Week 2 Alkanes and Alkenes Lecture 4 Alkanes Concepts Alkane root and ring names alkane branch names Boiling Point Trends Reaction Summary Write the general reactions in words Combustion Ques 1 Give the root names for all of the alkanes from one to 10 carbons Ques 2 a Give the branch names for carbons one to four b What is the structure of the special branch called isopropyl Ques 3 What are the ring structures and names for carbons three to six Ques 4 Write the IUPAC names for the following Ques 5 What is the general rule for melting and boiling points for a series of alkanes which increase in the number of carbons Ques 6 Which compound would have the lowest boiling point Write out the structures a heptane or pentane b pentane or cyclopentane c 2 methylbutante or pentane d Which compound in each a b c has the strongest intermolecular forces of attraction Explain why Ques 7 What is most significant about the saturated hydrocarbon portion of a molecule during a chemical reaction Ques 8 What is the combustion reaction of oxygen with butane Cigarette lighter Ques 9 What is the IUPAC name of each of the following Ques 10 What is the IUPAC name of each of the following Compounds in Gasoline http www elmhurst edu chm vchembook 515gasolinefs html Gasoline is a complex mixture of over 500 hydrocarbons that may have between 5 to 12 carbons Alkanes type compounds either straight chain or branched compounds are present is greatest amounts Smaller amounts of alkane cyclic and aromatic compounds are present Very small or trace amounts of alkenes are present in gasoline A few examples are given here Self Quiz on gasoline compounds http www elmhurst edu chm vchembook 515gasolinecpdQUIZ html Compound Paraffins or Alkanes n butane n pentane n hexane n heptane 2 methylbutane 2 2 dimethylpropane 2 2 dimethylbutane 2 2 dimethylpentane 2 2 3 trimethylbutane 2 2 4 trimethylpentane isooctane Octane Number Boiling Point 113 62 19 0 99 100 89 89 113 100 0 5 36 69 98 28 10 Olefins or Alkenes 1 pentene 152 2 methyl 2 butene 176 3 methyl 2 pentene 130 2 4 4 trimethyl 1 pentene164 Cycloalkanes or Naphthenes Cyclopentane 141 Methylcyclopentane 107 Cyclohexane 110 1 2 dimethylcyclohexane 104 1 4 dimethylcyclohexane 66 Aromatics 49 Ques 11 Write the Structures Benzene 99 Toluene or methylbenzene 124 m xylene or 1 3 dimethylbenzene 145 Ethylbenzene 124 Propylbenzene 127 Isopropylbenzene 132 What is the octane number http www elmhurst edu chm vchembook 514gasoline html Fuel octane requirements for gasoline engines vary with the compression ratio of the engine Engine compression ratio is the relative volume of a cylinder from the bottom most position of the piston s stroke to the top most position of the piston s stroke The higher an engine s compression ratio the greater the amount of heat generated in the cylinder during the compression stroke If fuel octane is too low for a given compression ratio the fuel prematurely and spontaneously ignites too early and the fuel charge EXPLODES rather than BURNS resulting in incomplete combustion The net effect is a loss in power possible engine damage and an audible knock or ping referred to as detonation The octane number of gasoline is a measure of its resistance to knock The octane number is determined by comparing the characteristics of a gasoline to isooctane 2 2 4 trimethylpentane and heptane Isooctane is assigned an octane number of 100 It is a highly branched compound that burns smoothly with little knock On the other hand heptane a straight chain unbranched molecule is given an octane rating of zero because of its bad knocking properties Straight run gasoline directly from the refinery distillation column has an octane number of about 70 In other words straight run gasoline has the same knocking properties as a mixture of 70 isooctane and 30 heptane Many of these compounds are straight chain alkanes Cracking isomerization and other refining processes can be used to increase the octane rating of gasoline to about 90 Anti knock agents may be added to further increase the octane rating See table above for octane ratings of the compounds in gasoline Ques 12 Octane rating versus type of compound Octane ratings with increasing carbon chain length Octane ratings with carbon chain branching Octane ratings increase in aromatics with same number of carbons What octane gasoline should you purchase For most automobiles use the lowest grade of 87 octane unless they specifically say to use a higher octane gasoline Using higher octane grades does not provide any extra power or extra mileage Boiling points and structures of hydrocarbons http www elmhurst edu chm vchembook 501hcboilingpts html The boiling points of organic compounds can give important clues to other physical properties and structural characteristics A liquid boils when its vapor pressure is equal to the atmospheric pressure Vapor pressure is determined by the kinetic energy of molecules Kinetic energy is related to temperature and the mass and velocity of the molecules K E 1 2 mv2 When the temperature reaches the boiling point the average kinetic energy of the liquid particles is sufficient to overcome the forces of attraction that hold molecules in the liquid state Vapor pressure is caused by an equilibrium between molecules in the gaseous state and molecules in the liquid state When molecules in the liquid state have sufficient kinetic energy they may escape from the liquid and turn into a gas Molecules with the most independence in individual motions achieve sufficient kinetic energy velocities to escape as gases at lower temperatures The vapor pressure will be higher more gas molecules are present and therefore the compound will boil at a lower temperature Boiling point principle Molecules which strongly interact or bond with each other through a variety of intermolecular forces can not move easily or rapidly and therefore do not achieve the kinetic energy necessary to escape the liquid state Therefore molecules with strong intermolecular forces will have higher boiling points This is a consequence of the increased kinetic energy needed to break the intermolecular bonds so that individual molecules may escape the liquid as gases The boiling point can be a rough measure of the amount of energy necessary to separate a liquid molecule from its nearest neighbors to form a gas molecule A variety of alkanes with the generic formula CnH2n 2 are given in the table above with names What is the general trend in the