Download ch04 - alkanes

Alkanes= saturated hydrocarbons
Chapter 4
Alkanes: Nomenclature,
Conformational Analysis, and an
Introduction to Synthesis
Simplest alkane = methane
CH 4
We can build additional alkanes by adding - CH 2 – units
C n H 2n+2 = saturated = no double bonds, no rings
Shapes of Alkanes
There is no limit on length of chain
“Straight-chain” alkanes have a zig-zag orientation when they
are in their most straight orientation
Polyethylene = an “infinite”
alkane
Branched alkanes
At least one carbon attached to more than two others
Many constitutional isomers are possible
Same molecular formula but different connectivity of atoms
Constitutional isomers have different physical properties
(melting point, boiling point, densities etc.)
5
“hexanes”
C6H 14
1
Source of alkanes = petroleum
The number of constitutional isomers possible for a given
molecular formula rise rapidly with the number of carbons
A complex mixture of organic compounds, largely alkanes
US uses 17 M barrels/day, 75% for energy, 8% chemicals
Refineries not only separate the compounds, but “reform” the
molecules by catalytic cracking and rearrangements
smaller alkanes
Crude
Petroleum
Refining
straightchain
alkanes
of
different
sizes
Cracking
Isomerization
Reforming
branched alkanes
Hyd rocarbon Fraction s from Petro le um
boi li ng range
of fraction ( o C)
s iz e range
A reaction of alkanes - Combustion
nam e and use
All hydrocarbons are combustible
C 6 to C7
n atural gas, bottl e d
gas, p etroche m icals
p e trole u m "ethe r".
s olve nts
l igroi n, sol ven ts
Heat of combustion is very high
be low 20
C 1 to C 4
20 to 60
C5 to C6
60 to 100
aromatics
40 to 200
C5 to C10
s traigh t-ru n gasol in e
175 to 325
C 12 to C 18
k e ros en e and je t fu el
250 to 400
C 12 and h igh e r
gas oil , fu el oil
and d ie se l oi l
non volati le
liq ui ds
C20 an d h igh er
non volati le
s oli ds
C 20 and hig he r
min e ral o il,
lu bri catin g oil
paraffi n wax,
asph al t, tar
Octane boosters (anti -knock additives)
Combine with oxygen and release energy
2C8 H18 + 25O2
16CO2( g) + 18H2O( l)
∆Hcomb = -5452 kJ/mol (or -47.8 kJ/g)
Examples of “reforming” to raise octane
Smoother engine performance if combustion is slowed
isooctane = 100
(2,2,5 -trimethylpentane)
(vs heptane = 0)
For decades, used Tetraethyl lead, now banned
For past decade, MTBE was used, now also banned
Some Octane Ra ting s of
Hydrocarbons and Addit iv es
Octa ne Rating
hepta ne
0
1-pentene
91
2,2,4-t rimethylpentane 10 0
benzene
10 6
met ha no l
1 07
ethanol
108
met hy l t-butyl ether
116
toluene
1 18
silica-alumina
CH3CH2CH2CH2CH2CH2CH3
catalyst, 500oC
heptane
20 atm H2
CH3 CH2 CH 2CH2CH2CH3
hexane
a cid catalyst
CH3
+ 4H2
CH3 CH2CH2CHCH3
CH3
+
CH3CH2 CHCH2 CH3
CH3
branched a lkanes
2
lNomenclature of Unbranched Alkanes
IUPAC Nomenclature
t Early chemicals were given “common” or “trivial” names based
on the source of the compound or a physical property
t The International Union of Pure and Applied Chemistry (IUPAC)
started devising a systematic approach to nomenclature in 1892
t The fundamental principle in devising the system was that each
different compound should have a unique unambiguous name
t The basis for all IUPAC nomenclature is the set of rules used for
naming alkanes
Nomenclature of Unbranched Alkyl groups
Nomenclature of Unbranched Alkanes – Learn the first ten!
The unbranched alkyl groups are obtained by removing
one hydrogen from the alkane and named by replacing
the -ane of the corresponding alkane with - yl
Nomenclature of Branched-Chain Alkanes (IUPAC)
Nomenclature of Branched -Chain Alkanes (IUPAC) – cont.
