Transcript Alkanes: Structure and Conformation

Alkanes: Structure and Conformation
• Compounds Contain Only C, H
 General Formula: CnH2n+2 (Saturated)
• Common Source of Alkanes: Petroleum
• Separation Technique: Fractional Distillation
• Boiling Point (Size) Method of Separating
• Basic Building Block of More Complex Organics
Alkane Shape: Straight Chains
Propane
Pentane
Butane
Heptane
• “Straight Chain” Better Termed “Unbranched”
• Actually Zig-Zag Structures (Tetrahedral Carbon Atoms)
• All Carbons sp3 Hybridized
Branched Alkanes: Simple
CH3
CH3
CH
CH
H3 C
H3C
CH3
Isobutane
CH3
C
H2
Isopentane
CH3
H3C
C
CH3
CH3
Neopentane
• Constitutional Isomers: Same Formula; Different Connectivity
 Butane and Isobutane: C4H10
 Pentane, Isopentane, Neopentane: C5H12
 Different Properties: BP, MP, Density, Refractive Index etc.
 Number of Constitutional Isomers Increases w/ # of Carbons
Alkane Nomenclature: The Rules
Unbranched Alkanes
#C
1
Name
Methane
#C
11
Name
Undecane
2
3
4
Ethane
Propane
Butane
12
13
14
Dodecane
Tridecane
Tetradecane
5
6
7
Pentane
Hexane
Heptane
15
16
17
Pentadecane
Hexadecane
Heptadecane
8
9
10
Octane
Nonane
Decane
18
19
20
Octadecane
Nonadecane
Eicosane
Alkyl Group Nomenclature
Unbranched Alkyl Groups
#C
1
Name
Methyl
#C
11
Name
Undecyl
2
3
4
Ethyl
Propyl
Butyl
12
13
14
Dodecyl
Tridecyl
Tetradecyl
5
6
7
Pentyl
Hexyl
Heptyl
15
16
17
Pentadecyl
Hexadecyl
Heptadecyl
8
9
10
Octyl
Nonyl
Decyl
18
19
20
Octadecyl
Nonadecyl
Eicosyl
Branched Alkanes
1. Locate Longest Continuous Chain (Parent Name)
2. Number Carbons in Chain; Begin @ End Nearest Substituent
3. Use Number of C on Parent Chain; Locate Substituents
4. Assign C Number to Each Substituent
5. Use Same C Number If Multiple Substituents
6. If Substituents Identical, Use Di-, Tri-, Tetra- Designations
7. Equal Length Chains Compete, Parent Chain Most Substituted
8. First Branches Equivalent; Choose Lowest Possible # Set
Branched Alkanes: Examples
1
3
3
4
5
6
5
8
3,5-dimethyloctane
4-isopropyl-3,5,6-trimethylnonane
2,3,5-trimethyl-4-propylheptane
9
Branched Alkanes: Examples
• Once Long Chain Found; Simplify w/ Alkyl Abbreviations
1
i
Pr
Me
Me
3
3
Me
4
5
6
5
8
Me
Me
4-isopropyl-3,5,6-trimethylnonane
3,5-dimethyloctane
Pr
Me
Me
Me
2,3,5-trimethyl-4-propylheptane
9
Branched Alkyl Groups
isopropyl
sec-butyl
neopentyl
(2,2 dimethylpropyl)
isobutyl
tert-butyl
• Can Name as Simple Alkane w/ “yl” Ending Replacing “ane”
Alkyl Halides
• Alkyl Halides Named as “Halo”-Alkane
 “Halo” = Fluoro, Chloro, Bromo, Iodo
Br
Br
I
2,4-dibromo-3-iodohexane
Cl
Cl
2,3-dichloropentane
Br
F
fluoropropane
Cl
6-bromo-3-chloro-4-isopropyloctane
Alcohols
Br
OH
I
5-bromo-4-iodo-3-hexanol
OH
OH
pentane-2,3-diol
OH
HO
propanol
Cl
6-chloro-5-isopropyl-3-octanol
(6-chloro-5-isopropyloctan-3-ol)
• Note the “-ol” Ending
• Use Same Di-, Tri-, Tetra- to Indicate Multiple
• We’ll Designate Alcohols as Priority Groups (Give Low #)
Monocyclic Alkanes/Alcohols
Lowest Number
Set
cyclopentane
cyclobutane
1,3-dimethylcyclohexane
tert-butylcyclooctane
1-ethyl-2-methylcycloheptane
• “Cyclo” Added  Indicates Cyclic Structure
Bicyclic Alkanes
bicyclo[2.2.2]octane
bicyclo[3.3.0]octane
bicyclo[2.2.1]heptane
8-methyl-bicyclo[4.3.0]nonane
bicyclo[4.4.0]decane
(decalin)
Terminal Alkenes
Feature an “ene” Ending
#C
1
Name
---
#C
11
Name
Undecene
2
3
4
Ethene
Propene
Butene
12
13
14
Dodecene
Tridecene
Tetradecene
5
6
7
Pentene
Hexene
