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Chapter 4-Alkanes and Cycloalkanes

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What are saturated hydrocarbons also known as?
-alkanes
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?
-stereoisomerism
-substituents
-root
-unsaturation
-ending
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?
-cyclopropane
A four carbon ring?
-cyclobutane
A five carbon ring?
-cyclopentane
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?
-isopropyl
-sec-butyl
-isobutyl
-tert-butyl
-isopentyl/isoamyl
-neopentyl
What is the long-handed name for isopropyl?
-1-methylethyl
For sec-butyl?
-1-methylpropyl
For isobutyl?
-2-methylpropyl
For tert-butyl?
-1,1-dimethylethyl
For isopentyl/isoamyl?
-3-methylbutyl
For neopentyl?
-2,2-dimethylpropyl
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?
-no
Are "sec" and "tert" also ignored?
-yes
Is "iso" ignored?
-no
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?
-bicyclo
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?
-petroleum
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
-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?
-cracking
-reforming
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?
-polymers
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
Why?
-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
Why?
-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?
-boat
-twist-boat
-half-chair
Does the boat conformation have angle strain?
-no: angles are 109.5 degrees
Does the boat conformation experience torsional strain?
-yes
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
Why?
-because they are the lowest in energy
How many substituents can each carbon in a cyclohexane ring bear?
-two
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
Why?
-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