CSUMB
ESSP 311 Organic Chemistry I
Ronald W. Rinehart, Ph.D.
Chapter 2 Alkanes
| Basic Organic Nomenclature by Dave Woodcock at Okanagan
University College [requires MDL Chime™; use Netscape™] http://www.molecularmodels.ca/nomenclature/nom1.htm |
| Organic Chemistry OnLine by Paul R.Young at the University of
Illinois at Chicago Alkanes and Cycloalkanes: Structure, Bonding, and Nomenclature tutorials [requires MDL Chime™; use Netscape™] http://www.chem.uic.edu/web1/OCOL-II/WIN/ALKANE/ALKANE.HTM |
| Exploring Alkanes by Gary Trammell
and Srinivas Vuppuluri at the University of Illinois at Springfield http://people.uis.edu/gtram1/organic/alkanesmenu.htm |
| Carey PowerPoint slides for chapter 2 [2.1 to 2.7,
hydrocarbon classes, functional groups, alkanes from C1 to C5, higher n-alkanes] from Columbia University can be
seen at http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/02_01_07.html |
| Carey PowerPoint slides for chapter 2 [2.8 to 2.12, IUPAC
nomenclature for alkanes and cycloalkanes] from Columbia University can be
seen at http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/02_08_12.html |
| Carey PowerPoint slides for chapter 2 [2.13 to 2.16,
sources, physical properties, chemical properties, redox in organic chem] from Columbia University can be
seen at http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/02_13_16.html |
| A set of PowerPoint slides on alkanes and
cycloalkanes
in PDF format by Paul R. Young of the University of Illinois at Chicago can be seen at http://www.chem.uic.edu/web1/PDF/CH2.PDF |
| Even though it was written for a course at a lower level,
there's lots of good stuff on hydrocarbons, alkanes, cycloalkanes, and
functional groups on my CHEM 30B "Alkanes" page http://www.mpcfaculty.net/ron_rinehart/30B/alkanes.htm |
Chapter 2. Alkanes.
To see a large number of
alkane and cycloalkane Chime structures
and particularly relevant to this section is his "Molecular Fragments:
Alkane Isomers" page at |
I. Introduction.
A. Classes of Organic Compounds
1. ~7,000,000 organic compounds.
2. Organic compounds can be characterized by
Functional Groups: the structural unit
responsible for a given molecule's chemical
behavior.
3. Functional Groups: See Tables 2.1,
2.2 or
click here
B. Types of Carbon Atoms in Organic Species:
► 1o (primary) carbons. C
bonded to 1 C (typically, a -CH3 group).
► 2o (secondary) carbons. C bonded
to 2 C's (e.g., a –CH2- group).
► 3o (tertiary)
carbons. C
bonded to 3 C's (typically, a CH group).
► 4o (quaternary) carbons. C bonded
to 4 C's.
The molecule shown below has six 1o
C, three 2o C, two 3o C, and one 4o
C.
There are four
different types of
1o
C (methyl groups, CH3-), three different types of 2o
C (methylene groups, -CH2-), and two different types of 3o
C (methinyl groups, >CH-) in this molecule. These distinctions will be important
when we discuss the free-radical halogenation of alkanes [Chapter 4], and also
when we consider 1H-NMR and 13C-NMR spectra [Chapter 13].
II. Classes of Hydrocarbons.
A. Hydrocarbons.
1. Compounds composed of carbon and hydrogen.
2. Can be aliphatic or aromatic
(determined by their structure and bonding.)
B. Aliphatic Hydrocarbons.
1. Aliphatic hydrocarbons are broken into the
following categories:
a) Alkanes.
sp3
hybrids. -C-
b) Cycloalkanes
sp3
hybrids. -C-
c) Alkenes.
sp2
hybrids. C=C
d) Alkynes.
sp hybrids. C≡C
C. Aromatic Hydrocarbons.
1. Most common example: benzene. 
III. Alkanes.
A. Bonding.
1. sp3 hybridization.
2. Sigma bonds only.
3. 109.5o C< bond angles.
