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CHAPTER 3: INTERMOLECULAR FORCES (ETC.)

TOOLS TO ANALYZE REACTIONS

ENERGY DIAGRAMS (FROM CHAPTER 6)

DEFINITIONS The graph represents a two-step sequence:

A à B

B à C

KINETICS VERSUS THERMODYNAMICS (CH. 6) Each graph plots two reactions overtop of one another: A + B à C and A + B à D

Kinetics Thermodynamics

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PREDICTING CHANGES IN ENTHALPY (CH. 6)

BOND DISSOCIATION ENERGIES

The “bond dissociation energy” (BDE) is the quantity of energy needed to break a bond into radicals. The BDE of H2O2 is 51 kcal/mol. Which is the best graphical representation of this quantity?

A B

USING BDE TO PREDICT DH˚

1)   EXAMPLE 1

CH4 + 2 O2 à CO2 + 2 H2O DH˚ = ??

O OH H

OH O H

H 51 kcal/mol

O OH H

OH O H

51 kcal/mol

H C

H

H

HO O

O OC OO

HO

H

HO

H

+

DH˚ = S BDE reactants

- S BDE products

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2)   EXAMPLE 2

CH3CH2Cl à CH2=CH2 + HCl DH˚ = ??

TABLE OF BOND DISSOCIATION ENERGIES The following table was summarized from the Smith Organic Chemistry textbook. On exception is the BDE for carbon dioxide is too low in the Smith appendix (BDE= 128). Reported BDE= 192 is from: http://www.cem.msu.edu/~reusch/OrgPage/bndenrgy.htm

Bond DH˚ Bond DH˚ Bond DH˚

H-Z bonds (kcal/mol) Z-Z bonds (kcal/mol) R-OH bonds (kcal/mol)

H-F 136 H-H 104 CH3-OH 93

H-Cl 103 F-F 38 CH3CH2-OH 94

H-Br 88 Cl-Cl 58 CH3CH2CH2-OH 92

H-I 71 Br-Br 46 (CH3)2CH-OH 96

H-OH 119 I-I 36 (CH3)3C-OH 96

HO-OH 51 R-H bonds R-X bonds

CH3-H 104 CH3-F 109 CH3CH2-F 107

CH3CH2-H 98 CH3-Cl 84 CH3CH2-Cl 81

(CH3)2CH-H 95 CH3-Br 70 CH3CH2-Br 68

(CH3)3C-H 91 CH3-I 56 CH3CH2-I 53

(CH3)2CH-F 106 (CH3)2CH-Br 68

(CH3)2CH-Cl 80 (CH3)2CH-I 53

R-R bonds Multiple bonds CH3-CH3 88 CH2=CH2 (s + p) 152 O=O (s + p) 119

CH3-CH2CH3 85 HC≣C-H (s + 2p) 200 O=C=O (s + p) 192

C C H Cl+H C

H

H

C

H

H

Cl H

H H

H

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FUNCTIONAL GROUPS

DEFINITION Functional Groups are grouping of atoms with characteristic reactivity and properties.

OH

HO

O

HO

O OH

OH + HBr Br + H2O

OH Br+ HBr + H2O

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GROUPINGS

Hydrocarbons Carbonyl (C=O) Containing

Alkane

CH3CH2CH3 Aldehyde

Alkene Ketone

Alkyne

Carboxylic Acid

Aromatic

Ester

Amide

Halogen, Oxygen or Nitrogen

Alkyl Halide

Ether

Alcohol

Amine

EXAMPLES

Viagra (for erectile dysfunction) Zocor (for lowering cholesterol)

R C

H

H

H RC

H

O

H

O

RR

RC

R

O O

R C C R H C C HR

COH

O

OH

O

R

RC

O

O

RO

O

RC

N

O

R

RNH2

O

R C

X

H

H

BrR O R O

R C

OH

H

H

OHR

NR

RNH2

N

NS

O O

O

N

HN

O

N

N

CH3

HO

O O

OH

HO COO

O

O

H3C

CH3

O

OHO

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INTERMOLECULAR FORCES

DIPOLE-DIPOLE FORCES Dipole-dipole forces are attractions between permanent dipoles (d+ and d- created when atoms in a bond have different electronegativity values).

1.   Weak and strong dipole-dipole forces

2.   Why do the dipole-dipole forces differ in strength?

3.   Molecular polarities1

1 Dipole moment image taken from Wade, Organic Chemistry, 8th edition, 2013, pp. 64.

H C

H

H

O C

H

H

H H3CC

CH3

O

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HYDROGEN BONDS Hydrogen bonds are an incredibly strong dipole-dipole force. They occur when a hydrogen atom (d+) in a polar bond interacts with an oxygen, nitrogen, or fluorine atom (d-) in a polar bond.

For each of the following: a.   Place d+, d- labels on the appropriate atoms in any polar bonds. b.   Draw a second identical molecule in each box, and show how it interacts with the first through the

strongest possible intermolecular force (IMF). Use dashed lines to show the IMF. c.   Point to and identify the type of intermolecular force present in each.

