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Organic Chemistry. William H. Brown Christopher S. Foote Brent L. Iverson. Alkanes and Cycloalkanes. Chapter 2. Structure. Hydrocarbon: a compound composed only of carbon and hydrogen Saturated hydrocarbon: a hydrocarbon containing only single bonds

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organic chemistry

Organic Chemistry

William H. Brown

Christopher S. Foote

Brent L. Iverson





Chapter 2

  • Hydrocarbon: a compound composed only of carbon and hydrogen
  • Saturated hydrocarbon:a hydrocarbon containing only single bonds
  • Alkane: a saturated hydrocarbon whose carbons are arranged in an open chain
  • Aliphatic hydrocarbon:another name for an alkane
  • Shape
    • tetrahedral about carbon
    • all bond angles are approximately 109.5°
drawing alkanes
Drawing Alkanes
  • Line-angle formulas
    • an abbreviated way to draw structural formulas
    • each vertex and line ending represents a carbon
constitutional isomerism
Constitutional Isomerism
  • Constitutional isomers: compounds with the same molecular formula but a different connectivity of their atoms
    • example: C4H10
constitutional isomerism8
Constitutional Isomerism
  • do these formulas represent constitutional isomers?
  • find the longest carbon chain
  • number each chain from the end nearest the first branch
  • compare chain lengths as well the identity and location of branches
constitutional isomerism9
Constitutional Isomerism

World population

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nomenclature iupac
Nomenclature - IUPAC
  • Suffix -ane specifies an alkane
  • Prefix tells the number of carbon atoms
nomenclature iupac11
Nomenclature - IUPAC
  • Parent name: the longest carbon chain
  • Substituent: a group bonded to the parent chain
    • alkyl group: a substituent derived by removal of a hydrogen from an alkane; given the symbol R-
nomenclature iupac12
Nomenclature - IUPAC

1.The name of a saturated hydrocarbon with an unbranched chain consists of a prefix and suffix

2. The parent chain is the longest chain of carbon atoms

3. Each substituent is given a name and a number

4. If there is one substituent, number the chain from the end that gives it the lower number

nomenclature iupac13
Nomenclature - IUPAC

5. If there are two or more identical substituents, number the chain from the end that gives the lower number to the substituent encountered first

  • indicate the number of times the substituent appears by a prefix di-, tri-, tetra-, etc.
  • use commas to separate position numbers
nomenclature iupac14
Nomenclature - IUPAC

6. If there are two or more different substituents,

  • list them in alphabetical order
  • number from the end of the chain that gives the substituent encountered first the lower number
nomenclature iupac15
Nomenclature - IUPAC

7. The prefixes di-, tri-, tetra-, etc. are not included in alphabetization

  • alphabetize the names of substituents first and then insert these prefixes
nomenclature common
Nomenclature - Common
  • The number of carbons in the alkane determines the name
    • all alkanes with four carbons are butanes, those with five carbons are pentanes, etc.
    • iso- indicates the chain terminates in -CH(CH3)2; neo- that it terminates in -C(CH3)3
classification of c h
Classification of C & H
  • Primary (1°) C: a carbon bonded to one other carbon
    • 1° H: a hydrogen bonded to a 1° carbon
  • Secondary (2°) C: a carbon bonded to two other carbons
    • 2° H: a hydrogen bonded to a 2° carbon
  • Tertiary (3°) C: a carbon bonded to three other carbons
    • 3° H: a hydrogen bonded to a 3° carbon
  • Quaternary (4°) C: a carbon bonded to four other carbons
  • General formula CnH2n
    • five- and six-membered rings are the most common
  • Structure and nomenclature
    • to name, prefix the name of the corresponding open-chain alkane with cyclo-, and name each substituent on the ring
    • if only one substituent, no need to give it a number
    • if two substituents, number from the substituent of lower alphabetical order
    • if three or more substituents, number to give them the lowest set of numbers and then list substituents in alphabetical order
  • Line-angle drawings
    • each line represents a C-C bond
    • each vertex and line ending represents a C
  • Example: name these cycloalkanes







