Chapter 3: Alkanes and their Stereochemistry
Coverage:
1. Survey of Functional Groups
2. Alkane Isomers
3. Alkyl Groups
4. Nomenclature of Alkanes
5. Properties of Alkanes
6. Conformations of Ethane, Propane, Butane
7. Special Topic: Refining of Petroleum
Goals:
1. Be able to recognize and identify the functional group(s) in a molecule.
2. Be able to predict the hybridization of an atom in a functional group.
3. Know the definition of constitutional and stereoisomers and be able to apply definitions
to alkanes.
4. Know the structures of alkyl groups up to 4 carbons.
5. Know the IUPAC system of nomenclature for naming alkanes.
6. Know trends in boiling points and melting points of alkanes, both branched and straight
chain.
7. Know the meaning of gauche, anti, eclipsed, staggered in conformations.
8. Be able to draw Newman projections of ethane, propane, butane and others.
9. Know the sources of hydrocarbons, particularly petroleum refining.
3-1
Classification of Organic Molecules
Functional Groups
1.
Hydrocarbons - molecules that possess only hydrogen and carbon
a1. Alkanes – noncyclic hydrocarbons with only C-C single bonds.
Formula CnH2n+2 where n is an integer.
Example:
Butane CH3CH2CH2CH3
C4H10
a2. Cycloalkanes – cyclic hydrocarbons with only C-C single bonds.
Formula CnH2n where n is an integer.
Example:
Cyclobutane
C4H8
b1. Alkenes – noncyclic hydrocarbons with C=C double bonds.
Formula CnH2n where n is an integer.
Example:
2-Butene CH3CH=CHCH3
C4H8
3-2
b2. Cycloalkenes – cyclic hydrocarbons with C=C double bonds.
Formula CnH2n-2 where n is an integer.
Example:
Cyclobutene
C4H6
c. Alkynes – noncyclic hydrocarbons with C=C triple bonds.
Formula CnH2n-2 where n is an integer.
Example:
Butyne CH3C=CCH3
C4H6
d. Aromatic Hydrocarbons – benzene and its derivatives
H
H C C C H
C
H
C
C
or
C6H6
H
H
3-3
2. Compounds containing Oxygen
a. Alcohols
R-OH
Functional Group OH
CH3OH methyl alcohol
b. Ethers
hydroxyl group
HOCH2CH2OH ethylene glycol (antifreeze)
R-O-R
CH3CH2-O-CH2CH3 Diethylether
c. Aldehydes and Ketones
O
O
R-C-H
R-C-R'
Functional Group
C=O carbonyl group
O
O
H
O
3-4
d.
O
Carboxylic Acids
O
R-C-OH
Functional Group
-CO2H
-COOH
-COH
Carboxyl Group
O
O
CH3-C-OH
+
H3O+
H2O
+
CH3CO-
Acetic Acid
e. Derivatives of Carboxylic Acids
O
R-C-OR'
Ester
O
R-C-Cl
Acid Chloride
O
R-C-NH2
Amide
3-5
3. Nitrogen-Containing Molecules
a. Amines
R-NH2
Functional Group
-NH2 amino group
CH3NH2
N
N
H
Amines are Lewis and Bronsted bases because of the nonbonded pair of electrons
..NH2
+
HCl
NH3+ Cl
b. Amides (see above)
c. Nitriles
R C N
Functional Group
C N
Cyano Group
3-6
Use of Wedges and Dashed Lines
H
H C
H
H
Solid Wedge indicates “coming toward you”
Dashed Line indicates “going away”
Narrow Line indicates “in the plane of paper”
3-7
Single versus Double Bonds
H
H
H
H
H
H
C
C
H
H
C
H
H
H
C
H
Ethane
The carbon-carbon single bond in ethane freely rotates at room temperature.
H
H
C
H
H
C
C
H
Ethylene
H
C
H
H
Pi bond broken
The carbon-carbon double bond in ethylene does not freely rotate at room temperature.
Why not?
Answer: The Pi bond would have to be broken. It will not break at room temperature
3-8
Isomers – different compounds with the same formula
a.
Constitutional (Structural) Isomers – isomers that differ in their bonding sequence
CH3
vs
CH3CHCH2CH3\
C5H12
C5H12
Br
Cl
CH3CH2CH2CH2CH3
Br
H
C=C
C=C
H
H
H
C2H2BrCl
Cl
C2H2BrCl
b. Stereoisomers – isomers that differ in their spatial orientation
Br
Cl
C=C
H
H
Cl
C=C
H
C2H2BrCl
Cis isomer
H
Br
C2H2BrCl
Trans isomer
3-9
Alkyl Groups
R is the general symbol for an alkyl group.
For example
R-OH represents an alcohol where R is an alkyl group.
R-Cl is an alkyl chloride.
Alkyl Groups – know the following
-CH3 methyl
-CHCH2CH3 sec-butyl
-CH2CH3 ethyl
CH3
-CH2CH2CH3 propyl
-CH2CH2CH2CH3 butyl
-CH2CHCH3
CH3
-C CH3
tert-butyl
CH3
isobutyl
CH3
3-10
Carbon Substitution
a. Primary carbon (1o) - carbon attached to one other carbon.
R-CH3 1o
b. Secondary carbon (2o) - carbon attached to two other carbons.
R-CH2-R
2o
c. Tertiary carbon (3o) – carbon attached to three other carbons
3o
R-CH-R
R
d. Quaternary (4o) – carbon attached to four other carbons.
R
4o
R C R
R
3-11
Alkane Nomenclature
1.
Find the parent hydrocarbon
a. Find the longest continuous chain of carbon atoms and name it accordingly.
