Chapter 11 Alkanes

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Chapter 11- Alkanes

• Homework 11.12, 11.14, 11.17, 11.19, 11.22,
  11.26, 11.30, 11.35, 11.46, 11.48, 11.59

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Alkanes

• Alkanes are the simplest type of organic compound
• They are members of a larger class called
  hydrocarbons
• Hydrocarbons- a compound composed of only carbon
  and hydrogen

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• Alkanes are saturated hydrocarbons, meaning they
  only contain carbon-carbon single bonds.
• A hydrocarbon that contain carbon-carbon double
  bonds, triple bonds or benzene rings is said to
  be unsaturated.
• Structure of Alkanes
• Each carbon has four bonds with bond angles of
  109.5o, and is tetrahedral in shape.
• As we add carbons, the structure gets more
  complex, but each carbon is still tetrahedral.

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Naming Alkanes
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Group Nomenclature
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Short hand

• We can condense the condensed formula further by
  putting like groups together in parenthesis with
  a subscript.

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Constitutional Isomers

• Constitutional Isomers- compounds that have the
  same molecular formula but different structural
  formulas.
• By different structural formulas, we mean that
  they can either have different types of bonds
  and/or differ in their connectivity.
• For CH4, C2H6, and C3H8, there are no
  constitutional isomers
• No variations!!

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• For C4H10, there are two
• These are different compounds and have different
  physical and chemical properties!!
• All compounds that have the same molecular
  formula but different structural formulas are
  constitutional isomers.

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Examples

• 2 alcohols C3H8O
• 2 1o amines C3H9N
• 2 aldehydes C4H8O
• 2 carboxylic acids C4H8O2

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• The ability of carbon atoms to form strong,
  stable bonds with other carbon atoms results in a
  staggering number of constitutional isomers.
• CH4- 1 C15H32- 4,347
• C5H12- 3 C25H52- 36,797,588
• C10H22- 75 C30H62- 4,111,846,763

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Nomenclature of Alkanes

• Ideally, every organic compound should have a
  name from which its structural formula can be
  drawn.
• The rules are established by the International
  Union of Pure and Applied Chemistry.
• They are called IUPAC names for short.

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Unbranched Alkanes

• IUPAC names for unbranched chains of carbon
  consist of two parts
• 1) Prefix- shows the number of carbons in the
  chain
• 2) Suffix- -ane, to show that the compound is an
  alkane
• Prefixes-

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Branched Alkanes

• The IUPAC name for alkanes with branched chains
  consist of a parent name, which shows the longest
  chain of carbons, and substituent names that
  indicate the groups attached to the parent chain.
• Alkyl Group- a substituent group derived from an
  alkane. It is named by dropping the ane and
  adding yl.

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Examples of Alkyl groups
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Rules of the IUPAC system

• 1)The name for an unbranched chain consists of a
  prefix showing the number of carbons and the
  ending ane.
• 2) For branched chains, find the longest
  continuous chain as the parent chain, and its
  name is the root name.
• 3) Give each substituent on the parent chain a
  name and number which corresponds to the carbon
  atom the substituent is bonded to. Use a hyphen
  to separate numbers and words.

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• 4) If there is only one substituent, number the
  parent chain from the end that gives the
  substituent the lowest number.
• example
• 5) If the same substituent occurs more than once,
  number the parent chain to give the lowest set of
  numbers and use di, tri, tetra, penta, hexa, etc.
  to indicate the number of times the substituent
  appears. Separate numbers with commas.

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Examples
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• 6) If there are more than one type of
  substituent, list them alphabetically and number
  the parent chain to obtain the lowest set of
  numbers. If there are different substituents in
  equal positions, give the one that come first
  alphabetically the lowest number.
• examples

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• 7) Do not include the numerical prefixes di, tri,
  tetra, etc. in alphabetizing. You also do not
  include sec- or tert- in alphabetizing.
• examples.

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SKIP

• We are skipping section B, common names, on page
  295.

