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Chapter 13 Introduction to Organic Chemistry: Alkanes
13 Alkanes
Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
STRUCTURES OF ALKANES
METHANE CH4
Bond Angle 109.5o Shape Tetrahedral
Can be illustrated as:
H
H C H
H
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Structural Formulas
Alkanes are written with structural formulas that are• expanded to show each bond.• condensed to show each carbon atom and its
attached hydrogen atoms.
Expanded Condensed H
H C H CH4 , methane
H
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Expanded and Condensed Structures
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Structural Formulas
Condensed formulas are written for expanded structural formula by showing each carbon and the attached hydrogen atoms.
Expanded Condensed H H H H │ │ │ │H─C ─C ─C ─C ─ H CH3─CH2─CH2─CH3
│ │ │ │ H H H H
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Names of Alkanes
The names of alkanes • are determined by the IUPAC (International Union of
Pure and Applied Chemistry) system.• end in –ane.• with 1-4 carbons in a chain use prefixes as follows.
Name # Carbons Structural Formula
Methane1 CH4
Ethane 2 CH3CH3
Propane 3 CH3CH2CH3
Butane 4 CH3CH2CH2CH3
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Names of Alkanes
Alkanes with 5-10 carbon atoms in a chain use Greek prefixes.Name # Carbons Structural Formula
Pentane 5 CH3CH2CH2CH2CH3
Hexane 6 CH3CH2CH2CH2CH2CH3
Heptane 7 CH3CH2CH2CH2CH2CH2CH3
Octane 8 CH3CH2CH2CH2CH2CH2CH2CH3
Nonane 9 CH3 CH2 CH2CH2CH2CH2CH2CH2CH3
Decane 10 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3
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Learning Check
A. Write the condensed formula for: H H H H H
H C C C C C H
H H H H H
B. What is its molecular formula?C. What is its name?
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Solution
A. CH3─CH2─CH2─CH2─CH3
B. C5H12
C. pentane
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Writing Structural Formulas
Carbon atoms in a chain
• maintain tetrahedral shape.• are connected in a zig-zag pattern.• are drawn as 2-dimensional. • Ccan be written in several conformations.
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Some Structures for Butane
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Hexane has Six Carbon Atoms
Hexane • is an alkane with six carbon atoms in a continuous
chain.• has a “zig-zag” look because each carbon atom is at
the center of a tetrahedron.• is represented by a ball-and-stick model as shown
below.
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Learning Check
Write the condensed structural formula for
A. ethane.
B. heptane.
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Solution
Write the condensed structural formula for
A. ethane CH3─CH3
B. heptane CH3─CH2─CH2─CH2─CH2─CH2─CH3
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Cycloalkanes
Cycloalkanes
• are cyclic alkanes.
• have two hydrogen atoms fewer than the open chain.
• are named by using the prefix cyclo- before the name of the alkane chain with the same number of carbon atoms.
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Cycloalkanes
The structural formulas of cycloalkanes are usually represented by geometric figures,
Cyclopropane CH2
CH2 CH2
CyclobutaneCH2 CH2
CH2 CH2
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More Cycloalkanes
Cyclopentane CH2
CH2 CH2
CH2 CH2
Cyclohexane CH2
CH2 CH2
CH2 CH2
CH2
CH2
CH2
CH2CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
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Learning Check
Name the following.
A. CH3─CH2─CH2─CH3
B.
C. CH3─CH2─CH2─CH2─CH2─CH2─CH2─CH3
D.
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Solution
Name the following:
A. CH3─CH2─CH2─CH3 butane
B. cyclopropane
C. CH3─CH2─CH2─CH2─CH2─CH2─CH2─CH3 octane
D. cyclohexane
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Types of alkanes
Alkanes
branched
Iso alkanes
Neo alkanes
straight
The alkanes in which one methyl group is attached to second carbon atom of normal chain of carbon atom is called isoalkanes
The alkanes in which two methyl group is attached to second carbon atom of normal chain of carbon atom is called neoalkanes
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Types of carbon atoms
Type of C atom
primary secondary Tertiary Quaternary
CH2
CH3
CH2
CH3
CH3
CH3
CH3C
CH
“classes of carbons”
primary carbon (1o) – a carbon bonded to one carbon
secondary carbon (2o) – a carbon bonded to two carbons
tertiary carbon (3o) – a carbon bonded to three carbons
quaternary carbon (4o) – a carbon bonded to four carbons
1o
4o
CH3 CH3CH3CHCH2CH2CCH3 1o
CH3 3o 2o
classification of hydrogens, halides – hydrogens or halides are classified by the carbon to which they are attached.
