basic of org chem
Post on 10-Apr-2018
232 Views
Preview:
TRANSCRIPT
-
8/8/2019 Basic of Org Chem
1/15
1 Basics of Organic Chemistry
Organic chemistry is the study of molecules based on carbon
Several interesting organic molecules:
N
N N
N
O
O
O
OH
H
H
NH
N
N
N
O
O
SN
N
OO
N
OO
O
O
caffeinetetrahydrocannabinol (THC)
sildenafil citrate (Viagra)
cocaine
1.1 Electron configuration of atoms
Most of this is assumed knowledge from high school and first year chemistry; the following is
only be a brief review of this material
Ground-state electron configuration - the electron configuration of lowest energy for an atom,
molecule, or ion
When determining the ground-state electron configuration of an atom, three rules are used:
1. Aufbau principle orbitals are filled in order of increasing energy, from lowest to highest
2. Pauli exclusion principle no more than two electrons may be present in an orbital; if two
electrons are present then their spins must be paired
1
-
8/8/2019 Basic of Org Chem
2/15
3. Hunds rule when orbitals of equivalent energy are available but there are not enough electrons
to fill all of them completely, then one electron is added to each equivalent orbital before a second
electron is added to any one of them; this is especially true of the p-orbitals
Ground state electron configuration for carbon: 1s2
2s2
2px1
2py1
2pz0
1.2 Lewis structures
Valence shell the outermost electron shell of an atom; contains the valence electrons
Lewis structure of an atom the symbol of an element surrounded by a number of dots equal
to the number of electrons in the valence shell of the atom
N
OB
H
Li Be
Na Mg
He
IA IIA VA VIA VIIA VIIIAIIIA IVA
Cl
F
S
Ne
Ar
C
SiAl P
Table 1.1 Lewis structures for the first 18 elements of the Periodic Table.
1.3 Electronegativity and chemical bonds
Electronegativity a measure of the force of an atoms attraction for electrons that it shares
with another atom in a chemical bond
o Plays an extremely important role in the reactivity of various functional groups in
organic chemistry
2
-
8/8/2019 Basic of Org Chem
3/15
Figure 1.1 The electronegativity values for some atoms commonly found in organic chemistry. Values are expressed in
Pauling units.
In most cases carbon atoms are more electropositive than the other atoms to which they are
bonded, but you should always refer to the electronegativity values to be sure
two types of chemical bonds:
1. ionic bonds
2. covalent bonds
ionic bond a chemical bond resulting from the electrostatic interactions between a cation and
an anion
o generally occur between a metal and a non-metal
o the electronegativity of the two atoms involved differs by more than 1.9 Pauling units
o NaCl, MgSO4, KOH
Covalent bond a bond resulting from the sharing of two or more electrons between a set of
atoms
o Non-polar covalent bond a covalent bond between two atoms whose electronegativity
is different by less than 0.5 Pauling units
H2, CH4
3
-
8/8/2019 Basic of Org Chem
4/15
o Polar-covalent bond a covalent bond between two atoms whose electronegativities are
different by 0.5-1.9 Pauling units
Si-C, C-O, H-Cl
o
Two ways of depicting polar covalent bonds:
Draw a plus sign over the more electropositive atom and then draw an arrow to
the more electronegative atom through the plus sign
Use the lower-case Greek letter delta (means slight in chemistry speak) and
include the charge that the atom possesses
H Cl H Cl2.1 3.0
+ -
Figure 1.2 The two different ways of depicting the polarity of a bond.
Using this notation it is possible to determine whether a molecule is polar in nature (e.g. H2O)
or non-polar (e.g. CCl4)
o It is possible for a molecule to be non-polar yet still have polar bonds
1.4 Drawing Lewis structures
1. determine the number of valence electrons in the molecule or ion count the number of valence
electrons contributed by each atom
2. determine the connectivity of the atoms in the molecule usually needs to be determined
experimentally for all but the most simple molecules
3. connect the atoms with single bonds and arrange the remaining electrons such that each atom
has a complete outer shell
bonding electrons valence electrons involved in forming a covalent bond (i.e., shared electrons)
4
-
8/8/2019 Basic of Org Chem
5/15
non-bonding electrons valence electrons not involved in forming covalent bonds (a.k.a. lone
pairs)
Formal charge the charge on an atom in a polyatomic ion or molecule
H O+
H
H
O O
O
HO
N+ O
OHFC = 6 - (2+(0.5 x 6))
= 1 FC = 6 - (6+(0.5 x 1))= -1
FCN = 5 - (0+(0.5 x 8))
= 1FCO = 6 - (6+(0.5 x 2))
= -1
H ydrogen chloride
M ethane A mmonia
W ater
H O
H
H
H NH C
H
H
H C l
H
H
H 2O ( 8 )
N H 3 (8)C H 4 (8 )
H C l ( 8 )
Figure 1.3 The Lewis structures of some common molecules. The number of valence
electrons is shown in parentheses.
# of valence
electrons in
unbonded atom
all
unshared
electrons
one half
of all shared
electrons
+Formal
charge=
Figure 1.4 The formal charge on some common molecules used in organic chemistry.
Most atoms prefer to have an octet in their valence shell (why?)
There are however exceptions to the octet rule (hydrogen sulphide 8, dimethylsulfoxide
10, and sulphuric acid 12)
5
-
8/8/2019 Basic of Org Chem
6/15
1.5 Functional Groups
This section is simply an introduction to common functional groups found in organic chemistry
It is important to recognize these functional groups
OH
OH OH
NH2
N
H
N
R
Figure 1.5 Common functional groups in organic chemistry.
