the double bond receive the numbers 1 and 2. Then attach a one-carbon substituent group to the carbon numbered 4. Here’s what your drawing should look like:

CH

H3C

3

HC

CH

CH

CH

U The name 4-butyl-3-ethylcyclopentyne reveals that the compound is a five-carbon cyclic alkyne, so begin by drawing a five-carbon ring with one triple bond. Number around the ring so the two carbons sharing the triple bond receive the numbers 1 and 2, and then attach a four-carbon (butyl) substituent to the fourth carbon and a two-carbon (ethyl) substituent to the third carbon. Your drawing should look like this one:

H3C

CH

H2CN

2 ^

H3C

3

CH

CH

CH

HC

CH

C

C