Chapter 10

An Introduction to Organic Chemistry:The Saturated Hydrocarbons

Denniston Topping Caret

5th Edition

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

10.1 The Chemistry of CarbonWhy are there so many organic compounds?1. Carbon forms stable, covalent bonds with other

carbon atoms• Consider three allotropic forms of elemental carbon

– Graphite in planar layers– Diamond is a three-dimensional network– Buckminsterfullerene is 60 C in a roughly spherical shape

Why are there so many organic compounds?

2. Carbon atoms form stable bonds with other elements, such as:

– Oxygen– Nitrogen– Sulfur– Halogen

• Presence of these other elements confers many new physical and chemical properties on an organic compound

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Why are there so many organic compounds?

3. Carbon atoms form double or triple bonds with:

– Other carbon atoms (double & triple)

– Oxygen (double only)– Nitrogen (double & triple)

• These combinations act to produce a variety of organic molecules with very different properties

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Car

bon CH2 O

CH2 NH

CH2 CH2

CH CH

CH N

double bonds

triple bonds

Why are there so many organic compounds?

4. Carbon atoms can be arranged with these other atoms; is nearly limitless

– Branched chains– Ring structures– Linear chains

• Two organic compounds may even have the same number and kinds of atoms but completely different structures and thus, different properties

– These are called isomers

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bon

CH3CH2CH2CH3CH2 CH2

CH2

CH2

CH2

Isomers

• Many carbon compounds exist in the form of isomers• Isomers are compounds with the same molecular formula but different structures

• An isomer example: both are C4H10 but have different structures

– Butane– Methylpropane 10

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Isomers

All have the same molecular formula: C4H8

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Important Differences Between Organic and Inorganic Compounds

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• Bond type–Organics have covalent bonds

• Electron sharing–Inorganics usually have ionic bonds

• Electron transfer

• Structure–Organics

• Molecules• Nonelectrolytes

–Inorganics• Three-dimensional crystal structures• Often water-soluble, dissociating into ions -

electrolytes

Important Differences Between Organic and Inorganic Compounds

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• Melting Point & Boiling Point–Organics have covalent bonds

• Intermolecular forces broken fairly easily–Inorganics usually have ionic bonds

• Ionic bonds require more energy to break

• Water Solubility–Organics

• Nonpolar, water insoluble–Inorganics

• Water-soluble, readily dissociate

Comparison of Major Properties of Organic and Inorganic Compounds

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Bonding Characteristics and Isomerism

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• One reason for the power of carbon is that it can form 4 covalent bonds

• It appears to have only 2 available electrons

• Carbon can hybridize its orbitals to move 2 electrons out of it 2s orbital

Hybrid Orbitals10

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• Each carbon-hydrogen bond in methane arises from an overlap of a C(sp3) and an H(1s) orbital

• 4 equivalent sp3 orbitals point toward the corners of a regular tetrahedron

• The 4 sp3 hybrid orbitals of carbon combine with the 1s orbitals on 4 H to produce methane – CH4

Families of Organic Compounds

• Hydrocarbons contain only carbon and hydrogen

• They are nonpolar molecules– Not soluble in water – Are soluble in typical nonpolar organic

solvents • Toluene• Pentane

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Families of Organic Compounds

• Hydrocarbons are constructed of chains or rings of carbon atoms with sufficient hydrogen atoms to fulfill carbon’s need for four bonds

• Substituted hydrocarbon is one in which one or more hydrogen atoms is replaced by another atom or group of atoms

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Division of the Family of Hydrocarbons

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Hydrocarbon Saturation

• Alkanes are compounds that contain only carbon-carbon and carbon-hydrogen single bonds– A saturated hydrocarbon has no double or

triple bonds

• Alkenes and alkynes are unsaturated because they contain at least one carbon to carbon double or triple bond

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Cyclic Structure of Hydrocarbons

• Some hydrocarbons are cyclic– Form a closed ring– Aromatic hydrocarbons contain a benzene

ring or related structure

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Common Functional Groups10

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10.2 Alkanes

• The general formula for a chain alkane is CnH2n+2

– In this formula n = the number of carbon atoms in the molecule

• Alkanes are saturated hydrocarbons– Contain only carbon and hydrogen– Bonds are carbon-hydrogen and carbon-carbon

single bonds

Formulas Used in Organic Chemistry

• Molecular formula - lists kind and number of each type of atom in a molecule, no bonding pattern

• Structural formula - shows each atom and bond in a molecule

• Condensed formula - shows all the atoms in a molecule in sequential order indicating which atoms are bonded to which

