Download Alkanes

Essential Organic Chemistry
Paula Yurkanis Bruice
Chapter 3
An Introduction to Organic Compounds:
Nomenclature, Physical Properties, and
Representation of Structure
Alkanes
Saturated hydrocarbons (Aliphatic)
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Hydrocarbons – Contain only C and H atoms.
Saturated – Only single bonds.
Aliphatic – “Fat” like.
Can be acyclic (no rings) or cyclic
(cycloalkanes).
Alkanes
Alkanes
Isomerism
Consider C4H10
These structures are constitutional isomers
Isomerism
Consider C5H12
These structures are constitutional isomers
Isomerism
Consider C6H14
These structures are constitutional isomers
Isomerism
 Isomerism – The phenomenon whereby certain
chemical compounds have structures that are
different although the compounds possess the
same elemental composition.
 Isomers – Two or more chemical substances
having the same elementary composition and
molecular weight but differing in structure.
3.1 Nomenclature of Alkyl Substituents
 Names and Formulas of Alkyl Groups:
Formula
Name
Formula
Name
CH3-
methyl
CH3CH2CH2CH2-
butyl
CH3CH2-
ethyl
(CH3)2CHCH2-
isobutyl
CH3CH2CH2-
propyl
CH3CH2CH(CH3)-
sec-butyl
(CH3)2CH-
isopropyl
(CH3)3C-
tert-butyl
Nonsystematic names; Common names
 Primary (1o) carbon – a carbon that is bonded to
only one other carbon.
 Secondary (2o) carbon– a carbon that is bonded
to two other carbons.
 Tertiary (3o) carbon– a carbon that is bonded to
three other carbons.
Nonsystematic names; Common names
Names of Some Alkyl Groups
3.2 IUPAC Nomenclature of Alkanes
 Determine the number of carbons in the longest
continuous carbon chain as the parent hydrocarbon.
 Number the chain so that the substituent gets the lowest
possible number.
 Substituent are listed in alphabetical order.
 When both directions lead to the same lowest number for
one of the substituents, the direction that gives the lowest
possible number to one of the remaining substituents is
chosen.
 If the same substituent numbers are obtained in both
directions, the first group cited receives the lower number.
 If a compound has two or more chains of the same length,
the parent hydrocarbon is the chain with the greatest
number of substituents.
3.2 IUPAC Nomenclature of Alkanes
 Numbers are used only for systematic names, never
for common names.
 A number and a word are separated by a hyphen;
numbers are separated by a comma.
 When the same alkyl group branch chain occurs more
than once, indicate this repetition by a prefix (di-, tri-,
tetra-, and so forth).
 di-, tri-, tetra-, sec-, and so on are ignored in
alphabetizing.
 iso and cyclo are not ignored in alphabetizing.
Examples
5
4
3
2
1
CH3
CH2
CH2
CH
CH3
CH3
2-methylpentane
parent alkane
alkyl group
Examples
1
2
3
4
CH3
CH
CH
CH3
CH3 CH3
2,3-dimethylbutane
4
CH3
3
CH2
CH3
2
1
C CH3
CH3
2,2-dimethylbutane
Examples
7
CH3
6
CH
CH3
5
CH2
CH3
CH
4
3
CH
CH3
CH
2
1
CH3
CH3
2,3,4,6-tetramethylheptane
Note: Number the chain so that the substituents get
the lowest possible numbers.
Examples
2
CH3
1
CH2
CH3
CH
CH2
3
4
CH2
5
CH3
6
3-methylhexane
Caution: Be careful to choose the longest chain as
the parent alkane.
Examples
8
7
6
5
CH2 CH3
4
CH3 CH2 CH2 CH2 C
3
2
1
CH CH CH3
CH3 Cl
CH3
3-chloro-4-ethyl-2,4-dimethyloctane
Note: Substituents are listed in alphabetical order.
Draw the Compounds
 3-ethylpentane
1
2
3
4
5
CH3
CH2
CH
CH2
CH3
CH2
CH3
 2,2,4-trimethylpentane
CH3
1
2
3
4
5
CH3
C
CH2
CH2
CH3
CH3
CH3
3.3 IUPAC Nomenclature of Cycloalkanes
IUPAC Nomenclature of Cycloalkanes
1. In the case of a cycloalkane with an attached alkyl
substituent, the ring is the parent hydrocarbon.
2. There is no need to number the position of a single
substituent on a ring.