1. Locate the longest continuous chain of carbons; this is the
parent chain and determines the parent name.
4.
7 carbons = a heptane
When two or more substituents are present, give each
substituent a number corresponding to its location on the
longest chain
¶ Substituents are listed alphabetically
2. Number the longest chain beginning with the end of the chain
nearer the substituent
3. Designate the location of the substituent
4-ethyl -3-methylheptane
3
Nomenclature of Branched -Chain Alkanes (IUPAC) – cont.
5.
6. When two chains of equal length compete to be parent,
choose the chain with the greatest number of substituents
When two or more substituents are identical, use the prefixes
di-, tri-, tetra- etc.
a) Commas are used to separate numbers from each other
b) Repeat the number if two substituents on same carbom
c) The prefixes are used in alphabetical prioritization
7. When branching first occurs at an equal distance from either
end of the parent chain, choose the name that gives the lower
number at the first point of difference
4-ethyl -2,3,3 -trimethylheptane
Nomenclature of Branched Alkyl Chains
How many butyl groups are possible?
Two alkyl groups can be derived from propane
4-isopropylheptane
The neopentyl group is a common branched alkyl group
Classification of Hydrogen Atoms
Hydrogens take their classification from the carbon they
are attached to
4
Nomenclature of Alkyl Halides
What type of hydrogens in
neopentane?
t In IUPAC nomenclature halides are named as substituents
on the parent chain
l Halo and alkyl substituents are considered to be of equal ranking
CH3
CH3-C-CH3
CH3
t In common nomenclature the simple haloalkanes are named
as alkyl halides
l Common nomenclature of simple alkyl halides is accepted by IUPAC
and still used
Nomenclature of Alcohols
In the IUPAC naming
system, there may be as
many as four components
to the name:
IUPAC Nomenclature of Alcohols
Locant indicates the position
of a substituent.
Prefix names the substituent group.
1. Select the longest chain containing the hydroxyl and
change the suffix ending of the parent alkane from -e to -ol
Pare nt is the pa rent a lka ne.
2. Number the parent to give the hydroxyl the lowest possible
Suffix names a key function.
number
Examples
3. The other substituents take their locations accordingly
OH
CH3 CH2CHCH 3
CH3 CHCH2 CH 2OH
CH3
parent
2-butanol su ffix
locant
parent
3-methyl-1 -buta no l
locant
prefix locantsuffix
l Examples
4-methyl-1-hexanol
t Alcohols with two hydroxyls are called diols in
IUPAC nomenclature and glycols in common
nomenclature
l Common Names of simple alcohols are still often used
and are approved by IUPAC
5
Nomenclature of cycloalkanes
Nomenclature of Substituted Cycloalkanes
t When one substituent is present it is assumed to be at
position one and is not numbered
t Ring compounds are very common
t Named as cyclo + base name for number of C in ring
t When two alkyl substituents are present the one with
alphabetical priority is given position 1
l Numbering continues to give the other substituent the lowest
number
l Hydroxyl has higher priority than alkyl = position 1
cyclobutane
cyclopentane
cyclooctane
Cycloalkyl group
t If other parts of the molecule are dominent or if a long chain
is attached to a ring with fewer carbons, the cycloalkane is
considered the substituent
Bicyclic Compounds – the Playground of Organic Chemists
l Bicyloalkanes contain 2 fused or bridged rings
l The alkane with the same number of total carbons is used
as the parent and the prefix bicyclo- is used
Bicyclic Compounds – the Playground of Organic Chemists
Beginning at bridgehead position, number around largest ring first,
then second largest ring
l The number of carbons in each bridge is included in the
middle of the name in square brackets
6
Nomenclature of Alkenes and Cycloalkenes
t Alkenes are named by finding the longest chain containing the
double bond and changing the name of the parent alkane from
-ane to -ene
t The compound is numbered to give one of the alkene carbons
the lowest number
Double bonds plus alcohol hydroxyl groups = alkenols
The hydroxyl is the group with higher priority and
must be given the lowest possible number
t The double bond of a cylcoalkene must be in position 1
and 2
Note that positions of both the C=C and the OH must be indicated
Vinyl and allyl groups
The vinyl and allyl groups are common names that
need to be readily recognized:
Alkene nomenclature – cis and trans
l If two identical groups occur on the same side of the
double bond the compound is cis
l If they are on opposite sides the compound is trans
l Several alkenes have common names which are
recognized by IUPAC
Alkynes = named similarly to alkenes
(4) Po sitions of substituents are determined by the usual rules .
