Unit 2

1
Unit 2 Alkanes and Chemical Reactions

• Structure and Stereochemistry of Alkanes
• Nomenclature of alkanes and cycloalkanes
• Physical Properties
• Conformational Analysis
• The Study of Chemical Reactions
• Kinetics and Thermodynamic Quantities
• Free Radical Halogenation
• Reactive Intermediates and Transition States

2
Hydrocarbons

• The simplest organic compounds are the
  hydrocarbons
• organic compounds that contain only carbon and
  hydrogen
• four general types
• alkanes
• alkenes
• alkynes
• aromatic hydrocarbons

3
Hydrocarbons

• Alkanes are often called saturated hydrocarbons
• they contain the maximum number of hydrogen atoms
  per carbon atom.
• Alkenes, alkynes, and aromatic hydrocarbons are
  called unsaturated hydrocarbons
• they contain fewer H atoms than an alkane with
  the same number of carbon atoms

4
Alkanes

• You must know the names and formulas for the 10
  simplest alkanes
• CH4 methane
• CH3CH3 ethane
• CH3CH2CH3 propane
• CH3CH2CH2CH3 butane
• CH3CH2CH2CH2CH3 pentane
• CH3CH2CH2CH2CH2CH3 hexane

5
Alkanes

• You must know the names and formulas for the 10
  simplest alkanes
• CH3CH2CH2CH2CH2CH2CH3 heptane
• CH3CH2CH2CH2CH2CH2CH2CH3 octane
• CH3CH2CH2CH2CH2CH2CH2CH2CH3 nonane
• CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3 decane

6
Alkanes

• The alkanes form a homologous series with a
  general molecular formula of CnH(2n2)
• Homologous series
• a series of compounds in which each member
  differs from the next member by a constant unit
• Alkanes differ from each other by -CH2-

7
Alkanes

• Example Which of the following are alkanes
• C2H6, C3H6, C5H12, C4H8

Example What is the formula for an alkane with
12 carbons?
8
Alkanes

• The previous alkanes are straight-chain alkanes
• all of the carbon atoms are joined in a
  continuous chain
• also called normal alkanes (n-alkanes)
• Alkanes containing 4 or more carbons can also
  form branched alkanes
• one or more of the carbon atoms form a branch
  or side-chain off of the main chain

9
Alkanes

• An example of a straight chain alkane
• C5H12 pentane
• Examples of branched alkanes
• C5H12

2-methylbutane
2,2-dimethylpropane
10
Alkanes

• The three structures shown previously for C5H12
  are structural (constitutional) isomers
• compounds with the same molecular formula but
  different bonding arrangements
• Structural isomers have different properties
• different melting points
• different boiling points
• often different chemical reactivity

11
Alkane Nomenclature

• Organic compounds can be named either using
  common names or IUPAC names.

pentane
2-methylbutane or isopentane
2,2-dimethylpropane or neopentane
12
Alkane Nomenclature
The common name for any alkane containing a CH3
group on the second carbon in the chain is
isoalkane.
iso
Isobutane (4 C total)
Isohexane (6 C total)
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Alkane Nomenclature

• Most of the time, organic chemists use the IUPAC
  names for organic compounds.
• LEARN THE RULES FOR EACH CLASS OF COMPOUNDS WE
  DISCUSS.

14
Alkane Nomenclature

• To name an alkane
• Find the longest continuous chain of carbon atoms
  and use the name of that chain as the base name
  of the compound
• the longest chain is often NOT written in a
  straight line

Base name heptane
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Alkane Nomenclature

• Number the carbon atoms in the longest chain
  starting at the end of the chain closest to a
  substituent
• a group attached to the main chain that has taken
  the place of a hydrogen atom on the main chain

A substituent
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Alkane Nomenclature

• Name and give the location of each substituent.
• Common substituents
• Halo group
• a halogen atom
• Halo groups are named using halo
• Cl chloro
• Br bromo
• I iodo
• F fluoro
• Nitro group
• -NO2

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Alkane Nomenclature

• Common substituents
• alkyl group
• A group that is formed by removing an H atom from
  an alkane
• the alkyl group attaches to the main chain at the
  carbon that has lost its H

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Alkane Nomenclature

• Alkyl groups are named by replacing the ane
  ending of the parent alkane with the yl ending.

