CHEM 3013
ORGANIC CHEMISTRY I
LECTURE NOTES
CHAPTER 3
1.
Alkanes
Hydrocarbons contain only carbon and hydrogen. Alkanes (also called paraffins) are
hydrocarbons which contain only single, sigma bonds between the carbons that make-up the
backbone.
SP3-1S sigma bond
H
H
H
C
normal bonds: in plane of page
dark wedge: out of page
dashed wedge: behind page
C
H
H
H
SP3-SP3 sigma bond
Bonding in ethane
2.
Normal alkanes
Normal alkanes (n-alkanes) are linear, unbranched alkanes. General Molecular Formula
CnH2n+2. Members of a homologous series have predictable chemical and physical properties
(mp, bp, density, reactivity). The first four straight chain alkanes have non-systematic names.
Thereafter, the alkanes are named using Greek number prefixes (penta, hexa, hepta, octa,
nona, deca...) followed by the suffix -ane
methane
ethane
propane
butane
pentane
hexane
heptane
octane
nonane
decane
......general
The n-alkanes
CH4
CH3CH3
CH3CH2CH3
CH3(CH2)2CH3
CH3(CH2)3CH3
CH3(CH2)4CH3
CH3(CH2)5CH3
CH3(CH2)6CH3
CH3(CH2)7CH3
CH3(CH2)8CH3
CH3(CH2)nCH3
A homologous series with
(CH2)n increasing by one
methylene unit for each
succeeding homolog.
CnH2n+2
1
2
3.
Isomers
Compounds with the same molecular formulas, but different structures are called isomers.
Isomers have different chemical and physical properties. Compounds which differ in their carbon
connectivity are called structural isomers or constitutional isomers. The number of possible
constitutional isomers increases dramatically as the number of carbon atoms increases.
Formula
Number of isomers
CH4
C2H6
C3H8
C4H10
C5H12
C6H14
C7H16
C8H18
C9H20
C10H22
.
.
C15H32
.
.
C40H82
1
1
1
2
3
5
9
18
35
75
4,347
62,491,178,805,831
Isomers of n-alkanes
CH3
H2
C
H 3C
n-pentane
CH3
H2
C
C
H2
CH3
CH
CH3 H3C
CH3
C
H2
isopentane
C
H 3C
CH3
neopentane
Non-systematic
common names
The structural isomers of C5H12
4. Alkyl groups
If a hydrogen atom is removed form an alkane, the structure that remains is called an alkyl
group. Alkyl groups are named by replacing the -ane ending of the parent alkane
with
-yl.
Removal of a hydrogen from methane generates a methyl group, removal from ethane generates an
ethyl group. Likewise, removal of a hydrogen from the terminal carbon of a straight-chain alkane
generates a n-alkyl group. Removal of an internal hydrogen generates a branched alkyl group.
3
CH4
Methane
CH3
methyl group
CH3CH3
Ethane
CH3CH2
ethyl group
CH3CH2CH3
Propane
CH3CHCH3
isopropyl group
CH3CH2CH2CH3
Butane
CH3CH2CHCH3
sec-butyl group
CH3CH2CH2
propyl group
CH3CH2CH2CH2
butyl group
Alkyl groups
6.
Alkane nomenclature
The IUPAC system divides a chemical name into three parts: prefix, parent, and
suffix.
The parent tells how many C atoms are in the main chain, the suffix identifies the functional groups
present in the molecule, and the prefix tells their position along the main chain.
Steps for naming complex, branched-chain alkanes
i.
Select the longest chain of carbon atoms and give the compound the
appropriate base name. (See Table 3.4, page 65.) If two or more longest
chains have the same number of carbons, select the one having the most
groups.
ii.
Number the longest chain starting from the end nearest a branch.
iii.
Give all side chains a group name by dropping the "ane" of the name
corresponding to the number of carbons in the side chain and add the new suffix
of "yl." All side chains are numbered starting with the carbon attached to the base
(longest) chain.
iv.
Precede each group name with a prefix which corresponds to the number of
times the group occurs along the chain. (di, tri, tetra, penta, etc.)
v.
Place each group name, including its prefix, in front of the base name in
alphabetical order. Use the first letter of the group name, but not the prefix,
to determine alphabetical order.
vi.
Place the number of the carbon, on which each group is attached, before the
group name. If a group occurs more than once on a chain, all the locations
of that
group must be placed before the group name.
vii.
Separate numbers from numbers with commas and numbers from words
with dashes. Your final name will be one continuous word.
Naming Complex Groups
Complex groups are side chains which have groups branching off the side chain.
i.
Find the longest chain in the group, starting with the point of attachment on
the base chain.
ii.
Number the group chain starting with the point of attachment on the base
chain.
iii.