  • Bicycloalkane: an alkane that contains two rings that share two carbons
  • Nomenclature
    • parent is the alkane of the same number of carbons as are in the rings
    • number from a bridgehead, along longest bridge back to the bridgehead, then along the next longest bridge, etc.
    • show the lengths of bridges in brackets, from longest to shortest
iupac general

Nature of Carbon-Carbon

Bonds in the Parent Chain

IUPAC - General
  • prefix-infix-suffix
    • prefix tells the number of carbon atoms in the parent
    • infix tells the nature of the carbon-carbon bonds
    • suffix tells the class of compound







all single bonds




one or more double bonds




one or more triple bonds





-oic acid

carboxylic acid

iupac general25
IUPAC - General

prop-en-e = propene

eth-an-ol = ethanol

but-an-one = butanone

but-an-al = butanal

pent-an-oic acid = pentanoic acid

cyclohex-an-ol = cyclohexanol

eth-yn-e = ethyne

eth-an-amine = ethanamine

  • Conformation: any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond
  • Newman projection: a way to view a molecule by looking along a carbon-carbon single bond
  • Staggered conformation:a conformation about a carbon-carbon single bond in which the atoms or groups on one carbon are as far apart as possible from the atoms or groups on an adjacent carbon
  • Eclipsed conformation: a conformation about a carbon-carbon single bond in which the atoms or groups of atoms on one carbon are as close as possible to the atoms or groups of atoms on an adjacent carbon
  • Torsional strain
    • also called eclipsed interaction strain
    • strain that arises when nonbonded atoms separated by three bonds are forced from a staggered conformation to an eclipsed conformation
    • the torsional strain between eclipsed and staggered ethane is approximately 12.6 kJ (3.0 kcal)/mol
  • Dihedral angle (Q): the angle created by two intersecting planes
  • Ethane as a function of dihedral angle
  • The origin of torsional strain in ethane
    • originally thought to be caused by repulsion between eclipsed hydrogen nuclei
    • alternatively, caused by repulsion between electron clouds of eclipsed C-H bonds
    • theoretical molecular orbital calculations suggest that the energy difference is not caused by destabilization of the eclipsed conformation but rather by stabilization of the staggered conformation
    • this stabilization arises from the small donor-acceptor interaction between a C-H bonding MO of one carbon and the C-H antibonding MO on an adjacent carbon; this stabilization is lost when a staggered conformation is converted to an eclipsed conformation
  • anti conformation
    • a conformation about a single bond in which the groups lie at a dihedral angle of 180°
  • Steric strain(nonbonded interaction strain):
    • the strain that arises when atoms separated by four or more bonds are forced closer to each other than their atomic (contact) radii will allow
  • Angle strain:
    • strain that arises when a bond angle is either compressed or expanded compared to its optimal value
  • The total of all types of strain can be calculated by molecular mechanics programs
    • such calculations can determine the lowest energy arrangement of atoms in a given conformation, a process called energy minimization
  • conformations of butane as a function of dihedral angle
anti butane
Anti Butane
  • Energy-minimized anti conformation
    • the C-C-C bond angle is 111.9° and all H-C-H bond angles are between 107.4 and 107.9°
    • the calculated strain is 9.2 kJ (2.2 kcal)/mol
eclipsed butane
Eclipsed Butane
  • calculated energy difference between (a) the non-energy-minimized and (b) the energy-minimized eclipsed conformations is 5.6 kJ (0.86 kcal)/mol
  • angle strain: the C-C-C bond angles are compressed from 109.5° to 60°
  • torsional strain: there are 6 sets of eclipsed hydrogen interactions
  • strain energy is about 116 kJ (27.7 kcal)/mol
  • puckering from planar cyclobutane reduces torsional strain but increases angle strain
  • the conformation of minimum energy is a puckered “butterfly” conformation
  • strain energy is about 110 kJ (26.3 kcal)/mol
  • puckering from planar cyclopentane reduces torsional strain, but increases angle stain
  • the conformation of minimum energy is a puckered “envelope” conformation