Vancomycin is an antibiotic (originated in 1956), and could “vanquish” every strain of gram-positive bacteria thrown at it. It targets a protein found on the surface of bacterial cell walls, forming five specific hydrogen bonds that allows it to lock onto the bacterium. Once attached to vancomycin, bacteria can no longer build and strengthen their cell walls (which are normally being assembled and disassembled constantly)- this eventually leads to bacterial death.

CN

HH

H

HH

HC

H

O

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LONDON DISPERSION FORCES (LDF) London Dispersion Forces (LDF’s) are temporarily induced weak dipoles from the polarization of electron clouds.

Electronic orientations at different times Temporarily induced dipoles

RELATIVE STRENGTH OF IMF

Type of Force Strength (kcal/mol) Type of Force Strength (kcal/mol)

LDF2 0 – 1 Hydrogen bonds3

Dipole-dipole forces2 0.5 - 2 O --- H-N 1.9

N --- H-N 3.1

O --- H-O 5.0

Covalent bonds 36-220 N --- H-O 6.9

Ionic Forces 400 F --- H-F 38.6

Weakest IMF Strongest IMF

2 https://en.wikipedia.org/wiki/Intermolecular_force 3 Larson, J. W.; McMahon, T. B. (1984). "Gas-phase bihalide and pseudobihalide ions. An ion cyclotron resonance determination of hydrogen bond energies in XHY- species (X, Y = F, Cl, Br, CN)". Inorganic Chemistry 23 (14): 2029–2033

HCH

H H

HCH

H H

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BOILING POINT TRENDS

WHAT HAPPENS DURING BOILING?

H2O (l) ⇋ H2O (g)

Which of these accurately represents gaseous water?

A B C

Thermodynamic data for water4: DH˚ = +9.720 kcal/mol DS˚ = +0.02605 kcal/mol·K At 25 ˚C (298.15 K): At 100 ˚C (373.15 K):

4 McMurry, J.E., Fay, R.C., Chemistry, 6th ed., Prentice Hall, 2012, pp. 359

Gas

Liquid HOH

H OH

HOH

HOH

HO H

H OH H

O H HO H

OOH

H

H

H

HOH

H OH

HOH

HOH

HO H

H OH H

O H HO H

H+ OH-OH-

H+

HOH

H OH

HOH

HOH

HO H

H OH H

O H HO H

HOH H

OH

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BOILING POINT COMPARISONS

HYDROCARBONS

B.p. (˚C)5

Graph6

LINEAR VERSUS BRANCHED

B.p. (˚C)

5 All boiling points in this chapter are from the Aldrich Handbook of Fine Chemicals, 2012-2014 6 Wade, L.G., Organic Chemistry, 8th ed., Pearson, 2013, pp. 96

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DIFFERENT FUNCTIONAL GROUPS

B.p. (˚C)

ALKYL HALIDES

CH3-I CH3-Cl

B.p. (˚C)

A DIFFICULT TO PREDICT COMPARISON

CH3CH2OH

B.p. (˚C)

OH

O

OH

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PROBLEM Rank the following in order of increasing boiling point and explain your answer.

B.p. (˚C)

SOLUBILITY

LIKE DISSOLVES LIKE

•   Polar solvents dissolve polar compounds well. •   Nonpolar solvents dissolve nonpolar or weakly polar compounds well. •   Polar / nonpolar compounds do not dissolve in each other well.

NH2 N CH3 N

CH3

H

Structure of wax (crayon)

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WHY DOES LIKE DISSOLVE LIKE?

POLAR COMPOUND / POLAR SOLVENT

WEAKLY POLAR COMPOUND / NONPOLAR SOLVENT

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NONPOLAR COMPOUND / POLAR SOLVENT

Thermodynamic data for transferring compounds from organic solvent into water, 25 ˚C.7,8

Compound DH (kcal/mol) –TDS (kcal/mol) DG (kcal/mol)

Butane -1.00 6.86 5.86

Pentane -0.50 7.46 6.96

Hexane 0.00 6.79 6.79

Explanation for unfavorable change in entropy9

7 (Butane data) Huque, E.M. J. Chem Educ. 1989, 66, 581-585 8 (Pentane, Hexane data) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd ed. Wiley: New York, 1980, pp. 21-41 9 Silverstein, T.P. J. Chem Educ. 1998, 75, 116-118. Graphic from Wade, L.G., Organic Chemistry, 8th ed., Pearson, 2013

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WATER SOLUBILITY

ALCOHOLS

# C Solubility10  (g solute per 100 g H2O)

1, 2, 3 CH3OH, CH3CH2OH, CH3CH2CH2OH miscible

4 7.99

5 2.25

6 0.60

7 0.17

“Water soluble” is semi-arbitrarily defined as when more than 3 grams of compound dissolves in 100 g water at 25 ˚C.

REQUIREMENTS FOR WATER SOLUBILITY

OTHER FUNCTIONAL GROUPS Solubility values10 are quoted as gram of solute per 100 g of water.

Solubility

Solubility

10 CRC Handbook of Chemistry and Physics, 84th edition, 2003-2004, pp. 8-93-109. All values are at 25 ˚C.

OH

OH

OH

OH

HOOH

Cl

OHO

H

O

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PROBLEM Rank the following compounds in order of increasing water solubility. Briefly explain your answer.

BIOMOLECULES Cholesterol:

Vitamins:

O

HO

HO

CH3

CH3