Chapter 12. Saturated Hydrocarbons
Sections
CHEM 121, Winter 2009, LA TECH
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12.1 Organic and Inorganic Compounds
12.2 Bonding Characteristics of the Carbon Atom
12.3 Hydrocarbons and Hydrocarbon Derivatives
12.4 Alkanes: Acyclic Saturated Hydrocarbons
12.5 Structural Formulas
12.6 Alkane Isomerism
12.7 Conformations of Alkanes
12.8 IUPAC Nomenclature for Alkanes
12.9 Line-Angle Formulas for Alkanes
12.10 Classification of Carbon Atoms
12.11 Branched-Chain Alkyl Groups
12.12 Cycloalkanes
12.13 IUPAC Nomenclature for Cycloalkanes
12.14 Isomerism in Cycloalkanes
12.15 Sources of Alkanes and Cycloalkanes
12.16 Physical Properties of Alkanes and Cycloalkanes
12.17 Chemical Properties of Alkanes and Cycloalkanes
Chemistry at a Glance: Properties of Alkanes and
Cycloalkanes
CHEM 121, Winter 2009, LA TECH
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Introduction
Organic chemistry is the study of the compounds
of carbon atoms consisting of single, double,
and triple covalent bonds with C, H, O, N, and
some metals
Biochemistry is the study of the compounds of
carbon atoms consisting of single, double, and
triple covalent bonds with C, H, O, N, and some
metals
CHEM 121, Winter 2009, LA TECH
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12.1 Organic and Inorganic Compounds
CHEM 121, Winter 2009, LA TECH
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Sections
Chapter 1: Covalent Bonding and Shapes of Molecules
1.
2.
3.
4.
5.
6.
7.
8.
Introduction
Electronic Structure of atoms
Lewis Model of bonding
Bond angles and molecular shapes
Polar and non-polar molecules
Resonance
Orbital overlap of covalent bonding
Functional groups
CHEM 121, Winter 2009, LA TECH
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Introduction
Organic chemistry is the study of the compounds
of carbon.
C is a small atom
• it forms four bonds consisting of single, double,
and triple bonds
• it forms strong bonds with C, H, O, N, and some
metals
Problem: Identify organic and inorganic
compounds:
NaCl, C2H5OH, CH3COOH, Na2CO3, CH3OK, KOH
CHEM 121, Winter 2009, LA TECH
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Atomic Orbitals
s orbital - a spherical-shaped atomic orbital; can
hold a maximum of 2 electrons
p orbital - a dumbbell-shaped atomic orbital; the
three p orbitals (px, py, pz) can hold a maximum
of 2 electrons each
Electrons always fill starting with the lowest-energy
orbital:
lower energy
higher energy
1s2 2s2 2p6 3s2 3p6
We will be concerned with only the valence
electrons which are the outermost electrons
involved in forming bonds.
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Shapes of s Atomic Orbitals
All s orbitals have the shape of a sphere, with
its center at the nucleus
• of the s orbitals, a 1s orbital is the smallest, a 2s
orbital is larger, and a 3s orbital is larger still
CHEM 121, Winter 2009, LA TECH
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Shapes of p Atomic Orbitals
• A p orbital consists of two lobes arranged in a
straight line with the center at the nucleus
CHEM 121, Winter 2009, LA TECH
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Electronic Structure of atoms (Review)
Ground state electronic configuration of atoms in core format
Carbon (C): ):
[He] 2s2, 2p2
or [He] 2s2, 2px13py13pz0
Potassium (K):
{Ar] 4s1
Phosphorous (P):
[Ne] 3s2, 3p3
Valence shell electronic configuration
Carbon (C): ):
3s2, 3p2
Potassium (K):
4s1
Phosphorous (P):
3s2, 3p3
How you get the electronic configuration of an atom from the
periodic table?
