Download Chapter 2: Structure and Properties of Organic Molecules

Chapter 2:
Structure and Properties of
Organic Molecules
Atomic Orbitals (2.1-2.5)
– Wave functions that represent the probability
of finding electrons in a specific region of space
s, p, d, f orbitals
– In organic chemistry, need to concentrate only
on s and p orbitals
33
p orbitals have a nodal plane
– Area of space where the probability of finding
electrons is almost zero
34
Ú Molecular orbitals (2.2)
are produced when atomic orbitals (either native or
hybridized) of different atoms interact
q Produces bonding and anti-bonding orbitals
Molecular Orbitals for H2
* antibonding molecular orbital
relative
energy
s atomic
orbital of
hydrogen
s atomic
orbital of
hydrogen
bonding molecular orbital
35
Ú In the case of p orbitals, the overlap can
take place in two different forms
36
37
Single Bonds (2.3)
All single bonds are sigma bonds and
all double or triple bonds contain only
one sigma bond
Ú Sigma bonds can be formed from
atomic orbitals, hybridized or native
or a combination
38
Ú 2 other types of bonds in organic
molecules:
Ú Pi (π
π) bond
Ú Hydrogen-bond (H-bond)
39
Ú π-bond: cannot exists if a σ-bond is not
already present
Ú Result from the overlap of p orbitals of two
atoms. The π-bond is always perpendicular
to the sigma bond connecting the nuclei.
Ú Occur in sp2 and sp hybridized atoms
(double/triple bonds)
40
4 electrons in the
bonding region between the nuclei
Ú Double bond
* first pair forms the sigma bond
* second pair forms the pi bond
Normal combination for a double
bond such as the one found in
ethylene
Example: Ethene or ethylene
H
H
C
H
C
H
41
Ú Triple Bond 6 electrons in the bonding
region between the nuclei
* first pair forms the sigma bond
* second/third pairs form the pi bonds
Normal combination for a triple bond such as
the one found in acetylene
42
Hybridization and Molecular shapes
(2.4)
Ú Molecular shapes are related to
hybridization of central atom
sp3: only single bonds; 109.5o angle
(methane)
sp2: double bond; 120o angle (ethylene)
sp: triple bond; 180o angle (acetylene)
43
Ú Atomic orbitals can combine to generate
new orbitals:
* Hybridization: combination of atomic orbitals of
the same atom producing new orbitals of lower
energy
* sp, sp2 and sp3 These orbitals explain the geometry of
molecules
Shape of sp Hybrid Orbitals
in phase
s
out-of-phase
p
sp3
one s orbital combines with 3 p orbitals
to form 4 new sp3 hybrid orbitals
44
45
46
Ú Only 3 general shapes are normally
found in organic molecules:
47
Rotation of Single Bond vs
Rigidity of Double Bond (2.7)
Ú Consider ethane: CH3-CH3
Each carbon is sp3
Many structures of ethane are possible.
They differ only by the position of one
methyl group in relation to the other one:
48
Ú CONFORMATIONS:
Structures that differ only by rotation
along a single bond
Ex: ethane
Both forms exist, and all other
structures in between. In fact
a real ethane molecule rotates
through all the conformations,
but the staggered conformation
is favoured.
49
Ú Consider ethylene: CH2=CH2
Ethylene is quite rigid and rotation along
the C=C is not allowed.
50
Ú In ethylene, rotation does not affect the
sigma bond, but the pi overlap is broken
Ú RULE: Rotation is allowed along single
bonds, but double/triple bonds are rigid
and cannot be twisted
51
Ú Because of this rigidity, compounds with
different substituent arrangements on a
double bond are different:
52
Ú these 2 compounds are different and
have different chemical/physical
properties. They are called:
Isomers
Compounds having the same
molecular formula, but different
structures
53
Structural Isomers (2.8)
Also known as “constitutional isomers,
they differ only by their bonding sequence
54
Example 2: C5H12 pentane has
3 structural isomers
CH3CH2CH2CH2CH3
CH3CHCH2CH3
CH3
pentane
isopentane
CH3
CH3CCH3
CH3
neopentane
55
Ú Problem:
How many structural isomers with a
molecular formula C6H14 can you draw?
