McMurry Chapter 1

Structure & Bonding

Organic Chemistry IS. Imbriglio

Organic Chemistry: What is it?

• 1770: Organic chemistry referred to compounds isolated from living things.

__________ belief that a “magic” force, present in plants and animals, is required to make organic compounds

• 1789: Lavoisier observed that organic compounds are composed primarily of __________________________

Organic Chemistry: What is it?• 1816: Michel Chevreul showed that one organic

compound could be converted into others.

• 1828: Friedrich Wohler converted an inorganic compound into an organic compound.

Organic Chemistry: the study of carbon compounds

• ________of all known compounds are composed of carbon (~30 million known compounds)

• ________of chemists define themselves as organic

• Organic chemistry is crucial to our way of life: Pharmaceutical, Petroleum, Materials/Polymer, OUR BODIES!

Why Carbon?What makes carbon so special? (3 things)

Why Carbon?• Carbon atoms form __________________ bonds

to other atoms (including other carbon atoms).

Incredible Structural Diversity!

O

OH

H

OH

H

H

OHOHH

H

OH

OH

HH

H OH

OH HO

HOHO

H

N

N NH

N

NH2

NH

NH

O

O

H2N CH C

CH3

OH

OH2N CH C

CH2

OH

O

HO

H H

H

CH3OH

CH3 H

CH3OH

H H

O

Carbohydrates Amino Acids

DNA Bases Hormones

Review of the AtomAtomic Structure:

– ___________ charged nucleus – dense and small – _______________ charged electrons in a cloud

around the nucleus– Diameter approximately 2 10-10 m

Review of the AtomOrbitals:- Three-dimensional shapes indicating where the

electron is most likely to be found – s, p, d, f- We will focus on s and p orbitals – carbon atoms do

not have d or f orbitals

Review of the AtomOrbitals:- Orbitals are organized into different shells and

subshells- In the ground state of an atom, electrons occupy

the lowest energy orbitals

Review of the AtomElectron Configurations:

– _________ electrons per orbital (max)– When two electrons occupy the same orbital, they

must have opposite spins (Pauli Exclusion Principle)– If two or more empty orbitals of equal energy are

available, electrons occupy each with spins parallel until all orbitals have one electron (Hund’s Rule)

• Write the electron configuration for oxygen.O

Review of Chemical Bonding• Atoms bond together

because the compound that results is more _________ than the separate atoms – bond formation is always exothermic.

• The Octet Rule: In most cases, atoms will _______ __________________ electrons to gain an octet.

Review of Chemical Bonding• An _____________ is the electrostatic attraction

between a cation (____) and an anion (____).• The two bonded atoms do not share electrons.

Review of Chemical Bonding• ___________________ occurs between atoms of similar

electronegativity – very important in organic chemistry.

eg. CCl4 is covalent . . . Why?

Large thermodynamic penalty for ionizationof carbon to C4+.

C

1s22s22px12py

1

C4+

1s2 = [He]

4e– H = +1480.7 kcal/mol(ionization potential)

+

Review of Chemical Bonding

• Instead of transferring electrons, each chlorine atom shares one valence electron with carbon so that every atom has a filled valence shell (an octet).

Lone Pairs: ____________________________________________

C 4 ClCCl

Cl

Cl

Cl

Lone Pairs

Review of Chemical Bonding• Covalently bonded compounds are represented by

Lewis or Kekulé structures.• Lone pairs will be included in this class.

Review of Chemical Bonding• The number of covalent bonds that a main group

element must form to achieve an octet equals eight minus its group number.

Keeping the common bonding patterns in mind, draw a valid Kekulé structure for each of the following formulas. Include all lone pairs.

CH3NH2 H2O2 C3H8

Review of Chemical BondingMultiple Covalent Bonds• Atoms can share more than one electron pair to gain

a full octet.– _________________: two electron pairs shared

between two atoms– _________________: three electron pairs shared

between two atoms

C CH

H H

H

ethylene acetylene

C C HH

Review of Chemical Bonding

C C C N N N O O F

Summary of Bonding (Neutral Atoms - Row 2)

C

Group 4A Group 5A Group 6A Group 7A

• Atoms will form combinations of single, double and triple bonds to gain a full octet.

4 total bonds 3 total bonds 2 total bonds 1 bond

Draw a valid Kekulé structure for each of the following formulas. Include all lone pairs.

CH3CO2H HCN

Review of Chemical BondingValence Bond Theory• Covalent bond forms when two atoms approach each

other closely so that a singly occupied orbital on one atom overlaps a singly occupied orbital on the other atom

• Electrons are paired in the overlapping orbitals and are attracted to nuclei of both atoms

• Carbon uses its four valence electrons to form four covalent bonds.

• Methane is a perfect tetrahedron – all of the C-H bonds are equivalent.

• The four orbitals that make those bonds must also be equivalent.

Review of Chemical Bonding

Review of Chemical BondingHybridization• Atomic orbitals (s, p) on the same atom can be

combined to form hybrid orbitals (sp, sp2, sp3) with geometries similar to those observed experimentally.

