1 Fall, 2009 Organic Chemistry I Alkanes Organic Chemistry I Alkanes Dr. Ralph C. Gatrone Department of Chemistry and Physics Virginia State University

Fall, Objectives Nomenclature Nomenclature Isomerization Isomerization Conformation Conformation Reactions Reactions

Fall, The Alkanes Paraffin hydrocarbons Paraffin hydrocarbons Aliphatic hydrocarbons Aliphatic hydrocarbons Saturated hydrocarbons Saturated hydrocarbons Class only contains C and H Class only contains C and H C has 4 bonds C has 4 bonds Every bond is a sigma bond to a C or H Every bond is a sigma bond to a C or H

Fall, Alkanes General formula is C n H 2n+2 Know names of CH 4 to C 12 H 26 (see table on next slide)

Fall, CarbonsName (C n H 2n+2 ) 1Methane CH 4 2Ethane C2H6C2H6C2H6C2H6 3Propane C3H8C3H8C3H8C3H8 4Butane C 4 H 10 5Pentane C 5 H 12 6Hexane C 6 H 14 7Heptane C 7 H 16 8Octane C 8 H 18 9Nonane C 9 H 20 10Decane C 10 H 22 11Undecane C 11 H 24 12Dodecane C 12 H 26

Fall, Methane, Ethane, Propane Only one compound regardless of point of attachment Consider butane

Fall, Butane C 4 H 10 C 4 H 10

Fall, Pentane C 5 H 12 C 5 H 12 Isomerism: constitutional isomers Same chemical formula different atom connections

Fall, Alkane Isomers Constitutional isomers Constitutional isomers Straight chain alkanes Straight chain alkanes Branched chain alkanes Branched chain alkanes Number of possible isomers increases with n (number of carbon atoms) Number of possible isomers increases with n (number of carbon atoms) C 6 has 5 isomers; C 8 has 18 isomers C 6 has 5 isomers; C 8 has 18 isomers

Fall, Constitutional Isomers

Fall, Pentane

Fall, Alkyl Groups portion of an alkane molecule portion of an alkane molecule remove one H from an alkane remove one H from an alkane general abbreviation “R” (for Radical) general abbreviation “R” (for Radical) an incomplete species or the “rest” of the molecule an incomplete species or the “rest” of the molecule Nomenclature of Alkyl Groups: Nomenclature of Alkyl Groups: replace -ane ending of alkane with -yl ending replace -ane ending of alkane with -yl ending CH 3 is “methyl” (from methane) CH 3 is “methyl” (from methane) CH 2 CH 3 is “ethyl” from ethane CH 2 CH 3 is “ethyl” from ethane must know name and structure of must know name and structure of propyl, isopropyl, butyl, sec-butyl, isobutyl, and propyl, isopropyl, butyl, sec-butyl, isobutyl, and tert-butyl (see next slide) tert-butyl (see next slide)

Fall, Alkyl Groups

Fall, Note on Alkyl Groups Type of C is based on number of C’s bonded to it Type of C is based on number of C’s bonded to it Type of H is based on type of C Type of H is based on type of C

Fall, Drawing Organic Structures Where a line ends: CH 3 Where two lines meet: CH 2 Hydrogens are not written. Every C has 4, repeat, 4 bonds

Fall, Alkane Nomenclature Alkane Nomenclature Name is based upon Name is based upon Prefix-Parent-Suffix Prefix-Parent-Suffix Rules Rules Name the longest possible chain Name the longest possible chain Number the carbons in the longest chain Number the carbons in the longest chain Numbers start at closest branching point Numbers start at closest branching point Substituents are numbered at their point of attachment Substituents are numbered at their point of attachment Substituents are named as alkyl group Substituents are named as alkyl group Molecule is named as a single word Molecule is named as a single word Substituents are placed alphabetically Substituents are placed alphabetically Complex substituents are named as compounds would be Complex substituents are named as compounds would be We will do some specific examples We will do some specific examples