Name all groups along the group chain according to the rules iii-vii above.
iv.
Enclose the entire group name, in parenthesis.
v.
Precede the parenthetical statement with the number of the carbon, on the
base chain, on which the group is attached.
4
6
2
7
5
2
3
8
not longest chain and
not lowest number
7
2-ethylheptane
not longest chain
1
6-methyloctane
not lowest number
6
4
1
6-ethylheptane
1
3
5
7
4
2
6
5
1
3
incorrect names
2
7
5
3
8
4
Correct Name
3-methyloctane
6
Findingthe longest chain and lowest position number
1
4
5
7
4,4,5,7-tetramethyloctane
Incorrect
8
8
5
4
2
1
2,4,5,5-tetramethyloctane
Correct!
Lowest number to groups along the principal chain
7.
Carbon Types
Carbons in an organic compound are classified as to their structural type in the molecule.
The structural type is determined by how many other carbons the carbon under consideration is
bonded to. Primary carbons, (1˚), are bonded to one other carbon. Secondary carbons,
(2˚) are bonded to two carbons. Tertiary carbons,(3˚), are bonded to three others.
Quaternary carbons,(4˚), are bonded to four others.
4
5
p
CH3
p
H2
C t CH s CH3
S CH t
H 3C
C
H2
q
p
C
p
H 3C
CH3
CH3
p
p = primary:
s = secondary:
t = tertiary:
q = quaternary:
( bonded to 1 C)
( bonded to 2 C)
( bonded to 3 C)
(bondedto 4 C)
p
Carbon Types
8.
Cycloalkanes
Alkanes formed into rings are called cycloalkanes, they have the general formula
CnH 2n . As a short hand, cycloalkanes are represented by polygons in skeletal drawings. The
smallest ring size possible is a three membered ring called cyclopropane. Although large rings are
possible, the most common ring sizes are 3-,4-,5-,6-, 7- and 8-membered rings. Larger ring sizes
are less common.
9.
Cycloalkane Nomenclature
Cycloalkanes are named by rules similar to those used for alkanes. The prefix Cyclo- is
used before the alkane name corresponding to the number of carbons in the ring. For example,
consider a 7-membered ring...a seven carbon alkane is heptane....thus a 7-membered cycloalkane
is cycloheptane
Steps for naming substituted cycloalkanes.
i.
Use the cycloalkane name as the parent name, i.e alkylcycloalkanes, unless an alkyl
side chain has more carbons than the ring...then it becomes a cycloalkylalkane
ii.
When more than one substituent is present, start at a point of attachment so as to
give the lowest order.
iii.
When two or more different substituents are present, number by alphabetical
priority.
iv.
Treat halogens like alkyl groups.
6
cyclohexane ethylcyclopentane
see rule i.
1,2-dimethyl-4-ethylcycloheptane
not
1-ethyl-3,4-dimethylcycloheptane
see rule ii.
2-cyclobutylheptane
see rule i.
1-ethyl-2-isobutylcyclobutane
not
1-isobutyl-2-ethylcyclobutane
see rule iii.
Cycloalkane Nomenclature
10.
Cis-Trans Stereoisomerism in Cycloalkanes
The lack of flexibility in cycloalkanes (complete rotation around C-C bonds is prevented)
has substantial consequences in that cycloalkanes have two sides, a "top" and a "bottom" side.
This leads to two distinct structures when the cycloalkane is substituted with various groups. This
is best illustrated in 1,2-dimethylcyclopropane. Note that the three carbons in cyclopropane lie in
the same plane (remember that you need three points to define a plane). One can imagine one
structure in which both methyls are on the same side of the ring plane, and another structure in
which they are on opposite sides of the plane of the ring. The connectivity of the methyls in the
two different structures is the same. However the orientation in space of the two different methyls
is different. These two structures are STEREOISOMERS - compounds which have the same
order of connectivity, but different three-dimensional geometry. We define the two different threedimensional orientations by using the prefix cis- (Latin for in the same side) or trans- (Latin;
across).
7
Stereoisomers - same connections but different 3-D geometry
H 3C
CH3
H
H
H
H 3C
H
H
H
CH3
H
H
H
H
H
CH3
H
cis-1,2-dimethylcyclopropane
(same sides of rintg plane
H
H
trans-1,2-dimethylcyclopropane
(opposite sides of ring plane)
Can be represented
CH3
Solid Wedge:
(above plane)
as
CH3
Dashed Wedge:
(below plane)
CH3
Dashed Wedge:
(below plane)
CH3
trans-1,2-dimethylcyclohexane
H
H
H
Can be represented
H
H
H 3C
H
as
CH3
cis-1,3-dimethylcyclohexane
Cis-Trans Stereoisomerism
CH3
Dashed Wedge:
(below plane)