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Appendix D. Carbon Compounds: An Introduction to Organic Chemistry

Appendix D. Carbon Compounds: An Introduction to Organic Chemistry

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Amines and Amides



Keywords

Questions and Problems

WHY DOES CARBON FORM SO MANY COMPOUNDS? The answer lies in its atomic structure. Carbon,

atomic number 6, has four valence electrons, and when it forms compounds by sharing these valence

electrons with other carbon atoms or atoms of other elements, it obeys the octet rule. It forms carbon

—carbon single bonds by sharing pairs of electrons:

When it combines with four hydrogen atoms to form methane, the carbon atom shares its four

valence electrons with the hydrogen atoms, thus forming four stable covalent bonds:



Carbon forms more compounds than any other element primarily because carbon atoms link with

each other in so many different ways. A molecule of a carbon compound may contain a single carbon

—carbon bond or thousands of such bonds. The carbon atoms can link in straight chains, branched

chains, or rings (Figure D.1). In addition to carbon—carbon single bonds, carbon can form carbon—

carbon double and triple bonds. In each of these different bonding patterns, the carbon atoms form

four covalent bonds.

Only the element carbon is able to form long chains of its atoms. Some elements, including oxygen

(O), nitrogen (N), and chlorine (Cl), form stable two-atom molecules; sulfur (S), silicon (Si), and

phosphorus (P) form unstable chains of from four to eight like atoms, but no element can form chains

as long as carbon. Even silicon and germanium (Ge), which are in the same group of the periodic

table as carbon, do not form long like-atom chains.



Figure D.1 Carbon atoms can join to form straight chains, branched chains, or rings. In addition to carbon–carbon single bonds, double

and triple bonds are also formed.



Different Forms of Carbon

Elemental carbon exists in two very different crystalline forms: diamond and graphite. In diamond,

each carbon atom is joined by strong covalent bonds to four other carbon atoms. Each of these carbon

atoms is also joined to four more carbon atoms and so on until a huge three-dimensional interlocking

network of carbon atoms is formed (Figure D.2a). This carbon—carbon bonding pattern accounts for

the stability and extreme hardness of diamond. For a diamond to undergo a chemical change, many

strong bonds within the crystalline structure must be broken.

Graphite is completely different from diamond. It is a soft, black, slippery material that is made of

hexagonal arrays of carbon atoms arranged in sheets (Figure D.2b). Each carbon atom in the array is

joined to three other carbon atoms by forming two single bonds and one double bond. Graphite is

slippery because the intermolecular attractive forces holding the sheets together are relatively weak,

and the sheets easily slide past each other. Graphite is used as a dry lubricant and, combined with a

binder, forms the “lead” in pencils.



Figure D.2 Two very different forms of carbon: (a) In diamond, the carbon atoms form a strong, three-dimensional network. © Joao

Virissimo/ShutterStock, Inc. (b) In graphite, the carbon atoms form sheets held together by weak attractive forces. © Grant Heilman

Photography/Alamy Images.



Compounds of Carbon with Other Elements



In addition to forming carbon–carbon bonds, carbon atoms form strong covalent bonds with other

elements, particularly with the nonmetal elements hydrogen (H), oxygen (O), nitrogen (N), fluorine

(F), and chlorine (Cl). Simple compounds in which carbon is combined with other elements are

shown in Figure D.3. Carbon also bonds with phosphorus (P), sulfur (S), silicon (Si), boron (B), and

the metals sodium (Na), potassium (K), calcium (Ca), and magnesium (Mg).

The enormous number of carbon compounds that exists can be divided into a relatively small

number of classes according to the functional group they contain. A functional group is a particular

arrangement of atoms that is present in each molecule of the class and that largely determines the

chemical behavior of the class. Examples of functional groups are —C-l, —OH, —COOH, and —

NH2. Before we examine the different functional groups, we study the hydrocarbons, the parent

compounds from which all carbon compounds are derived.



Hydrocarbons

Hydrocarbons are composed of the elements carbon and hydrogen. There are four classes of

hydrocarbons: alkanes, which contain —C—C—single bonds; alkenes, which contain one or more

—C=C—double bonds; alkynes, which contain one or more —C≡C—triple bonds; and aromatic

hydrocarbons, which contain one or more benzene rings. Complex mixtures of hydrocarbons, which

are present in enormous quantities in natural gas and oil (petroleum), are the source of many of the

organic compounds used by industry.