Fall, Example 1: Find the longest chain 2. Number the longest chain

Fall, Example Number 1 is nearest closest branch (C2) Identify substituents Name and number substituents Using position number on the longest chain

Fall, Example Assemble name Substituents are placed in name alphabetically

Fall, Example Note: isopropyl is placed using the letter i Also true for isobutyl sec-butyl and t-butyl are placed using the letter b

Fall, Using Complex Substituent An example An example

Fall, Example 1: Find the longest chain 2. Number the longest chain

Fall, Example Identify substituents Name using position on longest chain

Fall, Example How do we name the complex substituent?

Fall, Example Assemble name Place substituents alphabetically

Fall, Example

Fall, Example 2 Find longest chain Number from closest branch

Fall, Example 2 Assemble name 3,4-diisopropyl-2,6,6-trimethyloctane

Fall, Combining Substituents Substituents are combined Substituents are combined di = two, tri = three, tetra = four di = two, tri = three, tetra = four Alphabetized by substituent Alphabetized by substituent Not by the prefix di, tri, tetra, etc. Not by the prefix di, tri, tetra, etc.

Fall, Nomenclature Every exam will have two sections of nomenclature Every exam will have two sections of nomenclature First: you name the structure given First: you name the structure given Second: you draw the structure based upon the name given Second: you draw the structure based upon the name given Usually these sections represent 40% of test value Usually these sections represent 40% of test value Learn the rules. Follow the rules. Learn the rules. Follow the rules.

Fall, Chemical Properties of Alkanes Paraffinic Hydrocarbon (little affinity) Paraffinic Hydrocarbon (little affinity) Alkanes are unreactive Alkanes are unreactive Only sigma bonds C-C and C-H Only sigma bonds C-C and C-H Electrons are not available for reactions Electrons are not available for reactions

Fall, Chemical Properties

Fall, The Shape of Alkanes The three-dimensional shape of molecules result from many forces The three-dimensional shape of molecules result from many forces A molecule may assume different shapes, called conformations, that are in equilibrium at room temperature A molecule may assume different shapes, called conformations, that are in equilibrium at room temperature The conformational isomers are called conformers The conformational isomers are called conformers The systematic study of the shapes molecules is stereochemistry The systematic study of the shapes molecules is stereochemistry

Fall, Consider Ethane How do we represent three dimensional shape of the ethane molecule?

Fall, Three-Dimensional Shape: Ethane

Fall, Conformations of Ethane Conformers interconvert rapidly Conformers interconvert rapidly Molecular models are three dimensional objects that enable us to visualize conformers Molecular models are three dimensional objects that enable us to visualize conformers Representing three dimensional conformers in two dimensions is done with standard types of drawings Representing three dimensional conformers in two dimensions is done with standard types of drawings

Fall, Drawing Conformations Sawhorse representations Sawhorse representations –C-C bonds are at an angle to the edge of the page and all C-H bonds are shown Newman projections Newman projections –Bonds to front carbon are lines going to the center –Bonds to rear carbon are lines going to the edge of the circle

Fall, Dihedral Angle Four Bond Angle – the angle between C-H bonds on the front and back carbons as viewed in a Newman Projection Four Bond Angle – the angle between C-H bonds on the front and back carbons as viewed in a Newman Projection

Fall, Conformations of Ethane Free rotation is not free Free rotation is not free Energy barrier of 12kJ/mole is observed. Energy barrier of 12kJ/mole is observed.

