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1!
Chapter 4. Alkanes and Cycloalkanes
"Junha Jeon!
Department of Chemistry!University of Texas at Arlington!
Arlington, Texas 76019!"!
Chem 2321, Fall ‘13!
4.1 Introduction to Alkanes!
v Hydrocarbons: compounds that are only composed of hydrogen and carbon!
Introduction to Alkanes!
v Hydrocarbons: compounds that are only composed of hydrogen and carbon!
v Alkanes: Saturated hydrocarbons do NOT contain any pi bonds!
Introduction to Alkanes!
v Hydrocarbons: compounds that are only composed of hydrogen and carbon!
v Alkanes: Saturated hydrocarbons do NOT contain any pi bonds!
4.2 Common Names!
v Common names
v Nomenclature: systematic names of chemical compounds !
Nomenclature of Alkanes (IUPAC)!
v Common names
v Nomenclature: systematic names of chemical compounds !
2!
Parent Chain!
v Selection the parent chain: longest chain!!!!!!!!!!
Parent Names !
v Parent names for alkanes!!!!!!!!!!
Substituents!
v If two chains of equal length, choose the greater number of substituents (branched groups connected to the parent chain; side chains).!
!!!!!!!!!
Cycloalkanes!
v Cyclic alkanes (ring structures)!!!!!!!!!
Identifying and Naming Substituents!
v Identify the parent chain and substituents (side chains).!!!!!!v Naming of substituents: analogous to parent naming, yet add “yl.”!!!
Identifying and Naming Substituents!
v Identify the parent chain and substituents (side chains).!!!!!!v Naming of substituents: analogous to parent naming, yet add “yl.”!!!
3!
Alkyl Groups! Identifying and Naming Substituents: Which one?!
v Identify the parent chain and substituents: number of carbons!!!!!!!!
Complex Substituents! Complex Substituents!
1. Number the longest carbon chain within the substituent. Start with the carbon directly attached to the main chain.!
2. Name the substituent (in this case butyl).!
3. Name and number the substituent’s side group (in this case 2-methyl).!
Therefore, the name of this substituents is (2-methylbutyl). !!
Common Name in Complex Substituents! Common Name in Complex Substituents!
4!
Common Name in Complex Substituents! Systematic Name of an Alkane!
v the location of the methyl group is identified with a number, called a locant.!
Systematic Name of an Alkane!
v the location of the methyl group is identified with a number, called a locant.!
Guidelines!
1.!!!!!!2.!
Guidelines!
1.!!!!!!2.!
Guidelines!
3.!!!!!!
4.!
5!
Guidelines!
3.!!!!!!
4.!
Guidelines!
5. A prefix is used (di, tri, tetra, penta, etc.) if multiple substituents are identical.!1,1,3-trimethylcyclohexane!
!v List all substituents before the parent chain name in alphabetical order.!v Prefixes are not used for alphabetical purposes, except for the prefix “iso.”!
Assembling the Entire Name!
5. A prefix is used (di, tri, tetra, penta, etc.) if multiple substituents are identical.!1,1,3-trimethylcyclohexane!
v List all substituents before the parent chain name in alphabetical order.!v Prefixes (di, tri, tetra, penta, etc.) are not used for alphabetical purposes. Only two prefix that are used alphabetizing is "iso” as in isopropyl or isobutyl and “neo” as in neopentyl. However, the prefixes sec- and tert- are not used.!
1 3 5 7 92 4 6 8
135792468
6-Ethyl-4-methylnonane 4-Ethyl-6-methylnonane
parent
substituents
One more!
!
1 3 5 7 92 4 6 8
135792468
6-Ethyl-4-methylnonane 4-Ethyl-6-methylnonane
parent
substituents
One more!
!
1 3 5 7 92 4 6 8
135792468
6-Ethyl-4-methylnonane 4-Ethyl-6-methylnonane
parent
substituents
One more!
!
6!
One more!
!
1 3 5 7 92 4 6 8
135792468
6-Ethyl-4-methylnonane 4-Ethyl-6-methylnonane
parent
substituents
Assembling the Entire Name!
1. Identify the parent chain.!!2. Identify and name substituents.!!3. Number the parent chain and assign a locant to each substituent.!!4. Arrange substituents alphabetically.!!
Assembling the Entire Name!