Figure D.3 Some compounds in which carbon atoms are covalently bonded to other kinds of atoms.



Alkanes



The simplest alkane is methane (CH4), the major component of natural gas. The structure of the

molecule can be represented in a number of different ways (Figure D.4). The expanded structural

formula includes the four covalent bonds; the ball-and-stick and space-filling models show the spatial

arrangement of the atoms. As the ball-and-stick model indicates, the methane molecule is in the shape

of a tetrahedron. The space-filling model gives the most accurate representation of the actual shape of

the molecule. For simplicity, the condensed structural formula, which does not show the bonds or the

bond angles, is usually used to represent the molecule.

The two alkanes that follow methane are ethane (C2H6), and propane (C3H8) (Figure D.4).

Propane is a major component of bottled gas. The next member of the series is butane (C4H10), and

for this alkane, two structures are possible (Figure D.5). The four carbon atoms can be joined in a

straight line (n-butane), or the fourth carbon can be added to the middle carbon atom in the —C—C—

C—chain and form a branch (isobutane).

Alkanes are known as saturated hydrocarbons because for a given number of carbon atoms, they

contain the largest possible number of hydrogen atoms. The first 10 straight-chain saturated alkanes

are shown in Table D.1. The n- stands for normal and signifies straight chain. Notice that each alkane

differs from the one preceding it by the addition of a —CH2 group. A series of compounds in which

each member differs from the next member by a constant increment is called a homologous series.

The general formula for the alkane series is CnH2n+2, where n is the number of carbon atoms in a

member of the series. In a homologous series, the properties of the members change systematically

with increasing molecular weight. Table D.1 shows that in the alkane series, the boiling points of the

straight-chain alkanes rise quite regularly as the number of carbon atoms increases. The first four

alkanes are gases, and the remainder of those shown is liquids.



Figure D.4 Organic molecules can be represented in several ways, as shown here for the first three members of the alkane series.



Figure D.5 The two structural isomers of butane (C4H10).



EXAMPLE



D.1



Write the structural formula for the straight-chain alkane C5H12.

Solution

Write five carbon atoms linked together to form a chain:

C—C—C—C—C

Attach hydrogen atoms to the carbon atoms so that each carbon atom forms four covalent bonds.



Table D.1

The First Ten Straight-Chain Alkanes



EXAMPLE



D.2



Write the structural formula for the straight-chain hydrocarbon represented by CnH2n+2, where n =

7.

Solution

Write seven carbon atoms linked together to form a chain and attach hydrogen atoms to the carbon

atoms so that each carbon atom forms four covalent bonds.



We have described unbranched chains as straight chains. In fact, because of tetrahedral bonding, the

carbon atoms are not in a straight line (as shown in Table D.1), but are staggered as shown here and

in the ball-and-stick models for propane and n-butane in Figures D.4 and D.5.



Structural Isomerism

Compounds such as butane and isobutane that have the same molecular formula (C4H10), but the

different structures are called structural isomers. Because their structures are different (Figure D.5),

their properties—for example, boiling point and melting point—are also different.

All alkanes containing four or more carbon atoms form structural isomers. The predicted number

of possible isomers increases rapidly as the number of carbon atoms in the molecule increases. For

example, butane (C4H10) forms two isomers: Decane (C10H22) theoretically forms 75, and C30H62

forms over 400 million (Table D.2). Most of these isomers do not exist naturally and have not been

synthesized, but the large number of possibilities helps to explain the abundance of carbon

compounds. In isomers with large numbers of carbon atoms, crowding of atoms makes the structures

too unstable to exist.



Table D.2

Number of Possible Isomers for Selected Alkanes

Molecular Formula

C4H10

C5H12

C6H14

C7H16

C8H18

C9H20

C10H22

C15H32

C20H42

C30H62



EXAMPLE



Number of Possible Isomers

2

3

5

9

18

35

75

4,347

336,319

4,111,846,763



D.3



Give the structural and condensed formulas of the isomers of pentane.

Solution

Pentane has five carbon atoms. The carbon skeletons of the three possible isomers are as follows:



Attach hydrogen atoms so that each carbon atom forms four bonds. Each isomer has the same

condensed formula, C5H12.