Fall, Conformations of Ethane staggered staggered eclipsed

Fall, Energy Differences of Confomers Staggered conformation: lower in energy Staggered conformation: lower in energy Lower in energy = more stable Lower in energy = more stable Eclipsed conformation: higher in energy Eclipsed conformation: higher in energy

Fall, Energy Differences Hydrogens are close together in eclipsed conformations Hydrogens are close together in eclipsed conformations

Fall, Eclipsing Hydrogens Eclipsing H’s interact with each other Eclipsing H’s interact with each other Torsional strain Torsional strain 12kJ/mole 12kJ/mole There are 3 eclipsing hydrogens There are 3 eclipsing hydrogens 4kJ/mole each 4kJ/mole each

Fall, Propane CH 3 CH 2 -CH 3 – consider C-C bond shown CH 3 CH 2 -CH 3 – consider C-C bond shown Energy barrier is 14kJ/mole Two eclipsing H’s = 8kJ/mole Eclipsing H-CH 3 = 6kJ/mole

Fall, Propane

Fall, Butane CH 3 CH 2 -CH 2 CH 3 – consider C2-C3 bond CH 3 CH 2 -CH 2 CH 3 – consider C2-C3 bond

Fall, Energy of Butane Interactions

Fall, Conformations of Butane Anti – large groups are far apart (180 o ) Anti – large groups are far apart (180 o ) Eclipsed – large groups are close (0 o ) Eclipsed – large groups are close (0 o ) Gauche – large groups are close (60 o ) Gauche – large groups are close (60 o )

Fall, Butane Anti conformation Anti conformation Large groups are far apart Large groups are far apart All groups are staggered All groups are staggered Lowest energy conformation Lowest energy conformation

Fall, Butane Two methyl – H interactions (2 X 6kJ/mole) One H-H interaction (1 X 4kJ/mole) Total Energy = 16kJ/mole

Fall, Butane 2 H-H interactions – (2 X 4kJ/mole) 2 H-H interactions – (2 X 4kJ/mole) Total Energy is 19kJ/mole Total Energy is 19kJ/mole Methyl – methyl interaction = 11kJ/mole Methyl – methyl interaction = 11kJ/mole Highest energy conformation Highest energy conformation

Fall, Butane Staggered structure Staggered structure Energy is not equal to 0kJ/mole Energy is not equal to 0kJ/mole Methyl groups are close together Methyl groups are close together Interaction is 3.8kJ/mole Interaction is 3.8kJ/mole gauche gauche

Fall, Butane Eclipsing methyl groups Eclipsing methyl groups Two large groups trying to be in the same space Two large groups trying to be in the same space Steric Strain Steric Strain Gauche Gauche Two methyl groups 60 o apart Two methyl groups 60 o apart Two large groups trying to be in the same space Two large groups trying to be in the same space Steric Strain Steric Strain

Fall, Energy of Butane Interactions

Fall, Conformational Interactions Torsional Strain Torsional Strain Eclipsing groups Eclipsing groups Steric Strain Steric Strain Large groups occupying the same space Large groups occupying the same space

Fall, Preferred Conformations Molecule adopts the most stable (lowest energy) conformation where all bonds Molecule adopts the most stable (lowest energy) conformation where all bonds –Are staggered –Large groups are anti to each other At Room Temperature there is enough kinetic energy for all conformations to be in equilibrium At Room Temperature there is enough kinetic energy for all conformations to be in equilibrium LG

Fall, Energy and Equilibrium The relative amounts of the two conformers depend on their difference in energy The relative amounts of the two conformers depend on their difference in energy   E =  RT ln K R is the gas constant [8.315 J/(Kmol)], R is the gas constant [8.315 J/(Kmol)], T is the temperature (Kelvin) T is the temperature (Kelvin) K is the equilibrium constant between isomers K is the equilibrium constant between isomers

Fall, Determining the Value of K

Fall, The Calculation of K

Fall, How much of each conformer is present in solution? Calculate K Calculate K

Fall, How much of each conformer is present in solution? Calculate K Calculate K K = 2143 K = 2143 K = A/B = 2143/1 K = A/B = 2143/1 Calculate Percentages Calculate Percentages %A = (A/A+B ) X 100 %A = (A/A+B ) X 100 %A = (2143/ ) X 100 %A = (2143/ ) X 100 %A = 99.9% %A = 99.9%