157 terms

Chapter 4-Alkanes and Cycloalkanes

What are saturated hydrocarbons also known as?
What are saturated hydrocarbons/alkanes?
-hydrocarbons that lack pi bonds
-therefore have no double or triple bonds
What do the names of these compounds usually end with?
-the suffix "ane"
What is nomenclature?
-the system of naming chemical compounds
What does IUPAC stand for?
-the International Union of Pure and Applied Chemistry
What was IUPAC responsible for?
-developing a systematic method for naming compounds
What are names produced by IUPAC rules called?
-systematic names
What are the five parts of each systematic name?
What is the first step in naming an alkane?
-identifying the longest chain
What is this chain known as?
-the parent chain
What is the deciding factor if there is a competition between two chains of equal length?
-the chain with the greater number of substituents should be chosen
-(this helps to avoid having highly branched substituents)
What are substituents?
-groups connected to the parent chain
What is used to indicate the presence of a ring in the structure of an alkane?
-the term "cyclo"
What is a three carbon ring called?
A four carbon ring?
A five carbon ring?
After the parent has been identified, what is the next step in naming an alkane?
-to identify and name all substituents
How are substituents named?
-with the same terminology for naming parents; only the letters "yl" are added instead of "ane"
When an alkyl group is connected to a ring, what is generally treated as the parent?
-the ring
This is only true when...?
-the ring is comprised of more carbon atoms than the alkyl group
What are complex substituents?
-alkyl substituents that are not straight-chain alkyl groups
What is the first step in naming complex substituents?
-place numbers on the substituent, going away from the parent chain
How is the complex group then named?
-use a number to indicate where there is branching from the substituent
-combine the name of the primary substituent with the secondary substituent
-example: 2-methylbutyl
What are the 6 common substituent names that we should commit to memory?
What is the long-handed name for isopropyl?
For sec-butyl?
For isobutyl?
For tert-butyl?
For isopentyl/isoamyl?
For neopentyl?
What is a locant?
-a number assigned to parent alkanes that are used to help assemble the systematic name
What four guidelines are used to assign locants to substituents?
-if one substituent is present, it should be assigned the lower possible number
-when multiple substituents are present, numbers should be assigned so that the first substituent receives the lower number
-if there is a tie, then the second locant should be as low as possible
How are commas and hyphens used within alkane names?
-hyphens are used to separate numbers from letters
-commas are used to separate two numbers from each other
What is the next step after all substituents have been identified and assigned the proper locants?
-they are placed in alphabetical order
Are prefixes included as part of the alphabetization scheme?
Are "sec" and "tert" also ignored?
Is "iso" ignored?
In summary, what are the four steps that are required when naming an alkane?
-identify the parent chain
-identify and name the substituents
-number the parent chain and assign a locant to each substituent
-arrange the substituents alphabetically
What are compounds that contain two fused rings called?
-bicyclic compounds
For bicyclic systems, what term is introduced in the name of the parent?
What landmark components does each bicyclic compound have?
-two bridgeheads
What are bridgeheads?
-the two carbon atoms where the rings are fused together
How are bridgeheads used to number a bicyclic compound?
-the paths between each bridgehead should be identified (there will be three)
What is done after the paths are identified?
-for each path, count the number of carbon atoms, excluding the bridgeheads themselves
How is the parent then numbered?
-begin numbering along the longest path, go to the second longest path, and finally go along the shortest path
When naming the parent, which path is listed first?
-the longest path
What other factor must be taken into account?
-we must start numbering at the bridgehead that gives the substituent the lowest possible number
What three important names do pharmaceuticals have?
-trade names (marketing/brand names)
-generic names (shortened names)
-systematic IUPAC names
What is one tool that can be used to identify whether or not compounds are the same?
-using IUPAC rules to name each compound
-flipping model in 3D space
For an alkane, what does combustion describe?
-a reaction in which the alkane reacts with oxygen to produce CO2 and water
What does ΔH represent?
-change in enthalpy associated with a reaction
What is enthalpy?
-a measure of the exchange of energy between the system and its surroundings during any process
For a combustion process, what is -ΔH called?
-the heat of combustion
How does the heat of combustion for reactions with straight-chain alkanes and branched alkanes compare?
-the heat of combustion is higher for reactions with straight-chain alkanes than with branched alkanes
What can we conclude then?
-that branched alkanes are lower in energy (more stable) than straight-chain alkanes
What is one important method that helps to determine the relative stability of compounds?