(1) The name of the pa rent alkane is modified by dropping
the "ane" ending and adding "yne."
(2) The parent chain is numbered to give the carbons of
the alkyne lower numbers .
(3) The lo cation of the triple bond is g iven by the lower
positional number of the a lkynyl carbons.
5 4 3
2
CH 3CH2CH2 C
1-pen tyn e
1
CH
CH 3 C
CCH 3
2-butyne
4
3
C H3 C
5 4 3 2
CH3CH2CHC
CH 3
2 1
CCH 2C l
1-chloro -2 -butyn e
1
CH
3-methyl-1-pentyne
(5) In an alkynol, the a lcohol has priority in numbering.
4
HC
32 1
CCH2 CH2 OH
3-butyn-1-o l
1-alkynes are also called terminal alkynes
Terminal alkynes have an acidic H
R
C
CH pKa = 25
7
Physical Properties of Alkanes
Th e u nb ranch ed alk an es (CH4, CH3CH3, CH 3CH2CH3, CH3CH2CH2CH 3, etc.)
form a regul ar s eri es where each mem ber di ffers from th e n ext in order by CH2-. S uch a regu lar series is called a h omo logou s s eri es .
Melting points reflect size, but also packing factors
Propane: -188 oC
mp (oC)
The b oili ng poin ts of the u nbranche d alkane s incre ase more or
le ss regu larl y w i th in creasing si ze re flec tin g the i ncreasin g van
de r Waals attrac tion s .
pen tan e
bp (oC)
36
h exan e
hep tan e
69
98
Ethane: -183 oC
pentane
-130
hexane
-95
∆ = 35
heptane
-91
∆= 4
Methane: -182o C
octane
-57
∆ = 34
o ctan e
126
Sigma Bonds and Bond Rotation
Other physical properties of
hydrocarbons
t
Ethane has relatively free rotation around the carbon-carbon bond
t
The staggered conformation has C-H bonds on adjacent carbons as
far apart from each other as possible
l
The drawing to the right is called a Newman projection
Less dense than water
Immiscible with water and other highly polar solvents
Solvent for non -polar organic compounds
t The eclipsed conformation has all C-H bonds on adjacent
carbons directly on top of each other
How to draw a Newman projection
The potential energy diagram of the conformations of ethane
The staggered conformation is more stable than eclipsed by
12 kJ mol -1
Rotation rate
1011 /sec
Sight down the C-C bond (sigma bond is symmetric)
Energy barrier
called torsional
strain
Groups on front carbon intersect in center
Groups on back carbon end at circle
8
Bond Rotations in Propane
The co nformational
features are the same
for the two C-C bonds.
2 equivalent
C-C bonds
Analysis of the Extreme Conf ormations
H
H H
C
During o ne complete rotation around the internal bond in butane, the
three staggered conformations include two called "gauche" and one
called "a nti. "
H
C
H
C
H
H
CH3
H
H C
H
Consider as ethane with 1 H replaced by a CH3
H
H
120o
H
H C
C
rotation CH
3
CH3
H
o
H
H 120
CH3 C
C
H
CH3 rotation
H
H
C
CH3
The Newman proj ection formula s as viewed fro m the left are:
CH 3
CH 3
H
rotation
H
H
HH
H
H
H
H
CH 3
Barrier = 14 kJ/mol
H
eclipsed
H C
H
120 o
H
H
H
H
r otation
H
CH 3 rotation
C
CH3
H C
rotation CH
3
CH3
H
H
12 0o
H
H
H
H
rotation
H
CH3 ro tation
CH 3
anti
gauche
gauche
H
CH 3
steric
strain
H
CH 3
ga uche
3 minima and energy barriers between each (3 maxima)
H
H
C
CH3
H
H
CH 3
CH3
anti
H
Conformational Analysis of Butane
H
H
120o
H
C
CH3 C
H
CH3 rotation
H
120o
CH3
120 o
The stag gered anti is more stable than the two equivalent stagg ered gauche
conforma tions. In the a nti conformation, the t wo CH 3 groups are on
opposite sides of the structure. In t he ga uche co nformations, the two
groups are within van der W aals repulsive intera ction distance, and 3.8
kJ/mo l of steric strain energy is introduced.
