Che 163
Introductory Organic Chemistry
ALKANES
Summer Quarter 2010
What is Organic chemistry?
What is Organic chemistry?
The study of carbon and its compounds.
What is Organic chemistry?
The study of carbon and its compounds.
First we will concentrate on compounds just containing
carbon and hydrogen, these compounds are called
hydrocarbons.
What is Organic chemistry?
The study of carbon and its compounds.
First we will concentrate on compounds just containing
carbon and hydrogen, these compounds are called
hydrocarbons.
Hydrocarbon Classification
Hydrocarbons
Alkanes
Cycloalkanes
Alkenes
Cycloalkenes
Alkynes
1. Alkanes (saturated) hydrocarbons, or aliphatic hydrocarbons)
A. General formula of CnH2n+2
B. Examples
a. CH4
b. C2H6
c. C3H?
1. Alkanes
A. General formula of CnH2n+2
B. Examples
a. CH4
b. C2H6
c. C3H8
d. C4H?
1. Alkanes
A. General formula of CnH2n+2
B. Examples
a. CH4
b. C2H6
c. C3H8
d. C4H10
C. Draw Lewis Structures
CH4
C2H6
C3H8
1. Alkanes
A. General formula of CnH2n+2
B. Examples
a. CH4
b. C2H6
c. C3H8
d. C4H10
C. Draw Lewis Structures
CH4
C2H6
D. Polarity? Polar or nonpolar?
C3H8
1. Alkanes
A. General formula of CnH2n+2
B. Examples
a. CH4
b. C2H6
c. C3H8
d. C4H10
C. Draw Lewis Structures
CH4
C2H6
C3H8
D. Polarity? Polar or nonpolar? Nonpolar
1. Alkanes (Continued)
E. Draw three dimensional structures, bond angles and
hybridization.
CH4
C2H6
C3H8
F. There are two different structures for C4H 10
Structure 1
Structure 2
G. Types of carbon
1.
2.
3.
4.
Primary (1◦) Carbon connected to one carbon atoms.
Secondary (2◦) Carbon connected to two carbon atoms.
Tertiary (3◦) Carbon connected to three carbon atoms.
How many primary, secondary, and tertiary carbons in the
two different structures of C4H10
H
H H H H
H C H
H
H
H C C C C H
H C C C H
H H H H
H H H
Butane, C 4H10
Primary = ?
Secondary =
Tertiary =
Isobutane, C 4H10
Primary =
Secondary =
Tertiary =
=>
G. Types of carbon
1.
2.
3.
4.
Primary (1◦) Carbon connected to one carbon atoms.
Secondary (2◦) Carbon connected to two carbon atoms.
Tertiary (3◦) Carbon connected to three carbon atoms.
How many primary, secondary, and tertiary carbons in the
two different structures of C4H10
H
H H H H
H C H
H
H
H C C C C H
H C C C H
H H H H
H H H
Butane, C 4H10
Primary = 2
Secondary = ?
Tertiary =
Isobutane, C 4H10
Primary =
Secondary =
Tertiary =
=>
G. Types of carbon
1.
2.
3.
4.
Primary (1◦) Carbon connected to one carbon atoms.
Secondary (2◦) Carbon connected to two carbon atoms.
Tertirary (3◦) Carbon connected to three carbon atoms.
How many primary, secondary, and tertiary carbons in the
two different structures of C4H10
H
H H H H
H C H
H
H
H C C C C H
H C C C H
H H H H
H H H
Butane, C 4H10
Primary = 2
Secondary = 2
Tertiary = ?
Isobutane, C 4H10
Primary =
Secondary =
Tertiary =
=>
G. Types of carbon
1.
2.
3.
4.
Primary (1◦) Carbon connected to one carbon atoms.
Secondary (2◦) Carbon connected to two carbon atoms.
Tertiary (3◦) Carbon connected to three carbon atoms.
How many primary, secondary, and tertiary carbons in the
two different structures of C4H10
H
H H H H
H C H
H
H
H C C C C H
H C C C H
H H H H
H H H
Butane, C 4H10
Primary = 2
Secondary = 2
Tertiary = 3
Isobutane, C 4H10
Primary = ?
Secondary =
Tertiary =
=>
G. Types of carbon
1.
2.
3.
4.
Primary (1◦) Carbon connected to one carbon atoms.
Secondary (2◦) Carbon connected to two carbon atoms.
Tertiary (3◦) Carbon connected to three carbon atoms.