3. If the ring has two different substituents, they are
cited in alphabetical order and the number I
position is given to the substituent cited first.
Cycloalkanes
CH2
=
H2C CH2
Cyclopropane
H2C
H2C
CH2
CH
CH2
CH2
CH3
Methylcyclohexane
=
CH3
Cycloalkanes
CH2
H2C
CH CH2CH2CH2CH2CH2CH3
CH2
1-Cyclobutylhexane
(CH2)5CH3
or
Hexylcyclobutane
Cycloalkanes
H2C
H2C
CH2
CH
CH
CH2
CH2
6
CH3
1
5
=
CH3
4
1-Ethyl-2-methylcyclohexane
2
3
Name the Following Compounds
Methylcyclopropane
1,1-Dimethylcyclohexane
1,2-Dimethylcyclopentane
3-Cyclopropylpentane
3.4 IUPAC Nomenclature of Alkyl Halides
 Common name: alkyl group + halogen, with the
“ine” ending replaced by “ide” (fluoride, chloride,
bromide, iodide)
 IUPAC name: substituted alkanes, with the
substituent prefix for the halogens end with “O”
(fluoro, chloro, bromo, iodo)
IUPAC Nomenclature of Alkyl Halides
3.5 Classification of Alkyl Halides,
Alcohols, and Amines
 The number of alkyl groups attached to the carbon
to which the halogen is bonded determines whether
an alkyl halide is primary, secondary, or tertiary.
Classification of Alkyl Halides, Alcohols,
and Amines
 The number of alkyl groups attached to the carbon to
which the OH group is attached determines whether
an alcohol is primary, secondary, or tertiary.
Classification of Alkyl Halides, Alcohols,
and Amines
 The number of alkyl groups attached to the
nitrogen determines whether an amine is primary,
secondary, or tertiary.
3.6 Structures of Alkyl Halides, Alcohols,
Ethers, and Amines
Structures of Alkyl Halides, Alcohols,
Ethers, and Amines
Structures of Alkyl Halides, Alcohols,
Ethers, and Amines
Structures of Alkyl Halides, Alcohols,
Ethers, and Amines
3.7 Physical Properties of Alkanes, Alkyl
Halides, Alcohols, Ethers, and Amines
 Boiling Points (bp) – the temperature at which the
liquid form of the compound become a gas
(vaporizes).
 Induced-dipole-induced-dipole interactions
 van der Waals forces – in order for an alkane to
boil, the van der Waals forces must be overcome.
 The van der Waals forces that hold
alkane molecules together depends
on the area of contact between the
molecules.
Boiling Points of Alkanes
• Boiling points increase with increasing molecular weight within a
homologous series of alkanes.
Boiling Points of Alkanes
 Branching decreases the area of contact between
molecules.
 If two alkanes have the same molecular weight, the
more highly branched alkane will have a lower
boiling point.
Boiling Points of Ethers, Alcohols, and
Amines
1. Boiling points increase with increasing
molecular weight within a homologous series.
2. Ethers generally have higher boiling points than
alkanes of comparable molecular weight because
both van der Waals forces and dipole-dipole
interactions must be overcome.
Boiling Points of Ethers, Alcohols, and
Amines
3. Alcohols have much higher boiling points than
alkanes or ethers of comparable molecular
weight because, in addition to van der Waals
forces and dipole-dipole interactions, hydrogen
bonds have to be broken as well.
Boiling Points of Ethers, Alcohols, and
Amines
4. Primary and secondary amines also form
hydrogen bonds, so these amines have higher
boiling points than alkanes with similar
molecular weights.
Boiling Points of Alkyl Halides
5. Both van der Waals forces and dipole-dipoleinteractions must be overcome for alkyl halides
to boil.
6. As the halogen atom increases in size, the size of
its electron cloud increases, and the larger the
electron cloud, the stronger are the van der Waals
interactions.
Melting Points of Alkanes
 Melting Points (mp) – the temperature at which a
solid is converted into a liquid.
 The increase in mp is less regular than that in bp
because packing influences the mp of a compound.
 Packing – a property that determines how well the
individual molecules in a solid fit together in a
crystal lattice.
Melting Points of Alkanes
• Melting points increase with increasing molecular weight within
a homologous series of alkanes.
Melting Points of Alkanes
Solubility of Alkanes
• Solubility – “Like dissolves like”
• Alkanes are nonpolar, hydrophobic
• They are soluble in nonpolar solvents and insoluble
in water.