Ch04-alkanes-ques-12

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Chapter 4
Introduction to
Hydrocabons
Carbon Backbone,
Nomenclature, Physical &
Chemical Properties
HYDROCARBONS
• Compounds composed of only carbon and hydrogen atoms
(C, H). Each carbon has 4 bonds.
• They represent a “backbone” when other “heteroatoms”
(O, N, S, .....) are substituted for H. (The heteroatoms give
function to the molecule.)
• Acyclic (without rings); Cyclic (with rings); Saturated:
only carbon-carbon single bonds; Unsaturated: contains
one or more carbon-carbon double and/or triple bond
HYDROCARBONS
• Alkanes contain only single ( ) bonds and have the
generic molecular formula: [CnH2n+2]
• Alkenes also contain double ( + ) bonds and have the
generic molecular formula: [CnH2n]
• Alkynes contain triple ( + 2) bonds and have the
generic molecular formula: [CnH2n-2]
• Aromatics are planar, ring structures with alternating
single and double bonds: eg. C6H6
Types of Hydrocarbons
Each C atom is tetrahedral with sp3 hybridized orbitals. They only have single bonds.
Each C atom is trigonal planar with sp2 hybridized orbitals.
There is no rotation about the C=C bond in alkenes.
Question 4.1
• What is the hybridization of the starred
carbon in humulene (shown)?
•
A)
sp
•
B)
sp2
•
C)
sp3
•
D)
1s2 2s2 2p2
Question 4.2
• What is the hybridization of the starred
carbon of geraniol?
•
A)
sp
•
B)
sp2
•
C)
sp3
•
D)
1s2 2s2 2p2
Types of Hydrocarbons
Each C atom is linear with sp hybridized orbitals.
Each C--C bond is the same length; shorter than a C-C bond: longer than a C=C bond.
The concept of resonance is used to explain this phenomena.
Propane
It is easy to rotate about the C-C bond in alkanes.
Naming Alkanes
C1 - C10 : the number of C atoms present in the chain.
Each member C3 - C10 differs by one CH2 unit. This is called a homologous series.
Methane to butane are gases at normal pressures.
Pentane to decane are liquids at normal pressures.
Nomenclature of Alkyl Substituents
Examples of Alkyl Substituents
Constitutional or structural isomers have the same
molecular formula, but their atoms are linked
differently. Naming has to account for them.
Question 4.3
• How many hydrogens are in a molecule of
isobutane?
•
A)
6
•
B)
8
•
C)
10
•
D)
12
A compound can have more than one name, but a
name must unambiguously specify only one compound
C7H16 can be any one of the following:
Question 4.4
• How many isomeric hexanes exist?
•
A)
2
•
B)
3
•
C)
5
•
D)
6
Question 4.5
• The carbon skeleton shown at the bottom right
accounts for 9 carbon atoms. How many other
isomers of C10H22 that have 7 carbons in their
longest continuous chain can be generated
by adding a single carbon to various positions
on this skeleton?
•
A)
2
•
B)
3
•
C)
4
•
D)
5
Alkanes
(Different types of sp3 carbon atoms)
• Primary, 1o, a carbon atom with 3 hydrogen atoms:
[R-CH3]
• Secondary, 2o, a carbon atom with 2 hydrogen atoms:
[R-CH2-R]
• Tertiary, 3o, a carbon atom with 1 hydrogen atom:
• [R-CH-R]
R
• Quaternary, 4o, a carbon atom with 0 hydrogen atoms:
CR4
Different Kinds of sp3 Carbons and
Hydrogens
Question 4.6
• In 3-ethyl-2-methylpentane, carbon #3
(marked by a star) is classified as:
•
•
•
•
A)
B)
C)
D)
primary (1°)
secondary (2°)
tertiary (3°)
quaternary (4°)
Question 4.7
• How many primary carbons are in the
molecule shown at the bottom right?