Conformational Analysis of Butane
Rotation around C2 -C3 of butane gives three energy minima
The gauche conformation is less stable than the anti conformation by
3.8 kJ mol-1 because of repulsive van der Waals forces between
the two methyls
H
H
H
H
staggered
Conclusion: eclipsing of methyl with H adds very little to energy barrier
CH3
H
H
120o
H
H
CH 3
H
CH3
gauche
The Origin of Ring Strain in Cyclopropane :
Relative Stabilities of Cycloalkanes: Ring Strain
Angle Strain and Torsional Strain
t Heats of combustion per CH2 unit reveal cyclohexane has no
ring strain and other cycloalkanes have some ring strain
t
Angle strain is caused by bond angles different from 109.5o
t
Torsional strain is caused by eclipsing C-H bonds on adjacent C’s
l
l
C-C angle of 60 means orbital overlap is reduced
Cyclopropane has both high angle and torsional strain
H
H
C
H C
H
C H
H
Normal bond length sp 3 -sp3 C-C = 1.54 Å
Normal sp3 C-H =1.10 Å sp2 C-H=1.09 Å
9
Cyclobutane: Angle Strain and Torsional Strain
t Cyclobutane has considerable angle strain
t It bends to relieve some torsional strain
Cyclopentane
Th e i nternal an gles of a regu lar p en tagon
are 108o, close to the idealized tetrahedral
b ond an gles . Thu s, a plan ar cyclop en tan e
would have very littl e an gle strain.
108o
But a planar geometry would have very severe torsional strain (10
eclips ed H). Cons equently, the geometry of cyclopentane is bent.
H
The torsional strain is reduced in
t he bent structure. Four H are st ill
eclipsed, but 6 are st aggered.
H
H
H
H
H
H
H
H
H
bent or puckered
geomet ry
Cyclohexane: the perfect ring
Two types of H in chair cyclohexane
t Not 120o, the angle of simple hexagon
t Adopts a chair conformation with no ring strain!
t All bond angles are 109.5o and all C-H bonds are perfectly
staggered
H
6 H are axial
(up and down)
6 H are equatorial
(close to plane of ring)
H
H
Rotation about single bonds flips the ring to a boat form
t
The boat conformation is less stable because of flagpole interactions
and torsional strain along the bottom of the boat
2
1
H
H
H
H
H
Newman pro jections along
the C1-C6 a nd C3-C4 bonds
show the different orientations
of the axial and equatorial H.
H
6
H
1
H
5
H
3
2
CH2
CH2
5
4
H
H
H
H
H
H
3
H
eq
axial
H
Ring flipping leads to a second chair
Proceeds via boat (or slightly twisted boat)
Every axial position becomes equatorial, and vice versa
chair 1
boat
chair 2
Ring flipping occurs rapidly at room temperature
10
Conformational Energy Diagram for cyclohexane
Another view of energy barrier
half-chair
half-chair
42 kJ/mol
Flips about
100 x per sec
boat
99.9% of
molecules in
chair form
en ergetically preferred
conformation is the
tw is t boat
0
chair
chair
Note: kcal/mol x 4.2 = kJ/mol
Energy and Equilibrium
Drawing Cyclohexanes
t The C-C bonds and equatorial C-H bonds are all
drawn in sets of parallel lines
èThe axial hydrogens are drawn straight up and down
Note that 5 kJ/mol produces nearly a 90:10 preference
To draw chair cyclohexanes, follow these steps:
1.) Draw the carbon chair.
Cyclohexane structure can be elaborated
2.) Add the axial hydrogens.
Adamantane
C10 H16
3.) Draw the C1 and C4 equatorial hydrogens.
4.) Draw the remaining equatorial hydrogens.
Diadamantane
C14 H18
11
Substituted cyclohexanes
Diamond = “infinite” cyclohexane rings
Methyl cyclohexane is more stable with the methyl equatorial
l An axial methyl has an unfavorable 1,3-diaxial interaction with
axial C-H bonds 2 carbons away
More stable by
7.6 kJ/mol
95% eq
at R.T.
