Unit 2 Alkanes and Chemical Reactions

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
• hydrocarbons that contain only single bonds
• Examples
• Methane CH4
• ethane H H
• H C C H
• H H

4
Hydrocarbons

• Alkenes
• hydrocarbons that contain a C C double bond
• H2C CH2 (ethylene)
• Alkynes
• hydrocarbons that contain a C C triple bond
• H C C H (acetylene)

5
Hydrocarbons

• Aromatic hydrocarbons
• contain a planar ring structure in which the
  carbon atoms are connected by a combination of
  both s and p bonds

benzene
6
Hydrocarbons

• Alkanes are often called saturated hydrocarbons
• they contain the largest possible number of
  hydrogen atoms per carbon atom.
• Alkenes, alkynes, and aromatic hydrocarbons are
  called unsaturated hydrocarbons
• they contain less hydrogen than an alkane having
  the same number of carbon atoms

7
Alkanes

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

8
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-
• Homolog
• a member of a homologous series

9
Alkanes

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

Example What is the formula for an alkane with
12 carbons?
10
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

11
Alkanes

• An example of a straight chain alkane
• C5H12 CH3CH2CH2CH2CH3 pentane
• Examples of branched alkanes
• C5H12 CH3CHCH2CH3 CH3
• CH3 CH3CCH3
• CH3

2-methylbutane
2,2-dimethylpropane
12
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

13
Alkane Nomenclature

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

pentane
2-methylbutane or isopentane
2,2-dimethylpropane or neopentane
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Alkane Nomenclature
Any alkane containing a CH3 group on the second
carbon in the chain can be named as an 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.

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Alkane Nomenclature

• Alkane Nomenclature
• 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

CH3 - CH - CH3 CH2 - CH2 - CH2 - CH3
Base name hexane
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Alkane Nomenclature

• Alkane Nomenclature
• Number the carbon atoms in the longest chain
  beginning with 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

1
A substituent
2
CH3 - CH - CH3 CH2 - CH2 - CH2 - CH3
3
4
5
6
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Alkane Nomenclature

• Alkane Nomenclature
• Name and give the location of each substituent
  group
• Common substituents
• alkyl groups
• 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
• Halo group
• a halogen atom

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Alkane Nomenclature

• Halo groups are named using halo
• Cl chloro
• Br bromo
• I iodo
• F fluoro
• Alkyl groups are named by replacing the ane
  ending of the parent alkane with the yl ending

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Alkane Nomenclature
You must know the names and formulas for the
following common alkyl groups
21
IUPAC (Systematic) Names for Branched Alkyl Groups
propyl
(1-methylethyl)
22
Alkane Nomenclature

• You must know the common (nonsystematic) names
  and formulas for the following branched alkyl
  groups

butyl
butane
sec-butyl
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IUPAC Systematic Names for Branched Alkyl Groups
(1-methylpropyl)
(2,2-dimethylpropyl)
(1,1-dimethylethyl)
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Alkane Nomenclature

• You must know the common (nonsystematic) names
  and formulas for the following common branched
  alkyl groups

isobutyl
isobutane
tert-butyl or t-butyl
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Alkane Nomenclature

• You must know the common (nonsystematic) names
  and formulas for the following branched alkyl
  groups

neopentane
neopentyl
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Alkanes

• Alkane Nomenclature

1
Methyl group
2
CH3 - CH - CH3 CH2 - CH2 - CH2 - CH3
3
4
5
6
2-methylhexane
Note The position number and the name are
separated by a hyphen.
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Alkanes

• Alkane Nomenclature
• When two or more substituents are present, list
  them in alphabetical order
• isopropyl before methyl
• 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.

incorrect
correct
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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 branch point.

6
2
5
1
3
4
5
6
1
2
3
4
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 compounds

2-bromo-3-methylpentane
1-bromo-3-ethyl- 3,4-dimethylpentane
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Alkanes

• Example Name the following compound

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Alkanes
2,2,3,6 tetramethyl-4-(1-methylethyl)octane
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Alkanes

• You must also be able to write the structure of
  an alkane when given the IUPAC name.
• To do so
• 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 Write the condensed structure for the
  following compounds
• 3, 3-dimethylpentane
• 2-methyl-4-sec-butyloctane
• 1, 2-dichloro-3-methylheptane

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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