4. C-H bond length in CH4 = 109 pm.
5. C-C bond length in C2H6
= 153 pm.
C-H bond length in C2H6
= 111 pm.
6. Acyclic alkanes have the formula CnH2n+2
B. Nomenclature of Alkanes. IUPAC Rules.
|
IUPAC Nomenclature from ACD Labs Official IUPAC Nomenclature Home Page by G.P.
Moss at Queen Mary University of London
Nomenclature Shortcuts by Yorke E. Rhoades at Harvard University IUPAC nomenclature of hydrocarbons by Linda M. Sweeting at Towson University http://www.towson.edu/~sweeting/enrich/iupachc.pdf IUPAC nomenclature of simple organic compounds by
Linda M. Sweeting at Towson University http://www.towson.edu/~sweeting/enrich/iupac.pdf |
1. The names of the first ten [or even twenty -- see below] alkanes should be memorized:
|
methane |
CH4 |
hexane |
C6H14 or CH3(CH2)4CH3 |
|
ethane |
C2H6 or CH3CH3 |
heptane |
C7H16 or CH3(CH2)5CH3 |
|
propane |
C3H8 or CH3CH2CH3 |
octane |
C8H18 or CH3(CH2)6CH3 |
|
butane |
C4H10 or CH3CH2CH2CH3 |
nonane |
C9H20 or CH3(CH2)7CH3 |
|
pentane |
C5H12 or CH3(CH2)3CH3 |
decane |
C10H22 or CH3(CH2)8CH3 |
2. Alkyl groups are substituent groups which are attached to other atoms or parent chains.
a) They are named by dropping -ane
from the name of the corresponding alkane and
adding -yl.
b) Examples.
-CH3
methyl
-CH2CH3
ethyl
-CH2CH2CH3
propyl
3. IUPAC Rules: International Union of
Pure and Applied Chemistry.
a) Select the longest continuous
carbon chain as the "parent" structure and then
consider the attached groups.
methylpropane methylbutane
b) Where necessary indicate by
number the carbon to which the group is attached.
This number is called a
locant.
i) In numbering the
parent carbon chain start from the end closest to
the first
substituent.
3-methylpentane 2-methylpentane
c) If the same alkyl group occurs
more than once on the parent chain then indicate
this by the use of the
appropriate prefix (di-, tri-, tetra-, etc).
i) Indicate by number
the position of each group.
d) If there are several different
groups attached to the parent structure, then
list them in the name of the
compound in alphabetical order.
i) The prefixes di-,
tri-, etc are ignored when alphabetizing.
2,2,4-trimethyloctane 3,3,5-triethyl-4-methyloctane
e) Notice from the above examples that locant numbers are separated by
commas;
a number and a letter are separated by dashes.
f) When two chains are of equal
length, choose the chain with the greater number
of substituents.
4. Nomenclature of Cycloalkanes.
a) Cycloalkanes are named by adding
the prefix "cyclo" to the parent stem.
b) Notice the chemical formula of a
cycloalkane is CnH2n.
|
|
5. Name the following compounds.
a)
or
4,6-diethyl-3-methyloctane
b)
or
4-ethyl-3,5-dimethylheptane
c)
or
5,5-diethyldecane
d)
or
2,3,6,6-tetramethyloctane
e)
or 
1-ethyl-2-methyl-4-propylcyclopentane
f)
or
1,1-diethyl-3,3-dimethylcyclobutane
6. Draw structures for the following
compounds:
a) 4-ethyl-3,5-dimethylheptane
b) 4,6-diethyl-3-methyloctane
c) 1,1,3-tripropylcyclohexane
7. IUPAC rules for complex alkyl groups.
a) What difficulty arises in naming
the following compound?
i) Such alkyl groups are named with IUPAC rules or in some cases common names.
b) With IUPAC rules the substituent
group is named by taking the longest carbon
chain in the group as the parent
alkyl group and listing attached alkyl groups
with corresponding locants.
i) The above alkyl
group name is written in parentheses.
ii) In numbering the
parent chain of the alkyl group, the carbon of the
substituent group
attached to the main parent is carbon 1 of the
substituent alkyl
group.
iii) Now name the
compound in part (a).