Heptene
15
16
17
Pentadecene
Hexadecene
Heptadecene
8
9
10
Octene
Nonene
Decene
18
19
20
Octadecene
Nonadecene
Eicosene
Alkenes: Non-Terminal
trans-2-hexene
1,4-cyclohexadiene
cis-3-heptene
2,3-dimethyl-2-butene
Alkynes
propyne
2-methyl-3-hexyne
Br
Cl
2-bromo-5-chloro-3-heptyne
1-cyclohexylbutyne
Properties of Alkanes
• Boiling Point Increases Regularly for Unbranched
• Branching Lowers Boiling Points (Van der Waals, SA)
• Melting Point in Unbranched Increases Regularly Within
ODD or EVEN Numbered Series (Not Both)
• Density Less Than 1.0 g/mL (Less Than H2O)
• Solubility: Quite Insoluble in H2O; Less Dense  Float
 Non Polar (Like Dissolves Like)
 No Hydrogen Bonding
Sigma Bonds in Hydrocarbons
• Alkane Sigma (s) Bonds Formed From sp3 Hybridized C
• These Bonds Can Freely Rotate
• Temporary Shapes Adapted via Rotation: CONFORMERS
• Conformers can have Different Energies
• Need to Have a System for Depicting Various Conformations
 Newman Projections
 Sawhorse Formulas
Newman Projections/Sawhorse Formula
R
R
R
R
Newman Projection
Sawhorse Formula
• R Groups 180° Apart (Anti Conformation)
• 4 Atom Angle (3 Bond Angle) is a Dihedral (Torsional) Angle
• Having Large Groups Anti is Low in Energy
Newman Projections/Sawhorse Formula
R R
Newman Projection
R
R
Sawhorse Formula
• When R Groups 0° Apart (Ecclipsed Conformation)
• Having Large Groups Anti is High in Energy
• Can Adapt any Range of Conformations in Between
Conformational Analysis: Butane
Me
Me
Butane
Me Me
Energy
H Me
H
H
Me
Eclipsed
H
H
H
H
H Me
H
Eclipsed
H
H
Me
H
H
H
Me
H
Eclipsed
Me
Me
H
H
Me
H
H
H
H
H
Gauche
Me
H
H
Gauche
Me
H
H
H
H
Me
Me
Anti
Anti
• Can Plot these Points and Connect w/ Curve: PES
Cyclohexane Conformation
Ax
Ax
Eq
Ax
Eq
Eq
Eq
Eq
Ax
Ax
Ax
Eq
Eq
Eq
Eq
Eq
Eq
Eq
• Cyclohexanes Adapt Chair Conformations (Boats and Others)
• Ax: Axial (Straight up and Down on Chair)
• Eq: Equatorial (Parallel to Next Bonds over in Chair)
Cyclohexane Conformation: Cis
Me
Me
Me
Me
Ring Flip
Me
Me
• Methyls are Cis (Same); Two Energy Equivalent Chairs
• Ring Flips Interchange Chair Conformations
Cyclohexane Conformation: Trans
Me
Me
Me
Ring Flip
Me
Me
Minor
Me
Major
• Methyls are Cis (Same); Two Energy Inequivalent Chairs
• Diequatorial Much More Stable than Diaxial Conformation
Cyclohexane Conformation: Notes
• With Multiple Same Substituents; More Equatorial = Better
• Larger Groups Tend to Adopt Equatorial Positions
• tert-butyl Groups Nearly Always Equatorial
• Conformation Setters (Lock Ring)
• Set Other Groups Relative to tert-butyl
• Fused Rings (Decalin, for Example) Can be Drawn as Chairs
cis-Decalin
trans-Decalin
Reactions: Hydrogenation
H2
Pt, Pd, or Ni (catalyst)
Solvent, Pressure
Alkene
Alkane
2H2
Pt (catalyst)
Solvent, Pressure
Alkyne
Alkane
• Addition of Hydrogen (H2) Across a Multiple (p) Bond
• Ethanol (CH3CH2OH) is a Common Solvent
Reactions: Alkyl Halide Reducation
HBr
+ ZnBr2
Zn
Br
H
HBr
Br
Zn
H
+ ZnBr2
• Zn Transfers Electrons to C of Alkyl Halide
• Alkyl Halide is Reduced (Reduction = Gains Electrons)
• Zn: Good Two Electron Donor (Reductant, Reducing Agent)
Reactions: Alkylation of Terminal Alkynes
H3C
H
H3C
H
NaNH2
NH3
NaNH2
NH3
H3C
CH3Br
EtBr
H3C
H3C
CH3
H3C
Et
• NaNH2 (-NH2) to Deprotonate Alkyne (Acid/Base Reaction)
• Anion Reacts with Alkyl Halide (Bromide); Displaces Halide
• Alkyl Group Added to Alkyne
• Alkyl Halide Must be 1° or Me; No Branching at 2nd (b) Carbon