CHEM231 Chapters 1 + 2

Electronegativity Trend

Increases from left to right, bottom to top

S Orbital

Circle

P Orbital

Rounded figure eight

Dipole Moment

Measurement of dipole

How does electronegativity effect dipole moment?

Increased electronegativity leads to increased dipole moment

How does number of bonds effect dipole moment?

Increasing number of bonds increases dipole moment

What are the valence electrons of N, O, F?

5, 6, 7

How to get formal charge?

Valence electrons - electrons owned by atom

Line Drawing

Ends of lines and bends are carbons, hydrogen is inferred, only heteroatoms and their Hs are drawn

What is resonance?

Electron delocalization

Resonance

Moving around uneven electrons / lone pairs

Resonance hybrid

Even sharing with the use of a partial bond

Curved arrows

Shows the movement of electrons

Major / Minor Contributors

Major is usually more stable and more commonly found, minor is more unlikely

Resonance rules

1. Connectivity stays the same

2. Number of electrons and net charge is the same

3. Number of unpaired electrons is the same

4. Any structure exceeding the octet rule for 2nd row elements is not valid

How to predict the major contributor?

1. Structure with more covalent bonds contribute more

2. With octet satisfied, major structure has least amount of charge separation

3. With octet satisfied, major contributor has negative charge on most electronegative element

Increasing force of repulsion between electron pairs

Bonded pairs (least) → unshared pair to bonded pair → unshared pairs (most)

Molecular geometry: Methane (CH4)

109.5°, four bonded pairs, tetrahedral, tetrahedral

Molecular geometry: Water (H2O)

105°, two bonded pairs + two unshared pairs, tetrahedral, bent

Molecular geometry: Ammonia (NH3)

107°, three bonded pairs + one unshared pair, tetrahedral, trigonal pyramid

Molecular geometry: Boron trifluoride (BF3)

120°, three bonded pairs, trigonal planar, trigonal planar

Molecular geometry: Formaldehyde (H2CO)

120°, two bonded pairs + one double bond (one bonded pair), trigonal planar, trigonal planar

Molecular geometry: Carbon Dioxide (CO2)

180°, two double bonds (two bonded pairs), linear, linear

Reaction mechanisms: Dissociation

Bond broken

Reaction mechanisms: Formation

Bonds form

Reaction mechanisms: Substitution

Replacing something in a bond

Reaction mechanisms: Acid-Base Reactions

Acid or base being added in

Brønsted Acid

Proton donor

Brønsted Base

Proton acceptor

pKa =

-log (Ka)

Acidity strength: pKa

The more negative pKa → stronger the acid

Acidity strength: Periodic Table

More acidic down the column (F<Cl<Br<I), row follows electronegativity trend

Inductive effects

Inductive withdrawing groups near the conjugate base anion increases the acidity

Acid-Base Equilibria

Reaction forms weaker acid/base, equilibrium shifts to weaker acid

Lewis Acid

Electron acceptor

Lewis Base

Electron donor

Classes of Hydrocarbons: Aliphatic

Alkanes (Ethane), Alkenes (Ethylene), Alkynes (Acetylene)

Classes of Hydrocarbons: Aromatic

Arenes (Benzene)

“Saturated” Hydrocarbons

Alkanes (Ethane)

H2 (Nuclear Distance)

Ideal inter nuclear distance → 74 pm, potential energy → -435 kJ/mol (-104 kcal/mol)

kJ/mol or kcal/mol?

Interchangeable

Linear alkane formula

C(n)H(2n+2)

sp3 Hybridization

Sigma bond (single line), least energy (25:75)

sp2 Hybridization

1 sigma bond + 1 pi bond (double bond), mid energy (33:66)

sp Hybridization

1 sigma + 2 pi bonds, highest energy (50:50)

Butane Isomers (C4H10)

Constitutional isomers

• Same molecular formula

• Different connectivity

• Different physical properties

n-Alkanes

Condensed structural formula of higher n-alkanes

Pentane Isomers (C5H12)

Constitutional isomers: n-Pentane, isopentane, neopentane

Physical properties of alkanes

• Low melting points

• Poor solubility in water

• Boiling points lower than analogous amines or alcohols

• Boiling points change among isomers

Acidity of hydrocarbon C-H bonds

Alkanes are weakest acids known

Combustion of alkanes

Combustion relates to stability of isomers, higher kJ/kcal the more unstable

Oxidation

Increase in bonds to electronegativity elements

Reduction

Increase in bonds to less electronegative elements

IUPAC Names of Unbranched Alkanes

1. Methane

2. Ethane

3. Propane

4. Butane

5. Pentane

6. Hexane

7. Heptane

8. Octane

9. Nonane

10. Decane

11. Undecane

12. Dodecane

IUPAC Naming

1. Count parent chain (longest chain) (add cyclo- if it’s a shape)

2. Determine substituants (1-methyl, 2-ethyl, 3-propyl, 4-isopropyl)

3. Name alphabetically (#-wordchain)

4. Add di, tri, or tetra if multiple of one substituants (#,#-diwordchain)