• Hybrid Orbitals:– Are more directional (better bonding overlap

with orbitals on other atoms)– Minimize electron-electron repulsion (think

VSEPR)

Review of Chemical Bonding

• sp3 Hybridization: One s and three p orbitals combine to form four new sp3 hybrid orbitals.

The large lobes of the four sp3 orbitals are pointed 109.5 from each other – a tetrahedral geometry.

largelobe

backlobe

Review of Chemical Bondingsp3 Hybridization in Methane• Each sp3 orbital on carbon overlaps with a 1s orbital

on hydrogen to form four sigma bonds.• ___________________: cylindrically symmetrical bond

resulting from head-on overlap of two orbitals along the bonding axis

C

H

H HH

Orbital Picture Ball & Stick Model Space-Filling Model

Review of Chemical Bondingsp3 Hybridization in Ethane• The two carbon atoms are bonded together by sigma

overlap between the two sp3 orbitals.• Three sp3 orbitals on each carbon are used to form C-H

sigma bonds.

Review of Chemical Bonding• sp2 Hybridization: One s orbital combines with two p

orbitals to give three new sp2 hybrid orbitals

• One p orbital is left unhybridized.

• The large lobes of the sp2 orbitals are pointed 120 from each other – a trigonal planar geometry.

Review of Chemical Bondingsp2 Hybridization in Ethylene• Two sp2 orbitals overlap head-on to form a sigma bond

– Electrons in the sigma bond are along the bonding axis• Two p orbitals overlap side-by-side to form a pi bond

– Electrons in the pi bond occupy regions above and below the bonding axis (not cylindrically symmetrical)

C CH

H H

H

ethylene

Review of Chemical Bonding• sp Hybridization: One s and one p orbital combine to

form two new sp hybrid orbitals

• Two p orbitals left unhybridized

• The large lobes of the sp orbitals are pointed 180 from each other – a linear geometry

Review of Chemical Bondingsp Hybridization in Acetylene• Two sp orbitals overlap head-on to form a sigma bond• Two vertical p orbitals overlap side-by-side to form pi

bond• Two horizontal p orbitals overlap side-by-side to form

pi bond

acetylene

C C HH

Review of Chemical Bonding

Assigning Hybridization to Atoms• The hybridization of an atom in a molecule can

be determined by counting the number of hybrid orbitals the atom is using.

• Hybrid orbitals are used to:– Form sigma bonds (not pi bonds)– Hold lone pairs

Review of Chemical BondingAssigning Hybridization to Atoms• The number of hybrid orbitals on an atom must be

equal to the number of atomic orbitals that were “mixed together” to form the hybrid orbitals.

eg. One s orbital plus three p orbitals gives four sp3 orbitals

# hybrid orbitals hybridization geometry

4 sp3 tetrahedral

3 sp2 trigonal planar

2 sp linear

approx. bond angles

109°

120°

180°

Review of Chemical BondingAssigning Hybridization to Atoms• Heteroatoms also use hybrid orbitals to form

sigma bonds and hold lone pairs.• Below are examples of sp3-hybridized nitrogen

and oxygen.

Determine the hybridization of each non-hydrogen atom in the following molecules.

C C C CH

HH

O

CH3

C C NHH

H

Representing Molecules

Condensed Structures

Representing Molecules

• Lewis and Kekulé structures are only adequate for very small molecules.

C

CC

C

CC

CC

CC C

CC

C

CC

C

O

CH3 H

CH3OH

H

HH

H H HH

H HH

HH

HH

HH H

HH

H

Testosterone(not so easy)

CC

C

H H

Propane(pretty easy)

H

HH

H

HH

O

O

O

OO

O

O

O

OO

OH

H3C

H HH

HH

HO H2C H

O

H

CH3 CH3 CH3

HH

HH

H H

HH

H

Brevetoxin A (!!)

Just imagine...

Representing MoleculesSkeletal Structures (Line-Angle Formulas)

Rules for Drawing Skeletal Structures1. Do not draw carbon atoms. A carbon atom is

assumed to be at each intersection and at the end of each line.

2. Do not draw hydrogen atoms bonded to carbon. Assume enough C-H bonds to give each carbon a filled valence.

3. Draw all heteroatoms and attached hydrogens.

Representing Molecules

Convert the following Kekulé structures to skeletal structures. Show all lone pairs.

CC

OH

H H

H

HH

Ethanol

C

C C

CH H

HH

H

H H

H

Cyclobutane

CC

CCC

C CC

O

OH

H CH

HH

HH

CH

H

Ibuprofen

CC

C

HH

HH HH

H

HH

CC

O

C

H

H

HH

H

H

Acetone

Representing Molecules

• Dashes and Wedges: Molecules are not flat.

H

CH H

HCH4

109.5°H

H HH

lines

wedge

dashmethane

lines: in the plane of the paperdashes: going back into the paper (away from you)wedges: coming out of the paper (toward you)