Chapter 2: Alkanes

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Chapter 2 Alkanes

• Outline
• 2.1 Hydrocarbons 2.7 Chemical Properties
• 2.2 Alkanes
• 2.3 IUPAC Nomenclature
• 2.4 Cycloalkanes
• 2.5 Stereoisomerism in Cyclic Cmpds
• 2.6 Physical Properties

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Hydrocarbons

• Hydrocarbons are compounds which contain only
  carbon and hydrogen.
• a. alkanes only single bonds
• b. alkenes have carbon-carbon double bonds
• c. alkynes have carbon-carbon triple bonds

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Alkanes

• Alkanes are hydrocarbons with only single bonds
  also called saturated hydrocarbons.
• e.g. Methane Ethane Propane
• Draw structural formulas and condensed structural
  formulas.

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Alkanes (Contd)

• Table 2.1 Molecular and Structural formulas for
  the first 10 alkanes.
• Alkanes that are a continuous chain (no
  branching) are referred to as n-alkanes.
• Branched alkanes are also possible.
• e.g. Isobutane vs butane

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Alkanes (Contd)

• Types of Carbon
• a. quatenary bonds to four other carbons
• b. tertiary bonded to three other carbons
• c. secondary bonded to two other carbons
• d. primary bonded to one other carbon

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Alkanes (Contd)

• N-alkanes only have 1 and 2 carbons while
  branched alkanes have at least one 3 or 4
  carbon.
• Isomers for pentane and hexane (show on board).
• Free rotation around bonds.

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IUPAC Nomenclature of Alkanes and Alkylhalides

• IUPAC (International Union of Pure and Applied
  Chemistry)
• IUPAC names and common name
• Basis of IUPAC system
• group
• e.g. methyl group from methane
• alkyl groups

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Rules for Naming Alkanes

• I. Find the longest continuous chain of carbon
  atoms. Assign a parent name from Table 2.1.
• e.g. Longest chain is 5 carbons, parent name is
  pentane.
• II. Find whatever groups are not part of the
  longest continuous chain and name them as
  prefixes (substituents).
• e.g. methylpentane

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Rules (Contd)

• III. Assign numbers to groups by counting from
  one end of the chain. Start from the end that
  gives the lowest possible numbers to the groups.
• IV. If there are two or more of the same group,
  use the following prefixes
• 2 di- 5 penta-
• 3 tri- 6 hexa-
• 4 tetra-

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Rules (Contd)

• V. If there are two or more different groups, we
  put all of them into the prefix in alphabetical
  order.
• VI. The four halogens are named by the following
  prefixes
• F fluoro-
• Cl chloro-
• Br bromo-
• I iodo-

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Branched Alkyl Groups

• isopropyl secondary
• isobutyl primary
• sec-butyl secondary
• tert-butyl tertiary
• neopentyl primary

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Cycloalkanes

• Carbon atoms can be connected to each other to
  form rings. These are called cycloalkanes.
• The most common rings are 4, 5, and 6-membered
  rings.

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Rules for Naming Cycloalkanes

• VII. We may begin at any position and proceed
  either clockwise or counterclockwise, but we must
  end up with the lowest possible numbers.

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Examples of Naming Cyclic Alkanes
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Stereoisomerism in Cyclic Compounds

• In acyclic compounds, there is free rotation
  about all the carbon-carbon bonds.
• In cyclic compounds, this free rotation does not
  exist because the ring prevents it. The result
  is a new kind of isomer.
• These types of isomers have the same chemical
  formula and the same structural formula.

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Stereoisomers (Contd)

• To test, try to superimpose the two molecules on
  top of each other. If they superimpose, the
  molecules if not they are different and are
  called stereoisomers.
• Compounds that have the same structural formulas
  but different three-dimensional shapes (cannot be
  superimposed) are called stereoisomers.

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Stereoisomers (Contd)

• Example 1,2-Dimethylcyclopentane
• We use the prefixes cis- and trans- to tell the
  two molecules apart.
• Cis and trans isomers are found in all rings.
  All that is required is that two groups, the same
  or different, be substituted for hydrogens.
• Since cis and trans are different compounds, the
  have different physical and chemical properties.

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Physical Properties of Alkanes

• Physical State the melting point and boiling
  points increase as the molecular weight
  increases.
• Alkanes and cycloalkanes up to 5 carbons are
  gases at room temp. From 5-17 carbons are
  liquids and above 17 carbons are white, waxy
  solids. Are colorless, odorless, and tasteless.

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Physical Properties (Contd)

• Solubility Alkanes are nonpolar, therefore,
  they are not soluble in water.
• Density While alkanes have slightly different
  densities, they are all less dense than water.

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Chemical Properties

• Combustion Alkanes burn in the presence of
  oxygen to form carbon dioxide and water (complete
  combustion) incomplete combustion gives carbon
  monoxide and soot.
• C3H8 5O2 ? 3CO2 4H2O heat

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Chlorination

• Alkanes react with chlorine in the presence of
  light to produce chlorinated alkanes.
• CH4 Cl2 ? CH3Cl CH2Cl2 CHCl3
  CCl4
• This type of reaction is called a substitution
  reaction.

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Reactivity of Alkanes

• Combustion and chlorination are the only reaction
  alkanes undergo alkanes are generally chemically
  inert.

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Functional Groups

• A functional group refers to an atom or groups of
  atoms that substitutes for a hydrogen atom.
• e.g. In alcohols, the functional group is a
  hydroxy group OH.
• CH3-CH2-OH
• The general formula for alcohols is R-OH, where R
  represents any alkyl group.

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Important Functional Groups