Each C atom is tetrahedral with sp3 hybridized orbitals. They only have single
bonds. Question 2.1. • What is the hybridization of the starred carbon in
humulene ...
HYDROCARBONS
Chapter 2
• Compounds composed of only carbon and hydrogen atoms (C, H). Each carbon has 4 bonds.
Introduction to Hydrocabons
• They represent a “backbone” when other “heteroatoms” (O, N, S, .....) are substituted for H. (The heteroatoms give function to the molecule.)
Carbon Backbone, Nomenclature, Physical & Chemical Properties
• Acyclic (without rings); Cyclic (with rings); Saturated: only carbon-carbon single bonds; Unsaturated: contains one or more carbon-carbon double and/or triple bond
Types of Hydrocarbons
HYDROCARBONS • Alkanes contain only single (σ ) bonds and have the generic molecular formula: [CnH2n+2] • Alkenes also contain double (σ + π) bonds and have the generic molecular formula: [CnH2n] • Alkynes contain triple (σ + 2π) bonds and have the generic molecular formula: [CnH2n-2]
Each C atom is tetrahedral with sp3 hybridized orbitals. They only have single bonds.
• Aromatics are planar, ring structures with alternating single and double bonds: eg. C6 H 6 Each C atom is trigonal planar with sp2 hybridized orbitals. There is no rotation about the C=C bond in alkenes.
Question 2.1
Question 2.2
• What is the hybridization of the starred carbon in humulene (shown)?
• What is the hybridization of the starred carbon of geraniol?
•
A)
sp
•
A)
sp
•
B)
sp2
•
B)
sp2
•
C)
sp3
•
C)
sp3
•
D)
1s2 2s2 2p2
•
D)
1s2 2s2 2p2
1
Types of Hydrocarbons
Propane
It is easy to rotate about the C-C bond in alkanes. 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.
Naming Alkanes C1 - C10 : the number of C atoms present in the chain.
Each member C 3 - C 10 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
2
Constitutional or structural isomers have the same molecular formula, but their atoms are linked differently. Naming has to account for them.
Question 2.3 • 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 C7 H16 can be any one of the following:
Question 2.4 • How many isomeric hexanes exist? •
A)
2
•
B)
3
•
C)
5
•
D)
6
Question 2.5 • The carbon skeleton shown at the bottom right accounts for 9 carbon atoms. How many other isomers of C10H 22 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
3
Alkanes (Different types of sp3 carbon atoms)
Different Kinds of sp3 Carbons and Hydrogens
• Primary, 1o, a carbon atom with 3 hydrogen atoms: [RCH 3 ] • Secondary, 2o, a carbon atom with 2 hydrogen atoms: [RCH 2 -R] • Tertiary, 3o, a carbon atom with 1 hydrogen atom: [R-CHR] R • Quaternary, 4o, a carbon atom with 0 hydrogen atoms: CR4
Question 2.6 • 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°)
Nomenclature of Alkanes
Question 2.7 • How many primary carbons are in the molecule shown at the bottom right? •
A)
2
•
B)
3
•
C)
4
•
D)
5
3. Number the substituents to yield the lowest possible number in the number of the compound
1. Determine the number of carbons in the parent hydrocarbon
(substituents are listed in alphabetical order)
2. Number the chain so that the substituent gets the lowest possible number
4. Assign the lowest possible numbers to all of the substituents
4
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
6. If the same number is obtained in both directions, the first group receives the lowest number
7. In the case of two hydrocarbon chains with the same number of carbons, choose the one with the most substituents
8. Certain common nomenclatures are used in the IUPAC system
Question 2.7 • The correct structure of 3-ethyl-2methylpentane is: •
A)
B)
•
C)
D)
Cycloalkanes CnH2n
Cycloalkane Nomenclature
•
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
5
Cycloalkanes •
Name any alkyl groups on the ring in the usual way. A number is not needed for a single substituent. CH2CH3
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. H3 C CH3
Ethylcyclopentane CH2CH3 3-Ethyl-1,1-dimethylcyclohexane
Question 2.8
For more than two substituents,
• Which one contains the greatest number of tertiary carbons? •
A)
2,2-dimethylpropane
•
B)
3-ethylpentane
•
C)
sec-butylcyclohexane
•
D)
2,2,5-trimethylhexane
Naphtha (bp 95-150 °C)
2.17 Physical Properties of Alkanes and Cycloalkanes
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
6
Fig. 2.15
Question 2.9 • Arrange octane, 2,2,3,3-tetramethylbutane and 2-methylheptane in order of increasing boiling point. • A) 2,2,3,3-tetramethylbutane < octane < 2methylheptane • B) octane < 2-methylheptane < 2,2,3,3- tetramethylbutane • C) 2,2,3,3-tetramethylbutane < 2methylheptane < octane • D) 2-methylheptane < 2,2,3,3tetramethylbutane < octane
van der Waals Forces Weak Intermolecular Attractive Forces
Example of Intramolecular Forces: Protein Folding
10-40kJ/mol
Ion-dipole (Dissolving) 40-600kJ/mol
150-1000kJ/mol
700-4,000kJ/mol
0.05-40kJ/mol
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.
The boiling point of a compound increases with the increase in van der Waals force…and a Gecko uses them to walk!
Ion-Dipole & Dipole-Dipole Interactions: like dissolves like • Polar compounds dissolve in polar solvents & non-polar in non-polar
7
Intermolecular Forces Dipole-Dipole Forces (permanent dipoles)
Intermolecular Forces Dipole-Dipole Forces
5-25 kJ/mol
Boiling Points & Hydrogen Bonding
Hydrogen Bonding • Hydrogen bonds, a unique dipole-dipole (1040 kJ/mol).
