Handout #4, 5.12 Spring 2003, 2/14/02
Alkanes: Nomenclature and Conformational Analysis
Name
n-Alkane
Formula
(CnH2n+2)
Name
n-Alkyl
Substituent
# Constitutional
Isomers
CH4
C2H6
methane
ethane
–CH3 (–Me)
–C2H5 (–Et)
methyl
ethyl
1
1
C3H8
C4H10
propane
butane
–C3H7 (–nPr)
–C4H9 (–nBu)
propyl
butyl
1
2
C5H12
C6H14
pentane
hexane
–C5H11
–C6H13
pentyl
hexyl
3
5
C7H16
C8H18
heptane
octane
–C7H15
–C8H17
heptyl
octyl
C9H20
C10H22
nonane
decane
–C9H19
–C10H21
nonyl
decyl
C11H24
C12H26
undecane
dodecane
–C11H23
–C12H25
undecyl
dodecyl
C13H28
C14H30
tridecane
tetradecane
–C13H27
–C14H29
tridecyl
tetradecyl
C15H32
pentadecane
–C15H31
pentadecyl
9
18
etc.
Constitutional Isomers
(Be able to recognize these common fragments.)
methylMe
CH3
CH3
2
1 CH3
sec-butyl­
(1-methylpropyl-)
s
Bu
1
2 CH3
3
Pr
iso-butyl­
(2-methylpropyl-)
n-butyl­
nBu
CH3
tert-butyl­
(1,1-dimethylethyl-)
CH3
H3C
tBu
CH3
n
CH3
Et
iso-propyl­
(1-methylethyl-)
iPr
n-propyl­
ethyl­
CH3
1 CH3
2
sBu
1
2 CH3
3
CH3
neopentyl­
(2,2-dimethylpropyl)
1
3
2 CH3
H3C CH3
1
Conformational Analysis: The Fundamentals
Newman Projections
H
H
H
H
H
H
H
H
H
H
H
back carbon
(circle)
H
front carbon
(point)
θ = dihedral angle
Barrier to Rotation: torsional energy; energy required to twist a bond into a specific conformation;
Barrier to Rotation = Erel (highest energy conformer) – Erel (lowest energy conformer)
Ethane
H
H
H
H
H
H
staggered
θ = 60°
Erel = 0 kcal/mol
Propane
HH
H
H
H
H
H
H
H
Me
HH
H
H
H
H
staggered
θ = 60°
Erel = 0 kcal/mol
eclipsed
θ = 0°
Erel = 3.0 kcal/mol
Barrier to Rotation = 3.0 – 0 = 3.0 kcal/mol
Me
H
eclipsed
θ = 0°
Erel = 3.3 kcal/mol
Barrier to Rotation = 3.3 – 0 = 3.3 kcal/mol
H/H-eclipsing = 1.0 kcal/mol
H/Me-eclipsing = 1.3 kcal/mol
Butane
MeMe
H
H
H
H
H
fully eclipsed
(eclipsed)
θMe/Me = 0°
E rel = 6.0 kcal/mol
H
Me
H
HMe
Me
H
H
H
gauche
(staggered)
θMe/Me = 60°
Erel = 0.9 kcal/mol
H
Me
H
H
eclipsed
(eclipsed)
θMe/Me = 120°
E rel = 3.6 kcal/mol
Me
Me
H
H
anti
(staggered)
θ Me/Me = 180°
Erel = 0 kcal/mol
Barrier to Rotation = 6.0 – 0 = 6.0 kcal/mol
Me/Me-eclipsing = 4.0 kcal/mol*
Gauche Butane Interaction (gbi) = 0.9 kcal/mol
Numbers to Remember
H/H-eclipsing = 1.0 kcal/mol
H/Me-eclipsing = 1.3 kcal/mol
* In Wade, a Me/Me-eclipsing interaction is said to
be worth 3.0 kcal/mol. Either value will be
acceptable in this class, but most other sources
assign the value of 4.0 kcal/mol. A Me/Me-eclipsing
interaction is significantly higher than an H/H- or
H/Me-eclipsing interaction because the hydrogens
on the two methyl groups can actually bump into
each other (steric strain).
Me/Me-eclipsing = 4.0 kcal/mol*
Gauche Butane (gbi) = 0.9 kcal/mol
H
H
H
H/H
H H
H
H
H
H/Me
H
H
H
H
Me/Me
2
Cycloalkanes:
Names and Ring Strain
cyclopropane
(27.6 kcal/mol)
cyclobutane
(26.4 kcal/mol)
cyclopentane
(6.5 kcal/mol)
cyclohexane
no ring strain!
cycloheptane
(6.3 kcal/mol)
cyclooctane
(9.6 kcal/mol)
Substituted Cyclohexane
Drawing Cyclohexane Chairs
Steps 1–3: Draw three pairs of parallel lines as shown
X
equatorial: C–X bond
anti to ring bonds
axial: C–X bond gauche
to two ring bonds
(1)
(2)
(3)
Steps 4–6: Draw equatorial bonds parallel to ring bonds in bold
X
H
(4)
(5)
(6)
Steps 7,8: Draw axial bonds as vertical lines below and above ring.
H
• This means that axial groups
experience 1,3-diaxial
interactions; "bump into"
other axial groups.
X
• EQUATORIAL PREFERRED
A-Values to Remember
–X
(7)
(8)
Ring Flip
6
4
2
5
2
3
6
4
1
5
3
1
• Interconverts equatorial and axial substituents.
• Energetic barrier = 10 kcal/mol; occurs rapidly at room
temperature.
–CH3
–CH2CH3
–CH(CH3)2
–C(CH3)3
–CN
–OH
–Cl
–Br
A-Value
1.74
1.8
2.1
5.4
0.2
1.0
0.5
0.6
• A-Value = magnitude of
equatorial preference
3
Di-Substituted Cyclohexane: Which Conformer is Preferred?
1. Try to place all substituents equatorial.
Me
Me
Me
Me
1
3
Me
1
Me
3
Me
12
Me
Me
1
Me
2
2
1
3
preferred
Me
1
Me
preferred
2. If you can't, place the substituent with the largest A-value equatorial.
iPr
Et
iPr
Et
1
1
1
3
Me
Me
3
Et
3
Me
1
preferred
4
Et
1
Et
Et
4
iPr
4
1
preferred
REMEMBER: Midterm #1 is Friday, February 21, in Walker. It will cover reading and
lecture material through Tuesday, February 18. This means that not all of the
material on this handout will be covered on the exam.
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