Organic ChemistryOrganic ChemistryThe study of the compounds of

carbonOver 10 million compounds have

been identifiedC is a small atom

◦it forms single, double, and triple bonds◦it is intermediate in electronegativity

(2.5)◦it forms strong bonds with C, H, O, N, and

some metals

Schematic View of an Schematic View of an AtomAtom

◦a small dense nucleus, diameter 10-14 - 10-15 m, which contains positively charged protons and most of the mass of the atom

◦an extranuclear space, diameter 10-10

m, which contains negatively charged electrons

Electron Configuration of Electron Configuration of AtomsAtomsElectrons are confined to regions

of space called principle energy levels (shells)◦each shell can hold 2n2 electrons (n

= 1,2,3,4......)

Shell

Number of Electrons Shell

Can Hold

Relative Energiesof Electrons

in These Shells

3218 8 2

4321

higher

lower

Electron Configuration of Electron Configuration of AtomsAtoms

Aufbau Principle:Aufbau Principle: ◦orbitals fill in order of increasing energy

from lowest energy to highest energyPauli Exclusion Principle:Pauli Exclusion Principle:

◦only two electrons can occupy an orbital and their spins must be paired

Hund’s Rule:Hund’s Rule: ◦when orbitals of equal energy are

available but there are not enough electrons to fill all of them, one electron is added to each orbital before a second electron is added to any one of them

Electron Configuration of Electron Configuration of AtomsAtoms

Drawing electron

Lewis Dot StructuresLewis Dot StructuresGilbert N. LewisValence shell:Valence shell:

◦the outermost occupied electron shell of an atom

Valence electrons:Valence electrons: ◦electrons in the valence shell of an atom;

these electrons are used to form chemical bonds and in chemical reactions

Lewis dot structure:Lewis dot structure: ◦the symbol of an element represents the

nucleus and all inner shell electrons◦dots represent valence electrons

Struktur Lewis DotStruktur Lewis DotStruktur Dot Lewis untuk unsur 1-18

N OB

H

Li Be

Na

He

Cl

F

S

Ne

Ar

C

SiAl P

1A 2A 3A 4A 5A 6A 7A 8A

Mg ::

::

::

.

.

.

.

.

.

.

..

..

. .

.

.

.

:

:

:

::::::::

::::::.

:::

:

Lewis Model of BondingLewis Model of BondingAtoms bond together so that each atom

acquires an electron configuration the same as that of the noble gas nearest it in atomic number◦ an atom that gains electrons becomes an anionanion◦ an atom that loses electrons becomes a cationcation◦ the attraction of anions and cations leads to the

formation of ionic solidsionic solids◦ an atom may share electrons with one or more

atoms to complete its valence shell; a chemical bond formed by sharing electrons is called a covalent bondcovalent bond

◦ bonds may be partially ionic or partially covalent; these bonds are called polar covalent polar covalent bondsbonds

In chemistry, a bond is typically classified as one of two types:

covalentPurely covalent (non-polar): The bonding electrons are shared equally between the two bonding atoms.Polar covalent: The electrons are shared between the two bonding atoms, but unequally, with the electrons spending more time around the more electronegative atom.

IonicIonic: The electrons aren’t shared. Instead, the more electronegative atom of the two bonding atoms selfishly grabs the two electrons for itself, giving this more electronegative atom a formally negative charge and leaving the other atom with a formal positive charge. The bond in an ionic bond is an attraction of opposite charges.

ElectronegativityElectronegativityElectronegativity:Electronegativity:

◦a measure of an atom’s attraction for the electrons it shares with another atom in a chemical bond

Pauling scalePauling scale◦generally increases left to right in a

row◦generally increases bottom to top in

a column

Electronegativity:Electronegativity:

The ability of atoms in a molecule to attract electrons to itself. On the periodic chart, electronegativity increases as you go…

◦ …from left to right across a row.◦ …from the bottom to the top of a column.

Electronegativ

ity

increase

s

Electronegativity

increases

Formation of IonsFormation of IonsA rough guideline:

◦ ions will form if the difference in electronegativity between interacting atoms is 1.9 or greater

◦example: sodium (EN 0.9) and fluorine (EN 4.0)

◦we use a single-headed (barbed) curved arrow to show the transfer of one electron from Na to F

◦ in forming Na+F-, the single 3s electron from Na is transferred to the partially filled valence shell of F

Na + F Na+ F -

• •• •••

••

• •••

••

+ F-(1s22s22p6)Na+(1s22s22p6)F(1s22s22p5)+Na(1s22s22p63s1)

Covalent BondsCovalent BondsThe simplest covalent bond is that in H2

◦ the single electrons from each atom combine to form an electron pair

◦ the shared pair functions in two ways simultaneously; it is shared by the two atoms and fills the valence shell of each atom

The number of shared pairs◦one shared pair forms a single bond◦ two shared pairs form a double bond◦ three shared pairs form a triple bond

H H H-H+ • H0 = -435 kJ (-104 kcal)/mol•

Polar and Non-polar Covalent Polar and Non-polar Covalent BondsBonds

Although all covalent bonds involve sharing of electrons, they differ widely in the degree of sharing

We divide covalent bonds into◦Non-polar covalent bonds◦polar covalent bonds

Difference in ElectronegativityBetween Bonded Atoms Type of Bond

Less than 0.5

0.5 to 1.9Greater than 1.9

Nonpolar covalentPolar covalent

Ions form

Polar and Non-polar Covalent Polar and Non-polar Covalent BondsBonds

◦an example of a polar covalent bond is that of H-Cl

◦the difference in electronegativity between Cl and H is 3.0 - 2.1 = 0.9

◦we show polarity by using the symbols ++ and --, or by using an arrow with the arrowhead pointing toward the negative end and a plus sign on the tail of the arrow at the positive end

