Ch 4 - Alkanes Flashcards
a flexible molecule is one that can
adopt many different shapes, or conformations
alkanes and cycloalkanes
lack a functional group which allow them to change their three dimensional shape as a result of rotating C-C bonds
conformational analysis
the study of three dimensional shapes of molecules
alkane(saturated hydrocarbons)
hydrocarbon which lacks pie bonds(all single bonds)
- the name usually ends in “ane” - propane, butane, pentane
nomenclature
the system for naming chemical compounds
IUPAC
international union of pure and applied chemistry
- set up the Geneva rules in 1892 to standardize organic nomenclature
Systemic names
names produced by IUPAC rules
4 steps to naming Alkanes
- identify the parent chain
- identify and name the substituents
- number the parent chain and assign a locant to each substituent
- Arrange the substituents alphabetically
4 steps to naming Alkanes
Step 1: Select the parent chain
- identify the longest chain
- if 2 chains equal then the one with more substituents is chosen - substituent – groups connected to the parent chain
- meth – 1 carbon – methane
- eth – 2 carbon – ethane
- prop – 3 carbon – propane
- but – 4 carbon – butane
- pent – 5 carbon – pentane
- hex – 6carbon – hexane
- hept – 7 carbon – heptane
- oct – 8 carbon – octane
- non – 9 carbon – nonane
- dec – 10 carbon – decane
- cycloalkanes – “cyclo” is used to indicate the presence of a ring in the structure of an alkane
4 steps to naming Alkanes
Step 2: Naming Substituents
same naming as above except with “yl” group
alkyl group – the above smaller chained groups attached to the parent chain
- when an alkyl group is next to a ring the ring is the parent as long as the ring has more carbons than the alkyl group
4 steps to naming Alkanes
Step 3: Naming Complex Substituents(parent and assigning locants to substituents)
- When a substituent has a branch in it find the longest part and number each carbon going away from the parent chain
- This becomes a miniparent chain
- (2-methylbutyl) is a butyl group with a methyl group coming off the 2nd carbon
- must be in parentheses
4 steps to naming Alkanes
Step 4: Assembling the Systemic Name of an Alkane
- number the atoms of the parent chain
- locant – the location of a group off the parent chain identified by a number by a carbon atom along the parent chain
- Rules:
- if one substituent is present – assign the lowest number possible
- when multiple substituents present – assign so the lowest number is assigned first
- if tied then use the second substituent as lowest
- if still tied assign alphabetically by other atoms(Br then Cl etc)
- all above rules apply to cycloalkanes
- when a substituent appears more than once then a prefix is used to identify how many times
- 1,1,3-trimethylcyclohexane
- di = 2
- tri = 3
- tetra = 4
- penta = 5
- hexa = 6
- after all substituents are assigned to proper locants the name can be arranged alphabetically(excluding prefixes for alphabetizing)
Naming parent chain
meth
1 carbon – methane
Naming parent chain
eth
2 carbon – ethane
Naming parent chain
prop
3 carbon – propane
Naming parent chain
but
4 carbon – butane
Naming parent chain
pent
5 carbon – pentane
Naming parent chain
hex
6 carbon – hexane
Naming parent chain
hept
7 carbon – heptane
Naming parent chain
oct
8 carbon – octane
Naming parent chain
non
9 carbon – nonane
Naming parent chain
dec
10 carbon – decane
cycloalkanes
“cyclo” is used to indicate the presence of a ring in the structure of an alkane
Naming Substituents
methyl
1 carbon
Naming Substituents
ethyl
2 carbon
Naming Substituents
propyl
3 carbon
Naming Substituents
butyl
4 carbon
Naming Substituents
pentyl
5 carbon
Naming Substituents
hexyl
6 carbon
Naming Substituents
heptyl
7 carbon
Naming Substituents
octyl
8 carbon
Naming Substituents
nonyl
9 carbon
Naming Substituents
decyl
10 carbon
Naming Alkanes recap:
- identify the parent chain
- identify and name the substituents
- number the parent chain and assign a locant to each substituent
- arrange the substituents alphabetically
bicyclic
compounds containing two fused rings
bridgehead
the two point which fuse two rings of carbon together
start at one bridgehead and number the longest path, then the next longest, then the shortest path
- if there is a sub group anywhere number it in such a way that it is the lowest number possible
try to look at molecules from the IUPAC point of view(parent chain and groups on chain)
helps identify when two isomers may be drawn a little different but are actually the same
use the heat liberated from the combustion with oxygen
to produce CO2 and water
the deltaH standard is the change in enthalpy associated with
the complete combustion of 1 mol of the alkane in the presence of oxygen
heat of combustion
the negative deltaH standard
branched alkanes are lower in energy(more stable) than
straight chain alkanes
heats of combustion are an important way to
determine the relative stability of compounds
where do alkanes come from naturally?
