Chapter 2 ppt

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Alkanes and Cycloalkanes
Chapter 2
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Structure

• Hydrocarbon a compound composed only of carbon
  and hydrogen
• Saturated hydrocarbon a hydrocarbon containing
  only single bonds
• Alkane (aliphatic hydrocarbon) a saturated
  hydrocarbon whose carbons are arranged in an open
  chain
• Cycloalkane (alicycic hydrocarbon) an alkane
  whose carbons are arranged in a ring (cyclic
  arrangement)

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Classification of Hydrocarbons, Fig. 2-1
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2.1 Alkane Structure

• Shape
• tetrahedral about carbon
• all bond angles are approximately 109.5

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Drawing Alkanes

• Line-angle formulas
• an abbreviated way to draw structural formulas
• each vertex and line ending represents a carbon

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2.2 Constitutional (Chain) Isomerism

• Constitutional isomers compounds with the same
  molecular formula but a different connectivity of
  their atoms
• example C4H10

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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

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Constitutional Isomerism
World population is about 6,000,000,000
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Nomenclature IUPAC, Table 2-2

• Suffix -ane specifies an alkane
• Prefix tells the number of carbon atoms

Know these alkane names
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Nomenclature - IUPAC, Table 2-3

• 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-

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2.3 A. 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

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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

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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

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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

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Nomenclature - IUPAC, Table 2-3

• Alkyl groups

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B. 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

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C. 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
• Quarternary (4) C a carbon bonded to four other
  carbons

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2.4 Cycloalkanes

• General formula CnH2n
• five- and six-membered rings are the most common
• A. 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

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Cycloalkanes

• Line-angle drawings
• each line represents a C-C bond
• each vertex and line ending represents a C

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Cycloalkanes

• Example name these cycloalkanes

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Bicycloalkanes, Fig. 2-4

• B. Bicycloalkane an alkane that contains two
  rings that share two carbons

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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

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2.5 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
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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

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2.6 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

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A. Conformations, Fig. 2-5

• 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

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Conformations, Fig. 2-6

• 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

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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

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Conformations, Fig. 2-7

• Dihedral angle (Q) the angle created by two
  intersecting planes

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Conformations, Fig. 2-8

• Ethane as a function of dihedral angle

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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

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Conformations

• anti conformation
• a conformation about a single bond in which the
  groups lie at a dihedral angle of 180

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Conformations, Fig. 2-9

• conformations of butane as a function of dihedral
  angle

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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

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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

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Eclipsed Butane, Fig. 2-10

• calculated energy difference between (a) the
  non-energy-minimized and (b) the energy-minimized
  eclipsed conformations is 5.6 kJ (0.86 kcal)/mol

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B. Cyclopropane, Fig. 2-12

• 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

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Cyclobutane, Fig. 2-13

• 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

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Cyclopentane, Fig. 2-14

• 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

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Cyclohexane, Fig. 2-15

• 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

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Cyclohexane

• In a chair conformation, six H are equatorial and
  six are axial

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Cyclohexane, Fig. 2-20

• 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

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Cyclohexane, Fig. 2-18

• 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

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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

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Cyclohexane, Fig. 2-19
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Methylcyclohexane, Fig. 2-21

• Equatorial and axial methyl conformations

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?G0 axial ---gt equatorial, Table 2-4

• given the difference in strain energy between
  axial and equatorial conformations, it is
  possible to calculate the ratio of conformations
  using the following relationship

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Isomers, Fig. 2-22

• Relationships among isomers

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2.7 Cis,Trans Isomerism

• Stereoisomers compounds that have
• the same molecular formula
• the same connectivity
• a different orientation of their atoms in space
• Cis,trans isomers
• stereoisomers that are the result of the presence
  of either a ring (this chapter) or a
  carbon-carbon double bond (Chapter 5)

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A. Cis,Trans Isomers

• 1,2-Dimethylcyclopentane

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Cis,Trans Isomerism

• 1,4-Dimethylcyclohexane

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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

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Cis,Trans Isomerism

• cis-1,4-Dimethylcyclohexane

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B. Cis,Trans Isomerism

• The decalins

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Steroids

• The steroid nucleus
• Cholestanol

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Bicycloalkanes

• Norbornane drawn from three different perspectives

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Bicycloalkanes

• Adamantane

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2.8 Physical Properties, Fig. 2-23

• 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)

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Physical Properties

• Low-molecular-weight alkanes (methane....butane)
  are gases at room temperature
• Intermediate 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

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Physical Properties, Table 2-6

• Constitutional isomers have different physical
  properties

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2.9 Reactions A. 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

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B. Heat of Combustion, Table 2-7

• Heat of combustion for constitutional isomers

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Heats of Combustion, Fig. 2-24

• For constitutional isomers kJ (kcal)/mol

-5470.6 (-1307.5)
-5465.6 (-1306.3)
-5458.4 (1304.6)
-5451.8 (1303.0)

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Heat of Combustion, Fig. 2-25

• strain in cycloalkane rings as determined by
  heats of combustion

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2.10 Sources of Alkanes

• A. Natural gas
• 90-95 methane
• B. Petroleum
• gases (bp below 20C)
• naphthas, including gasoline (bp 20 - 200C)
• kerosene (bp 175 - 275C)
• fuel oil (bp 250 - 400C)
• lubricating oils (bp above 350C)
• asphalt (residue after distillation)
• C. Coal

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Gasoline

• Octane rating the percent 2,2,4-trimethylpentane
  (isooctane) in a mixture of isooctane and heptane
  that has equivalent antiknock properties

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Synthesis Gas

• A mixture of carbon monoxide and hydrogen in
  varying proportions which depend on the means by
  which it is produced

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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

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Alkanes andCycloalkanes
End Chapter 2