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Cycloalkanes

• A hydrocarbon that contains carbon atoms joined
  to form a ring are called cyclic hydrocarbons.
• When all carbons of the ring are saturated, the
  hydrocarbon is called a cycloalkane.
• Examples

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Naming

• To name a cycloalkane, use the prefix cyclo and
  the parent name that corresponds to the number of
  carbons in the ring.
• Then name each substituent on the ring, if there
  are any.
• If there is only one substitituent, there is no
  need to give the locate or position number.

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• If there are two substituents, number the ring
  beginning with the substituent that comes first
  alphabetically.
• Examples

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Shapes of Alkanes and Cycloalkanes

• Organic Molecules are 3-D
• Alkanes
• Free rotation about each C-C bond
• Conformation- Any 3-D arrangement of atoms in a
  molecule that results from rotation about a
  single bond
• There are an infinite number of conformations due
  to this rotation
• Molecules will spend most of there time in the
  lowest energy conformations

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Shapes of Alkanes and Cycloalkanes

• Cycloalkanes
• Rotation is restricted due to ring structure
• Cyclopentane- most staple conformation is the
  envelope conformation
• 4 carbons in a plane and 1 carbon that is bent
  out of the plane.
• All bond angles are 109.5o

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Shapes of Alkanes and Cycloalkanes

• Cyclohexane- most stable in the chair
  conformation, all bond angles 109.5o
• The 12 C-H bonds are arranged in two different
  orientations
• 6 are Axial bonds, the other 6 are Equatorial
  bonds.
• Axial bonds extend straight up or down
• Equatorial bonds extend out from the parameter of
  the ring

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Cis/Trans Isomerism

• Cycloalkanes with substituents on two or more
  carbons of the ring show a type of isomerism
• Cis/trans isomers have
• Same Molecular formula
• Same order of attachment
• Different arrangement of their atoms in space due
  to the restricted rotation around the C-C single
  bonds of the ring

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Cis/Trans Isomerism

• The prefix cis- indicates that the substituents
  are on the same side of the ring
• The prefix trans- indicates that the substituents
  are on the opposite side of the ring.
• Cis/trans isomers differ in the orientation of
  their atoms in space, so they are called
  stereoisomers
• Cis/trans are only one form of stereoisomers

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Physical Properties of Alkanes

• Most important is the almost complete lack of
  polarity in cycloalkanes and alkanes
• Melting/Boiling Points
• In general, both increase as you increase MW
• 1-4 carbonsgases at RT
• 5-17 carbons.liquids at RT
• 18 carbons..white, waxy solids
• As branching increases, the surface area
  decreases, so London Forces are weaker, which
  lead to lower MP/BP

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Physical Properties of Alkanes

• Solubility
• Alkanes are not soluble in water
• They are soluble in each other
• Reactions of Alkanes
• The most important chemical property of alkanes
  is their inertness
• They are quite unreactive toward any of the
  normal ionic reaction conditions.

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Reactions of Alkanes

• 1) Combustion
• Oxidation of hydrocarbons is the basis for their
  uses as energy sources for heat and power
• CH4 2O2 CO2 2H2O 212
  kcal/mol
• CH3CH2CH35O2 3CO24H2O 530 kcal/mol

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Reactions of Alkanes

• 2) Reactions with Halogens Halogenations
• CH4 Cl2 heat or light CH3Cl HCl
• In IUPAC naming, the name for haloalkanes comes
  from naming the halogen atom as a substituent and
  alphabetizing it along with other substituents.
• F Fluoro, Cl Chloro, Br Bromo, I Iodo

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Sources of Alkanes

• Major sources Natural Gas and Petroleum
• Natural gas consists of about 90-95 methane,
  5-10 ethane, and some propane, butane, and
  2-methylpropane
• Petroleum is a thick, viscous liquid mixture of
  1000s of compounds, most of them hydrocarbons,
  formed from the decomposition of marine plants
  and animals

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Sources of Alkanes

• The fundamental separation process in refining
  petroleum is fractional distillation
• Practically all crude petroleum that enters a
  refinery goes to distillation units, where it is
  heated to temperatures as high as 370 to 425oC
  and separated into fractions
• Each fraction contains a mixture of hydrocarbons
  that boil within a particular range.