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3. Organic Compounds: Alkanes and Cycloalkanes

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Title: 3. Organic Compounds: Alkanes and Cycloalkanes


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2.12 Drawing Chemical Structures
  • Representations of Structural Formulas
  • Dot formulas are more cumbersome to draw than
    dash formulas and condensed formulas
  • Lone-pair electrons are often (but not always)
    drawn in, especially when they are crucial to
    the chemistry being discussed

3
  • Dash formulas
  • Each dash represents a pair of electrons
  • This type of representation is meant to emphasize
    connectivity and does not represent the
    3-dimensional nature of the molecule

4
  • Condensed structures C-H and C-C and single
    bonds aren't shown but understood
  • If C has 3 Hs bonded to it, write CH3
  • If C has 2 Hs bonded to it, write CH2 and so
    on. The compound called 2-methylbutane, for
    example, is written as follows
  • Horizontal bonds between carbons aren't shown in
    condensed structuresthe CH3, CH2, and CH units
    are simply but vertical bonds are added for
    clarity

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  • Condensed Structural Formulas
  • In these representations, some or all of the dash
    lines are omitted
  • In partially condensed structures all hydrogens
    attached to an atom are simply written after it
    but some or all of the other bonds are explicitly
    shown
  • In fully condensed structure all bonds are
    omitted and atoms attached to carbon are written
    immediately after it
  • For emphasis, branching groups are often written
    using vertical lines to connect them to the main
    chain

7
  • Three-Dimensional Formulas
  • The conventions for this are
  • Bonds that lie in the plane of the paper are
    indicated by a simple line
  • Bonds that come forward out of the plane of the
    paper are indicated by a solid wedge
  • Bonds that go back out of the plane of the paper
    are indicated by a dashed wedge

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Classes of Compounds
  • Classification based on functional group
  • Three broad classes
  • Hydrocarbons
  • Compounds containing oxygen
  • Compounds containing nitrogen
    gt

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Hydrocarbons
  • Alkane single bonds, sp3 carbons
  • Cycloalkane carbons form a ring
  • Alkene double bond, sp2 carbons
  • Cycloalkene double bond in ring
  • Alkyne triple bond, sp carbons
  • Aromatic contains a benzene ring
    gt

11
Compounds Containing Oxygen
  • Alcohol R-OH
  • Ether R-O-R'
  • Aldehyde RCHO
  • Ketone RCOR'

12
Cyclic ethers and ketones
THF (tetrahydrofuran)
cyclopentanone
13
Carboxylic Acids and Their Derivatives
  • Carboxylic Acid RCOOH
  • Acid Chloride RCOCl
  • Ester RCOOR'
  • Amide RCONH2

14
Cyclic esters
Cyclic ester
(cyclic ether and cyclic ketone)
15
Compounds Containing Nitrogen
  • Amines RNH2, RNHR', or R3N
  • Amides RCONH2, RCONHR, RCONR2
  • Nitrile RCN

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Summary of Important Families of Organic
Compounds
17
  • Summary (cont.)

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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 in it
    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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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, and
    2,2-dimethylpropane
  • 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 Isomers
  • Isomers that differ in how their atoms are
    arranged in chains are called constitutional
    isomers
  • They must have the same molecular formula to be
    isomers

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3.4 Naming Alkanes
  • Compounds are given systematic names by a process
    that uses
  • Prefix-Parent-Suffix
  • Follows specific rules
  • Named as longest possible chain
  • 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 as compounds would
    be

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Names of Small Hydrocarbons
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  • Nomenclature of Branched-Chain Alkanes (IUPAC)
  • Locate the longest continuous chain of carbons
    this is the parent chain and determines the
    parent name.
  • Number the longest chain beginning with the end
    of the chain nearer the substituent
  • Designate the location of the substituent
  • When two or more substituents are present, give
    each substituent a number corresponding to its
    location on the longest chain
  • Substituents are listed alphabetically

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  • When two or more substituents are identical, use
    the prefixes di-, tri-, tetra- etc.
  • Commas are used to separate numbers from each
    other
  • The prefixes are used in alphabetical
    prioritization
  • When two chains of equal length compete to be
    parent, choose the chain with the greatest number
    of substituents
  • When branching first occurs at an equal distance
    from either end of the parent chain, choose the
    name that gives the lower number at the first
    point of difference

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3.3 Alkyl Groups
  • Alkyl group remove one H from an alkane (a part
    of a structure)
  • Naming replace -ane ending of alkane with -yl
    ending
  • ?CH3 is methyl (from methane)
  • ?CH2CH3 is ethyl from ethane
  • See Table 3.4 for a list