• Line formula - assume a carbon atom at any location where lines intersect– Assume a carbon at the end of any line – Each carbon in the structure is bonded to the correct number of

hydrogen atoms

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The Tetrahedral Carbon Atom

(a) Lewis dot structure(b)The tetrahedral shape around the carbon atom(c) The tetrahedral carbon drawn with dashes and

wedges(d)The stick drawing of the tetrahedral carbon

atom(e) Ball and stick model of methane

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Drawing Methane and Ethane

C

H

HH

C

H

HH

109.5 o

Staggered form of ethane

H

H

HH

in plane

in front of plane

behind plane

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anes

Comparison of Ethane and Butane Structures

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anes

Names and Formulas of the First Ten Straight-Chain Alkanes

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anes

Structural Isomers• Constitutional/Structural Isomers differ in how

atoms are connected– Two isomers of butane have different physical

properties– The carbon atoms are connected in different

patterns

ButaneBp –0.4 oCMp –139 oC

IsobutaneBp –12 oC

Mp –145 oC

CH3 CH2 CH2 CH3 CH3 CH CH3

CH3

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Comparison of Physical Properties of Five Isomers of Hexane

Compare the basic linear structure of hexane– All other isomers have one or more carbon atoms

branching from the main chain– Branched-chain forms of the molecule have a much

smaller surface area• Intermolecular forces are weaker• Boiling and melting points are lower than straight chains

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Physical Properties of Organic Molecules

1. Nonpolar

2. Not water soluble

3. Soluble in nonpolar organic solvents

4. Low melting points

5. Low boiling points

6. Generally less dense (lighter) than water

7. As length (molecular weight) increases, melting and boiling points increase as does the density

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Properties of Alkanes10

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-250

-200

-150

-100

-50

0

50

100

150

200

0 1 2 3 4 5 6 7 8 9 10

Number of Carbons in Chain

Temperature

Melting PointBoiling Point

Properties of Alkanes

• Most of the alkanes are hydrophobic: water hating

• Straight chain alkanes comprise a homologous series: compounds of the same functional class that differ by a –CH2- group

• Nonpolar alkanes are:– Insoluble in water (a highly polar solvent)– Less dense than water and float on it

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Alkyl Groups

• An alkyl group is an alkane with one hydrogen atom removed

• It is named by replacing the -ane of the alkane name with -yl

• Methane becomes a methyl group

or CH3H C

H

H

H

H C

H

H

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Alkyl Groups

• All six hydrogens on ethane are equivalent

• Removing one H generates the ethyl group

• All 3 structures shown at right are the same

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Names and Formulas of the First Five Alkyl Groups

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Alkyl Group Classification

• Alkyl groups are classified according to the number of carbons attached to the carbon atom that joins the alkyl group to a molecule

• All continuous chain alkyl groups are 1º

• Isopropyl and sec-butyl are 2º groups

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Iso- Alkyl Groups• Propane: removal of a hydrogen generates

two different propyl groups depending on whether an end or center H is removed

n-propyl isopropyl

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Sec- Alkyl Groups

• n-butane gives two butyl groups depending on whether an end (1º) or interior (2º) H is removed

sec-butyln-butyl

CH3 CH2 CH2 CH3

CH3 CH CH2 CH3CH3 CH2 CH2 CH2

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Structures and Names of Some Branched-Chain Alkyl Groups

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CH3 CH CH3

CH3

CH3 CH CH2

CH3

CH3 C CH3

CH3

More Alkyl Group Classification

• Isobutane gives two butyl groups depending on whether a 1o or 3o H is removed

isobutyl t-butyl

1o C 3o C

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Nomenclature

• The IUPAC (International Union of Pure and Applied Chemistry) is responsible for chemical names

• Before learning the IUPAC rules for naming alkanes, the names and structures of eight alkyl groups must be learned

• These alkyl groups are historical names accepted by the IUPAC and integrated into modern nomenclature

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Carbon Chain Length and Prefixes

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IUPAC Names for Alkanes

1. The base or parent name for an alkane is determined by the longest chain of carbon atoms in the formula

– The longest chain may bend and twist, it is seldom horizontal

– Any carbon groups not part of the base chain are called branches or substituents

– These carbon groups are also called alkyl groups

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IUPAC Names for Alkanes

• Rule 1 applied – Find the longest chain in each molecule

• A=7 B=8

CH3

CH2

CH2CH2CH CH2CH3

CH3

CH3CH

CH2

CH3

CH2CH

CH2CH2

CH2

CH3

CH3

A

B

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IUPAC Names for Alkanes

2. Number the carbon atoms in the chain starting from the end with the first branch

– If both branches are equally from the ends, continue until a point of difference occurs

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IUPAC Names for Alkanes

Number the carbon atoms correctly

• Left: first branch is on carbon 3

• Right: first branch is on carbon 3 (From top) not carbon 4 (if number from right)

CH3

CH2

CH2CH2CH CH2CH3

CH3

CH3CH

CH2

CH3

CH2CH

CH2CH2

CH2

CH3

CH3

1

2

3 4 5

6 7 8

this branch would be on C-4if you started at correct C-8

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123

45

6

7

IUPAC Names for Alkanes

3. Write each of the branches/substituents in alphabetical order before the base/stem name (longest chain)

– Halogens usually come first– Indicate the position of the branch on the main

chain by prefixing its name with the carbon number to which it is attached

– Separate numbers and letters with a hyphen– Separate two or more numbers with commas