56
ÚStereoisomerism (2.8)
compounds that differ by the 3D
arrangements of atoms (or groups) in
space. They are called:
Stereoisomers
57
Ú Problem: what is the relationship
between A/B, A/C and B/C?
H
H
H3C
CH3
A
H
H3C
B
CH3
H
CH3
H
H
CH3
C
58
Problem: Draw all the possible isomers of
formula C5H10 and identify the structural
and stereoisomers.
59
Polarity of molecules (2.9)
Bond polarity can range from:
Non-polar covalent (C-C)
Polar covalent (C-O)
Ionic (NH4+ -Cl)
The polarity of bonds is related to
electronegativity of atoms
60
Ú Electronegativity of some important
elements (for more examples see Figure 17)
61
Example:
Bond polarity is the most important topic
to remember in order to understand
organic chemistry
62
The dipole moment
the dipole moment of a bond is the
measure of its polarity Ǻ (in Debye: D)
63
Ú For example:
64
The molecular dipole moment is the
dipole moment of the molecule taken as a
whole. It is a good indicator of the
overall polarity of molecules.
Its value is the vector sum of individual
bond dipole moments
65
Examples:
H
O
H
formaldehyde
2.3D
O
C O
carbon dioxide
0
D
66
Ú Because of bond dipole, partial positive
and negative charges are induced
within a neutral molecule. These
induced charges will be important in
reactivity as well as in interactions that
exist between molecules.
H Cl
H O
H
H N H
H
67
Intermolecular Forces (2-10)
3 major kinds of attractive forces cause
molecules to associate into solids or liquids:
Ú dipole-dipole forces (polar molecules)
Ú London forces (affect all molecules)
Ú Hydrogen bonds (for molecules with NH or
OH groups)
68
Ú Dipole-dipole forces:
attractive intermolecular forces resulting
from the attraction of the positive and
negative ends of polar molecules
The greater the forces, the higher will be
the boiling point of a given compound
69
O
cyclopentane
no dipole moment
bp: 49oC
tetrahydrofuran
dipole moment exists
bp: 65oC
70
Ú London dispersion forces:
In non-polar molecules (alkanes), this is
the principal attractive force
They arise from temporary dipole
moments created by the proximity of other
molecules
71
Ú Result: molecules with larger surface
area will have a higher boiling/melting
point
72
Hydrogen bonding
Not a true bond, but a very strong form of
dipole-dipole attraction. An H atom can
participate in H-bonding if it is bonded to
O or N (-OH, -NH)
73
Ú Hydrogen bonds form between the
hydrogen and the lone pair of the
heteroatom from another molecule
Ú result: the stronger the H-bond, the higher
the boiling/melting point
– alcohols (OH) have higher boiling points
than amines (NH) of similar molecular
weights, because the OH bond is more
strongly polarized than NH
OH
Ethanol
bp= 78oC
NH2
Propyl amine
bp= 49oC
74
Nomenclature
(will be covered as we introduce new functional
groups
Ú Types of Functional Groups
The following slides are to help you
recognize organic compounds
See Inside Front Cover of Book
75
Ú 3 general classes of organic
compounds:
A) Hydrocarbons (contains only C, H)
B) Compounds containing “O”
C) Compounds containing “N”
76
Ú Hydrocarbons: compounds composed
only of carbon and hydrogen. They
include:
– alkanes (saturated hydrocarbons)
– alkenes (double bond)
– alkynes (triple bond)
– Aromatics (double and single bonds
in conjugation)
77
Ú Alkanes:
General formula
CnH2n + 2
Names: ends with the suffix “anes”
First part of the names refer to the
number of carbon atoms.