•
A)
2
•
B)
3
•
C)
4
•
D)
5
Nomenclature of Alkanes
1. Determine the number of carbons in the parent hydrocarbon
8
7
6
5
4
3
2
8
1
CH3CH2CH2CH2CHCH2CH2CH3
7
6
5
4
4
CH3CH2CH2CH2CHCH2CH3
CH3
2
2
1
CH3CH2CH2CHCH2CH2CH3
CH2CH2CH3
3
3
CH2CH2CH2CH3
1
5
6
7
8
2. Number the chain so that the substituent gets the lowest
possible number
1
2
3
4
5
CH3CHCH2CH2CH3
CH3
2-methylpentane
1
2
3
4
5
6
7
8
CH3CH2CH2CHCH2CH2CH2CH3
CHCH3
CH3
4-isopropyloctane
CH3
CH3CHCH2CH2CH3
common name: isohexane
systematic name: 2-methylpentane
3. Number the substituents to yield the lowest possible number
in the number of the compound
CH3CH2CHCH2CHCH2CH2CH3
CH3
CH2CH3
5-ehtyl-3-methyloctane
not
4-ethyl-6-methyloctane
because 3<4
(substituents are listed in alphabetical order)
4. Assign the lowest possible numbers to all of the substituents
CH3 CH3
CH3CH2CHCH2CHCH3
CH3
CH3
2,4-dimethylhexane
CH3CH2CH2C
CC
H
CH2CH3
H 3
2C
CH3 CH3
3,3,4,4-tetramethylheptane
CH3
CH3CH2CHCH2CH2CHCHCH2CH2CH3
CH2CH3
CH2CH3
3,3,6-triethyl-7-methyldecane
5. When both directions lead to the same lowest number for one
of the substituents, the direction is chosen that gives the lowest
possible number to one of the remaining substituents
CH3
CH3
CH3CHCH2CHCH3
CH3
CH3
2,2,4-trimethylpentane
not
2,4,4-trimethylpentane
because 2<4
CH2CH3
CH3CH2CHCHCH2CHCH2CH3
CH3
6-ethyl-3,4-dimethyloctane
not
3-ethyl-5,6-dimethyloctane
because 4<5
6. If the same number is obtained in both directions, the first
group receives the lowest number
Cl
CH2CH3
CH3CH2CHCH3
r
B
2-bromo-3-chlorobutane
not
3-bromo-2-chlorobutane
CH3CH2CHCH2CHCH2CH3
CH3
3-ethyl-5-methylheptane
not
5-ethyl-3-methylheptane
7. In the case of two hydrocarbon chains with the same number of
carbons, choose the one with the most substituents
3
4
5
6
CH3CH2CHCH2CH2CH3
2 CHCH3
1 CH3
3-ethyl-2-methylhexane
(two substituents)
1
2
3
4
5
6
CH3CH2CHCH2CH2CH3
CHCH3
CH3
3-isopropylhexane
(one substituent)
8. Certain common nomenclatures are used in the IUPAC system
CH3CH2CH2CH2CHCH2CH2CH3
CHCH3
CH3
4-isopropyloctane
or
4-(1-methylethyl)octane
CH3CH2CH2CH2CHCH2CH2CH2CH2CH3
CH2CHCH3
CH3
5-isobutyldecane
or
5-(2-methylpropyl)decane
Question 4.7
• The correct structure of 3-ethyl-2methylpentane is:
•
A)
B)
•
C)
D)
CnH2n
Cycloalkane Nomenclature
Cycloalkanes
•
Cycloalkanes are alkanes that contain a ring
of three or more carbons.
•
Count the number of carbons in the ring,
and add the prefix cyclo to the IUPAC name of
the unbranched alkane that has that number of
carbons.
Cyclopentane
Cyclohexane
Cycloalkanes
•
Name any alkyl groups on the ring in the
usual way. A number is not needed for a single
substituent.
CH2CH3
Ethylcyclopentane
Cycloalkanes
•
Name any alkyl groups on the ring in the
usual way. A number is not needed for a single
substituent.
•
List substituents in alphabetical order and
count in the direction that gives the lowest
numerical locant at the first point of difference.
H3C CH3
CH2CH3
3-Ethyl-1,1-dimethylcyclohexane
For more than two substituents,
CH3
CH3CH2CH2
CH3
H 3C
CH2CH3
4-ethyl-2-methyl-1-propylcyclohexane
not
1-ethyl-3-methyl-4-propylcyclohexane
because2<3
not
5-ethyl-1-methyl-2-propylcyclohexane
because 4<5
CH3
1,1,2-trimethylcyclopentane
not
1,2,2-trimethylcyclopentane
because1<2
not
1,1,5-trimethylcyclopentane
because 2<5
Question 4.8
• Which one contains the greatest number
of tertiary carbons?
•
A)
2,2-dimethylpropane
•
B)
3-ethylpentane
•
C)
sec-butylcyclohexane
•
D)
2,2,5-trimethylhexane
Physical Properties of Alkanes
and Cycloalkanes
Naphtha
(bp 95-150 °C)
Kerosene
(bp: 150-230 °C)
C5-C12
Light gasoline
(bp: 25-95 °C)
C12-C15
Crude oil
Gas oil
(bp: 230-340 °C)
Refinery gas
C1-C4
C15-C25
Residue
Question 4.9
Arrange octane, 2,2,3,3-tetramethylbutane and
2-methylheptane in order of increasing boiling
point.