1o
CH3CH3CHCH2CH2CH3
1o 3o 2o 2o 1o
CH3CH2CHCH3 sec-butyl bromide 2o bromide Br
CH3CH3CCH3 tert-butyl chloride 3o chloride Cl
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Nomenclature
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Isomers of Butane
Isomers• have the same
molecular formula.• have different atom
arrangements.• of butane (C4H10) are a
straight chain and a branched chain.
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Alkyl groups
Alkyl groups are• alkanes that are missing one H. • substituents attached to carbon chains.• named with a –yl ending.
H
H C CH3 methyl
H H H H C C CH3 CH2 ethyl H H
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Naming Substituents
In the IUPAC system,• a carbon branch is
named as an alkyl group.
• halogen atoms are named as halo.
TABLE 10.6
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Alkanes with Substituents
CH3
CH3 CH CH3 methylpropane
methyl groups
CH3 CH3
CH3 CH CH2 CH CH3 2,4-dimethylpentane
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Naming Alkanes
Give the name of CH3 CH3
CH3─CH─CH─CH3
STEP 1 Name the longest continuous chain. CH3 CH3
CH3─CH─CH─CH3
butane
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Naming Alkanes
Give the name of CH3 CH3
CH3─CH─CH─CH3
STEP 2 Number chain from end nearest substituent .
CH3 CH3
CH3─CH─CH─CH3
1 2 3 4
STEP 3 Locate substituents and name alphabetically. 2,3-dimethylbutane
Note: when more substituents present on same carbon atom, its gets priority
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Learning Check
Write the name of
Cl CH3
CH3─CH2─CH─CH─CH3
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Solution
STEP 1 Longest chain is pentane.
STEP 2 Number chain from end nearest substituent. Cl CH3
CH3─CH2─CH─CH─CH3
5 4 3 2 1
STEP 3 Locate substituents and name alphabetically. 3-chloro-2-methylpentane
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Learning Check
Give the IUPAC name for each of the following:
A. CH3 CH3
| |
CH3─CH─CH2 ─CH─CH3
B. Cl CH3 | |
CH3─CH2─CH─CH2─C─CH2─CH3
|
Cl
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Solution
A. CH3 CH3
| | CH3─CH─CH2 ─CH─CH3 2,4-dimethylpentane
1 2 3 4 5
B. Cl CH3 more substituents = C3 | |
CH3─CH2─CH─CH2─C─CH2─CH3
|
Cl 7 6 5 4 3 2 1
3,5-dichloro-3-methylheptane
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Learning Check
Draw the condensed structural formula for 2-bromo-4-chlorobutane.
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Solution
2-bromo-4-chlorobutaneSTEP 1 Longest chain has 4 carbon atoms.
C─C─C─CSTEP 2 Number chain and add substituents.
Br
C ─ C ─ C ─ C ─ Cl 1 2 3 4
STEP 3 Add hydrogen to complete 4 bonds to each C. Br
CH3─CH─CH2─CH2─Cl
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Naming Cycloalkanes with Substituents
The name of a substituent is placed in front of thecycloalkane name.
methylcyclobutane CH3
chlorocyclopentane Cl
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Learning Check
Name each of the following.
1. CH3
CH2─CH3
2.
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Solution
Name each of the following.
1. methylcyclopropane
2. ethylcyclohexane
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Properties
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Conformation of alkanes
ETHANE. Molecular formula C2H6
Structural formula: CH3 CH3 or H H
H C C H
H H
PROPANE.