How many primary, secondary, and tertiary carbons in the
two different structures of C4H10
H
H H H H
H C H
H
H
H C C C C H
H C C C H
H H H H
H H H
Butane, C 4H10
Primary = 2
Secondary = 2
Tertiary = 3
Isobutane, C 4H10
Primary = 3
Secondary = ?
Tertiary =
G. Types of carbon
1. Primary (1◦) Carbon connected to one carbon atoms.
2. Secondary (2◦) Carbon connected to two carbon atoms.
3. Tertiary (3◦) Carbon connected to three carbon atoms.
4. How many primary, secondary, and tertiary carbons in the
two different structures of C4H10
H
H H H H
H C H
H
H
H C C C C H
H C C C H
H H H H
H H H
Butane, C 4H10
Primary = 2
Secondary = 2
Tertiary = 3
Isobutane, C 4H10
Primary = 3
Secondary = 0
Tertiary = ?
G. Types of carbon
1.
2.
3.
4.
Primary (1◦) Carbon connected to one carbon atoms.
Secondary (2◦) Carbon connected to two carbon atoms.
Tertiary (3◦) Carbon connected to three carbon atoms.
How many primary, secondary, and tertiary carbons in
the two different structures of C4H10
H
H H H H
H C H
H
H
H C C C C H
H C C C H
H H H H
H H H
Butane, C 4H10
Primary = 2
Secondary = 2
Tertiary = 3
Isobutane, C 4H10
Primary = 3
Secondary = 0
Tertiary = 1
Constitutional Isomers (Structural Isomers) are different
compounds of the same formula. The different structures
from the previous slide for the formula C4H10 is an example
of Constitutional isomers.
How many isomers are there of an alkane containing five
carbons (C5H10)?
NOMENCLATURE
1. Common system
a. Works best for low molecular weight hydrocarbons
b. Steps to give a hydrocarbon a common name:
1. Count the total number of carbon atoms in the
molecule.
2. Use the Latin root from the following slide that
corresponds to the number of carbon atoms followed
by the suffix “ane”.
3. Unbranced hydrocarbons use the prefix normal, or n-,
4. Branched hydrocarbons use specific prefixes, as
shown on a subsequent slide
Latin
Hydrocarbon
Roots
Latin
Hydrocarbon
Roots
Number of Latin
Carbons
Root
Number of Latin
Carbons
Root
Examples
H H H H
H C
C
C
H
C
H H H H
n-butane
H
H
C H
H
H
1
meth
12
dodec
2
eth
13
tridec
3
prop
14
tetradec
H H H
4
but
15
pentadec
isobutane
5
pent
16
hexadec
6
hex
17
heptadec
7
hept
18
octadec
8
oct
19
nonadec
9
non
20
eicos
10
dec
21
unicos
11
undec
22
doicos
H C
C
C
H
H
H H C H H
H C C C H
H H C H H
H
neopentane
2. Systematic System of Nomenclature (IUPAC)
•Find the longest continuous chain of carbon atoms.
•Use a Latin root corresponding to the number of carbons in the
longest chain of carbons.
•Follow the root with the suffix of “ane” for alkanes
•Carbon atoms not included in the chain are named as
substituents preceding the root name with Latin root followed
by “yl” suffix.
•Number the carbons, starting closest to the first branch.
•Name the substituents attached to the chain, using the carbon
number as the locator in alphabetical order.
•Use di-, tri-, etc., for multiples of same substituent.
• If there are two possible chains with the same number of
carbons, use the chain with the most substituents.
Substituent Names (Alkyl groups)
Systematic Nomenclature continued.
H3C
CH CH2
CH3
CH3
H3C CH2
C
CH
CH2
CH2
CH3
CH3
H3C
CH CH2
CH3
CH3
H3C CH2
C
CH3
CH
CH2
CH2
CH3
Which one?
Systematic Nomenclature continued.
H3C
CH CH2
CH3
CH3
H3C CH2
C
Which one?
CH
CH2
CH2
CH3
CH3
H3C
The one with the most
number of substituents
CH CH2
CH3
CH3
H3C CH2
C
CH3
CH
CH2
CH2
CH3
Systematic Nomenclature continued.
H3C
CH CH2
CH3
CH3
H3C CH2
C
Which one?
CH
CH2
CH2
CH3
CH3
H3C
The one with the least
number of substituents
CH CH2
CH3
CH3
H3C CH2
C
CH3
CH
CH2
CH2
CH3
The top structure has
four substituents and
the bottom has three
Substituents.
Systematic Nomenclature continued.
H3C
CH CH2
CH3
CH3
H3C CH2
C
Which one?
CH
CH2
CH2
CH3
CH3
H3C
The one with the least
number of substituents
CH CH2
CH3
CH3
H3C CH2
C
CH3
Name = ?