EXAMPLE



D.4



Give the structural formulas of the possible isomers of hexane (C6H14).

Solution



Nomenclature of Alkanes

Organic compounds are named according to rules established by the International Union of Pure and

Applied Chemistry (IUPAC). Table D.1 shows that except for the first four members of the family, the

first part of the name of an alkane is derived from the Greek name for the number of carbon atoms in

the molecule. The suffix -ane means that the compound is an alkane.

Before we can follow the IUPAC rules for naming branched-chain alkanes, we must consider the

names of the groups that are formed when one hydrogen atom is removed from the formula of an

alkane. For example, when an H atom is removed from methane (CH4) a —CH3 or methyl group is

formed. Similarly, removal of an H atom from ethane (C2H6) gives —C2H5 or an ethyl group.

Because the groups are derived from alkanes, they are called alkyl groups. Examples of some

common alkyl groups are given in Table D.3.



Table D.3

Some Common Alkyl Groups



Branched-chain alkanes are named by applying the following rules:

1. Determine the name of the parent compound by finding the longest continuous chain of carbon

atoms. Consider the following example:



The longest chain contains five carbon atoms; therefore, the parent name is pentane.

Sometimes, because of the way in which a formula is written, it is not easy to recognize the

longest chain. For example, the alkane shown below is not a hexane (six carbon atoms in the

longest chain) as at first might be supposed but is a heptane (seven carbon atoms in the longest

chain).



2. Number the longest chain beginning with the end closest to the branch. Use these numbers to

designate the location of the groups (or substituents) at the branch.

Applying this rule to the previous examples, the carbon atoms are numbered and named as

follows:

a.



b.



In the names of compounds, the numbers are separated from words by a hyphen, and the parent name

is placed last. In compound a, a methyl group is attached to carbon number 2; in compound b, a

methyl group is attached to carbon number 3. Thus the names of the compounds are 2-methylpentane

in a and 3-methylheptane in b.

EXAMPLE



D.5



Name the following compound:



Solution

1. Determine the name of the parent compound by finding the longest continuous chain of carbon

atoms. The longest chain is five carbons. Therefore, the parent compound is a pentane.

2. Number the longest chain beginning with the end closest to a branch. Use these numbers to



designate the location of the groups (or substituents) at the branch. The methyl is on the 2

position.

3. The compound is 2-methylpentane.

EXAMPLE



D.6



Name the following:



Solution

2-methylhexane

3. When two or more substituents are present on the same carbon atom, use the number of that

carbon atom twice. List the substituents alphabetically. Thus, the compound shown here is 3ethyl-3-methylhexane:



4. When two or more substituents are identical, indicate this by the use of the prefixes di-, tri-,

tetra-, and so on. Commas are used to separate numbers from each other.

Thus, the following compounds are 2,2,4-trimethylhexane and 3-ethyl-2, 4-dimethyloctane:



EXAMPLE



D.7



Name the following compound:



Solution

1. Determine the name of the parent compound by finding the longest continuous chain of carbon

atoms. The longest chain is seven carbon atoms; therefore, the parent compound is a heptane.

2. Number the longest chain beginning with the end closest to a branch. Use these numbers to

designate the location of the groups (or substituents) at the branch. There are methyl groups at

positions 2, 4, and 6. Trimethyl indicates three methyl groups.

3. The name of the compound is 2,4,6-trimethylheptane.

EXAMPLE



D.8



Name the following compound:



Solution

2,2,5,5-tetra methyl hexane



Reactions of Alkanes

Alkanes are relatively unreactive. Like all hydrocarbons, alkanes undergo combustion, but for this to

occur, a flame or spark is required to initiate bond cleavage. When burned in air, alkanes form carbon

dioxide and water as shown below for methane and butane.



Once initiated, the reactions are exothermic and produce a considerable amount of heat. They are the

source of the energy that we obtain when natural gas, gasoline, and fuel oil are burned.



Alkenes

Alkenes are hydrocarbons that have one or more carbon–carbon double bonds (—C=C—). The two

simplest are ethene (C2H4), commonly called ethylene, and propene (C3H6), commonly called

propylene. In forming the double bond, the two carbon atoms share two pairs of electrons to acquire

the stable octet configuration.



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Appendix D. Carbon Compounds: An Introduction to Organic Chemistry

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