All other compounds are named
based on the corresponding alkanes
78
#C
1
2
3
4
5
6
7
8
9
10
Name
methane
ethane
propane
butane
pentane
hexane
heptane
octane
nonane
decane
Formula
CH4
C2H6
C3H8
C4H10
C5H12
C6H14
C7H16
C8H18
C9H20
C10H22
79
pentane
heptane
Ú If the alkane molecule forms a ring, it is a
“cycloalkane” and is formula is: CnH2n
(same as an alkene)
80
Ú If a molecule contains a non alkane part, the
non alkane portion is referred to as:
Functional group
Ú The alkane portion is then referred to as the
Alkyl group
alkyl
OH
pentanol
functional
group
81
Ú Alkyl group: the alkane portion of a
molecule minus one H atom which was
removed to allow bonding to the functional
group
Ú Names of Alkyl groups: derived from the
alkane
[alkane name - ane] + yl = alkyl
82
Ú When the structure of the alkyl group is of
no concern, it is often replaced by “R’ as a
generalization
83
Ú Alkenes: hydrocarbons with a C=C
General formula CnH2n (same as
cycloalkanes)
The C=C is the most reactive part of an
alkene: it is the functional group
Names: [alkane – ane] + ene =alkene
ethane
ethene
cyclohexane cyclohexene
84
Ú Alkene may have stereochemistry and can be
labeled as cis or trans.
Ú In small rings (4-8 carbon atoms) containing a
double bond, the term “cis” is omitted since the
double bond is always in that geometry.
85
Ú In alkenes where there is more than one
possibility for the position of the double
bond, the location of the C=C must be
specified
86
Ú Alkynes
Hydrocarbons with a C-C triple bond
The triple bond is the functional group
Names: [alkane – ane] + yne = alkyne
87
Ú Aromatic Hydrocarbons:
Aromatic hydrocarbons are derived from
“benzene”
Benzene is a 6-carbon cyclic structure
containing 3 double bonds. Each double
bond is separated from the next one by a
single bond…a conjugated system where
the electron are delocalized
88
Ú Names: alkyl group first + benzene
Ú The symbol Ar represents aryl groups the
same way R represented alkyl groups
89
Compounds Containing “O”
–
–
–
–
–
Alcohols
Ethers
Aldehydes and Ketones
Carboxylic Acids
Carboxylic Acid Derivatives
90
Ú Alcohols: compounds containing the “OH”
group, the hydroxyl group
Ú Names: [alkane – e] + ol = alkanol
Position of the “OH” group must be
specified
91
Ú
Ethers: made of 2 alkyl groups bonded to
the same “O”
Ú
Names: [alkyl + alkyl] + ether or
[dialkyl] + ether
If alkyl groups are different, use
alphabetical order.
92
Ú
Aldehydes and Ketones
For both of these compounds, the functional
group is the carbonyl group: C=O
Ú
Ketones: 2 alkyl groups attached to C=O
Ú
Aldehydes: 1 alkyl group and one H attached to
C=O
93
Ketones: [alkane – e] + one = alkanone
C=O position must be specified
Aldehydes: [alkane – e] + al = alkanal OR
[alkane – e] + aldehyde = alkanaldehyde
not necessary to specify the position of
C=O (always #1 unless with an acid
derivative)
O
propanone
(acetone)
O
O
H
2-pentanone
ethanal
(acetaldehyde))
O
H
butanal
94
Ú
Carboxylic acids: molecules containing
the “carboxyl group”
O
OH
COOH
CO2H
carboxyl group
Ú
Names: [alkane – e] + oic acid = alkanoic
acid
95
Ú
Carboxylic acid derivatives
All acid derivatives contain the carbonyl
group bonded to an heteroatom (Cl, O,
N).
96
Ú Compounds Containing “N”
– Amines
– Amides
– Nitriles
97
Ú Amines: alkylated derivatives of ammonia
(NH3)
Ú Amines are basic. They are classified
based on the number of alkyl groups
attached to “N”.
R NH2
primary
amine
R
R N
R NH
R
R
secondary tertiary
amine
amine
R
R N+ R
R
quaternary
ammonium
salt
98
Ú Names: [alkyl + alkyl] + amine OR
(di) or (tri) + amine
99
Study Problems:
Ú 2-35
What is the relationship between the
following pairs of compounds (same,
structural or stereoisomers).
100
Ú 2-41
Which compound in each of the following
pair has the higher boiling point? Why?
101
Ú 2-42
Circle and name the functional groups in
the following compounds.
Cl
O
OH
CHO
102
Ú 2-37
Predict whether the following compounds
have a dipole moment or not. Use an
arrow to show the direction of the dipole
when present.
O
103