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Alkane Nomenclature
20
Alkane Nomenclature
21
Alkane Nomenclature

• Complex alkyl substituents
• Use the longest alkyl chain of the substituent as
  the base name of the substituent
• Number the substituent chain with the head
  carbon as carbon 1
• List substituents on the alkyl chain with the
  appropriate numbers
• Use parentheses around the name of the group

22
Alkane Nomenclature
Methyl group
3-methylheptane
Note Separate numbers from letters using a
hyphen. Separate groups of numbers using commas.
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Alkanes

• Alkane Nomenclature
• When two or more substituents are present, list
  them in alphabetical order
• isopropyl before methyl
• t-butyl or sec-butyl before chloro
• When more than one of the same substituent is
  present (i.e. two methyl groups), use prefixes to
  indicate how many. Give the location of each as
  well.
• Di two
• Tri three
• Tetra four
• Penta five
• Hexa six

Know these.
Note Ignore these prefixes when alphabetizing.
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Alkane Nomenclature

• Additional rules
• When there are two longest chains of equal
  length, use the chain with the greater number of
  substituents.

correct
incorrect
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Alkane Nomenclature

• Additional rules
• If each end of the longest chain has a
  substituent the same distance from the end, start
  with the end nearer to the second substituent.

correct
incorrect
3-chloro-2,5-dimethylhexane
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Alkanes

• Example Name the following compounds

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Alkanes

• Example Name the following compound

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Alkanes

• You must also be able to write the structure of
  an alkane when given the IUPAC name.
• Identify the main chain and draw the carbons in
  it.
• Identify the substituents (type and ) and attach
  them to the appropriate carbon atoms on the main
  chain.
• Add hydrogen atoms to the carbons to make a total
  of 4 bonds to each carbon

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Alkanes

• Example Draw the structure for the following
  compounds
• 3, 3-dimethylpentane
• 4-sec-butyl-2-methyloctane
• 1,2-dichloro-3-methylheptane
• 2-nitropropane

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Alkane Nomenclature

• Example Draw the structural isomers of hexane
  (C6H14). Name each isomer.
• Use a systematic approach to draw structural
  isomers
• Draw the unbranched isomer for the first
  structure.
• For other structures, remove 1 or more carbons
  (and/or functional groups) from the unbranched
  isomer and reposition to make unique compounds

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Types of Carbon Atoms

• Primary carbon (1o)
• a carbon bonded to
• one other carbon
• Secondary carbon (2o)
• a carbon bonded to
• two other carbons
• Tertiary carbon (3o)
• a carbon bonded to
• three other carbons

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Physical Properties

• Solubility
• Alkanes are nonpolar
• hydrophobic
• do not dissolve in water
• soluble in nonpolar or weakly polar organic
  solvents
• Density
• varies from 0.5 - 0.8 g/mL
• less dense than water (1.0 g/mL)
• Alkanes float on water

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Physical Properties

• Boiling Point
• In general, boiling point increases as the
  molecular weight of the alkane increases
• larger molecules have greater surface area and
  higher London dispersion forces of attraction
• must be overcome for vaporization and boiling to
  occur

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Physical Properties

• Boiling Point (cont)
• Given the same total number of carbon atoms
• BP (branched) lt BP (n-alkane)
• Branched alkanes are more compact.
• less surface area
• smaller London dispersion forces
• lower BP

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Physical Properties

• Melting Points
• In general, melting point increases as MW
  increases
• irregular, sawtooth pattern

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Physical Properties

• Melting Point
• Alkanes with odd number of carbons have lower
  than expected melting points (compared to the
  previous alkane with an even number of carbons)
• Even carbons
• better packing in solid structure
• higher MP
• Odd carbons
• do not pack as well
• lower MP

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Physical Properties

• Melting Points
• Given the same total number of carbon atoms
• MP (branched) gt MP (n-alkane)
• branched alkanes have more compact structure
• better packing
• higher MP

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Sources Uses of Alkanes

• Alkanes are derived primarily from petroleum and
  petroleum by-products
• Refining via fractional distillation gives useful
  mixtures of alkanes
• C2 - C4 liquified petroleum gas
• C4 - C9 gasoline
• C8 - C16 kerosene
• C10 - C18 diesel
• C16 heavy/mineral oil

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Reactions of Alkanes

• Catalytic Cracking
• converts alkanes into more valuable mixtures of
  smaller alkanes and alkenes
• alkane smaller alkanes
  alkenes
• C12H26

