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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

slide2

Alkanes

and

Cycloalkanes

Chapter 2

structure
Structure
  • 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
structure5
Structure
  • 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

is about

6,000,000,000

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
cycloalkanes
Cycloalkanes
  • 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
cycloalkanes20
Cycloalkanes
  • Line-angle drawings
    • each line represents a C-C bond
    • each vertex and line ending represents a C
cycloalkanes21
Cycloalkanes
  • Example: name these cycloalkanes
bicycloalkanes

Bicyclo[4.4.0]decane

Bicyclo[4.3.0]nonane

Bicyclo[2.2.1]heptane

(Decalin)

(Hydrindane)

(Norbornane)

Bicycloalkanes
  • Bicycloalkane: an alkane that contains two rings that share two carbons
bicycloalkanes23
Bicycloalkanes
  • 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

Suffix

Class

Infix

-e

hydrocarbon

-an-

all single bonds

-ol

alcohol

-en-

one or more double bonds

-al

aldehyde

-yn-

one or more triple bonds

-amine

amine

-one

ketone

-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

conformations
Conformations
  • 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
conformations27
Conformations
  • 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
conformations28
Conformations
  • 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
conformations29
Conformations
  • 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
conformations30
Conformations
  • Dihedral angle (Q): the angle created by two intersecting planes
conformations31
Conformations
  • Ethane as a function of dihedral angle
conformations32
Conformations
  • 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
conformations33
Conformations
  • anti conformation
    • a conformation about a single bond in which the groups lie at a dihedral angle of 180°
conformations34
Conformations
  • 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
conformations35
Conformations
  • 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
cyclopropane
Cyclopropane
  • 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
cyclobutane
Cyclobutane
  • 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
cyclopentane
Cyclopentane
  • 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
cyclohexane
Cyclohexane
  • 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
cyclohexane42
Cyclohexane
  • In a chair conformation, six H are equatorial and six are axial
cyclohexane43
Cyclohexane
  • 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
cyclohexane44
Cyclohexane
  • 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
cyclohexane45
Cyclohexane
  • 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
methylcyclohexane
Methylcyclohexane
  • 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)
isomers
Isomers
  • 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
steroids
Steroids
  • The steroid nucleus
  • Cholestanol
bicycloalkanes57
Bicycloalkanes
  • Norbornane drawn from three different perspectives
bicycloalkanes58
Bicycloalkanes
  • 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

8

C

O

9

H

O

2

2

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
gasoline
Gasoline
  • 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