1. Identify the parent chain.!!2. Identify and name substituents.!!3. Number the parent chain and assign a locant to each substituent.!!4. Arrange substituents alphabetically.!!!
Practice Problem"!
Bicyclic System!
v Bicyclic compounds: containing two fused ring!!!!!!v Prefix: bicyclo– (the above should be bicycloheptane)!
v Bridgeheads: Identifying the constitution!of compounds!
!!!!
Bicyclic System: Bridgehead!
v Bicyclic compounds: containing two fused ring!!!!!!v Prefix: bicyclo– (the above should be bicycloheptane)!
v Bridgeheads: Identifying the constitution!of compounds!
!!!
Bicyclic System!
v Bicyclic compounds: containing two fused ring!!!!!!v Prefix: bicyclo– (the above should be bicycloheptane)!
v Bridgeheads: Identifying the constitution!of compounds!
!v Bicyclo[2.2.1]heptane!!!
7!
Bicyclic System: Substituents!
v To number the bicyclo parent chain, start at a bridgehead carbon and number the longest carbon chain connecters first.!
v Without violating the rule above, give the substituents the lowest numbers possible.!
!!!!!!
v 6-Methylbicyclo[3.2.1]octane !!
Bicyclic System: Substituents!
v To number the bicyclo parent chain, start at a bridgehead carbon and number the longest carbon chain connecters first.!
v Without violating the rule above, give the substituents the lowest numbers possible.!
!!!!!!
v 6-Methylbicyclo[3.2.1]octane !!
Name?! Name? Two Bridgehead Carbons!
●
▲
4.3 Constitutional Isomers of Alkanes! Constitutional Isomer?!
8!
▲
●
▲
●
Yes!
Single bonds freely rotate.
Constitutional Isomer?!
Yes!
4.4 Relative Stability of Isomers!
Stable = low potential energy = low reactivity != little energy will be released upon reacting
v The relative stability of constitutional isomers can be measured by combustion experiment: Heat of combustion (–ΔH°) – change in enthalpy
Relative Stability of Isomers!
Stable = low potential energy = low reactivity != little energy will be released upon reacting
v The relative stability of constitutional isomers can be measured by combustion experiment: Heat of combustion (–ΔH°) – change in enthalpy
Relative Stability of Isomers!
Stable = low potential energy = low reactivity != little energy will be released upon reacting
v The relative stability of constitutional isomers can be measured by combustion experiment: Heat of combustion (–ΔH°) – change in enthalpy
Relative Stability of Isomers!
Experimental result: branched alkanes are lower in energy (i.e. more stable)!than straight-chain alkanes
v The relative stability of constitutional isomers can be measured by combustion experiment: Heat of combustion (–ΔH°) – change in enthalpy
4.6 Three-Dimensional Representation: Newman Projections!
v To predict rotation about C–C single bonds allows a compound to adopt a variety of possible three- dimensional shapes, called conformations.!
v Newman projection will help to visualize conformational variation of a molecule.!
45°! 45°!
9!
4.6 Three-Dimensional Representation: Newman Projections!
v To predict rotation about C–C single bonds allows a compound to adopt a variety of possible three- dimensional shapes, called conformations.!
v Newman projection will help to visualize conformational variation of a molecule.!
45°! 45°!
4.6 Three-Dimensional Representation: Newman Projections!
v To predict rotation about C–C single bonds allows a compound to adopt a variety of possible three- dimensional shapes, called conformations.!
v Newman projection will help to visualize conformational variation of a molecule.!
45°! 45°!
Three-Dimensional Representation: Newman Projections!
v To predict rotation about C–C single bonds allows a compound to adopt a variety of possible three-dimensional shapes, called conformations. (model)!
v Newman projection will help to visualize conformational variation of a molecule.!
45°! 45°!
Three-Dimensional Representation: Newman Projections!
Practice: Newman Projections! 4.7 Rotational Conformations!
v Dihedral Angle (torsional angle): 0 to 180° ––> an infinite !
number of possible conformations with different energies. !
10!
Rotational Conformations!
v Dihedral Angle (torsional angle): 0 to 180° ––> an infinite !
number of possible conformations with different energies. !
v The lowest and highest conformations:
dihedral angle = 0°!
Conformational Analysis of Ethane!
torsional!stain!