D
SiO2 or Al2O3 catalyst
D
SiO2

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Reactions of Alkanes

• Hydrocracking
• converts higher alkanes into shorter alkanes and
  eliminates N and S impurities
• Alkane shorter
  alkanes
• C12H26

D
H2, SiO2 or Al2O3 catalyst
D

H2, SiO2
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Reactions of Alkanes

• Combustion
• a rapid, exothermic redox reaction that converts
  hydrocarbons into carbon dioxide and water
• alkane O2 CO2 H2O
• 2 C6H14 19 O2 12 CO2 14 H2O

(unbalanced)
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Reactions of Alkanes

• Halogenation
• a reaction in which a halogen atom is substituted
  for a hydrogen atom on an alkane
• alkane X2 mixture of alkyl
  halides
• CH4 Cl2 CH3Cl CH2Cl2 CHCl3 CCl4

D or hu
hu
unbalanced
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Conformations of Alkanes

• The simplest alkane, CH4, is perfectly
  tetrahedral
• bond angle 109.5
• C-H bond length 1.09 A
• free rotation of the C-H bond

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Conformations of Alkanes

• Ethane
• Two carbons
• overlapping sp3 hybrid orbitals form a sigma bond

45
Conformations of Alkanes

• The two methyl groups are relatively free to
  rotate about the sigma bond between the two
  carbon atoms
• sigma bond maintains its overlap at all times
• The different arrangements formed by rotation
  around a single bond are called conformations.
• Conformer
• a specific conformation
• a conformational isomer

46
Conformations of Alkanes

• Conformers are often drawn using Newman
  projections
• a way of drawing a molecule looking straight down
  the bond connecting two carbon atoms
• front carbon atom is represented by three lines
  joined together in a Y shape
• back carbon is represented by a circle with three
  bonds pointing out from it

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Conformations of Alkanes
View from this end

3-D structure of one conformer of ethane
Newman projection
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Conformations of Alkanes

• An infinite number of conformations are possible
  for ethane and higher alkanes.
• The dihedral angle (q) can have an infinite
  number of values
• angle between the C-H bonds on the front and back
  carbons

q
49
Conformations of Alkanes
Important conformations for ethane
Eclipsed conformation
Staggered conformation
Skew conformation
Molecules are constantly rotating through all
possible conformations.
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Conformations of Alkanes

• The conformation of ethane changes constantly at
  room temperature.
• Conformations may have different energies.
• Lowest energy conformer is most favored.
• Highest energy conformer is least favored.
• Conformational analysis
• the study of the energies of different
  conformations
• helps predict which conformation are favored and
  which reaction may occur

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Conformations of Alkanes

• Staggered conformation of ethane
• lowest energy most favored
• electron clouds in the C - H bonds separated as
  much as possible
• Eclipsed conformation of ethane
• highest energy least favored
• electron clouds of C - H bonds are closest
  together

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Conformations of Alkanes

• As ethane rotates from the staggered conformation
  towards the eclipsed conformation
• potential energy increases due to torsional
  strain
• resistance to rotation or twisting about a bond

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Conformations of Alkanes
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Conformation of Alkanes

• Butane
• 4 carbon chain held together by end-to-end
  overlap of sp3 hybrid orbitals on the carbon
  atoms
• tetrahedral geometry around each carbon
• free rotation about any C - C bond
• many conformers of differing energies are
  possible
• Newman projections of butane are drawn by looking
  down the central C2 - C3 bond.

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Conformations of Alkanes
Totally eclipsed (0o)
Gauche (60o)
Eclipsed (120o)
Anti (180o)
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Conformation of Alkanes

• Totally eclipsed conformer of butane
• highest energy due to steric hinderance between
  the methyl groups
• Steric hinderance
• interference between two bulky groups that are
  close enough together so that their electron
  clouds repel each other

57
Conformations of Alkanes
58
Conformations of Alkanes

• Eclipsed conformer of butane
• second highest in energy due to repulsion of the
  methyl group on one carbon and the hydrogen atom
  on the other
• All staggered conformers (gauche and anti) of
  butane are lower in energy than any of the
  eclipsed conformers.
• Anti conformer of butane
• lowest energy because methyl groups are furthest
  apart

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Conformations of Alkanes