Destabilization of axial groups due to steric interactions
Equatorial groups are free of torsional strain
In axial methylcyclohexane there are two gauche interactions
H
H
H 4
H 5
H
CH 3
H
H 6
H
3
H
2
H
view a long t he C 1-C 2 bo nd
gauche
CH3
2
1
CH 2
6
1 H
H
CH2
H
H
3
H
5
H 4
view along the C1-C2 bond
H
3 H
H
H
H 6
1H
H
2 CH
3
H
eq uatorial-meth ylcyclohexane
3
CH2
H
2
6
CH2
1
CH3
H anti
H
H
axial-methylcyclohe xane
Each gauche interaction
introduces 3.75 kJ/mol of
steric strain
view along the C 3 -C 2 bond
CH g auche
3
H
2
3
H
CH3
4
CH2
anti
H
CH 2
H
H
2
1
CH2
3
4
CH2
H
1
Disubstituted cycloalkanes
The larger the group, the greater the equatorial preference
t-butylcyclohexane is >99.9% equatorial
view al on g the C 3-C 2 b on d
Equivalent to a
staggered butane
With two substituents on the ring in different positions,
configurational isomers arise
Cis: groups on same side of ring
Trans: groups on opposite side of ring
e.g.: 1,2 -dichlorocyclobutane
trans
isomer
cis
isomer
Cl
H
Cl
H
Cl
Cl
Cl
H
H
Cl
Cl
Cl
12
For 3, 4, or 5 membered rings, can treat as if planar
Three options for pairs of cis-trans stereoisomers
1,3-dimethylcyclopentanes
H
H
2
1
Configurational Isomers in Disubstituted Cyclohexanes
H
C H3
3
C H3
CH 3 H
cis
CH3
tr ans
Preferred comformations
Cis-1,4-dimethylcyclohexane exists in an axial-equatorial
conformation
Trans-1,4-dimethylcylohexane prefers trans-diequatorial conformation
Note that both are on the same side of ring
upper side in this projection
Note methyls on opposite sides of ring
Confirming whether cis or trans
Is diequatorial always trans?
Test one: identify each substituent as on the upper or lower side of ring
Di-equatorial isomer
CH3
H
u pp er b on d
CH3
low er b on d
up p er b on d
Since the two bo nds to the
CH3 g ro ups are "upper",
meaning sa me side, they
a re "cis."
low er b on d
H
H
Test Two: Flatten the Ring
H
1,3
CH3
H
CH3
trans
1,4
CH3
CH3
H
H
CH3
CH3
H
1,2
H
cis
Since the CH3 groups are
on the sa me side, they a re
"cis."
CH3
CH3
trans
H
SQUEEZE IT
13
Trans 1,2 dimethylcyclohexane: an energy analysis
CH3
H
2
The bicyclic decalin system exists in non-interconvertible cis
and trans forms
H
1 H
1
CH3
CH3
1,2-diaxial
Each axial m ethyl group
generates 2 x 3.75 = 7.5 kJ/m ol
of strain energy, for a total
of 15 kJ/mol.
Bicyclic and Polycyclic Alkanes
2 CH
3
H
1,2-diequatorial
There is one gauche
interaction from the
interacting CH 3 groups:
H
But the energy difference between the
1,2 -axial and the 1,2-diequa torial
co nformat ions is only 3 ga uche
intera ctions or 3 x 3.7 5 = 1 1.25 kJ/m ol.
CH2
1
CH2
2 CH
3
gauche
CH3
H
Synthesis of Alkanes and Cycloalkanes
Steroid backbone
1. Hydrogenation of Alkenes and Alkynes
cholesterol
Synthesis of Alkanes and Cycloalkanes
2. Reduction of Alkyl Halides
Synthesis of Alkanes and Cycloalkanes
3. Alkylation of Terminal Alkynes
l Alkynes can be subsequently hydrogenated to alkanes
14
Retrosynthetic Analysis-Planning Organic Synthesis
l The synthetic scheme is formulated working backward
from the target molecule to a simple starting material
l Often several schemes are possible
15