CHEM 121, Winter 2009, LA TECH
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Excited State Valence Electron Configuration
Carbon (C):
Ground state:
2s2, 2p2 or 2s2, 2px13py13pz0
electron
promotion
E
1s2 2s2
Excited State:
CHEM 121, Winter 2009, LA TECH
2p2
2s1, 2p3
1s2 2s1
2p3
or 2s1, 2px13py13pz1
1-11
Lewis Model of bonding (Review)
"octet rule“
atoms tend to gain, lose or share electrons so as to
have eight electrons in their outer electron shell
“Lewis structure of atoms”
Shows only valence electrons, is a convenient way
of representing atoms to show their chemical
bonding pattern.
CHEM 121, Winter 2009, LA TECH
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Lewis structure of atoms (Review)
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Cations and Anions
Cations
•What elements lose electrons? And how many?
•What is the positive charge on their cations?
•Anions
•What elements gain electrons?
•What is the positive charge on their anions?
•Covalent bonds
•How many covalent bonds are formed?
•What elements share electrons?
C
N
Na+
O
CHEM 121, Winter 2009, LA TECH
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Covalent or Ionic
Identify covalent and ionic compounds:
NaCl, C2H5OH, CH3COOH, Na2CO3, CH3OK, KOH
Covalent :
Ionic:
CHEM 121, Winter 2009, LA TECH
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Ionic model of bonding model (Review)
Ionic bond - results from the electrostatic attraction
between a cation and an anion of two atoms typically
involves a metal and a nonmetallic element.
Anion: An atom that gains electrons becomes a negative
ion
Cation: An atom that loses electrons becomes a positive
ion
+
Na +
2
Li +
CHEM 121, Winter 2009, LA TECH
F
S
Na
2 Li
F
+
-
S
21-16
Covalent model of bonding (Review)
Covalent bonds - results from the sharing of electrons
between two atoms typically involves two
nonmetallic elements
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Electronegativity (Review)
Is the attraction of an atom for its valence electrons
increases
i
n
c
r
e
a
s
e
s
H
2.1
Li
1.0
Na
0.9
CHEM 121, Winter 2009, LA TECH
Be
1.5
Mg
1.2
B
2.0
Al
1.5
C
2.5
Si
1.8
N
3.0
P
2.1
O
3.5
S
2.5
F
4.0
Cl
3.0
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Nonpolar/polar-covalent and ionic bonds. (Review)
We classify chemical bonds as polar covalent, nonpolar
covalent and ionic based on the difference in
electronegativity between the atoms
D i fference in
Electronegativity
Betw een Bonded Atoms
less than 0.5
0.5 to 1.9
greater than 1.9
CHEM 121, Winter 2009, LA TECH
Ty pe o f Bo nd
no npo lar co val ent
po lar coval ent
ioni c
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Classify following bonds nonpolar-covalent,
polar-covalent or ionic bonds
N-H nonpolar-covalent, polar-covalent or ionic bonds
O-H nonpolar-covalent, polar-covalent or ionic bonds
C-H nonpolar-covalent, polar-covalent or ionic bonds
C-F nonpolar-covalent, polar-covalent or ionic bonds
Na-Cl nonpolar-covalent, polar-covalent or ionic bonds
Al-Cl nonpolar-covalent, polar-covalent or ionic bonds
CHEM 121, Winter 2009, LA TECH
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Drawing Lewis structure molecules and ions
(Review)
1) Predict arrangement of atoms.
a) H is always a terminal atom.
b) Halogens and oxygen are often terminal.
c) The central atom of binary compounds is usually written
d) first and has the lowest subscript.
e) Most organic compounds have more than two central atoms.
f)
These are mainly C, but N, O and S can also be central atoms.
2) Total number of valence electrons (e-)
a) Add all valence electron of atoms in the molecule from the
formula.
b) Add the ion charge for negative ions or subtract for positive ions.
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Drawing Lewis structure molecules and ions (Review)
3) Draw the skeletal structure by connecting the atoms with
single bonds.
4) Give each of the atoms an octet (8 e-). Adding unshared
pairs of electrons
5) Count the total number of e- used through step 4 and
compare to the number calculated in step 2.
a) If it results in zero, the structure is correct.
b) For every two electrons too many, another bond is added
(minimize formal charges).