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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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3.6 Cycloalkanes
  • Cycloalkanes are alkanes that have carbon atoms
    that form a ring (called alicyclic compounds)
  • Simple cycloalkanes rings of ?CH2? units, (CH2)n,
    or CnH2n
  • Structure is shown as a regular polygon with the
    number of vertices equal to the number of Cs (a
    projection of the actual structure)

cyclobutane
cyclohexane
cyclopentane
cyclopropane
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Complex Cycloalkanes
  • Naturally occurring materials contain cycloalkane
    structures
  • Examples chrysanthemic acid (cyclopropane),
    prostaglandins (cyclopentane), steroids
    (cyclohexanes and cyclopentane)

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6. Alkenes Structure and Reactivity
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Alkene - Hydrocarbon With Carbon-Carbon Double
Bond
  • Also called an olefin but alkene is better
  • Includes many naturally occurring materials
  • Flavors, fragrances, vitamins

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6.3 Naming of Alkenes
  • Find longest continuous carbon chain containing
    the double bond and name the compound using the
    suffix ene
  • Number carbons in chain so that double bond
    carbons have lowest possible numbers
  • Rings have cyclo prefix

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Names of Small Alkene
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Many Alkenes Are Known by Common Names
  • Ethylene ethene
  • Propylene propene
  • Isobutylene 2-methylpropene
  • Isoprene 2-methyl-1,3-butadiene

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6.3 Degree of Unsaturation
  • Relates molecular formula to possible structures
  • Degree of unsaturation number of multiple bonds
    or rings
  • Formula for saturated a acyclic compound is
    CnH2n2
  • Each ring or multiple bond replaces 2 H's

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Example C6H10
  • Saturated is C6H14
  • Therefore 4 H's are not present
  • This has two degrees of unsaturation
  • Two double bonds?
  • or triple bond?
  • or two rings
  • or ring and double bond

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Degree of Unsaturation With Other Elements
  • Organohalogens (X F, Cl, Br, I)
  • Halogen replaces hydrogen
  • C4H6Br2 and C4H8 have one degree of unsaturation
  • Oxygen atoms - if connected by single bonds
  • These don't affect the total count of H's

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Degree of Unsaturation and Variation
  • Compounds with the same degree of unsaturation
    can have many things in common and still be very
    different

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If C-N Bonds Are Present
  • Nitrogen has three bonds
  • So if it connects where H was, it adds a
    connection point
  • Subtract one H for equivalent degree of
    unsaturation in hydrocarbon

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Summary - Degree of Unsaturation
  • Count pairs of H's below CnH2n2
  • Add number of halogens to number of H's (X
    equivalent to H)
  • Don't count oxygens (oxygen links H)
  • Subtract N's - they have two connections

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6.4 Cis-Trans Isomerism in Alkenes
  • The presence of a carbon-carbon double can create
    two possible structures
  • cis isomer - two similar groups on same side of
    the double bond
  • trans isomer similar groups on opposite sides
  • Each carbon must have two different groups for
    these isomers to occur

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Cis, Trans Isomers Require That End Groups Must
Differ in Pairs
  • 180rotation superposes
  • Bottom pair cannot be superposed without breaking
    CC

X
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6.6 Sequence Rules The E,Z Designation
  • Neither compound is clearly cis or trans
  • Substituents on C1 are different than those on C2
  • We need to define similarity in a precise way
    to distinguish the two stereoisomers
  • Cis, trans nomenclature only works for
    disubstituted double bonds

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Develop a System for Comparison of Priority of
Substituents
  • Assume a valuation system
  • If Br has a higher value than Cl
  • If CH3 is higher than H
  • Then, in A, the higher value groups are on
    opposite sides
  • In B, they are on the same side
  • Requires a universally accepted valuation

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E,Z Stereochemical Nomenclature
  • Priority rules of Cahn, Ingold, and Prelog
  • The Cahn-Ingold-Prelog convention is used to
    assign the groups of highest priority on each
    carbon
  • Compare where higher priority group is with
    respect to bond and designate as prefix
  • E -entgegen, opposite sides
  • Z - zusammen, together on the same side

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Ranking Priorities Cahn-Ingold-Prelog Rules
  • Must rank atoms that are connected at comparison
    point
  • Higher atomic number gets higher priority
  • Br gt Cl gt O gt N gt C gt H

In this case,The higher priority groups are
opposite (E )-2-bromo-2-chloro-propene
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Extended Comparison
  • If atomic numbers are the same, compare at next
    connection point at same distance
  • Compare until something has higher atomic number
  • Do not combine always compare

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Dealing With Multiple Bonds
  • Substituent is drawn with connections shown and
    no double or triple bonds
  • Added atoms are valued with 0 ligands themselves
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