Answer:
5-isobutylnonane or
5-(2-methylpropyl)nonane
8. Common names of some complex alkyl groups.
a) Alkyl groups with (CH3)2CH-
(on the otherwise unbranched group) whose point
of attachment to the parent is
at the opposite end is named by adding the
prefix "iso" to the name of the
alkyl group containing all carbons.
|
structure |
|
|
|
|
common name |
isopropyl |
isobutyl |
isopentyl or isoamyl |
|
IUPAC name |
(1-methylethyl) |
(2-methylpropyl) |
(3-methylbutyl) |
b) Groups with 4 carbons or more in
a straight chain but with the point of
attachment to the parent at the
second carbon gets the prefix "sec-".
|
structure |
|
|
|
|
common name |
sec-butyl |
sec-pentyl (or sec-amyl) |
sec-hexyl |
|
IUPAC name |
(1-methylpropyl) |
(1-methylbutyl) |
(1-methylpentyl) |
c) The group -C(CH3)3
is known as the tert-butyl (or t-butyl) group.
i) Name this using
IUPAC rules: (1,1-dimethylethyl)
ii) Adding an extra
carbon to this results in the t-pentyl (or t-amyl)
group:
(1,1-dimethylpropyl)
d) Some other important common
names:
|
structure |
|
|
|
common name |
neopentyl (NOT neoamyl) |
neohexyl |
|
IUPAC name |
(2,2-dimethylpropyl) |
(2,2-dimethylbutyl) |
9. Name the following compounds.
a)
b)
Answers:
a) 1,1-dimethyl-2-(1,2-dimethylpropyl)cyclopentane
b)
7-sec-butyl-5-ethyl-3,3-dimethyldecane
a better name is
5-ethyl-3,3,8-trimethyl-7-propyldecane -- at C-7, the "main" chain continues
upward. Why? Given a choice
such as this, select the main chain to include as many of the
substituents as possible.
C. Isomers.
1. As the formula of an alkane increases in
number of carbon atoms, the number of
possible constitutional isomers grows.
|
name |
# of constitutional isomers |
condensed formula of ‘normal’ isomer |
|
|
1 |
methane |
1 |
CH4 |
|
2 |
ethane |
1 |
CH3CH3 |
|
3 |
propane |
1 |
CH3CH2CH3 |
|
4 |
butane |
2 |
CH3CH2CH2CH3 |
|
5 |
pentane |
3 |
CH3(CH2)3CH3 |
|
6 |
hexane |
5 |
CH3(CH2)4CH3 |
|
7 |
heptane |
9 |
|
|
8 |
octane |
18 |
|
|
9 |
nonane |
35 |
|
|
10 |
decane |
75 |
|
|
11 |
undecane |
|
|
|
12 |
dodecane |
|
|
|
13 |
tridecane |
|
|
|
14 |
tetradecane |
|
|
|
15 |
pentadecane |
4347 |
|
|
16 |
hexadecane |
|
|
|
17 |
heptadecane |
|
|
|
18 |
octadecane |
|
|
|
19 |
nonadecane |
|
|
|
20 |
eicosane |
366,319 |
|
|
22 |
docosane |
|
|
|
30 |
triacontane |
|
|
|
40 |
tetracontane |
6.249…x 1012 |
|
|
100 |
hectane |
|
CH3(CH2)98CH3 |
|
n |
gen. alkane |
|
CH3(CH2)n-2CH3 |
2. Draw all possible constitutional isomers
for alkanes of the formula:
i) C5H12
ii) C6H14
iii) C4H8
D. Physical Properties.
1. Intermolecular vs intramolecular forces of
attraction.
2. Recall 3 major forces of attraction:
a) Hydrogen bonding
b) Dipole-dipole
c) London (dispersion) forces:
induced dipole-induced dipole
3. Intermolecular force of attraction between
alkanes: London forces.
a) These weak forces result in low
boiling points for alkanes.