Multiple bonds form only with C, N, O and S.
Total number of bonds to neutral atoms:
4 bonds to C
3 bonds to N, P
2 bonds to O, S
1 bond to H, F, Cl, Br, I
CHEM 121, Winter 2009, LA TECH
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Calculation of formal charges of atoms in the Lewis structure
1. For a neutral molecule, the sum of the formal charges equals zero. For
a polyatomic ion, the sum of the formal charges equals the charge on
the ion.
2. Formal charge of each atom is calculated by:
(group #) - (# unshared e-) - ½ (# shared e-)
3. Formal charges are shown as + or - on the atom with that charge.
4. An atom with the same number of bonds as its group number has no
formal charge.
5. In a molecule if two different elements can be assigned a negative
charge, then the more electronegative element gets the charge; the
same sign should not be given to bonded atoms.
CHEM 121, Winter 2009, LA TECH
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Types of electrons
Bonding pairs
Two electrons that are shared between two atoms.
A covalent bond.
Unshared (nonbonding ) pairs
A pair of electrons that are not shared between two
atoms. Lone pairs or nonbonding electrons.
oo
oo
oo
H Cl
oo
Unshared
pair
Bonding pair
CHEM 121, Winter 2009, LA TECH
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Lewis Structures (Review)
H
O
H
H 2O (8)
Water
H
Cl
HCl (8)
Hydrogen chloride
H
H
C
H
H
CH 4 (8)
Methane
H
N
H
H
N H 3 (8)
Ammonia
•How many bonding electron pairs are in the molecule?
•How many bonding electron pairs are in each atom?
•How many nonbonding electron pairs are in the molecule?
CHEM 121, Winter 2009, LA TECH
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Draw Lewis structure of molecules
CHCl3
C2H4
C3H8O
CH3CH2CH2OH
CH3CH2OCH3
CH3CO2H
CH3CHO
CHEM 121, Winter 2009, LA TECH
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Draw Lewis structure and assign formal charges
CH3NH3+
H
H
H
C
N
H
H
+
H
CH3O-
CHEM 121, Winter 2009, LA TECH
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Valence-Shell Electron-Pair Repulsion (VSEPR) model
(Review)
For predicting shapes of molecules and polyatomic Ions
based on the repulsion of valence pairs of electrons
making them as far apart as possible around an
atom of a Lewis structure.
1) Draw the Lewis structure for the molecule or ion.
2) Determine the number of bonding and unshared pairs attached to
the central atom.
One single, double or triple bond counted as a bonding pair
3) Choose the appropriate case from the given chart.
CHEM 121, Winter 2009, LA TECH
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Predict the bond angles of molecules from their Lewis
structures. (Review)
Molecule
Bonding pairs
unshared
pairs
Shape
H2O
two
two
bent
NH3
three
one
Trigonal pyramid
CH2O
three
none
Trigonal planar
CHCl3
four
none
tetrahedral
CH4
four
none
tetrahedral
CCl4
four
none
tetrahedral
CHEM 121, Winter 2009, LA TECH
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Polar and nonpolar molecules
In sert elpot of
ammon ia
(page 19)
In sert elpot of
acetylene
(page 20)
O
N
H
In sert elpot of
formaldeh yd e
(page 20)
H
H +
A mmon ia
(p olar)
CHEM 121, Winter 2009, LA TECH
C
H + H
H C C H
Formald ehyde
(p olar)
Acetylene
(nonpolar)
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Molecular Shape and Polarity (Review)
Molecule
Bonding pairs and
unshared pairs
Electron pair
distribution
Polarity
H2 O
four
asymmetric
polar
NH3
four
asymmetric
polar
CH2O
three
asymmetric
polar
CHCl3
four
asymmetric
polar
CH4
four
symmetric
Non-polar
CCl4
four
symmetric
Non-polar
CHEM 121, Winter 2009, LA TECH
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Curved arrow Electron pushing
Curved arrow: a symbol used to show the
redistribution of valence electrons
In using curved arrows, there are only two
allowed types of electron redistribution:
• from a bond to an adjacent atom
• from an atom to an adjacent bond
Electron pushing by the use of curved arrows is
also used in explaining reaction mechanisms
CHEM 121, Winter 2009, LA TECH
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Drawing Curved Arrows
To show the movement of electrons in breaking and forming
bonds. The tail of the arrow is started at the site of electron
density (negative character such as a pi bond or lone pair of
electrons) and proceeds to the arrowhead which is drawn to
the site of electron deficiency (positive character).