CH
CH2
CH2
CH3
The top structure has
four substituents and
the bottom has three
Substituents.
Systematic Nomenclature continued.
H3C
CH CH2
CH3
CH3
H3C CH2
C
Which one?
CH
CH2
CH2
CH3
CH3
H3C
The one with the least
number of substituents
CH CH2
CH3
CH3
H3C CH2
C
CH
CH2
CH3
Name = ?
heptane
CH2
CH3
The top structure has
four substituents and
the bottom has three
Substituents.
Systematic Nomenclature continued.
H3C
CH CH2
CH3
CH3
H3C CH2
C
Which one?
CH
CH2
CH2
CH3
CH3
H3C
The one with the least
number of substituents
CH CH2
CH3
CH3
H3C CH2
C
CH
CH2
CH2
CH3
CH3
Name =
3,3,5-trimethyl-4-propylheptane
The top structure has
four substituents and
the bottom has three
Substituents.
Another Example:
CH3
CH3
H3C CH CH CH2
CH2
CH CH3
CH2CH3
Name = 3-ethyl-2,6-dimethylheptane
Another Example:
CH3
CH3
H3C CH CH CH2
CH2
CH CH3
CH2CH3
Name = 2,6-dimethyl-3-ethylheptane
Notice substituents are in alphabetical order; di, tri,
etc. do not participate in the alphabetical order
Line Structures
A quicker way to write sturctures
CH3
CH3
H3C CH CH CH2
CH2
CH CH3
CH2CH3
ethyl
methyl
(A line structure of the above
condensed structure)
methyl
(Condensed Structure)
Complex Substituents
•If the branch has a branch, number the carbons from the point of
attachment.
•Name the branch off the branch using a locator number.
•Parentheses are used around the complex branch name.
3
1
1
2
1-methyl-3-(1,2-dimethylpropyl)cyclohexane
Alkane Physical Properties
Solubility: hydrophobic (not water soluble)
Density: less than 1 g/mL (floats on water)
Boiling points increase with increasing carbons (little
less for branched chains) due to dispersion forces
being larger.
Melting points increase with increasing carbons (less for
odd-number of carbons).
Boiling Points of Alkanes
Branched alkanes have less surface area contact,
so weaker intermolecular forces.
Melting Points of Alkanes
Branched alkanes pack more efficiently into a crystalline
structure, so have higher m.p.
Reactions of Alkanes
I. Combustion
reaction
H
heat
H
C
H
CO2 + H2O
+ O2
H
heat
CO2 + H2O
+ O2
II. Cracking reaction
heat
+
catalyst
III. Halogenation reaction (substitution reaction)
sun
+ HCl
+
+ Cl2
Cl
Butane
2-chlorobutane
Cl
1-chlorobutane
Sample problem:
Which isomer of C5H12 has the most monochloro isomers?
Problem solving process:
Step 1 draw the isomers of C5H12
Step 2 react each isomer with chlorine
Step 3 count the products
Sample problem:
Which isomer of C5H12 has the most monochloro isomers?
Problem solving process:
Step 1 draw the isomers of C5H10
Step 2 react each isomer with chlorine
Step 3 count the products
+ Cl2
+ Cl2
+ Cl2
Sample problem:
Which isomer of C5H12 has the most monochloro isomers?
Problem solving process:
Step 1 draw the isomers of C5H10
Step 2 react each isomer with chlorine
Step 3 count the productsCl
2-chloropentane
+ Cl2
+
+
+ Cl2
+ Cl2
Cl
Cl
1-chloropentane
3-chloropentane
Sample problem:
Which isomer of C5H12 has the most monochloro isomers?
Problem solving process:
Step 1 draw the isomers of C5H10
Step 2 react each isomer with chlorine
Step 3 count the products
Cl
Cl
+ Cl2
+
+
2-chloropentane
Cl
Cl
1-chloropentane
3-chloropentane
Cl
+ Cl2
Cl
Cl
1-chloro-3-methylbutane 2-chloro-3-methylbutane 2-chloro-2-methylbutane 1-chloro-2-methylbutae
+ Cl2
Sample problem:
Which isomer of C5H12 has the most monochloro isomers?
Problem solving process:
Step 1 draw the isomers of C5H10
Step 2 react each isomer with chlorine
Step 3 count the products
Cl
Cl
+ Cl2
+
+
Cl
2-chloropentane
3-chloropentane
1-chloropentane
Cl
Cl
+ Cl2
Cl
Cl
1-chloro-3-methylbutane 2-chloro-3-methylbutane 2-chloro-2-methylbutane 1-chloro-2-methylbutae
Cl
+ Cl2
1-chloro-2,2-dimethylpropane
Sample problem:
Which isomer of C5H12 has the most monochloro isomers?