Molecular formula: C3H8
Structural formula: CH3 CH2 CH3 or H H H
H C C C H
H H HBoth ethane and propane are “straight” chain molecules
BUT!! Bonds are NOT 90o molecules are NOT STRAIGHT!!!
Schematic formula
Molecular formula: C4H10 - can have two different structures
“Straight” chain. CH3 CH2 CH2 CH3 BUTANE
Branched chain CH3 CH CH3
CH3
METHYL PROPANE
Isomers
Compounds that have the same molecular formula but different structural formula.
branch
BUTANE & ISOMERS.
Schematic formula:
Names & Structures of three different isomers C5H12.
Examples
2- methylbutane
CH3
CH3 CH CH2 CH3
CH3
CH3 C CH3
CH3
2,2 – dimethyl propane
TASK: illustrate the structures of:
2-methylpentane.
2,3 – dimethylbutane.
2,2,3 -trimethylpentane
CH3CH(CH3)CH2CH2CH3
CH3CH(CH3)CH(CH3)CH3
CH3C(CH3)2CH(CH3)CH2CH3
THE RULES FOR NAMING ORGANIC COMPOUNDS
1. Choose the longest unbroken chain of Carbon atoms and assign a name for the carbon chain using the prefixes; meth-1, eth-2 etc.
2. Identify any carbon chain branches (alkyl groups). These are assigned names using the same prefixes as above along with the suffix “-yl” – methyl, ethyl etc.
3. Identify the functional groups present in the molecule. Assign a prefix or suffix according to their homologous series. These will be written in front of the name of the carbon chain.
4. There is an order of precedence, to decide the suffix for the carbon chain:
COOH / C=C > OH > Br / Cl
5. Number the Carbon atoms in the longest chain so that the branches/functional groups have the lowest number possible. Allocate a number for every group/branch no matter how many times it occurs. Where groups are on the same carbon write their names in alphabetical order.
6. Numbering takes precedence "wins" over alphabetical spelling. Prefixes are used for groups that occur more than once.
Di – 2 Tri – 3 Tetra – 4 Penta – 5 etc.
7. The final name is written as one word with commas between numbers, hyphens separating numbers from words.
Give the names of the following alkanes
3-methyl pentane
2,4-dimethylpentane
2,2,4-trimethyl
hexane
(a) CH3 CH2 CH CH2 CH3
CH3
(b) CH3 CH CH2 CH CH3
CH3 CH3
(c) CH3 C(CH3)2 CH2 CH(CH3) CH2 CH3
(d) CH3CH2CH(CH3)C(CH3)3 2,2,3-trimethylpentane
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Chapter 13 Introduction to Organic Chemistry: Alkanes
13 Properties of Alkanes
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Some Properties of Alkanes
Alkanes are• nonpolar.• insoluble in water.• less dense than water.• flammable in air.
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Some Properties of Alkanes
Alkanes with 1-4 carbon atoms are
• methane, ethane, propane, and butane.
• gases at room temperature.
• used as heating fuels.
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Some Properties of Alkanes
Alkanes with 5-8 carbon atoms are• liquids at room temperature.• pentane, hexane, heptane, and octane.• very volatile.• used to make gasoline.
Alkanes with 9-17 carbon atoms • are liquids at room temperature• have higher boiling points.• are found in kerosene, diesel, and jet fuels.
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Some Properties of Alkanes
Alkanes with 18 or more carbon atoms• have high molar masses.• are waxy solids at room temperature.• used in waxy coatings of fruits and
vegetables.
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Melting and BoilingPoints of Hydrocarbons
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Combustion
In combustion reactions,• alkanes react with oxygen. • CO2, H2O and energy are produced.
Alkane + O2 CO2 + H2O + heat
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Learning Check
Write a balanced equation for thecomplete combustion of propane.
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Solution
Unbalanced equation
C3H8 + O2 CO2 + H2O
Balance C
C3H8 + O2 3CO2 + H2O
Balance H
C3H8 + O2 3CO2 + 4H2O
Balance O
C3H8 + 5O2 3CO2 + 4H2O (Balanced)
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CH4 + Cl2 → CH3Cl + HCl
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