Welcome to organic chemistry

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Welcome

to

Organic Chemistry

Organic Chemistry• Jons Jakob Berzelius – (1779 – 1848)

– organic – contained the vital force and were obtained from living things

– Inorganic – do not contain the vital force and were from non living things

Scientists believed that is was impossible to create compounds that contained the vital force.

• Problem arose when Friedrich Wohler produced urea

NH4+NCO- NH2 C NH2

OHeat

ammonium cyanate

Urea

Review

• Atomic number• Atomic mass• Isotope• Atomic weight – amu = 1/12 mass of C12 • Molecular weight – sum of atomic

weights of all atoms present

Review

• Waves and Particles– Particles have wave properties – waves have particle properties– What do you do when you can’t see any more?????????? …………….. You rely on mathematics

• Schrodinger equation tells us the most likely energy and electron possesses

• The closer the shell is to the nucleus the lower the energy

• Practice an electronic configuration

• Re-visit this section in Gen. Chem. if needed.

Covalent Bond

Review

• Chemical Bonding– Ionic– Covalent– Polar Covalent

Electronegativity is my new Organic Chemistry Friend – I know everything and anything about it……………….

Electronegativity

• Attraction an atom has for a pair of shared electrons

• Dipole – molecule with a positive and negative end

reported in debye (D)

Electrostatic Potential Maps

Lewis Structures

• Count valence electron

• Place least electronegative in center

• Draw dashes to form bonds

• Fill valence on outer atoms

• Put extra electrons in pairs on center atom

• Make sure center atom has octet

• Form double or triple bonds if needed

Lewis Structures

Basic Shapes

# of atoms bonded # of lone pair electrons

to center on center atom Shape

4 0Tetrahedral

3 0 Triangular

3 1 Pyramidal

2 0 Linear

2 1 or 2 Bent

Lewis Structures

Practice the Following

CO2

CH2O

SiCl4

PCl3 SO2

Formal Charge

FC = # valence – ( lone pairs + ½ bonded e- )

Practice

NO2- NH4+ CH2O

When two structures are possible, the one with more negative

numbers on the electronegative atoms is correct

CHN , CS2

Atomic Orbitals

Traveling wave – wave that travels through space (light – water)

Standing wave – wave that is confined to a limited space (guitar string)

Electrons acts like a standing wave

but in three dimensions

Atomic Orbital Theory

• Wave equation predicts probability of e- location

Atomic Orbital Theory

• Orbitals are actually solutions to the Wave Equation

Degenerate Orbitals• Simultaneous Roots (equal in energy)

"Many people tell you that an expert is someone who knows a great deal about his subject. To this I would object that no one can ever know very much about any subject. I would much prefer the following definition: an expert is someone who knows some of the worst mistakes of his subject, and how to avoid them."

- Werner Heisenberg

Molecular Orbital Theory

• An overlap of atomic orbitals yields … – Molecular orbitals

Bonding• Bond distance occurs at an energy

minimum• Direct overlap of orbitals results in a (sigma)

bond.– Cylindrically symmetrical– e- attracted to both nuclei

MO Theory

• Overlap of wave equation shows– reinforcement and…– cancellation

MO Theory• Molecular Orbital Theory predicts a

bonding orbital and a or antibonding orbital.

• Results: one 1s orbital and one 1s

orbital.

2p overlapMolecular Orbitals

Atomic Orbitals Atomic Orbitals

2p overlap

Bond order = ½ (# e- in bond MOs - # e- in anti bond MOs)

e-

e-

e-

e-e-

e-

1s

2s

2p

E

EN

RG

Y

Carbon Atom

CH

HH

H

1sE

EN

RG

Y

Carbon sp3

sp3

C C

H

H

H

H

1sE

EN

RG

Y

Carbon sp2

sp2

2p

C C HH

2p

1sE

EN

RG

Y

Carbon sp

sp

Bonding in Methane

• 4 Covalent bonds– All identical bonds (4 bonds)– Bond angles of 109o – Non-polar molecule since e- density uniform

Bonding in Ethane - CH3CH3

• Ethane has sp3 hybridization about each C orbitals– Nearly tetrahedral shape about each C

(All single bonds in organic chem are bonds!)

Bonding in Ethene - CH2CH2

• Overlap of the unhybridized, parallel orbitals yields a bond (along with the bonds you would expect).

Bonding in Ethyne - CHCH

• Each C forms a bond and 2 bonds

Representation of Acetylene

• Shape is referred to as “linear” (180o)

Bonding in Cations and Anions

• Cations have too few electrons• Anions have extra electrons• Unpaired e- are unhybridized

• Consider the methyl cation, CH3+

CH3.