Problem solving process:
Step 1 draw the isomers of C5H10
Step 2 react each isomer with chlorine
Step 3 count the products
Cl
Cl
+ Cl2
+
+
Cl
2-chloropentane
3-chloropentane
1-chloropentane
Cl
Cl
+ Cl2
Cl
Cl
1-chloro-3-methylbutane 2-chloro-3-methylbutane 2-chloro-2-methylbutane 1-chloro-2-methylbutae
Cl
+ Cl2
1-chloro-2,2-dimethylpropane
Winner!
Conformers of Alkanes
•Structures resulting from the free rotation of
a C-C single bond
•May differ in energy. The lowest-energy
conformer is most prevalent.
•Molecules constantly rotate through all the
possible conformations.
Ethane Conformers
Staggered conformer has lowest energy.
Dihedral angle = 60 degrees
H
H
H
Dihedral angle
H
H
H
Newman
projection
model
sawhorse
Ethane Conformers (2)
Eclipsed conformer has highest energy
Dihedral angle = 0 degrees
=>
Conformational Analysis
•Torsional strain: resistance to rotation.
•For ethane, only 12.6 kJ/mol
=>
Propane Conformers
Note slight increase in torsional strain
due to the more bulky methyl group.
Butane Conformers C2-C3
Highest energy has methyl groups eclipsed.
Steric hindrance
Dihedral angle = 0 degrees
totally eclipsed (methyl groups)
=>
Butane Conformers (2)
Lowest energy has methyl groups anti.
Dihedral angle = 180 degrees
Staggered-anti
=>
Butane Conformers (3)
•Methyl groups eclipsed with hydrogens
•Higher energy than staggered conformer
•Dihedral angle = 120 degrees
Eclipsed (hydrogen and methyl)
=>
Butane Conformers (4)
•Gauche, staggered conformer
•Methyls closer than in anti conformer
•Dihedral angle = 60 degrees
Staggered-gauche
=>
Conformational Analysis
Cycloalkanes
•Rings of carbon atoms (-CH2- groups)
•Formula: CnH2n
•Nonpolar, insoluble in water
•Compact shape
•Melting and boiling points similar to branched
alkanes with same number of carbons
•Slightly unsaturated compared to alkanes
Naming Cycloalkanes
•Count the number of carbons in the cycle
•If the bonds are single then use the suffix “ane”
•First substituent in alphabet gets lowest number.
•May be cycloalkyl attachment to chain.
cyclopropane
cyclobutane
CH 3
mehtylcyclopropane
cyclopentane
H3CH 2C
cyclohexane
cycloheptane
CH 3
1-ethyl-2-methylcyclobutane
2-cyclopropylheptane
Cis-Trans Isomerism
(a type of stereoisomerism)
Cis: like groups on same side of ring
Trans: like groups on opposite sides of ring
Cycloalkane Stability
• 6-membered rings most stable
• Bond angle closest to 109.5
• Angle (Baeyer) strain
• Measured by heats of combustion per -CH2 -
Heats of Combustion/CH2
Alkane + O2  CO2 + H2O
697.1
658.6 kJ
Long-chain
686.1
664.0
658.6
662.4
663.6 kJ/mol
Cyclopropane
• Large ring strain due to angle compression
• Very reactive, weak bonds
=>
Cyclopropane (2)
Torsional strain because of eclipsed hydrogens
Cyclobutane
• Angle strain due to compression
• Torsional strain partially relieved by ring puckering
=>
Cyclopentane
• If planar, angles would be 108, but all
hydrogens would be eclipsed.
• Puckered conformer reduces torsional strain.
Cyclohexane
• Combustion data shows it’s unstrained.
• Angles would be 120, if planar.
• The chair conformer has 109.5 bond angles
and all hydrogens are staggered.
• No angle strain and no torsional strain.
Chair Conformer
Boat Conformer
Conformational Energy
Axial and Equatorial Positions
Monosubstituted Cyclohexanes
1,3-Diaxial Interactions
Disubstituted Cyclohexanes
Cis-Trans Isomers
Bonds that are cis, alternate axial-equatorial
around the ring.
CH3
CH3
One axial, one equatorial
=>
Bulky Groups
• Groups like t-butyl cause a large energy
difference between the axial and equatoria
l conformer.
• Most stable conformer puts t-butyl equatorial
regardless of other substituents.
=>
End of Chapter 2