Number Carbons
1
2
3
4
5
6
7
8
9
10
Name
methane
ethane
propane
butane
pentane
hexane
heptane
octane
nonane
decane
Named as a hexane
3-12
b. If two different chains of equal length are present, choose the one with the
larger number of branch points.
Named as hexane with 2 substituents not hexane with 1 substituent
2. Number the atoms in the main chain
a. Begin at the end nearer the first branch point.
4
6
2
1
3
5
b. If there are branch points at equal distances away, begin at the end nearer the second
branch point.
9
8
5
1
3
7
6
4
2
Named as a nonane with branch points
at carbons 3, 4, 7.
3-13
3. Identify and number the substituents.
9
8
5
1
3
7
6
4
2
3-ethyl
4-methyl
7-methyl
4. Write the name as a single word.
a. Use prefixes such as mono, di, tri, tetra, etc. to indicate multiple substituents.
b. List substituents alphabetically, ignoring prefixes.
c. Use dashes to separate substitutents, use commas to separate numbers.
The above example would have the name
3-ethyl-4,7-dimethylnonane
1
2
Name this alkane:
Answer: 5-sec-butyldecane
3
4
7
8
5
6
9
10
3-14
Physical Properties of Alkanes
1. Combustion - Alkanes are inflammable, i.e. they burn.
CH4
+
2 O2
CO2
+ 2 H2O
2. Boiling and melting points
a. Both bp and mp increase with increasing carbon number for straight-chain
alkanes with formula CnH2n+2
Carbon Number
C1 - C4
C5 – C16
C17 – C30
C30 – C50
>C50
Physical State
gases
liquids
oils and greases
paraffin waxes
plastics (polyethylene)
b. Branching tends to raise the melting point and lower the boiling point.
3-15
b. Branching tends to lower the boiling point and raise the melting point
BP
60oC
MP -154oC
58oC
50oC
-135oC
-98oC
Explanation:
MP Branching reduces the flexibility of the molecule which reduces the entropy term
S in the equation Tmp = H/S. Since S is in the denominator, Tmp increases.
BP Branching reduces surface area (more compact structure), and therefore London
dispersion forces which control boiling point for these molecules.
3. Solubility – alkanes are nonpolar molecules and therefore insoluble in water, which
is polar. Alkanes are hydrophobic.
4. Densities – alkanes are less dense than water, with densities near 0.7 g/mL. Therefore
3-16
they float on water, e.g. Exxon Valdez oil spill.
1. Conformation of Ethane
CH3CH3
H
H
H
C
H
Rotate 60o
H
C
2
H
C
1
H
H
H
H
H
H
H
Sawhorse
C
H
H
H
H
H
H
Newman
projection
H
H
HH
H
Staggered Conformation
Lower Energy
More Stable
Eclipsed Conformation
Higher Energy
Less Stable
The staggered conformation is more stable by 3.0 kcal/mol
3-17
Dihedral Angle – the angle defined by the C-H bond on the front carbon
and the C-H bond on the back carbon in a Newman projection.
600
0
H 0
H
H
H
H
H
H
H
HH
H
H
3.0 kcal/mol
0.0 kcal/mol
Eclipsed
Staggered
Eclipsed
Staggered
Eclipsed
Staggered
Eclipsed
3-18
Why is the staggered conformation more stable than the eclipsed conformation?
In the eclipsed conformation, the C-H bonds are closer, resulting in a repulsion
of the electron clouds.
This effect is referred to torsional strain. Torsional strain exists anytime C-H
bonds are eclipsed.
1.0 kcal/mol
H 1.0 kcal/mol
H
H
H
Total Energy: 3.0 kcal/mol
HH
1.0 kcal/mol
3-19
2. Conformations of Propane
H
H
H
C
H
H
Rotate
600
C
1
H
C1
2
CH3
H
H
H
C
2
H
CH3
H
H
H
H
CH3
Rotate 600
H
More stable
Less stable
0.0 kcal/mol
3.3 kcal/mol
1.0 kcal/mole
1.0 kcal/mol
1.3 kcal/mol
The additional 0.3 kcal/mol energy is due
to steric strain.
Steric strain – electronic repulsion that
occurs when two atoms or groups are
forced together.
3-20
3. Conformations of Butane
H
H3C
H
C2
H
H
60o
H3C
C
C2
CH3
H
CH3
H
CH3
H
H
CH3
H
H
HCH3
Anti Conformation
Staggered
180o dihedral
H
H
H
3
CH3
H
H
C
H
60o
H
H
3
H
H
Eclipsed
H
H
H
H
H
H3C
H
H
H
Anti
H
CH3
CH3
Gauche Conformation
Staggered
60o dihedral
H3CCH3
H3C
CH3
H
HCH3
3-21
There are two staggered conformations of
different energy.
There are two eclipsed conformations of
different energy.
Remember, staggered conformations are
more stable than eclipsed conformatiions
Quicktime Movie
3-22
Petroleum Refining
3-23
1. The first step in petroleum refining is fractional distillation.
3-24
2. Upstream processing of the distillates.
a. Catalytic hydrocracking – produces small alkanes from large alkanes by
adding hydrogen.
H2, heat
Si-Al catalyst
b. Catalytic cracking – produces small alkenes and alkanes by cracking in the absence
of hydrogen.
heat
Si-Al catalyst
c. Catalytic Reforming – the alkanes and cycloalkanes are upgraded to higher octane number
by conversion into aromatic compounds.
-H2
catalyst, heat
-3H2
catalyst, heat
3-25
Octane Number
Straight-chain hydrocarbons have low octane and make poor fuels.
Octane Number = 0
heptane
Branched Alkanes burn more slowly and reduce the knocking in the engine.
Octane Number = 100
2,2,4-trimethylpentane
3-26