NEGATIVE TO POSITIVE!
Arrows can be drawn from:
tail
1) lone pair
bond
2) bond
lone pair
3) bond
bond
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head
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Resonance
For many molecules and ions with double
bonds, two or more Lewis structure could be
written
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Orbital Overlap Model
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Hybrid Atomic Orbitals
Hybridization is the mixing up of two or more
atomic orbitals
There are three types of hybrid atomic orbitals for
carbon
sp3 (one s orbital + three p orbitals give four sp3
orbitals)
sp2 (one s orbital + two p orbitals give three sp2
orbitals)
sp (one s orbital + one p orbital give two sp orbitals)
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s and p hybrids
Four sp3 hybrids
Three sp2 hybrids
Two sp hybrids
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 and  bonds
Overlap of hybrid orbitals can form two types
of bonds, depending on the geometry of the
overlap
 bonds are formed by “direct” overlap
s1 s1
p1
p1
s1
p1
 bonds are formed by “parallel” overlap of
unhybrid p prbitlas
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 and  bonds in single and multiple bonds
single bond - one shared pair of electrons between
two atoms; a  bond
double bond - two shared pairs of electrons
between two atoms; one s bond and one  bond
triple bond - three shared pairs of electrons
between two atoms; one s bond and two p bonds
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Bond Properties
Bond strength:
strongest
C C > C
C
weakest
> C C
Bond length:
longest
C
C > C
CHEM 121, Winter 2009, LA TECH
shortest
C
> C
C
1-40
Counting  and  bonds in Lewis structure
Hybridization
Types of
Bonds to Carbon
sp 3
fou r s igma bond s
sp 2
three sigma bonds
and on e pi bond
Example
HH
H-C-C-H
Ethan e
HH
H
H
C
H
sp
CHEM 121, Winter 2009, LA TECH
tw o sigma b on ds
and tw o p i bonds
N ame
H-C
Ethylene
C
H
C-H Acetylene
1-41
Predicting hybridization of atoms in a Lewis
structure
Count sigma bonds and unshared electrons around
the atom
If the total number of pairs:
2 sp hybridization
3 sp2 hybridization
4 sp3 hybridization
CHEM 121, Winter 2009, LA TECH
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Functional Groups in Organic Compounds
Functional group: an atom or group of atoms within a
molecule that shows a characteristic set of physical and
chemical properties
Functional group
• divide organic compounds into classes
• the sites of characteristic chemical reactions
• the basis for naming organic compounds
CHEM 121, Winter 2009, LA TECH
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Common Functional Groups
Alcohol(carbon, hydrogen and
oxygen)
CH3-CH2-OH
Thiol (thioalcohol) thio means
sulfur
CH3-CH2-SH
Amine
CH3-CH2-NH2
Ether
CH3-CH2-O-CH2-CH3
H H
H- C-C- O- H
H H
An alcohol
(Ethanol)
CH3 N H
H
(a 1° amine)
CHEM 121, Winter 2009, LA TECH
CH3 N H
CH3
(a 2° amin e)
CH3 N CH3
CH3
(a 3° amine)
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Common Functional Groups (continued)
Carbonyl group
or unit
Aldehyde
ketones
O
Carboxylic acids
CH3 -C-O-H
or
CH3 COOH
or
CH3 CO2 H
Acetic acid
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Classification of organic compounds
Class
CHEM 121, Winter 2009, LA TECH
Functional group
Example
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