Organic Chemistry

1
Organic Chemistry

• William H. Brown
• Christopher S. Foote
• Brent L. Iverson

2
Alkanes and Cycloalkanes
Chapter 2
3
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

4
Hydrocarbons
5
Structure

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

6
Drawing Alkanes

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

7
Constitutional Isomerism

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

8
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

9
Constitutional Isomerism
World population is about 6,000,000,000
10
Nomenclature - IUPAC

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

11
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-

12
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

13
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

14
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

15
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

16
Nomenclature - IUPAC

• Alkyl groups

17
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

18
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

19
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

20
Cycloalkanes

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

21
Cycloalkanes

• Example name these cycloalkanes

22
Bicycloalkanes

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

23
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

24
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
25
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

26
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

27
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

28
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

29
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

30
Conformations

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

31
Conformations

• Ethane as a function of dihedral angle

32
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

33
Conformations

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

34
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

35
Conformations

• conformations of butane as a function of dihedral
  angle

36
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

37
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

38
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

39
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

40
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

41
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

42
Cyclohexane

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

43
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

44
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

45
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

46
Cyclohexane
47
Methylcyclohexane

• Equatorial and axial methyl conformations

48
?G0 axial ---gt 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

49
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)

50
Isomers

• relationships among isomers

51
Cis,Trans Isomers

• 1,2-Dimethylcyclopentane

52
Cis,Trans Isomerism

• 1,4-Dimethylcyclohexane

53
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

54
Cis,Trans Isomerism

• cis-1,4-Dimethylcyclohexane

55
Cis,Trans Isomerism

• The decalins

56
Steroids

• The steroid nucleus
• Cholestanol

57
Bicycloalkanes

• Norbornane drawn from three different perspectives

58
Bicycloalkanes

• Adamantane

59
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)

60
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

61
Physical Properties

• Constitutional isomers have different physical
  properties

62
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

63
Heat of Combustion

• Heat of combustion for constitutional isomers

64
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)

65
Heat of Combustion

• strain in cycloalkane rings as determined by
  heats of combustion

66
Sources of Alkanes

• Natural gas
• 90-95 methane
• 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)
• Coal

67
Gasoline

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

68
Synthesis Gas

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

69
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

70
Alkanes andCycloalkanes
End Chapter 2