CH3CH2OH CH3CH2F CH3CH2CH3
78oC -32oC -42oC
4. Boiling Point vs Branching. More branching
results in less contact points between
molecules: less intermolecular forces of attraction
and a lower boiling point.
36oC 28oC 9oC
5. Boiling Point vs Molecular Weight. The
boiling points of alkanes increase with
increasing molecular weight: higher alkanes
have a greater number of atoms which can
attract to each other via
London
forces.
a) C1-C4 are all gases are room
temperature and 1 atmosphere pressure.
CH4 CH3CH3
CH3CH2CH3
-160oC
-89oC -42oC
6. Melting Point vs Branching. Greater symmetry
results in a better fit in the crystal
lattice (pack together better) and a higher
melting point:
-129oC
-160.5oC -16.6oC
7. Solubility in Water.
a) "Like dissolves
like."
b) Alkanes (nonpolar) and water
(polar) do not mix.
|
For greater
appreciation of the importance of the role of intermolecular attractive forces
Colby College's
Shockwave tutorial on Physical Properties is at |
E. Chemical Properties of Alkanes.
1. Alkanes are relatively unreactive.
2. Combustion.
(Table 2.5)
| Dr. Ron's magic formula for balancing
combustion equations for any HYDROCARBON since C + O2 à CO2 and 2H2 + O2 à 2H2O, it can readily be shown that CxHy + (x +
¼·y)O2
à
xCO2 + ½yH2O |
|
Combustion from the Virtual Textbook
of Organic Chemistry by William Reusch at Michigan State U http://www.cem.msu.edu/~reusch/VirtualText/funcrx1.htm#combust |
CnH2n+2 + O2
à
H2O + CO2 + energy
CH4 + 2O2
à
2H2O + CO2 + 890 kJ
C7H16 + 11 O2
à 8H2O + 7CO2
+ 4817 kJ
a) Add ~653 kJ/mole for each -CH2-
b) Heat of combustion can be used as
an indicator of relative stabilities of isomeric alkanes.
c) The isomer with the lowest heat
of combustion is the most stable.
d) Branching decreases the heat of
combustion. The more compact shape
maximizes intramolecular forces
of attraction which stabilizes (and
therefore decreases the potential
energy of) the molecule.
C8H18 + 25/2 O2 à 8CO2 + 9H2O
CH3(CH2)6CH3
(CH3)2CH(CH2)4CH3
5471 kJ
(CH3)3C(CH2)3CH3
5466 kJ
(CH3)3CC(CH3)3
5458 kJ
5452 kJ
 |
8CO2 + 9H2O
e) Notice the same product in each case: The difference
in the heats of
combustion must be due to a difference in potential energy of the isomeric
forms of C8H18.
i) (Compare the 8CO2
+ 9H2O to four identical bricks at the same ground
level. Which
released the most energy upon falling? It depends
upon the height at
which the brick was released.
IV. Oxidation-Reduction in Organic Chemistry.
|
If you're really rusty on the subject, see my PowerPoint review of oxidation-reduction
(redox) reactions at For a summary of the basic take-home message, see the redox table |
A. The concepts of oxidation and reduction are important in understanding organic
chemistry
and biochemistry.
B. Oxidation.
1. Oxidation is the loss of electrons. (OIL)
2. Oxidation of carbon is the decrease of bonds
to H atoms:
C2H6 à
C2H4 + H2
3. Oxidation of carbon is the increase of bonds
to O atoms:
C + O2
à
CO2
H3C-OH
à
H2C=O
C. Reduction.
1. Reduction is the gain of electrons. (RIG)
2. Reduction of carbon is the increase of bonds
to H atoms:
C2H4 + H2 à
C2H6
3. Reduction of carbon is the decrease of bonds
to O atoms:
(CH3)2C=O + H2 à
(CH3)2CHOH
Many thanks to Rod Oka of
MPC for generously sharing his "Lecture Companion" outline, reproduced here
in edited form by permission, with
web references and other goodies added by me.
Structures drawn using CS ChemOffice ChemDraw™ and MDL IsisDraw™
updated 9/15/07