  • strain energy is about 42 kJ (6.5 kcal)/mol
  • Chair conformation: the most stable puckered conformation of a cyclohexane ring
    • all bond C-C-C bond angles are 110.9°
    • all bonds on adjacent carbons are staggered
  • In a chair conformation, six H are equatorial and six are axial
  • For cyclohexane, there are two equivalent chair conformations
    • all C-H bonds equatorial in one chair are axial in the alternative chair, and vice versa
  • Boat conformation: a puckered conformation of a cyclohexane ring in which carbons 1 and 4 are bent toward each other
    • there are four sets of eclipsed C-H interactions and one flagpole interaction
    • a boat conformation is less stable than a chair conformation by 27 kJ (6.5 kcal)/mol
  • Twist-boat conformation
    • approximately 41.8 kJ (5.5 kcal)/mol less stable than a chair conformation
    • approximately 6.3 kJ (1.5 kcal)/mol more stable than a boat conformation
  • Equatorial and axial methyl conformations
g 0 axial equatorial
G0 axial ---> equatorial
  • given the difference in strain energy between axial and equatorial conformations, it is possible to calculate the ratio of conformations using the following relationship
cis trans isomerism
Cis,Trans Isomerism
  • Stereoisomers: compounds that have
    • the same molecular formula
    • the same connectivity
    • a different orientation of their atoms in space
  • Cis,transisomers
    • stereoisomers that are the result of the presence of either a ring (this chapter) or a carbon-carbon double bond (Chapter 5)
  • relationships among isomers
cis trans isomers
Cis,Trans Isomers
  • 1,2-Dimethylcyclopentane
cis trans isomerism52
Cis,Trans Isomerism
  • 1,4-Dimethylcyclohexane
cis trans isomerism53
Cis,Trans Isomerism
  • trans-1,4-Dimethylcyclohexane
    • the diequatorial-methyl chair conformation is more stable by approximately 2 x (7.28) = 14.56 kJ/mol
cis trans isomerism54
Cis,Trans Isomerism
  • cis-1,4-Dimethylcyclohexane
  • The steroid nucleus
  • Cholestanol
  • Norbornane drawn from three different perspectives
  • Adamantane
physical properties
Physical Properties
  • Intermolecular forces of attraction (example)
    • ion-ion (Na+ and Cl- in NaCl)
    • ion-dipole (Na+ and Cl- solvated in aqueous solution)
    • dipole-dipole and hydrogen bonding
    • dispersion forces (very weak electrostatic attraction between temporary dipoles)
physical properties60
Physical Properties
  • Low-molecular-weight alkanes (methane....butane) are gases at room temperature
  • Higher molecular-weight alkanes (pentane, decane, gasoline, kerosene) are liquids at room temperature
  • High-molecular-weight alkanes (paraffin wax) are semisolids or solids at room temperature
physical properties61
Physical Properties
  • Constitutional isomers have different physical properties
oxidation of alkanes
Oxidation of Alkanes
  • Oxidation is the basis for their use as energy sources for heat and power
    • heat of combustion: heat released when one mole of a substance in its standard state is oxidized to carbon dioxide and water
heat of combustion
Heat of Combustion
  • Heat of combustion for constitutional isomers
heats of combustion









Heats of Combustion
  • For constitutional isomers [kJ (kcal)/mol]

-5470.6 (-1307.5)

-5465.6 (-1306.3)

-5458.4 (1304.6)

-5451.8 (1303.0)


heat of combustion65
Heat of Combustion
  • strain in cycloalkane rings as determined by heats of combustion
sources of alkanes
Sources of Alkanes
  • Natural gas
    • 90-95% methane
  • Petroleum
    • gases (bp below 20°C)
    • naphthas, including gasoline (bp 20 - 200°C)
    • kerosene (bp 175 - 275°C)
    • fuel oil (bp 250 - 400°C)
    • lubricating oils (bp above 350°C)
    • asphalt (residue after distillation)
  • Coal
  • Octane rating:the percent 2,2,4-trimethylpentane (isooctane) in a mixture of isooctane and heptane that has equivalent antiknock properties
synthesis gas
Synthesis Gas
  • A mixture of carbon monoxide and hydrogen in varying proportions which depend on the means by which it is produced
synthesis gas69
Synthesis Gas
  • Synthesis gas is a feedstock for the industrial production of methanol and acetic acid
    • it is likely that industrial routes to other organic chemicals from coal via methanol will also be developed