(12 kJ/mol)!
Conformational Analysis of Ethane: Torsional Strain Energy! Conformational Analysis of Ethane: Torsional Strain Energy!
v Torsional strain can be explained !
via i) VSEPR theory: minimizing electrostatic repulsion between electron pairs !
ii) Molecular Orbital Theory: in the staggered conformation, the bonding! and antibonding MOs of neighboring carbons overlap.!
Conformational Analysis of Ethane: Torsional Strain Energy!
v Torsional strain can be explained !
via i) VSEPR theory: minimizing electrostatic repulsion between electron pairs !
ii) Molecular Orbital Theory: in the staggered conformation, the bonding! and antibonding MOs of neighboring carbons overlap.!
Conformational Analysis of Propane!
11!
Conformational Analysis of Propane!
= ? kJ/mol !
Conformational Analysis of Propane!
= ? kJ/mol !
= 4 kJ/mol !
Conformational Analysis of Propane!
= 6 kJ/mol !
= 4 kJ/mol !
Conformational Analysis of Butane!
Three Staggered Conformations!
3.8 kJ/mol!
Anti Conformation!
anti conformation!dihedral angle!
= 180°!
gauche interaction!dihedral angle!
= 60°!
gauche interaction!dihedral angle!
= 60°!
12!
Two Degenerated Gauche Conformations!
anti conformation!dihedral angle!
= 180°!
gauche interaction!dihedral angle!
= 60°!
gauche interaction!dihedral angle!
= 60°!
3.8 kJ/mol!
Three Eclipsed Conformations!
Two Degenerated Eclipsed Conformations! Eclipsing Interaction of Two Methyl Groups!
Eclipsing Interaction of Two Methyl Groups! Relative Energy Costs of Butane’s Rotational Conformations!
13!
4.9 Cycloalkanes!
v sp3 hybridization
v Angle Strain vs. Stability!
!
Can you remember?!
Stable = low potential energy = low reactivity != little energy will be released upon reacting
v The relative stability of constitutional isomers can be measured by combustion experiment: Heat of combustion (–ΔH°) – change in enthalpy
Cycloalkanes: Heats of Combustion per CH2 Group! Cyclopropane!
v Angle strain:!a. Bond angles of 60° cause
electron pair repulsion in adjacent bonds!
b. Inefficient sigma bond overlap!: bent bond (banana bond)
v Torsional strain:!Eclipsing C–H bonds all the way around the ring—see Newman projection!
!
!
Cyclopropane!
v Angle strain:!a. Bond angles of 60° cause
electron pair repulsion in adjacent bonds!
b. Inefficient sigma bond overlap!: bent bond (banana bond)
v Torsional strain:!Eclipsing C–H bonds all the way around the ring—see Newman projection!
!
!
Cyclobutane!
v Angle strain: less than cyclopropane!
v Torsional strain: morethan cyclopropane!
14!
Cyclobutane!
v Angle strain: less than cyclopropane!
v Torsional strain: more than cyclopropane – a slightly puckered conformation!
Cyclopentane!
v only 5 kJ/mol less stable than cyclohexane per CH2 group!
Cyclopentane!
v Angle strain: less than cyclopropane and cyclobutane – close to the optimal
value!
v Torsional strain: the minimal but significant!
Cyclopentane!
v Angle strain: less than cyclopropane and cyclobutane – close to the optimal
value!
v Torsional strain: the minimal but significant!
4.10 Conformations of Cyclohexane!
In both chair and Boat conformations,"
v Angle strain (ring strain): zero (109.5°)!!
In only chair conformation,"
v Torsional strain: zero! – all adjacent C-H bonds are staggered.
Conformations of Cyclohexane!
In both chair and Boat conformations,"
v Angle strain (ring strain): zero (109.5°)!!
In only chair conformation,"
v Torsional strain: zero! – all adjacent C-H bonds are staggered.
Demo!chem 3-D via RDC!
!Utilize a molecular model set!!
15!
Chair Conformation!
"
v No Angle strain (ring strain): 109.5°!
"
v No Torsional strain: all adjacent C-H bonds are staggered.
Boat Conformation!
"
v No Angle strain (ring strain): 109.5°!
v Two types of Torsional strain:!
1. eclipsed C-H bonds!
