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CHAPTER 3: INTERMOLECULAR FORCES
FUNCTIONAL GROUPS
DEFINITION Functional Groups are grouping of atoms with characteristic reactivity and properties.
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 (erectile dysfunction) Zocor (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 (δ+ and δ- created when atoms in a bond have different electronegativities).
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 in a polar bond (δ+) interacts with an oxygen, nitrogen or fluorine atom in a polar bond (δ-).
Use dashed lines to show the IMF’s between NH3 molecules.
Use dashed lines to show the IMF’s between these two molecules. Label each IMF.
LONDON DISPERSION FORCES (LDF) London Dispersion Forces (LDF’s) are temporarily induced weak dipoles from the polarization of electron clouds.
H3CO
H
HN
HH
HC
H
OHO
H
CH4 CH4 CH4 CH4 CH4 CH4
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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
BOILING POINT TRENDS
WHAT HAPPENS DURING BOILING?
H2O (l) ⇋ H2O (g)
Thermo Data for H2O:4 Sign Means At certain T’s ΔG
ΔH= +9.720 kcal/mol At 25 ˚C (298.15 K), TΔS= +7.767 kcal/mol
ΔS= +0.02605 kcal/mol·K At 100 ˚C (373.15 K), TΔS=
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 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
HOH
H OH
HOH
HOH
HO H
H OH H
O H HO H
HOH
H OH
HOH
HOH
HO H
H OH H
O H HO H
OOH
H
H
H
HOH H
OH
H+ OH-OH-
H+
A B C
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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)
OO
H
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
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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 molecule from organic solvent into water.7,8
Compound ΔH (kcal/mol) TΔS (kcal/mol) ΔG (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
