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8/8/2019 Chapter 1 Notes Organic Chemistry
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STRUCTURE AND BONDING
McMurry Chapter 1
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Organic compounds are compounds containing carbon
Carbon neither readily gives up nor readily accepts electrons
Carbon shares electrons with other carbon atoms as well as with several differentkinds of atoms
ORGANIC CHEMISTRY
SB-1
STRUCTURE OF THE ATOM AND ATOMIC ORBITALS
SB-2
electrons housed in orbitals that are grouped in shells of increasing size and energy
an atomic orbital can hold two electrons at most
a list of orbitals occupied by an atoms electrons describes the atomsground-state electron configuration
atom consists of nucleus surrounded by electron cloud
nucleus electron cloud
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ELECTRON CONFIGURATIONS OF SOME2ND ROW ELEMENTS
SB-3
Atomic # 5 6 7 8 9 10
use the following rules to assign electron configuration:
Aufbau Principle : the lowest energy orbitals are filled first
Pauli Exclusion Principle : a maximum of two spin paired electronsmay be placed in each orbital
Hunds Rule : one electron is added to each degenerate (equal energy)orbital before a second electron is added
The outermost shell is called the valence shell.
Electrons in the valence shell are the ones involved in bonding.
LEWIS STRUCTURES OF ATOMS
SB-4
Lewis structures are concerned only with valence electrons
nucleus represented by symbol of element of interest
valence electrons represented by dots surrounding nucleus
B C N O F Ne
Atoms in the same group have the same number of valence electrons.
The number of valence electrons is the same as the group number.
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ELECTRON CONFIGURATIONS OF SOME2ND ROW ELEMENTS
SB-5
Neon (a noble gas) has a full valence shell = 8 electrons (an octet).
For 2 nd row elements, a valence shell octet = great stability.
THE OCTET RULE
SB-6
Atoms form bonds to achieve a stable octet (noble gas configuration)
Atoms can donate, accept, or share electrons to achieve a noble gaselectronic configuration
exceptions to the octet rule, e.g., P and S (may have expanded octets)
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VALENCY
Atoms in organic compounds form a fixed number of bonds.
Carbon can form one or more bonds to other carbon atoms.
Alkane Alkene Alkyne
SB-7
Organic Nomenclature: McMurry 3.4, 4.1, 6.2, 10.1, 13.1Extra help/practice available through SI workshop(s)
Which of the following line bond structures is correct?
C
H
H
H
C C H
H
C
H
H
H
C C H C
H
H
H
C C H
H
C
H
H
H
C C H
H H
A B C D
SB-8
1. A and B2. A and C3. B and D4. Only B5. Only C
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THE NATURE OF CHEMICAL BONDS.A CLOSER LOOK AT ATOMIC STRUCTURE
SB-9
Quantum Mechanics
mathematical description of bonding that takes into account the wave nature of electrons wave equation is solved to yield a series of wave functions ( ) for the atom
describes a series of states with different energies for each electron
2 gives the probability of finding an electron in a particular location in space
plots of 2 in three dimensions generate electron clouds called orbitals
four types of orbitals: s, p, d, f; only s and p orbitals are important in organicchemistry
atomic orbital : region in space where the probability of finding an electronis large
s orbital p orbitals
wave functions may be positive, negative or zero
Heisenberg uncertainty principle : the position and momentum of an electron cannotsimultaneously be known
VALENCE BOND THEORY
SB-10
Example: Bonding in H 2
two atomic orbitals overlap
result is a -bond
all purely single bonds are -bonds
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SHAPE OF ORGANIC COMPOUNDS. VSEPR THEORY
3D Shape
Virtually all molecules possess a 3-dimensional shape which isoften not accurately represented by drawings
Note that a dashed line indicates abond is behind the page
Note that a wedge indicates a bond iscoming forward
One explanation is Valence Shell Electron Pair Repulsion (VSEPR)
four bonds to carbon have tetrahedral arrangement
SB-13
Electrons pairs want to be as far away from one another as possible