1.6 Resonance and the Movement of Electrons
Electrons are not static, not even when involved in covalent bonds in molecules
This movement of electrons is known asresonance
Generally involves the movement of electrons into, or out of, pi bonds
H
O
R R
O
R O
O
R R NH
O
R
Alcohols:
primary secondary tertiary
Amines:
primary secondary tertiary
Carbonyl Compounds:
aldehyde ketone ester amide
6
-
8/8/2019 Basic of Org Chem
7/15
Ethanoate ion
(Acetate ion)
C
O
O
C H 3C
O
O
C H 3
-
-
and
the oxygen atoms in the acetate ion are not different from each other
the negative charge is shared between the two oxygen atoms
the two molecules are resonance contributors
resonance only depicts the movement of electrons, not atoms
o sigma bonds remain intact
resonance imparts stability to a molecule
o the more resonance forms are possible, the more stable a molecule is (i.e., benzene)
Pauls Rule of Happy Molecules for the most part, organic molecules are happiest if they are
neutral, content if they possess partial charges, and usually ticked off (reactive) if they possess a
full formal charge
usecurly arrows to denote the movement of electrons
o understanding the use of curly arrows is extremely important in understanding organic
chemistry an excellent way to keep track of your electrons
N
O
O
N
O
O
-
-
Ethanoate ion
(equivalent
contributing
structures)
C
O
O
C H 3 C
O
O
C H 3
-
Nitrite ion
(equivalent
contributing
structures)
-
7
-
8/8/2019 Basic of Org Chem
8/15
reaction arrow- goesfrom reagents to products
curly arrow- depictsthe movement ofelectrons
resonance arrow- placedbetween two resonanceforms of a molecule
equilibrium arrow- placedbetween the reagents andproducts in equilibrium reactions
Figure 1.6 Arrows used in organic chemistry and what they mean.
1.7 Molecular orbital theory of covalent bonding
1.7.1 Atomic s and p orbitals
Atomic orbitals represent the probability of finding electrons in a given space around the
nucleus of an atom
o The part of an orbital in which there is zero probability of finding an electron is called a
node
Figure 1.7 The shape of the s atomic orbital.
8
-
8/8/2019 Basic of Org Chem
9/15
s orbitals are symmetric in all three planes
Figure 1.8 The shape of atomic p orbitals depicting the two ways of denoting the phase of the orbital
lobes and showing the nodal plane.
Figure 1.9 The three, mutually perpendicular p orbitals.
1.7.2 Formation of molecular orbitals
Molecular orbital (MO) theory the theory in which electrons in molecules occupy
molecular orbitals formed by the combination of the atomic orbitals of the atoms that make
up the molecule
Three rules for applying MO theory to the formation of covalent bonds:
1. the number of molecular orbitals formed is equal to the number of atomic orbitals combined
2. molecular orbitals are arranged in order of increasing energy
9
-
8/8/2019 Basic of Org Chem
10/15
3. molecular orbitals are filled with electrons using the same principles used for filling atomic
orbitals (Aufbau principle, Pauli exclusion principle, Hunds rule)
Figure 1.10 The combination of two 1s orbitals to form a bond.
Sigma () bond a bond in which the electron density is concentrated between the two nuclei
and along the axis joining them
Figure 1.11 The molecular orbital energy diagram for the hydrogen molecule, H2.
10
-
8/8/2019 Basic of Org Chem
11/15
Figure 1.12 The molecular orbital energy diagram of two atomic p orbitals coming together to form a sigma bond.
bonding molecular orbital molecular orbital formed between two atomic orbitals that have
the same phase
anti-bonding molecular orbital molecular orbital formed between two atomic orbitals that
have different phases
pi () bond a bond formed between the overlap of parallel p orbitals on adjacent atoms
11
-
8/8/2019 Basic of Org Chem
12/15
Figure 1.13 The molecular orbital energy diagram for the formation of a pi bond.
Sigma bonds are stronger than pi bonds
1.7.3 Hybridization of atomic orbitals
The second-period elements use 2s and 2p atomic orbitals to form molecular orbitals
Hybrid orbital orbital formed by the combination of two or more types of atomic orbitals
12
-
8/8/2019 Basic of Org Chem
13/15
3.7.1.1sp3 Hybrid orbitals
Figure 1.14 Four tetrahedral sp3 orbitals.
sp3 atomic orbitals are made up of one s orbital and 3 p orbitals
consist of two lobes, one larger than the other
the bond angle between the orbitals is 109.5
3.7.1.2sp2Hybrid orbitals
Figure 1.15 Orbitals contributing to the bonding structure of ethene. Unhybridized p orbital is not shown.
Consists of one s orbital and 2 p orbitals
13
-
8/8/2019 Basic of Org Chem
14/15
The third p orbital is not involved in hybridization and lies perpendicular to the plane of the sp 2
orbitals
A pair of sp2 orbitals involved in forming a bond and a pair of sp2 orbitals involved in
forming a bond
o sp2 orbitals involved in the formation of double bonds
The bond angle in sp2 systems is 120
3.7.1.3sp Hybrid orbitals
Figure 1.16 The orbitals contributing to the bonding structure of ethyne (a.k.a. acetylene).
sp orbitals result in one bond and two bonds to carbon
14
-
8/8/2019 Basic of Org Chem
15/15
o Produce carbon-carbon triple bonds
The bond angle between these orbitals is 180
Table 1.2 Bond lengths and bond strengths for ethane, ethene, and ethyne.
Molecule Bond Bond Length (pm) Bond Strength (kJ/mol)
Ethane C-C 153.2 368
Ethene (ethylene) C-C 133.9 611
Ethyne (acetylene) C-C 121.2 837
15
top related