Intermolecular Forces Hydrogen Bonding
8
DNA: Size, Shape & Self Assembly
Intermolecular Forces
http://www.umass.edu/microbio/chime/beta/pe_alpha/atlas/atlas.htm
Views & Algorithms
10.85 Å
10.85 Å
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).
Gecko: toe, setae, spatulae
Boiling Points of Alkanes
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
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
http://micro.magnet.fsu.edu/primer/java/electronmicroscopy/magnify1/index.html
Induced dipole-Induced dipole Attractive Forces +– • •
Induced dipole-Induced dipole Attractive Forces +–
+–
two nonpolar molecules center of positive charge and center of negative charge coincide in each
•
+–
movement of electrons creates an instantaneous dipole in one molecule (left)
9
Induced dipole-Induced dipole Attractive Forces + •
–
Induced dipole-Induced dipole Attractive Forces +
+–
temporary dipole in one molecule (left) induces a complementary dipole in other molecule (right)
•
•
–
+
Boiling Points •Increase with increasing number of carbons •
more atoms, more electrons, more opportunities for induced dipole-induced dipole forces
•Decrease with chain branching •
Induced dipole-Induced dipole Attractive Forces –
–
the result is a small attractive force between the two molecules
branched molecules are more compact with smaller surface area—fewer points of contact with other molecules
–
temporary dipole in one molecule (left) induces a complementary dipole in other molecule (right)
Induced dipole-Induced dipole Attractive Forces +
+
–
•
+
–
+
the result is a small attractive force between the two molecules
Intermolecular Forces London Dispersion Forces
Which has the higher attractive force?
10
Boiling Points
Question 2.10 • Which alkane has the highest boiling point? •
A)
hexane
•
B)
2,2-dimethylbutane
•
C)
2-methylpentane
•
D)
2,3-dimethylbutane
•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
2.18
Chemical Properties: Combustion of Alkanes •All alkanes burn in air to give carbon dioxide and water.
2,2,3,3-Tetramethylbutane: bp 107°C
Heats of Combustion Heptane
Heats of Combustion •Increase with increasing number of carbons
4817 kJ/mol 654 kJ/mol
Octane
5471 kJ/mol
• more moles of O2 consumed, more moles of CO2 and H2O formed
654 kJ/mol Nonane
6125 kJ/mol What pattern is noticed in this case?
11
Figure 2.17
Heats of Combustion 5471 kJ/mol
5471 kJ/mol
5 kJ/mol 5466 kJ/mol
+
8 kJ/mol
5466 kJ/mol
25 O2 2
5458 kJ/mol
5458 kJ/mol
+
6 kJ/mol
5452 kJ/mol
25 O2 2
25 + O2 2
+
25 O2 2
5452 kJ/mol 8CO2 + 9H2O
What pattern is noticed in this case?
Heat of Combustion Patterns
Important Point
•Increase with increasing number of carbons
•Isomers can differ in respect to their stability.
• more moles of O2 consumed, more moles of CO2 and H2O formed
•Equivalent statement: –Isomers differ in respect to their potential energy.
•Decrease with chain branching • branched molecules are more stable (have less potential energy) than their unbranched isomers
Differences in potential energy can be measured by comparing heats of combustion. (Worksheet problems)
O
2.19 Oxidation-Reduction in Organic Chemistry Oxidation of a carbon atom corresponds to an increase in the number of bonds to the carbon atom and/or a decrease in the number of hydrogens bonded to the carbon atom. See examples on the board.
O
increasing oxidation state of carbon O H H H
C H -4
H H
C
H
C
H
C
HO
C
OH
OH
H
OH
H -2
0
+2
+4
12
HC
increasing oxidation state of carbon H
H
H
H
C
C
H
H
H C
H
C
H
-3
• How to calculate the oxidation state of each carbon in a molecule that contains carbons in different oxidation states?
CH
CH3CH2 OH
H
-2
-1
Table 2.5 How to Calculate Oxidation Numbers • 1. Write the Lewis structure and include unshared electron pairs.
H H
H
C
C
H
H
••
O ••
H
Table 2.5 How to Calculate Oxidation Numbers • 3. For a bond between two atoms of the same element, assign the electrons in the bond equally.
C 2 H 6O
H
H
••
••
••
••
H •• C H
C H
••
•• O •• ••
H
Table 2.5 How to Calculate Oxidation Numbers • 2. Assign the electrons in a covalent bond between two atoms to the more electronegative partner.
H
H
••
••
••
••
H •• C H
C
•• •• O •• ••
H
H
Table 2.5 How to Calculate Oxidation Numbers • 3. For a bond between two atoms of the same element, assign the electrons in the bond equally.
H
H
••
••
••
••
H
H
H •• C• • C
•• •• O •• ••
H
13
Table 2.5 How to Calculate Oxidation Numbers • 4. Count the number of electrons assigned to each atom and subtract that number from the number of valence electrons in the neutral atom; the result is the oxidation number.
H
H
••
••
••
••
H
H
H •• C• • C
•• •• O •• ••
Each H C of CH3 C of CH2O O
= = = =
H
+1 -3 -1 -2
Generalization Oxidation of carbon occurs when a bond between carbon and an atom which is less electronegative than carbon is replaced by a bond to an atom that is more electronegative than carbon. The reverse process is reduction. oxidation C X C Y reduction X less electronegative than carbon Y more electronegative than carbon
Examples
Question 2.11
Oxidation CH4 + Cl2
CH3Cl + HCl
Reduction CH3Cl + 2Li
CH3Li + LiCl
• To carry out the reaction shown below we need: • • CH3OH → H2C=O • •
A)
an oxidizing agent
•
B)
a reducing agent
14
