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The Nature of Organic Compounds: Alkanes and Cycloalkanes
11 million known organic compounds
Grouped into few dozen families
Simplest family: Alkanes
Functional Group (FG): A group of atoms within a molecule that has a
characteristic chemical behavior. A functional group behaves in nearly the same
way in every molecule, regardless of the size and complexity, for example:
H
H
H
C
C
H
KMnO4
OH
H
H
CH3 H
H
H
H
C
C
C
OH
+
H
C
CH3 H
KMnO4
OH
H
H
O
H
H3 C
H
H
H
H
+
H
H
H3 C
H
H
H
H
O
C
C
OH
H
Table 3.1 lists many common functional groups.
FGs based on type of Carbon-Carbon Bonds:
C
Alkane
C
C
Alkene
C
C
Alkyne
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FGs with Carbon Singly Bonded to an Electronegative Atom:
C
C
Alkyl halide
Alcohol
Ether
C
C
N
C
S
SH
Thiol
Amine
C
O
OH
X
C
C
Sulfide
In all cases, the bonds are polar. C: δ+ and electronegative atom: δ -
FGs with a Carbon-Oxygen double bonded (Carbonyl Groups)
Present in some of the most important organic compounds.
C
H
C
C
C
OH
C
C
C
O
O
O
Aldehyde
Carboxylic Acid
Ketone
O
C
C
C
O
Ester
C
N
C
Cl
C
C
O
O
Amide
Acid Chloride
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Alkanes and Alkane Isomers
Alkanes are also known as
Saturated hydrocarbons
Only carbon-carbon single bonds
C-C
Only C and H
General formula: CnH2n+2 n = integer
CH4, C2H6, C3H8, C8H18, C12H26
• structure: sp3, td
• bond angle: 109°
Carbons can be:
• straight-chain (normal alkanes)
H2
H2
C
C
H3C
C
H2
CH3
n-pentane
• branched (branched-chain alkanes)
CH3
CH3
CH
H3 C
CH
C
H2
CH3
2,4-Dimethylpentane
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Isomers: molecules with the same formula, but different structure.
• Constitutional isomers (geometrical isomers)
• Conformational
• Stereoisomers
Constitutional Isomers: Have the same formula but a different
connection of atoms, for example: C4H 10
CH 3
H 3C
CH
CH3
H3C
2-Methylpropane
(Isobutane)
H
H
C
C
H
H
CH 3
Butane
Constitutional isomers are not restricted to alkanes, for example:
H3C CH 2OH
C 2H 6O
H3C O CH3
C2H 6O
ethanol
diethyl ether
Problem: Draw structures to meet the following description:
a) Four isomers with the formula C 8H 18
(There are many more!)
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b) Two isomers with the formula C4H8O2
Names of Straight or Normal Alkanes.
No of C
1
2
3
4
5
6
7
Name
methane
ethane
propane
butane
pentane
hexane
heptane
Formula
CH4
C2 H6
C3 H8
C4H10
C5H12
C6H14
C7H16
Alkyl
-CH3
-CH2CH3
-CH2CH2CH3
- CH2 CH2CH2CH3
-CH2CH2CH2 CH2 CH3
- CH2 CH2CH2CH2CH2 CH3
- CH2 CH2CH2CH2CH2 CH2 CH3
Alkyl name
Methyl (Me)
Ethyl (Et)
Propyl (Pr)
Butyl (Bu)
pentyl
hexyl
heptyl
n-alkyl groups
H3C
CH2
CH 3
H3C
CH 2
CH2
Propyl
Propane
branched alkyl groups
H 3C
CH 2
Propane
CH 3
H3 C
CH
CH 3
isopropyl
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Carbon atoms can be classified as primary (1°), secondary (2°), tertiary (3°) and
quaternary (4°) by the number of alkyl groups attached.
H
R
C
R
H
H
R
C
H
R
C
R
H
R
C
H
R
R
R
1o
2o
3o
4o
R
CH3
H 2C
CH
CH2
CH2CH3
C
CH3
CH3
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Nomenclature of Branched-Chain Alkanes
IUPAC: International Union of Pure Applied Chemistry
Prefix
Parent
Location of substituents
Suffix
number of carbons
FGs
Rules:
1) Longest continuous chain of carbons determines the parent name
CH 3CH2CH 2CH 2CHCH3
CH3
CH 3CH2CH2 CH 2CHCH3
CH2
CH2
hexane
heptane
2) Number the longest chain beginning with the end of the chain nearest the
branching.
6
5
4
3
2 1
CH 3CH2CH2CH2CHCH3
CH 3
7
6
5
4
3
CH 3CH2CH 2CH 2CHCH3
2 CH 2
1 CH 2
3) Use the numbers obtained by application of rule 2 to designate the location of
substituents.
2-Methylhexane
3-Methylheptane
Numbers are separated from words by a dash.
4) When two or more substituents are present, give each substituent a number
corresponding to its location on the longest chain.
1
2 3
4 5
6
CH3CHCH2CHCH2CH3
CH3
CH2
CH3
4-Ethyl-2-methylhexane
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List groups alphabetically. Disregard prefixes such as: di, tri, sec-, tertExample: ethyl precedes dimethyl
tert-butyl precedes ethyl
but, ethyl precedes isobutyl
5) If two or more substituents are present on the same carbon, use the number twice
CH3
1
2 3 4 5 6
CH3CH2CCH2CH2CH3
CH2
CH3
3-Ethyl-3-methylhexane
6) If 2 or more substituents are identical, use prefixes di, tri, tetra, etc.