H Cl+ -

H Cl

Polar Covalent BondsPolar Covalent BondsBond dipole moment (Bond dipole moment ():):

◦a measure of the polarity of a covalent bond◦ the product of the charge on either atom of a

polar bond times the distance between the nuclei

◦Table below shows average bond dipole moments of selected covalent bonds

H-OH-NH-C

H-S C-I

C-FC-ClC-Br C-N

-

C-O

C=N

C=O

-

Bond Dipole (D) Bond

1.4

1.51.51.41.2

Bond

Bond Dipole (D)

1.30.3 0.7

0.20.7

2.3

3.5

Bond Dipole (D)Bond

Lewis StructuresLewis StructuresTo write a Lewis structure

◦determine the number of valence electrons◦determine the arrangement of atoms◦connect the atoms by single bonds◦arrange the remaining electrons so that

each atom has a complete valence shell◦show a bonding pair of electrons as a

single line◦show a nonbonding pair of electrons as a

pair of dots◦ in a single bond atoms share one pair of

electrons, in a double bond they share two pairs of electrons, and in a triple bond they share three pairs of electrons

Lewis StructuresLewis Structures

In neutral molecules◦ hydrogen has one bond◦ carbon has 4 bonds and no lone pairs◦ nitrogen has 3 bonds and 1 lone pair◦ oxygen has 2 bonds and 2 lone pairs◦ halogens have 1 bond and 3 lone pairs

H2O (8) NH3 (8) CH4 (8) HCl (8)

C2H4 (12) C2H2 (10) CH2O (12) H2CO3 (24)

H-O-H H-N-HH

H-C-HH

HH-Cl

H-C C-HH

HC O

H

HC C

H

HO O

CH H

O

Ethylene

Hydrogen chlorideMethaneAmmoniaWater

Carbonic acidFormaldehydeAcetylene

Formal ChargeFormal Charge Formal charge:Formal charge: the charge on an atom in a molecule or a

polyatomic ionTo derive formal charge

1. write a correct Lewis structure for the molecule or ion

2. assign each atom all its unshared (nonbonding) electrons and one-half its shared (bonding) electrons

3. compare this number with the number of valence electrons in the neutral, unbonded atom

Number of valence electrons in the neutral, unbonded atom

All unsharedelectrons

One half of all shared electrons

+Formalcharge

=

Structure and Bonding

Formal ChargeFormal ChargeDraw Lewis structures, and show which atom

in each bears the formal charge

ClO2- CH3COO-

HCOO- BH4- CH3NH2

C3H4

Exceptions to the Octet Exceptions to the Octet RuleRule

Molecules containing atoms of Group 3A elements, particularly boron and aluminum

Aluminum chloride

:

:

:

F B

F

F

Cl Al

Cl

Cl

6 electrons in the valence shells of boron

and aluminum

Boron trifluoride

: :

: :

: :

::

::

:

::

::

Exceptions to the Octet Exceptions to the Octet RuleRule

Atoms of third-period elements have 3d orbitals and may expand their valence shells to contain more than 8 electrons◦phosphorus may have up to 10

:

:

Phosphorus pentachloride

Phosphoricacid

P

ClCl Cl

Cl ClCH3-P-CH3

CH3

Trimethyl-phosphine

H-O-P-O-H

O

O-H

:

:

::

:

::

::

::

::

::

:

:

::

:

:

Exceptions to the Octet Exceptions to the Octet RuleRule

◦sulfur, another third-period element, forms compounds in which its valence shell contains 8, 10, or 12 electrons

:

::H-S-H CH3-S-CH3 H-O-S-O-H

O

OO

Sulfuricacid

Hydrogensulfide

Dimethylsulfoxide

::

::

: : : :

: :

VSEPRVSEPRBased on the twin concepts that

◦ atoms are surrounded by regions of electron density◦ regions of electron density repel each other

HC C

H

O C

C

H

NH

HC

HH

O

H

CH C H

H

O

4 regions of e- density(tetrahedral, 109.5°)

3 regions of e- density(trigonal planar, 120°)

2 regions of e- density(linear, 180°)

H

CH H

HN

H HH

:

::

::

::

:

H HO

CH N

VSEPR ModelVSEPR Model Example:Example: predict all bond angles for these

molecules and ions

(a) NH4+ (b) CH3NH2

(f) H2CO3 (g) HCO3-(e) CH3CH=CH2

(i) CH3COOH(h) CH3CHO

(d) CH3OH

(j) BF4-

Polar and Nonpolar Polar and Nonpolar MoleculesMoleculesTo determine if a molecule is

polar, we need to determine ◦if the molecule has polar bonds◦the arrangement of these bonds in

spaceMolecular dipole moment (Molecular dipole moment ():): the

vector sum of the individual bond dipole moments in a molecule◦reported in debyes (D)

Polar and Nonpolar Polar and Nonpolar MoleculesMoleculesthese molecules have polar bonds,

but each has a zero dipole moment

O C O

Carbon dioxide = 0 D

B

F

F

F

Boron trifluoride = 0 D

C

Cl

ClClCl

Carbon tetrachloride = 0 D

Polar and Nonpolar Polar and Nonpolar MoleculesMoleculesthese molecules have polar bonds and are polar

molecules

N

HH

H

O

H H

Water = 1.85D

Ammonia = 1.47D

directionof dipolemoment

directionof dipolemoment

Polar and Nonpolar Polar and Nonpolar MoleculesMolecules

◦formaldehyde has polar bonds and is a polar molecule

Formaldehyde = 2.33 D

directionof dipolemoment H H

C

O