crude oil in the earth
cracking
C-C bonds of larger Alkanes are broken producing alkanes suitable for gasoline
- tend to be straight chains which increase knocking in the engine
reforming
the goal is to convert straight chain alkanes into branched and aromatic hydrocarbons
conformation
rotation of a C-C single bond allows a compound to adopt a variety of possible 3D shapes
Newman projection
drawing type designed to show the conformation of a molecule
sawhorse
Newman projection drawing after 45degrees of rotation
newman projections represent a snapshot
after 90 degrees of rotation where one carbon in directly in front of another
dihedral angle(torsional angle)
the angle of separation of two atoms in a Newman projection
- the value of a dihedral changes as the C-C bond rotates - can be any value between 0 and 180 degrees
there are an infinite number of conformations since
dihedral angles can be forever changing
staggered conformation
where atoms in a Newman projection are as far apart as possible
- lowest in energy
eclipsed conformation
where atoms in a Newman projection are as close as possible
- highest in energy
degenerate
equivalent energy of all conformations of the same type in Ethane
– all staggered conformations are the same amount of energy
- All eclipsed conformation are the same amount of energy
torsional strain
the difference in energy between staggered and eclipsed conformations of Ethane
the torsional strain in ethane is
12kJ/mol
- 4kJ/mol for each eclipsed H/H which means we can use this as a baseline for other torsional strains
- propane has a torsional strain of 14kJ/mol or 4(H/H),4(H/H), and 6(H/CH3)
the three eclipsed orbitals are not degenerate(the same)for butane
one has higher energy
the three staggered orbitals are not degenerate(the same) for butane
one is lower than the other two
anti conformation
dihedral staggered conformation at 180 degrees for butane
- represents the lowest energy conformation of butane - 3.8kJ/mol lower than the other two orbitals
two types of interactions for staggered conformations
- anti
- gauche
Anti conformation type
Methyl(CH3) groups are farthest apart
Gauche conformation type
methyl groups experience a gauche interaction
- electron clouds get close together and repel each other causing a need for more energy to keep together
gauche interactions
type of steric interaction
- different than torsional strain - when methyl groups are closer than 180 degree together and their electron clouds repel each other(trying to occupy the same region of space) creating an unfavorable interaction requiring more energy
H/H = 4kJ/mol
4kJ/mol
- torsional strain - eclipsed conformation
H/CH3 = 6kJ/mol
6kJ/mol
- torsional strain - eclipsed conformation
CH3/CH3 = 11kJ/mol
11kJ/mol
- torsional strain + steric interaction - eclipsed conformation
CH3/CH3 = 3.8kJ/mol
- steric interaction
- staggered conformation
angle strain
the increase in energy associated with a bond angle that has deviated from the preferred 109.5 degrees
- proposed by Adolph von Baeyer
cyclopropane has high energy
angle strain(small bond angles) and torsional strain(eclipsing H’s)
cyclobutane has less
angle strain than cyclopropane but has more torsional strain
cyclopentane has much less
angle and torsional strain than cyclobutane or cyclopropane
2 often used cyclohexane formations
- chair conformation
- boat conformation
both chair and boat conformations have bond angles close to 109.5 degrees and
possess very little angle strain
significant difference between chair and boat conformations:
- Chair conformation has not torsional strain
- boat conformation has two sources of torsional strain
to alleviate torsional strain boat conformation can
twist into a twist boat
flagpole interactions
steric interactions experienced by H’s on either side of a cyclohexane ring
the most important cyclohexane conformation is
the chair conformation
the lowest energy conformations are the
chair(and mirrored chair) conformations
chair is
3 sets of 2 parallel lines
each carbon atom in a cyclohexane ring can bear two substituents
- axial position
- equatorial position
axial position
group parallel to a vertical axis passing through the center of the ring(cyclohexane)
- 6(3 up and 3 down) lines from the carbons
equatorial position
group positioned approximately along the equator of the ring
- 6(2 right,2 left,1 forward,1backward) lines from the carbons
only one substituent can be in either an axial position or equatorial
possibilities are in equilibrium with each other
ring flip
a conformational change accomplished only through a rotation of all C-C single bonds
- the axial should become equatorial
when two chair conformations are in equilibrium the
lower energy conformation will be favored
1,3-diaxial interactions
the substituents electron cloud is trying to occupy the same region of space as the H’s causing steric interactions
- 1,3 describes the distance between the substituent and each H - most 1,3 interactions are gauche interactions
the chair conformation will generally favor
the conformation with the equatorial substituent
a wedge line is
UP
a dashed line in
DOWN
if the two groups compete with each other then the one with less 1,3-diaxial interactions is better as its lower energy
- 1,3-diaxial interactions
- Cl 2kJ/mol
- OH 4.2 kJ/mol
- CH3 7.6 kJ/mol
- CH2CH3 8.0kJ/mol
- CH(CH3)2 9.2kJ/mol
- C(CH3)3 22.8kJ/mol
1,30diaxial interactions
Cl
2 kJ/mol
1,30diaxial interactions
OH
4.2 kJ/mol
1,30diaxial interactions
CH(CH3)2
7.6 kJ/mol
1,30diaxial interactions
CH3
8.0 kJ/mol
1,30diaxial interactions
CH2CH3
9.2 kJ/mol
1,30diaxial interactions
C(CH3)3
22.8 kJ/mol
cis and trans are used to signify the
relative spatial relationship of similar substituents
cis
two groups are on the same face of the ring
trans
two groups are on opposite faces of the ring from each other
Haworth projections
planar representations and do not represent conformations
- dark bolded area to the front and groups above and below
Stereoisomers
different compounds with different physical properties, and they cannot be interconverted via a conformation change
- cis-1,2-Dimethylcyclohexane and trans-1,2-dimethylcyclohexane
a stereoisomer will be more stable if
all groups can be in equilateral positions
norborane
bicycle[2.2.1]heptane
- commonly encountered in bicyclic systems - six membered ring locked into a boat conformation by a CH2 group that serves as a bridge
many naturally occurring compounds are polycyclic systems
steroids(4 fused rings)
- 3 six membered rings and one five membered ring
