3. Organic Compounds: Alkanes and Their Stereochemistry

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3. Organic Compounds Alkanes and Their
Stereochemistry

• Based on
• McMurrys Organic Chemistry, 7th edition, Chapter
  3

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Families of Organic Compounds

• Organic compounds can be grouped into families by
  their common structural features
• We shall survey the nature of the compounds in a
  tour of the families in this course
• This chapter deals with alkanes, compounds that
  contain only carbons and hydrogens, all connected
  exclusively by single bonds

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3.1 Functional Groups

• Functional group - collection of atoms at a site
  within a molecule with a common bonding pattern
• The group reacts in a typical way, generally
  independent of the rest of the molecule
• For example, the double bonds in simple and
  complex alkenes react with bromine in the same
  way (See Figure 3.1)

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Double Bond as Functional Group
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Survey of Functional Groups

• Table 3.1 lists a wide variety of functional
  groups that you should recognize
• As you learn about them in each chapter it will
  be easier to recognize them
• The functional groups affect the reactions,
  structure, and physical properties of every
  compound in which they occur

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Types of Functional Groups Multiple
CarbonCarbon Bonds

• Alkenes have a C-C double bond
• Alkynes have a C-C triple bond
• Arenes (or aromatic hydrocarbons) have special
  bonds that are represented as alternating single
  and double C-C bonds in a six-membered ring

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Multiple CarbonCarbon Bonds
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Functional Groups with Carbon Singly Bonded to an
Electronegative Atom

• Alkyl halide C bonded to halogen (C-X)
• Alcohol C bonded O of a hydroxyl group (C?OH)
• Ether Two Cs bonded to the same O (C?O?C)
• Amine C bonded to N (C?N)
• Thiol C bonded to SH group (C?SH)
• Sulfide Two Cs bonded to same S (C?S?C)
• Bonds are polar, with partial positive charge on
  C (?) and partial negative charge (??) on
  electronegative atom

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Groups with a CarbonOxygen Double Bond (Carbonyl
Groups)

• Aldehyde one hydrogen bonded to CO
• Ketone two Cs bonded to the CO
• Carboxylic acid ?OH bonded to the CO
• Ester C-O bonded to the CO
• Amide C-N bonded to the CO
• Acid chloride Cl bonded to the CO
• Carbonyl C has partial positive charge (?)
• Carbonyl O has partial negative charge (?-).

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Problem 3.1 Identify the Functional Groups
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3.2 Alkanes and Alkane Isomers

• Alkanes Compounds with C-C single bonds and C-H
  bonds only (no functional groups)
• The formula for an alkane with no rings (acyclic)
  must be CnH2n2 where the number of Cs is n
• Alkanes are saturated with hydrogen (no more can
  be added)
• They are also called aliphatic compounds

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A saturated fat (glyceryl stearate)
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Alkane Isomers

• CH4 methane, C2H6 ethane, C3H8 propane
• The molecular formula of an alkane with more than
  three carbons can give more than one structure
• C4 (butane) butane and isobutane
• C5 (pentane) pentane, 2-methylbutane
  (isopentane), and 2,2-dimethylpropane (neopentane)

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Methane, ethane, propane
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Butanes (C4H10)
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Pentanes (C5H12)
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Alkane Isomers
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Alkane Isomers

• Alkanes with Cs connected to no more than 2
  other Cs are straight-chain or normal alkanes
• Alkanes with one or more Cs connected to 3 or 4
  Cs are branched-chain alkanes

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Constitutional (Structural) Isomers

• Isomers that differ in how their atoms are
  arranged in chains are called constitutional (or
  structural) isomers
• Compounds other than alkanes can be
  constitutional isomers of one another
• They must have the same molecular formula to be
  isomers

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Constitutional (Structural) Isomers
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Condensed Structures of Alkanes

• We can represent an alkane in a brief form or in
  many types of extended form
• A condensed structure does not show bonds but
  lists atoms, such as
• CH3CH2CH3 (propane)
• CH3C(CH3)2CH3 (2,2-dimethylpropane)

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Condensed Structures of Alkanes
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3.3 Alkyl Groups

• Alkyl group remove one H from an alkane (a part
  of a structure)
• General abbreviation R (for Radical, an
  incomplete species or the rest of the molecule)
• Name replace -ane ending of alkane with -yl
  ending
• ?CH3 is methyl (from methane)
• ?CH2CH3 is ethyl from ethane