• Consider the Methyl Radical, CH3.

CH3-

• Consider the Methyl Anion, CH3-

sp3 Water H2O

e-

e-

e-

e-e-

e- H O

H

e-

e-

1s

2s

2p

EN

RG

Y

Oxygen Atom

1sE

EN

RG

Y

Water sp3

sp3

Ammonia• Ammonia, NH3 has one pair of “unbonded” e-

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

1s

2s

2p

EN

RG

Y

Nitrogen Atom

1sE

EN

RG

Y

Ammonia

sp3

Ammonium Ion

• Ammonium ion, NH4+, has 8 valence e-

• Final shape is tetrahedron

Bonding - HX

• Hydrogen halides– F is in second series, Cl in third, etc.– Longer bonds are weaker bonds

Determining Bond Hybridization

• Count e- pairs as one “thing”

• Count a bond as one “thing”– Double bonds count as only one thing– Triple bonds count as one only thing

Acid / Base

Acid• Anything that increases the hydronium ion

concentration• Anything that donates a hydrogen• Anything that accepts a pair of electrons

Base • Anything that increases the hydroxide ion

concentration• Anything that accepts a hydrogen• Anything the donates a pair of electrons

Bronsted-Lowry Definitions

• Proton donors/acceptors– HCl + H2O H3O+ + Cl-

acid base acid base

conjugateconjugate

acid base

Acid / Base

What is?????

pH

Ka

pKa

Acids and Bases

• pH is a measure of [H+]

• The more loosely held H+ , the more acidic

Acid Dissociation Constants

• Weak acids don’t dissociate completely!

• We measure dissociation using a Ka.

HA H+ + A-

Ka = [H+] [A- ] [HA]

(a weak acid)

Acid Dissociation Constants

• Stronger acids have a LARGER Ka

• Weaker acids have a smaller Ka

HA H+ + A-

Ka = [H+] [A- ] [HA]

(a weak acid)

• Name Formula Ka

phosphoric acid H3PO4 7.5 x 10-3

hydroflouric acid HF 3.5 x 10-4 formic acid HCOOH 1.8 x 10-4

lactic acid CH3CHOHCOOH 1.4 x 10-4

acetic acid CH3COOH 1.8 x 10-5

carbonic acidH2CO3 4.3 x 10-7

boric acid H3BO3 7.3 x 10-10

ammonium ion NH4+ 5.6 x 10-10

phenol C6H5OH 1.3 x 10-11

bicarbonate ion HCO3- 2.2 x 10-13

Acid Dissociation Constants

pKa

• pKa = - log (Ka)– The larger the pKa, the weaker the acid

– The smaller the pKa, the stronger the acid

Strength of Acids

• Equilibrium favors the weaker acid / base

Strength of Acids

weakest acid - CH4 < NH3 < H2O < HF - strongest acid

weakest acid - HF < HCl < HBr < HI - strongest acid

Atoms Connected To H Are More Acidic

Electronegativity

Size

1.20 - How Structure Affects AcidityWhy does size affect base stability more so than electronegativity?

Identifying trends - valence electrons of F-, Cl-, Br-, and I- are found in 2sp3, 3sp3, 4sp3, and 5sp3 shells respectively.

There is significantly more volume in a 5sp3 orbital than a 2sp3 orbital

The negative charge in I- is thus spread out over a much larger volume of space (see potential map), making it more stable, even though it is the least electronegative of the halide series.

Organic Acid (Carboxylic Acid)

R O

O

H

acidic H

1.18 - Organic Acids and BasesAcid and base behavior of carboxylic acids:

As an acid, a proton is donated (in reality (as indicated with arrows), the basic hydroxide ion takes the proton, and leaves the electrons from the O-H bond).

As a base, a lone pair of electrons from the sp2 oxygen of the carboxylic acid takes a proton from a hydronium ion (note also the electrons from the bond returning as a lone pair to oxygen)

1.21 - How Substituents Affect Acidity

Why the change in acidity?

Hydrogen is replaced with a more electronegative atom, which pulls the bonding electrons towards itself

Inductive electron withdrawal - pulling electrons through sigma () bonds

Inductive electron withdrawal stabilizes the conjugate base by decreasing the electron density about the oxygen atom

RULE - acid strength is (still) determined by stability of the conjugate base…but new atoms substituted for others (substituents) play a role in this stability

1.21 - How Substituents Affect AcidityRULE - the effect of a substituent on the acidity of a compound decreases as the distance between the substituent and the acidic proton increases