chapter 1 notes organic chemistry

8/8/2019 Chapter 1 Notes Organic Chemistry

1/22

STRUCTURE AND BONDING

McMurry Chapter 1


2/22

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


3/22

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.


4/22

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)


5/22

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


6/22

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


7/22


8/22

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


9/22

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


10/22

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


11/22

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


12/22

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


13/22

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


14/22

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


15/22

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


SB-28

sideways overlap of p-orbitals is also possible

result is a -bond



16/22

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.


17/22

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


18/22



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


1 x 4 + 3 x 1 + 1 x 7 = 14



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

..

....



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


1 x 4 + 3 x 6 + 2 = 24



Recall valency: C 4 bonds; O 2 bonds if neutral


Extra electrons are nonbonding;distribute tosatisfy octet rule

+/- any existing charge

Answer:

C

O

O O....

..

.. ....

.. ..2-

There are some exceptions that require experience


19/22

O

SO O

O

2-



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


1 x 6 + 4 x 6 + 2 = 32




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


20/22


SB-37

Condensed structures


SB-38

Bond-line structures


21/22

1. 142. 153. 16

4. 175. 18

How many hydrogen atoms are present in thenaturally-occurring terpene -terpinene shown

below?

SB-39


SB-40

3-D Structures

used to emphasize tetrahedral arrangement at carbon


22/22

chapter 1 notes organic chemistry

Documents