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 Octane Boiling Ques. 11: Write the Structures
Number Point
Paraffins or Alkanes
n-butane 113 -0.5
n-pentane 62 36
n-hexane 19 69
n-heptane 0 98
2-methylbutane 99 28
2,2-dimethylpropane 100 10
2,2-dimethylbutane 89
2,2-dimethylpentane 89
2,2,3-trimethylbutane 113
2,2,4-trimethylpentane 100
(isooctane)
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 49
Methylcyclopentane 107
Cyclohexane 110
1,2-dimethylcyclohexane 104
1,4-dimethylcyclohexane 66
Aromatics
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 melting and boiling data? As the chain length (numbers of carbons) increases the melting and boiling points of the alkanes gradually increase for these compounds.
The reason that longer chain molecules have higher boiling points is that longer chain molecules become wrapped around and enmeshed in each other much like the strands of spaghetti. More energy is needed to separate them than short chain molecules which have only weak forces of attraction for each other.
Ques. 13: Name some of the compounds in the table and state whether the compound will be a gas, liquid, or solid state at room temperature (20oC). Hint: If the boiling point is below 20oC, then the liquid has already boiled and is a gas. At room temperature, the lighter alkanes are gases (methane = -164 bp - a gas); the midweight alkanes are liquids; and the heavier alkanes are solids, or tars (triacontane, C30H62 = 450 bp - a solid).
Ques. 14: What is the boiling point trend in terms of the molecular weights or chain length of the compounds?
http://www.elmhurst.edu/~chm/vchembook/513refining.html
Petroleum refining is the process of separating the many compounds present in crude petroleum. This process is called fractional distillation where the crude oil is heated; the various of the compounds boil at different temperatures and change to gases; and are later recondensed back into liquids.
Ques. 15: The refining principle based upon boiling points which is used is that the longer the carbon chain, the __?___ the temperature the compounds will boil and the __?_ position in the distilling column.
The crude petroleum is heated and changed into a gas. The gases are passed through a distillation column which becomes cooler as height increases. See the figure above. When a compound in the gaseous state cools below its boiling point, it condenses into a liquid. The liquids may be drawn off the distilling column at various heights. Although all fractions of petroleum find uses, the greatest demand is for gasoline. One barrel of crude petroleum contains only 25-35% gasoline. Transportation demands require that over 50% of the crude oil be converted into gasoline. To meet this demand some petroleum fractions must be converted to gasoline. This may be done by "cracking" - breaking down large molecules of heavy heating oil; "reforming" - changing molecular structures of low quality gasoline molecules; or "polymerization" - forming longer molecules from smaller ones.
Conversion Oil Refining: For example if decane is heated to about 500 C the covalent carbon-carbon bonds begin to break during the cracking process. Many kinds of compounds including alkenes are made during the cracking process. Alkenes are formed because there are not enough hydrogens to saturate all bonding positions after the carbon-carbon bonds are broken.
Week # 2 Lecture # 5 and 6 - Alkenes, Alkynes, Aromatics
Concepts: Saturated, unsaturated, aromatics, benzene, cis/trans isomers
Reaction Summary: Write the general reactions in words.
Combustion:
Hydrogenation:
Hydration:
Halogenatiion (2 types):
Polymerization:
Ques. 16: Classify the following according to functional group, types of bonding, saturated and unsaturated: 1-butene, 2-butene, 1-butyne, propane, and propene. Write the structures.
Ques. 17: Write all of the structures for three isomers for C2H4Cl2? Identify the cis and trans isomers.
Ques. 18: Write the IUPAC names of the following. Identify those that are cis and trans isomers.
Ques. 19. Write the IUPAC names for the following:
Ques. 20: Write the structures of cyclohexane, cyclohexene, 1,3-cyclohexadiene, and benzene.
Ques. 21: a. Write all of the structures for dibromobenzene?
b. What is the the name for a benzene ring that is used as a branch?
Ques. 22: Addition Reactions
Reaction Name / Word Equation / Example1. Hydrogenation (H2) / Alkene + hydrogen à alkane / Propene + H2
2. Halogenation (X2) / Alkene + diatomic halogen
à Dihaloalkane / propene and Br2
3. Halogenation (HX) / Alkene + HX (X = Cl, Br, I)
à Haloalkane
*Markovnikov's Rule / 3-methyl-2-pentene + HCl
4. Hydration(HOH / Alkene + water à alcohol
*Markovnikov's Rule / 1-butene + HOH
5. Polymerization / Add several monomers together
à polymer / Ethene à ethylene
Ques. 23: State and Memorize Markovnikov's Rule.