Commas are used to separate numbers from each other:
1
CH 3
3
4
CH 3
3
CH 3-CH-CH-CH-CH 3
CH 3-CH-CH-CH3
2
CH 3
1
4
2
5
CH 3
CH3
2,3-Dimethylbutane
2,3,4-Trimethylpentane
7) When 2 chains of equal length compete for selection as the base, choose the
chain with the greater number of substituents.
CH3
CH2
CH2
7
6
5
CH3
3
CH3 -CH2 -CH-CH-CH-CH-CH3
4
CH3
2
1
CH3
2,3,5-Trimethyl-4-propylheptane and not 4-sec-Butyl-2-3-dimethylheptane
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8) When branching first occurs at an equal distance from either end, choose the
name that gives the lower number at the first point of difference.
6
5
CH 3
4
2
1
CH 3 -CH-CH2 -CH-CH-CH 3
3
CH 3
CH 3
2,3,5-Trimethylhexane and not 2,4,5-Trimethylhexane
9) If a substituent of the main chain has sub-branching, name the substituent by
applying previous rules and begin numbering at the point of attachment. The
complex substituent is set off in parenthesis when writing the complete name of
the compound.
CH3 CH3
9
8
7
6
5
CH3 -CH 2-CH 2-CH 2-CH-CH-CH-CH3
4
CH 2
3 CH2
CH3 CH 3
2
CH
CH3
CH-CH-CH3
1
CH3
1
2-Methyl-5- (1,2-dimethylpropyl) nonane
2
3
5- (1,2-dimethylpropyl)
For historical reasons, some simpler branched-chain alkyl groups have
nonsystematic names and IUPAC accepts these names.
CH3-CH-CH3
Isopropyl (i-Pr)
CH3
CH3 -CH2-CH-CH 3
CH3 -CH-CH2
CH3
CH3 -C
CH3
Sec-Butyl (sec-Bu)
Isobutyl
CH3
CH3 -CH-CH2-CH 2
CH3
Isopentyl
CH3 -C
CH2
CH3
Neopentyl
tert-Butyl (t-Bu)
CH3
CH3 -CH2 -C
CH3
tert-Pentyl
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Examples
Properties of Alkanes
Physical properties:
♦ The molecules are non-polar or very weakly polar
♦ The forces holding together non-polar molecules are van der Waals forces.
These intermolecular forces, which operate only over very small distances,
result from induced polarization of the electron clouds in molecules.
♦ Within a family: The larger the molecule the stronger the intermolecular forces.
greater number of Carbons > b.p. > m.p.
n-alkanes
gases
<4 C
5-17 C liquids
>18 C solids
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♦ Branched-chain isomers have lower bp than straight chain isomers
>branches <bp
>branches: molecule tends to approach a sphere, so < contacts.
n-pentane and neo-pentane
Chemical properties:
♦ Inertness to many chemicals (also called paraffins = slight affinity)
C-C and C-H are strong bonds, resistant to breaking.
C and H have almost the same electronegativity, therefore little polarization.
♦ No unshared electrons for acid attack
♦ Characteristic reactions:
Halogenation
C
H
+
X2
hν
C
X
+
HX
heat
mixture
Combustion
nC 12H2
+
8O2
5CO2
+
6H2O
∆H = - 845 Kcal/mole
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Pyrolysis (or cracking)
400-600°
Alkane
H2 + smaller alkanes
Cycloalkanes (or alicyclic compounds)
alkanes in which the carbon atoms are arranged in a ring.
General formula: C nH2n
Properties: Similar to alkanes
• Non polar
• Chemically inert to most reagents
Cycloalkanes have less freedom of rotation around C-C
Kinds: monocyclic, bicyclic, polycyclic
Nomenclature:
A. Monocyclic:
Cycloalkanes with only one ring are named by attaching the
prefix cyclo to the name of the corresponding alkane with the
same number of carbons.
Examples: cyclopropane
cyclobutane
cyclopentane
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1) Use the cycloalkane name as the parent
methylcyclopentane and NOT cyclopentylmethane
If the alkane chain contains greater number of carbons than the ring, designate
the ring as the substituent.
1-cyclopropyl butane and NOT butylcyclopropane
2) When more than one substituent is present. Number the ring
beginning with one substituent in the way that gives the next
substituent the lower number possible.
1,3-Dimethylcyclohexane and NOT 1,5-Dimethylcyclohexane
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B. Bicyclic: Compounds with two rings.
The name of the alkane corresponding to the total # of
carbon atoms is used as “base name”
bridge head
H
C
H2C
CH2
CH2
H2C
CH2
C
H
bridge carbon
Bicyclo(2.2.1)heptane or norbornane
Bridge head carbon: carbon common to both rings
Bridge carbon: carbon connecting bridge heads
H
C
H2 C
CH2
C
H
Bicyclo (1.1.0) butane
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Stereoisomers: Atoms connected in the same order, but different spatial
orientation.
Example:
OH
OH
OH
OH
Cis-1,2-cyclopentanediol
trans-1,2-cyclopentanediol
Do not interconvert without breaking a bond.
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