Structure & Reactivity

Alkanes – Molecules w/o functional Groups

• Hydrocarbons– Alkanes, Alkenes, Alkynes.

• Functional Groups; Aromatics– Polar bonds create chemical reactivity– Haloalkanes, Alcohols,Phenols, Ethers,

Carbonyls, Aldehydes, Ketones, Carboxylic Acids, Anhydrides, Esters, Amides, Nitriles, Amines, Thiols

• “R” – residue (Alkyl Group)– R-OH – an alcohol

– R-NH2 – an amine

ALCOHOLS

H

R C OH

H

1o CH3CH2CH2CH2OH Butanol

2o ALCOHOLS R C OH

R

H

CH3CH2CHCH3

OH

2-Butanol

1-

R C OH3o ALCOHOLS

R

R

CH3 C CH3

CH3

OH

tert- Butanolor

2-Methyl 2-Propanol

1o AMINES R NH2 CH3CH2CH2 NH2 Propylamine

2o AMINES R NHR

CH3CH2 NH

CH3

Ethyl,Methylamine

3o AMINES R N RR

CH3 N CH3

CH3

Tri-methylamine

Alkanes

– Only single bonds, C, H– Straight chained, branched, cyclic– IUPAC Nomenclature “International Union of

Pure & Applied Chemistry”

– Homologous series of Alkanes CH3(CH2)nCH3

• -(CH2)- methylene group

– Constitutional Isomers (branched alkanes)

Types of Carbon in Organic Molecules

• Primary C – connected to only one additional C (Methyl group)

• Secondary C - connected to two additional C (-CH2-)

• Tertiary C - connected to three additional C (Isopropyl group)

• Quaternary C - connected to four additional C (tert-Butyl group)

Alkanes

• Bond angles, Molecular Shapes

Alkanes

• Physical Properties– Gases – liquids – solids

Intermolecular Forces• A: Ionic compounds (salts)

– very strong Coulomb attraction

• B:Polar compounds (e.g. Haloalkanes)– Dipole-dipole interaction

• C:Nonpolar compounds (alkanes)– Very weak London forces

Bond Rotation - Conformations

• Freedom of rotation about a C-C single bond• Newman Projection Formulas• Potential Energy Diagrams of Bond Rotation

Bond Rotation - Conformations

• Newman Projection Formulas

Bond Rotation - Conformations

• Newman Projection Formulas

Bond Rotation - Conformations

• Potential Energy Diagrams of Bond Rotation in Ethane

Bond Rotation - Conformations

• Potential Energy Diagrams of Bond Rotation in Propane

Bond Rotation - Conformations

• Potential Energy Diagrams of Bond Rotation of Butane

Kinetics & Thermodynamics

• Chemical Thermodynamics– Changes in energy during a reaction,

determines the extent to which a reaction goes to completion

• Chemical Kinetics– Velocity, rate of a reaction (change in

concentration of reactants/product)

• Reaction may be under thermodynamic or kinetic control

Equilibrium

• State of a reaction when there is no more change in reactant and product conc.

• Equilibrium constant K– A B A + B C + D– K = [B]/[A] K = [C][D]/[A][B]– Large k value, reaction goes to completion

Gibbs Standard Free Energy Change

Go = -RT ln K (in kcal/mol)

• Negative Go - release of energy

• Free energy change – changes in bond strength (enthalpy H) & degree of order (entropy S)

Go = Ho – T So

Enthalpy Change Ho

• Sum of strength of bonds broken – sum of strength bonds formed

• Negative Ho - heat releasing, exothermic• Positive Ho - heat absorbing, endothermic• CH4 + 2O2 CO2 + 2H2O Ho = -213 kcal/mol

– 1 mol methane = 16g– 213 kcal/16g = 13.3 kcal/g– Fats: 9 kcal/g– Alcohol: 7 kcal/g– Sugars: 4 kcal/g

Entropy Change S

• Value of S increases with increasing disorder

• Nitroglycerin • 4 C3H5N3O9 6N2 + 12 CO2 + 10 H2O + O2 +

energy (lots of it! as heat!)

Activation Energy

• Most exothermic reactions do not occur spontaneously

• Bond breaking precedes bond formation

• Reaching of Transition State requires Activation Energy (input)– E.g. gasoline, wood, H2/O2

Reaction Rates k = rate constant

• A + B C rate: k=[A][B] [mol/Ls]– Dependent on 2 molecules “second order”

• A B rate: k[A] [mol/Ls]– Dependent on 1 molecule “first order”

Temperature Effects on Rx rates

• Arrhenius Equation• k = A e-Ea/RT (A = max. rate constant)• More molecules have sufficient energy to

overcome Ea

• Approx. 10oC increase 2-3x increased rate

• At extremely high temperature Ea/RT approaches 0, e-Ea/RT = 1

• A maximum rate of particular reaction

Review of Acids & Bases

• BrØnsted & Lowry Definition:– Acid = H+ donor– Base = H+ acceptor

• Water (can behave as both) pure H2O is “neutral”

• H2O + H2O H3O+ + OH-

• Kw = [H3O+][OH-] = 10-14 mol2/L2

• [H3O+]= 10-7 mol/L (1.8g/l water = 0.00000018%)

– 1.8 parts per trillion

• pH = -log [H3O+]= 7

Review of Acids & Bases

• Acidity of Acids– HA + H2O H3O+ + A-

– K = [H3O+][A-]/[HA][H20]

– In aqueous solution [H2O] constant 55 mol/L

– Acidity constant Ka

– Ka = K[H20] = [H3O+][A-]/[HA]

– pKa = -log Ka ( pKa = pH + pA- -pHA)

– pKa = pH where 50% of acid is dissociated [A-] = [HA]

– “weak acids” pKa > 4

Review of Acids & Bases

• Basicity of Bases

• A- + H2O OH- + HA

• K’ = [OH-][HA]/[A-][H20]

• Kb = K’[H2O] = [OH-][HA]/[A-]

• Ka x Kb = Kw = 10-14

• NH3: pKb = 4.74 pKa: 9.26

Reasons for Acid/Base Strengths

• Increasing size of anion A- allows better distribution of negative charge – HI>HBr>HCl>HF

• Electronegativity of the element to which H is attached:– HF>H2O>H3N>H4C

• Resonance favors dissociation– Acetic acid, sulfuric acid

Review of Acids & Bases

• Lewis Acids-Bases• Electron Pair Acceptors – Acids

– BH3, Carbocation, AlCl3, MgCl2

• Electron Pair Donators – Bases– OH-, R-OH, RNH2

• Important concept for many organic Rx– Conversion of a Haloalkane in to an Alcohol:

– (CH3)3C-Cl (CH3)3C+ (carbocatioin) + Cl –

– (CH3)3C+ + H2O (CH3)3C-OH + H+