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IUPAC Names for Alkanes

Name : 4-ethyl-2-methylhexane

CH3CH2CH CH2CH CH3

CH3CH2

CH3

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IUPAC Names for Alkanes

• Hyphenated and number prefixes are not considered when alphabetizing groups– Name the compound below

– 5-sec-butyl-4-isopropylnonane

CH CHCH3

CH3 CH2 CH2 CH3

CHCHCH3

CH2 CH3

CH2 CH2 CH2 CH3

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IUPAC Names for Alkanes

• When a branch/substituent occurs more than once– Prefix the name with

• di• tri• tetra

– Then list the number of the carbon branch for that substituent to the name with a separate number for each occurrence

• Separate numbers with commas

• e.g., 3,4-dimethyl or 4,4,6-triethyl

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IUPAC Names for Alkanes

5-ethyl-2,3-dimethylheptaneethyl>dimethyl10

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Practice: IUPAC Name

6-ethyl-6-isobutyl-3,3-dimethyldecane

CH3CCH3

CH2

CH3

CH2CH2C CH2CH3

CH2

CH CH3CH3

CH2CH2CH2CH3

Name

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anes 1

2

3 4 5 6

7 8 9 10

10.3 Cycloalkanes

• Cycloalkanes have two less hydrogens than the corresponding chain alkane– Hexane=C6H14; cyclohexane=C6H12

• To name cycloalkanes, prefix cyclo- to the name of the corresponding alkane– Place substituents in alphabetical order before

the base name as for alkanes– For multiple substituents, use the lowest

possible set of numbers; a single substituent requires no number

Cycloalkane Structures10

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Cyclopropane

Cyclobutane

Cyclohexane

Type of Formula: Structural Condensed Line

Naming a Substituted Cycloalkane

Name the two cycloalkanes shown below• Parent chain 6 carbon ring 5 carbon ring

cyclohexane cyclopentane• Substituent 1 chlorine atom a methyl group

chloro methyl• Name Chlorocyclohexane Methylcyclopentane

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cis-trans Isomers in Cycloalkanes

• Atoms of an alkane can rotate freely around the carbon-carbon single bond having an unlimited number of arrangements

• Rotation around the bonds in a cyclic structure is limited by the fact that all carbons in the ring are interlocked– Formation of cis-trans isomers, geometric isomers, is a

consequence of the lack of free rotation

• Stereoisomers are molecules that have the same structural formulas and bonding patterns, but different arrangements of atoms in space– cis-trans isomers of cycloalkanes are stereoisomers

whose substituents differ in spatial arrangement

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cis-trans Isomers in Cycloalkanes

• Two groups may be on the same side (cis) of the imagined plane of the cycloring or they may be on the opposite side (trans)

• Geometric isomers do not readily interconvert, only by breaking carbon-carbon bonds can they interconvert

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10.4 Conformations of Alkanes• Conformations differ only in rotation about carbon-

carbon single bonds • Two conformations of ethane and butane are shown

– The first (staggered form) is more stable because it allows hydrogens to be farther apart and thus, the atoms are less crowded

Two Conformations of Cyclohexane

Chair form (more stable) Boat form

E=equitorial A=axial

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10.5 Reactions of Alkanes

• Alkanes, cycloalkanes, and other hydrocarbons can be: – Oxidized (by burning) in the presence of excess

molecular oxygen, in a process called combustion

– Reacted with a halogen (usually chlorine or bromine) in a halogenation reaction

Alkane Reactions

The majority of the reaction of alkanes are combustion reactions

– Complete CH4 + 2O2 CO2 + 2H2O Complete combustion produces

– Carbon dioxide and water

– Incomplete 2CH4 + 3O2 2CO + 4H2O

• Incomplete combustion produces – Carbon monoxide and water– Carbon monoxide is a poison that binds

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HalogenationHalogenation is a type of substitution reaction, a reaction that results in a replacement of one group for another

– Products of this reaction are: • Alkyl halide or haloalkane• Hydrogen halide

– This reaction is important in converting unreactive alkanes into many starting materials for other products

– Halogenation of alkanes ONLY occurs in the presence of heat and/or light (UV)

HH +Br2

heat orlight +HBr

BrH

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Petroleum Processing

Fraction Boiling Pt Range ºC Carbon size Typical uses

Gas -164-30 C1-C4 Heating, cooking

Gasoline 30-200 C5-C12 Motor fuel

Kerosene 175-275 C12-C16 Fuel for stoves, diesel and jet engines

Heating oil Up to 375 C15-C18 Furnace oil

Lubricating oil

350 and up C16-C20 Lubrication, mineral oil

Greases Semisolid C18-up Lubrication, petroleum jelly

Paraffin (wax)

Melts at 52-57 C20-up Candles, toiletries

Pitch / tar Residue in boiler High Roofing, asphalt paving

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