-heats of combustion
What is the main source of alkanes?
What does petroleum literally mean?
-rock oil
What is petroleum a mixture of?
-hundreds of hydrocarbons
What type of molecule are most of these hydrocarbons?
-alkanes have varying numbers of carbons and degrees of branching
How was the first petroleum obtained separated into its various components?
-through distillation
What is distillation?
-the process by which the components of a mixture are separated from each other based on differences in their boiling points
Today, how is petroleum (crude oil) separated into commercially available products?
-through refining
What is currently the most important product that results from refining?
-the gasoline fraction
How many carbons does the gasoline fraction contain?
-between 5 and 12
What percentage of the crude oil does this fraction represent?
-only 19% of the crude oil (which is not enough to meet demand)
What two processes are used to increase the yield of gasoline from every barrel of crude oil?
What is cracking?
-the process by which C-C bonds of larger alkanes are broken, producing smaller alkanes suitable for gasoline
How can cracking be achieved?
-at high temperature (thermal cracking) or with the aid of catalysts (catalytic cracking)
What does cracking generally yield?
-straight-chain alkanes
What is one of the drawbacks of these straight-chain alkanes?
-they tend to give rise to preignition/knocking in automobile engines
What is reforming?
-a process that is used to convert straight-chain alkanes into branched alkanes and aromatic compounds
Why is it desirable to produce branched alkanes and aromatic compounds?
-because these show less of a tendency for knocking
What occurs after cracking and reforming?
-the straight-chain, branched, and aromatic alkanes are blended
By how much does the combination of cracking and reforming increase gasoline yield by?
-from 19 to 47% for every barrel of crude oil
At room temperature, what state are small alkanes (with 1-4 carbons) in?
-are gases
Medium size alkanes (with 5-12 carbons)?
-are liquids
Large alkanes (with 13-20 carbons)?
-are oils
Extra-large alkanes (with 20-100 carbons)?
-are solids like tar and wax
What are supersized alkanes called?
What is this trend explained by?
-higher molecular weight alkanes experience greater London dispersion forces
What is a conformation?
-a 3D shape that can be adopted by a compound as a result of rotation about single bonds
What is a newman projection?
-a drawing style that is designed to show the conformation of a molecule
What is the torsional angle?
-the angle between two groups in a Newman projection
What is torsional angle also known as?
-the dihedral angle
Is the torsional angle/dihedral angle static?
-no, it changes as the C-C bond rotates
What can the value for the dihedral angle between two groups be?
-any value between 0 and 180 degrees
What two conformations require special attention?
-the staggered conformation and the eclipsed conformation
What is important about the staggered conformation?
-it is the lowest-energy conformation
What exactly is the staggered conformation?
-a conformation in which nearby groups in a Newman projection have a dihedral angle of 60 degrees
What is important about the eclipsed conformation?
-it is the highest-energy conformation
What exactly is the eclipsed conformation?
-a conformation in which groups are eclipsing each other in a Newman projection
What is the difference in energy between staggered and eclipsed conformations of ethane?
-12 kJ/mol
What is one key similarity between all of the staggered conformations and between all of the eclipsed conformations?
-they are degenerate
What is torsional strain?
-the difference in energy between staggered and eclipsed conformations of ethane
What is the current believed cause of torsional strain?/Why are staggered conformations more stable than eclipsed conformations?
-because it is believe that the staggered conformation possesses a favorable interaction between an occupied, bonding MO and an unoccupied, antibonding MO
What is this interaction responsible for?
-lowering the energy of the staggered conformation
Why isn't this interaction present in eclipsed conformation?
-because the favorable overlap is temporarily disrupted
What does this result in?
-an increase in energy
What is the contribution of each eclipsing H-H interaction to the torsional strain of ethane?
-4 kJ (since there are 3 eclipsing groups and the torsional strain equals 12 kJ)
What is the contribution of each eclipsing H-CH3 interaction?
-6 kJ
How do the eclipsed conformations of butane differ from those of ethane and propane?
-the three eclipsed conformations are not degenerate-->one eclipsed conformation is higher in energy than the other two eclipsed conformations
How do the staggered conformations of butane differ from those of ethane and propane?
-the three staggered conformations are not degenerate-->one staggered conformation is lower in energy than the other two eclipsed conformations
What is the dihedral angle of the conformation of butane that has the lowest energy level?
-180 degrees
What is the name of this conformation?
-the anti conformation
How are the methyl groups positioned in the anti conformation?