THE STRUCTURE OF ETHANE
SB-14
each carbon forms -bonds to three hydrogens by overlap of C sp 3 orbitalsand H s orbitals
C-C -bond formed by end-to-end overlap of two sp 3 orbitals
molecule is approximately tetrahedral around each carbon
because a -bond has cylindrical symmetry along the bond axis, there isrelatively free rotation of groups joined by a single bond
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THE STRUCTURE OF ETHENE
SB-15
ethene (C 2H4) contains a carbon-carbon double bond and is in the class oforganic compounds called alkenes
the geometry around each carbon is called trigonal planar (all atoms ina single plane; bonds point toward corners of a regular triangle)
explained by orbital hybridization:
electron promotion
orbital hybridization three hybrid
sp2
orbitals hybridization of the 2s orbital and two 2p orbitals yields three sp 2 hybrid
orbitals one 2p orbital remaining
THE STRUCTURE OF ETHENE
SB-16
2p orbital is perpendicular to plane of sp 2 orbitals
sp 2 orbitals used to form single ( ) bonds
2p orbital used to form -bond
combination of -bond and -bond yields double bond typical of alkenes
note difference in C-C bond length for ethane compared to ethene
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THE STRUCTURE OF ETHYNE
SB-17
ethyne (C 2H2) contains a carbon-carbon triple bond and is in the class oforganic compounds called alkynes
the geometry around each carbon is linear with bond angles of 180
explained by orbital hybridization:
electron promotion
orbital hybridization
two hybridsp orbitals
hybridization of the 2s orbital and one 2p orbital yields two sp hybridorbitals
two 2p orbitals remaining
THE STRUCTURE OF ETHYNE
SB-18
2p orbitals are perpendicular to each other and to plane of sp orbitals
sp orbitals used to form single ( ) bonds
2p orbitals used to form -bonds
combination of -bond and two -bonds yields triple bond typical of alkynes
note difference in C-C bond length for ethane compared to ethene comparedto ethyne
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What is the hybridization of the carbonatom in the following molecule?
1. s 2. p 3. sp 4. sp 2
5. sp 3
SB-19
HYBRIDIZATION OF NITROGEN,
OXYGEN, PHOSPHORUS AND SULFUR
SB-20
orbital hybridization four hybrid
sp 3 orbitals
Nitrogen
nitrogen is sp 3-hybridized with 1 e-pair trigonal pyramidal
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HYBRIDIZATION OF NITROGEN,OXYGEN, PHOSPHORUS AND SULFUR
SB-21
Oxygen
orbital hybridization four hybrid
sp 3 orbitals
oxygen is sp 3-hybridized with 2 e-pairs bent
HYBRIDIZATION OF NITROGEN,
OXYGEN, PHOSPHORUS AND SULFUR
SB-22
Phosphorus
phosphorus is sp 3-hybridized with no e-pairs tetrahedral
Sulfur
sulfur is sp 3-hybridized with 2 e-pairs bent
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Given the ground state electronic configuration of boron, predict itshybridization state and geometry in its neutral compounds, e.g. BF 3.[Hint: BF 3 readily accepts a pair of electrons to form a stable octet.]
1. sp 3, tetrahedral2. sp 2, trigonal planar3. sp 3, trigonal pyramidal4. sp 2, trigonal pyramidal
SB-23
RELATIONSHIP BETWEEN
HYBRIDIZATION AND VSEPR THEORY
SB-24
Hybridization ~ Bond Angle Molecular Geometry
sp 180 linear
sp 2 120 trigonal planar
sp 3 109.5 tetrahedral
Lone Pairs
0
0
0
sp 2 120 bent 1
sp 3 109.5 trigonal pyramidal 1
sp 3 109.5 bent 2
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BOND LENGTHS OF ETHANE, ETHENE AND ETHYNE
SB-25
C-H bond length also decreases in the series
C-C bond length decreases with increase in number of bonds
110 pm
2s orbitals closer to the nucleus than 2 p orbitals
bond length decreases with an increase in s-character of bond
hybridized orbital with more % s character closer to the nucleus than orbitalwith less s character
sp 3 (25% s-character) sp 2 (33% s-character) sp (50% s-character)
A SECOND PICTURE OF BONDING.MOLECULAR ORBITAL (MO) THEORY
SB-26
LCAO: mathematic operation by which wave functions are added or subtracted
number of MOs that result must always equal the number of AOs used
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SB-27
-bonds can also be formed by end-to-end overlap of p-orbitals
Is there another way by which p-orbitals may overlap?