• A) 2,2,3,3-tetramethylbutane < octane
< 2-methylheptane
• B) octane < 2-methylheptane < 2,2,3,3tetramethylbutane
• C) 2,2,3,3-tetramethylbutane < 2-methylheptane
< octane
• D) 2-methylheptane < 2,2,3,3- tetramethylbutane
< octane
Crude Oil and Uses of Alkanes
•
The gasoline fraction of crude oil only makes up
about 19%, which is not enough to meet demand.
van der Waals Forces
Weak Intermolecular Attractive Forces
The boiling point of a compound increases with
the increase in van der Waals force…and a
Gecko uses them to walk!
Gecko: toe, setae, spatulae
6000x Magnification
Full et. al., Nature (2000)
5,000 setae / mm2
600x frictional force; 10-7
Newtons per seta
Geim, Nature Materials
(2003)
Glue-free Adhesive
100 x 10 6 hairs/cm2
http://micro.magnet.fsu.edu/primer/java/electronmicroscopy/magnify1/index.html
Intermolecular Forces
Ion-Dipole Forces (40-600 kJ/mol)
• Interaction between an ion and a dipole (e.g. NaOH and
water = 44 kJ/mol)
• Strongest of all intermolecular forces.
Ion-Dipole & Dipole-Dipole Interactions:
like dissolves like
• Polar compounds dissolve in polar solvents
& non-polar in non-polar
Intermolecular Forces
Dipole-Dipole Forces
(permanent dipoles)
5-25 kJ/mol
Intermolecular Forces
Dipole-Dipole Forces
Boiling Points &
Hydrogen Bonding
Hydrogen Bonding
• Hydrogen bonds, a
unique dipole-dipole (1040 kJ/mol).
Intermolecular Forces
London or Dispersion Forces
• An instantaneous dipole can induce another dipole in an
adjacent molecule (or atom).
• The forces between instantaneous dipoles are called
London or Dispersion forces ( 0.05-40 kJ/mol).
Boiling Points of Alkanes
•
governed by strength of intermolecular
attractive forces
•
alkanes are nonpolar, so dipole-dipole
and dipole-induced dipole forces are
absent
•
only forces of intermolecular attraction
are induced dipole-induced dipole forces
Boiling Points
•Increase with increasing number of carbons
•
more atoms, more electrons, more
opportunities for induced dipole-induced
dipole forces
•Decrease with chain branching
•
branched molecules are more compact with
smaller surface area—fewer points of contact
with other molecules
Intermolecular Forces
London Dispersion
Forces
Which has the higher
attractive force?
Question 4.10
• Which alkane has the highest boiling
point?
•
A)
hexane
•
B)
2,2-dimethylbutane
•
C)
2-methylpentane
•
D)
2,3-dimethylbutane
Boiling Points
•Increase with increasing number of carbons
• more atoms, more electrons, more
opportunities for induced dipole-induced
dipole forces
Heptane
bp 98°C
Octane
bp 125°C
Nonane
bp 150°C
Boiling Points
•Decrease with chain branching
• branched molecules are more compact with
smaller surface area—fewer points of contact
with other molecules
Octane: bp 125°C
2-Methylheptane: bp 118°C
2,2,3,3-Tetramethylbutane: bp 107°C
Sources and Uses of Alkanes
•
Gasoline is a mixture of straight, branched, and
aromatic hydrocarbons (5–12 carbons in size).
–
Large alkanes can be broken down into smaller
molecules by CRACKING.
–
Straight chain alkanes can be converted into branched
alkanes and aromatic compounds through
REFORMING.
After using these processes, the yield of gasoline is
about 47% rather than 19%.
–
Chemical Properties:
Combustion of Alkanes
•All alkanes burn in air to give
carbon dioxide and water.
Heats of Combustion
Heptane
4817 kJ/mol
654 kJ/mol
Octane
5471 kJ/mol
654 kJ/mol
Nonane
6125 kJ/mol
What pattern is noticed in this case?
Heats of Combustion
•Increase with increasing number of carbons
• more moles of O2 consumed, more moles
of CO2 and H2O formed
Heats of Combustion
5471 kJ/mol
5 kJ/mol
5466 kJ/mol
8 kJ/mol
5458 kJ/mol
6 kJ/mol
5452 kJ/mol
What pattern is noticed in this case?
ENERGY Diagrams /
Reaction Coordinate
Diagrams
5471 kJ/mol
25
+
O2
2
5466 kJ/mol
5458 kJ/mol
+
25
O2
2
5452 kJ/mol
+
8CO2 + 9H2O
25
O2
2
+
25
O2
2
Heat of Combustion
Patterns
•Increase with increasing number of carbons
• more moles of O2 consumed, more moles
of CO2 and H2O formed
•Decrease with chain branching
• branched molecules are more stable
(have less potential energy) than their
unbranched isomers
Important Point
•Isomers can differ in respect to their stability.
•Equivalent statement:
–Isomers differ in respect to their potential energy.
Differences in potential energy can
be measured by comparing heats of
combustion. (Worksheet problems)

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