Ques. 24. Complete the following and write the names of the reactants and the products:
a) Hydrogenation: CH3CH2CH=CH2 + H2 ®
b) Halogenation: CH3CH=CH2 + Br2 ®
c) Hydration: CH2=CH2 + HOH ®
d) Halogenation: CH3CH=CH2 + HBr ®
e) Combustion: CH3CH2CH2CH3 + O2 ®
f) Hydration:
g) Halogenation:
Ques. 25: The reactants needed to produce the compound chlorocyclopentane are:
Write the structure of the product given and work backwards.
Ques. 26: Write the IUPAC names of the following structures:
Ques. 27: Which structure above is the product of the reaction of 2-pentene with hydrogen (H2)? Write the reaction first.
Ques. 28: Which structure above is the product of the reaction of 1-pentene with HBr? Write the reaction first.
Ques. 29: Which structure above is the product of the reaction of 1-pentene with bromine (Br2)? Write the reaction first.
Ques. 30: Write the IUPAC names of the following:
Ques. 31: Which structure above is the product of the hydration reaction of 2-methyl-2-butene? Write the reaction first.
Ques. 32: Which structure above is the product of the hydration reaction of 2-methyl-1-butene? Write the reaction first.
Ques. 33: Which structure is the product of the hydration reaction of cyclopentene?
Write the reaction first.
Ques. 34: Which structure above is the product of the hydration reaction of methylcyclopentene? Write the reaction first.
Ques. 35. Write the general name of each reaction, write the structure of the product, and name the reactants and products.
c. Three repeating units of propene (polypropylene)
d. Three repeating units of chloroethene (polyvinylchloride-PVC)
Ques. 36: Write the structures for the products and give the names of both.
Application of Cis/Trans Isomers to Vision
or how do we see on a molecular level?
Conversion of Vitamin A into Cis-Retinal:
http://www.elmhurst.edu/~chm/vchembook/533cistrans.html
Vitamin A, trans-retinol, is carried to the rods in the eyes from storage in the liver. First it is converted to cis-retinol by a process of isomerization, which means that the trans isomer is converted to a cis isomer. The molecule must break the pi bond, rotate on the single bond, and reform the pi bond.
The cis-retinol, an alcohol, is then oxidized to cis-retinal, an aldehyde.
Cis-Trans Isomerization of Retinal:
Photochemical events in vision involve the protein opsin and the cis/trans isomers of retinal. The cis-retinal fits into a receptor site of opsin. Upon absorption of a photon of light in the visible range, cis-retinal can isomerize to all-trans-retinal. In the cis-retinal, the hydrogens (light gray in the molecular model on the left) are on the same side of the double bond (yellow in the molecular model).
In the trans-retinal, the hydrogens are on opposite sides of the double bond. In fact, all of the double bonds are in the trans-configuration in this isomer: the hydrogens, or hydrogen and -CH3, are always on opposite sides of the double bonds (hence, the name "all-trans-retinal"). Note how the shape of the molecule changes as a result of this isomerization. The molecule changes from an overall bent structure to one that is more or less linear. All of this is the result of trigonal planar bonding (120o bond angles) about the double bonds.
This photochemical reaction is best understood in terms of molecular orbitals, orbital energy, and electron excitation. In cis-retinal, absorption of a photon promotes a p electron in the pi bond to a higher-energy orbital. This excitation "breaks" the pi component of the double bond and is temporarily converted into a single bond. This means the molecule can now rotate around this single bond, which it does by swiveling through 180o.
The double bond then reforms and locks the molecule back into position in a trans configuration of the all-trans-retinal. This isomerization occurs in a few picoseconds (10-12 s) or less. Energy from light is crucial for this isomerization process: absorption of a photon leads to breaking the double bond and consequent isomerization about half the time (in the dark it almost never happens.)
Protein Geometry Changes Following Retinal Isomerization
As we shall see below, the isomerization of retinal has an important effect on special proteins in the rod cell: the isomerization event actually causes the proteins to change their shape. This shape change ultimately leads to the generation of a nerve impulse.