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3.3 Alkyl Groups
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3.3 Alkyl Groups
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Alkyl Groups
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Types of Alkyl groups

• a carbon at the end of a chain (primary alkyl
  group)
• a carbon in the middle of a chain (secondary
  alkyl group)
• a carbon with three carbons attached to it
  (tertiary alkyl group)

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Types of Alkyl groups
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Types of Alkyl groups
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Types of Hydrogens
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Problem 3.8 Primary, secondary, tertiary, or
quaternary carbons?
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3.4 Naming Alkanes IUPAC

• Compounds are given systematic names by a process
  that uses

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3.4 Naming Alkanes IUPAC

• Follows specific rules
• Named as longest continuous chain of Cs
• Carbons in that chain are numbered in sequence
• Substituents are numbered at their point of
  attachment
• Compound name is one word (German style)
• Complex substituents are named similarly

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1. Find the Parent longest continuous carbon
chain
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2. Number the atoms in the chain
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3. Identify number the substituents
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4. Write the name
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Practice prob. 3.2 IUPAC name?
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Solution
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Practice prob. 3.3 structure?

• 3-isopropyl-2-methylhexane
• C-C-C-C-C-C
• Two substituents isopropyl methyl
• Add hydrogens to complete the structure

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Solution
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Problem 3.11 IUPAC names?
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Problem 3.39 Draw structures

• 2-methylheptane
• 4-ethyl-2,2-dimethylhexane
• 4-ethyl-3,4-dimethyloctane
• 2,4,4-trimethylheptane
• 3,3-diethyl-2,5-dimethylnonane
• 4-isopropyl-3-methylheptane

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3.5 Properties of Alkanes

• Called paraffins (low affinity compounds) because
  they are relatively unreactive
• They will burn in a flame, producing carbon
  dioxide, water, and heat

CH4 2 O2 ? CO2 2 H2O heat
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3.5 Properties of Alkanes

• They react with Cl2 in the presence of light to
  replace Hs with Cls (not easily controlled)

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Physical Properties

• Boiling points and melting points increase as
  size of alkane increases
• Forces between molecules (temporary dipoles,
  dispersion) are weak

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Physical Properties
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3.6 Conformations of Ethane

• Stereochemistry concerned with the 3-D aspects of
  molecules
• ? bonds are cylindrically symmetrical
• Rotation is possible around C-C bonds in
  open-chain molecules

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Conformers

• Conformation- Different arrangement of atoms
  resulting from bond rotation
• Conformations can be represented in 2 ways

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Conformations of Ethane
staggered conformation
eclipsed conformation
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Conformations of Ethane
staggered conformation
eclipsed conformation
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Representing Conformations

• Sawhorse representations show molecules at an
  angle, showing a molecular model
• C-C bonds are at an angle to the edge of the page
  and all C-H bonds are shown
• Newman projections show how the C-C bond would
  project end-on onto the paper
• Bonds to front carbon are lines going to the
  center
• Bonds to rear carbon are lines going to the edge
  of the circle

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Newman Projections
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Ethanes Conformations
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Torsional Strain

• We do not observe perfectly free rotation
• There is a barrier to rotation, and some
  conformers are more stable than others
• Staggered- most stable all 6 C-H bonds are as
  far away as possible
• Eclipsed- least stable all 6 C-H bonds are as
  close as possible to each other

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3.7 Conformations of Other Alkanes

• The eclipsed conformer of propane has 3
  interactions two ethane-type H-H interactions,
  and one H-CH3 interaction

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Conformations of Other Alkanes
Conformational situation is more complex for
larger alkanes Not all staggered conformations
has same energy, and not all eclipsed
conformations have same energy
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Conformations of Butane

• Anti conformation- methyl groups are 180 apart
• Gauche conformation- methyl groups are 60 apart
• Which is the most energetically stable?

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Conformations of Butane
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Steric Strain

• Steric strain- repulsive interaction occurring
  between atoms that are forced closer together
  than their atomic radii allow

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Staggered Conformations of Butane
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Conformations of Butane
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Conformations of Butane
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Eclipsed Conformations of Butane
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Gauche conformation steric strain
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1-chloropropane
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Hydrocarbon Chains Staggered
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Gasoline Octane Ratings