-they are positioned so that they achieve maximum separation from each other
What are the two other staggered conformations known as?
-gauche conformations
Why are the two guache conformations higher in energy than the anti conformation?
-because their electron clouds are repelling each other as they try to occupy the same region of space
What are these unfavorable interactions known as?
-gauche interactions
What are these a form of?
-steric hindrance
Why is one of the eclipsed conformations of butane higher in energy than the others?
-in the highest energy conformation, the methyl groups are eclipsing each other
What is angle strain?
-the increase in energy associated with a bond angle that has deviated from the preferred angle of 109.5 degrees
What value is used to accurately compare rings of different sizes?
-the heat of combustion per CH2 group
Describe the relative energy level of cyclopropane
-very high
What does this make cyclopropane?
-highly reactive and very susceptible to ring-opening reactions
What two main factors contribute to the high energy of cyclopropane?
-angle strain
-torsional strain
What produces angle strain?
-the small bond angles (60 degrees) of cyclopropane
What produces torsional strain?
-eclipsing hydrogen molecules
Why does the molecule exhibit such significant torsional strain?
-because the ring is locked in an eclipsed conformation
Compare the angle strain of cyclobutane to the angle strain of cyclopropane
-cyclobutane exhibits less angle strain than cyclopropane because its bond angles are larger
-because the bond angles of cyclobutane are larger than those of cyclopropane
Compare the torsional strain of cyclobutane to cyclopropane
-cyclobutane exhibits more torsional strain than cyclopropane
-because there are four sets of eclipsing H's rather than just three
What can cyclobutane do to alleviate some of this additional torsional strain?
-cyclobutane can adopt a slightly puckered conformation
Compare the angle strain of cyclopentane versus cyclopropane and cyclobutane
-much less angle strain
-angles (at 108 degrees) are very close to the optimal value
What is a contributor to its instability relative to cyclohexane?
-minimal but significant torsional strain
What can cyclopentane do to alleviate some of the torsional strain it experiences?
-it adopts an "envelope" conformation
What is the lowest-energy conformation of cyclohexane?
-the chair conformation
Why is it the lowest-energy conformation?
-it has no angle strain: angles are at 109.5 degrees
-it has no torsional strain: all H's are staggered, none are eclipsed
What are the three other conformations of cyclohexane?
Does the boat conformation have angle strain?
-no: angles are 109.5 degrees
Does the boat conformation experience torsional strain?
Between how many C-C bonds?
-2 bonds
Involving how many hydrogen pairs?
-4 pairs
What additional type of strain is present in the boat conformation?
-steric strain
As a result of?
-flagpole interactions
What are flagpole interactions?
-the steric interactions that occur between the flagpole hydrogen atoms in a boat conformation
How can the boat alleviate some of this torsional strain?
-by twisting into the "twist boat" conformation
In what conformation does cyclohexane spend the majority of tis time?
-in the two chair conformations
-because they are the lowest in energy
How many substituents can each carbon in a cyclohexane ring bear?
What are the two groups said to occupy?
-either an axial position or an equatorial position
What is the location of the axial position?
-parallel to a vertical axis passing through the center of the ring
What is the location of the equatorial position?
-positioned approximately along the equator of the ring
What is a ring flip?
-a conformational change that is accomplished only through a rotation of all C-C single bonds
What will the lower energy chair conformation of a molecule typically be?
-the chair conformation that has more groups in equatorial positions
-because interactions called 1,3-diaxial interactions de-stabilize molecules with groups in equatorial positions
What are 1,3 diaxial interactions?
-steric interactions that occur in the chair conformation of a di- or polysubstituted cyclohexane when groups located at C1 and C3 both occupy axial positions
What type of interactions are these?
-gauche interactions
What occurs when groups are in equatorial positions?
-the 1,3-diaxial interactions are absent
For this reason, what will the equilibrium between two chair conformations generally favor?
-the conformation of the equatorial substituent
What is the term "cis" used to signify?
-that two groups are on the same side of a ring
What is the term "trans" used to signify?
-that two groups are on opposite sides of the ring
What does HDPE stand for?
-high-density polyethylene
What does LDPE stand for?
-low-density polyethlyene
Describe the structure and relative strength of HDPE?
-HDPE is a linear molecule, and therefore has a strong, solid structure
-used for plastic bottles (Odwalla included)
Describe the structure and relative strength of LDPE?
-LDPE is branched and cannot regularly pack
-therefore it is weaker than HDPE
-used for shopping bags, bread bags