-bonds can also be formed by end-to-end overlap of one s-orbital and onep-orbital
A SECOND PICTURE OF BONDING.MOLECULAR ORBITAL (MO) THEORY
SB-28
sideways overlap of p-orbitals is also possible
result is a -bond
A SECOND PICTURE OF BONDING.MOLECULAR ORBITAL (MO) THEORY
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RELATIVE ENERGIES OF - AND -ORBITALS
SB-29
-bonding orbital is lower in energy than -bonding orbital
lower energy = greater stability
-bonds are more difficult to break than -bonds
LEWIS STRUCTURES OF ATOMS
SB-30
Lewis structures are concerned only with valence electrons
nucleus represented by symbol of element of interest
valence electrons represented by dots surrounding nucleus
B C N O F Ne
Atoms in the same group have the same number of valence electrons.
The number of valence electrons is the same as the group number.
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DRAWING CHEMICAL STRUCTURES.LEWIS STRUCTURES OF MOLECULES
SB-31
representation of a molecule showing electron pairs as a pairof dots or as a dash
only valence electrons are indicated
number of valence electrons is determined by position inperiodic table (valence number = group number)
12 3 4 5 6 7
8
LEWIS STRUCTURES OF MOLECULES
drawing Lewis structures
SB-32
Example: C 2H4
determine total number of valence electrons required to satisfy octet rulefor carbon and to provide each hydrogen with two electrons
2 x 8 + 4 x 2 = 24
determine total number of electrons available from valence shell of eachatom
2 x 4 + 4 x 1 = 12
the difference represents the number of electrons that must be shared (12)
the number of shared electrons / 2 = number of bonds required (6)
Recall valency: C 4 bonds; H 1 bond
Answer:
C C
H
H
H
H
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LEWIS STRUCTURES OF MOLECULES
drawing Lewis structures
SB-33
Try: CH 3Fdetermine total number of valence electrons required to satisfy octet rulefor carbon and fluorine, and to provide each hydrogen with two electrons
2 x 8 + 3 x 2 = 22
determine total number of electrons available from valence shell of eachatom
1 x 4 + 3 x 1 + 1 x 7 = 14
the difference represents the number of electrons that must be shared (8)
the number of shared electrons / 2 = number of bonds required (4)
Recall valency: C 4 bonds; H 1 bond; F 1 bond
Answer:
CH
H
H
F
Note: 14 electrons available; 8 used; 6 remaining
Extra electrons are nonbonding electrons ;distribute to satisfy octet rule
..
....
LEWIS STRUCTURES OF MOLECULES
drawing Lewis structures
SB-34
Try: CO 32-
determine total number of valence electrons required to satisfy octet rulefor carbon and oxygen 1 x 8 + 3 x 8 = 32
determine total number of electrons available from valence shell of eachatom
1 x 4 + 3 x 6 + 2 = 24
the difference represents the number of electrons that must be shared (8)
the number of shared electrons / 2 = number of bonds required (4)
Recall valency: C 4 bonds; O 2 bonds if neutral
Note: 24 electrons available; 8 used; 16 remaining
Extra electrons are nonbonding;distribute tosatisfy octet rule
+/- any existing charge
Answer:
C
O
O O....
..
.. ....
.. ..2-
There are some exceptions that require experience
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O
SO O
O
2-
LEWIS STRUCTURES OF MOLECULES
drawing Lewis structures
SB-35
Try: SO 42-
determine total number of valence electrons required to satisfy octet rulefor sulfur and oxygen 1 x 8 + 4 x 8 = 40
determine total number of electrons available from valence shell of eachatom
1 x 6 + 4 x 6 + 2 = 32
the difference represents the number of electrons that must be shared (8)
the number of shared electrons / 2 = number of bonds required (4)
Note: 32 electrons available; 8 used; 24 remaining
Extra electrons are nonbonding;distribute tosatisfy octet rule
+/- any existing charge
Answer: 2-
X OSO OO2-
REPRESENTATION OF STRUCTURAL FORMULAS
SB-36
Lewis structures
relatively free rotation around single bonds
used to emphasize connectivity
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REPRESENTATION OF STRUCTURAL FORMULAS
SB-37
Condensed structures
REPRESENTATION OF STRUCTURAL FORMULAS
SB-38
Bond-line structures
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1. 142. 153. 16
4. 175. 18
How many hydrogen atoms are present in thenaturally-occurring terpene -terpinene shown
below?
SB-39
REPRESENTATION OF STRUCTURAL FORMULAS
SB-40
3-D Structures
used to emphasize tetrahedral arrangement at carbon
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