2. flagpole interactions!
Twisted Boat Conformation!
To alleviate the torsional strain in the Boat conformation,!
!
!
!
!
!
!
or other possible conformations…. !
Conformational Analysis of Cyclohexane!
v Ground state !conformation
4.11 Drawing Chair Conformations!
v It is critical to draw a CHAIR properly; Use three sets of parallel lines.!!!!
Drawing Chair Conformations!
v It is critical to draw a CHAIR properly; Use three sets of parallel lines.!!!!
16!
v SIX of the atoms attached to the chair are axial: Axial groups point straight up and down alternating around the ring. !
!!!!!!!
Drawing Chair Conformations!
v The other SIX atoms attached to a chair are in equatorial positions: angles parallel to the sets of parallel lines making up the chair itself!
!!!!!v Overall, !!
Drawing Chair Conformations!
v The other SIX atoms attached to a chair are in equatorial positions: angles parallel to the sets of parallel lines making up the chair itself!
!!!!!v Overall, !!
Drawing Chair Conformations! 4.12 Monosubstituted Cyclohexane!
v Begin with the ground state conformation (energetically most stable):!!Chair conformation (two possibilities)!
!
ring flip"
require 45 kJ/mol!!
Ring Flip !
no direct way!
Ring Flip !
v Flipping a chair is the result of C–C single bonds rotating only.!
v Affecting the axial or equatorial position of the substituents.!!
Newman projection!
17!
Stability of Chair Conformations!
v 1,3-diaxial interactions:!steric hindrance !
(electron repulsion)!X = CH3!
1!
3!
3’!
Stability of Chair Conformations!
1!
3!
3’!
1,3-Diaxial Interaction: Gauche Interaction! Equatorial Substituent: Anti Interaction!
equatorial!
Steric Hindrance!
v Conformational equilibrium depends upon the steric hindrance, resulting from
1,3-diaxial interaction.!!
1,3-Diaxial Interactions!
X =!
18!
4.13 Disubstituted Cyclohexane --- Additional Feature!
v Configuration: the spatial arrangement of the atoms of a molecular entity – the
three-dimensional orientation of substituents!!
XY
Disubstituted Cyclohexane --- Additional Feature!
v Configuration: the spatial arrangement of the atoms of a molecular entity – the
three-dimensional orientation of substituents (cf. conformation, see p41)"!
XY
Disubstituted Cyclohexane --- Additional Feature!
v Configuration: the spatial arrangement of the atoms of a molecular entity – the
three-dimensional orientation of substituents (cf. conformation, see p41)"!
XY
XY
XY
XY
XY
1,2-Disubstituted Cyclohexane: Chair Conformations!
1! 2!
1,3-Disubstituted Cyclohexane: Chair Conformations!
?
1,3-Disubstituted Cyclohexane: Chair Conformations!
1,3-diaxial interaction!
19!
1,3-Disubstituted Cyclohexane: Chair Conformations!
?
1,3-Disubstituted Cyclohexane: Chair Conformations!
cis–trans Stereoisomerism"
Haworth projections!
v cis: two groups are on the same side.!
v trans: two groups are on opposite sides."
More generally, !cis-trans Isomerism: differ in the positions of atoms (or groups) relative to a
reference plane
cis–trans Stereoisomerism"
v cis: two groups are on the same side.!
v trans: two groups are on opposite sides."
More generally, !cis-trans Isomerism: differ in the positions of atoms (or groups) relative to a
reference plane – different compounds
cis–trans Stereoisomerism" cis–trans Stereoisomerism"
20!
4.15 Polycyclic Systems: Dacaline!
v Decalin: a bicyclic system composed of two fused six-membered rings!
!
!
!
!
!
!
!
v cis- and trans-decalin: a stereoisomeric relationship !
!– not interconvertible by ring flip!
Dacaline in Steroids!
Dacaline in Steroids! Norbornane!
Diamond Structure!
gauchebutane
antibutane
Diamond Structure is Useful for Conformational Analysis!
HH
H H HH
HH H H
HHH
H H H H H
H
H
gauchebutane
antibutane
21!
Anti Conformation!
anti conformation!dihedral angle!
= 180°!
gauche interaction!dihedral angle!
= 60°!
gauche interaction!dihedral angle!
= 60°!
