Noncovalent Molecular Forces Part 1

1
Noncovalent Molecular Forces - Part 1

• Lecture Supplement
• Take one handout from the stage

2
Chemistry 14C Part 3
Structure Controls Everything
?

Influence antibiotic behavior
3
Noncovalent Molecular Forces

• Attractive forces (other than covalent bonding)
  between atoms or molecules
• Why should I study this?

Noncovalent forces control association of
molecules
Molecular organization into larger structures
membranes, etc. Molecular recognition
substrate/enzyme docking, etc.
4
How Do We Measure Noncovalent Forces?
Consider evaporation of water...

• Stronger attractions more energy required to
  break
• more energy
  needed for evaporation
• higher boiling
  point
• Boiling point is useful approximation of
  attractive forces
• Boiling point easily measured

add energy

• Noncovalent attractive force disrupted
• Covalent bonds still intact

5
How Do We Measure Noncovalent Forces?

• Boiling point temperature at which vapor
  pressure of substance ambient pressure
• Melting point influenced by...
• Attractive forces
• Crystal packing

What Kinds of Noncovalent Forces Occur?
Consider these substances NaCl bp 1413 oC H2O
bp 100 oC BrF bp 20 oC Ar bp -186 oC
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NaClbp 1413 oC

• What attractive force is operating?
• What is the nature of association between Na and
  Cl?

Cl Na
3.0
EN
0.9

• DEN 3.0 - 0.9 2.1
• Attractive force ionic bond cation-anion
• electrostatic
  (opposite charges attract)

• Evaporation or melting separating opposite
  charges
• NaCl evaporates as Na and Cl- not NaCl
• NaCl heat of vaporization 188 kcal mol-1
• High bp and mp typical of ionic compounds

7
Ionic versus Covalent Bonds

• Chemical bond sharing of electron pair
• Ionic bond highly unequal sharing of electron
  pair
• Covalent bond approximately equal sharing of
  electron pair

• ? bond length amplifies polarity

8
BrFbp 20 oC (liquid at room temperature)

• What attractive force is operating?
• What is the nature of association between Br and
  F?

DEN 4.0 - 2.8 1.2 not
ionic polar covalent
F Br
4.0
EN
2.8
d-
d

• Attractive force dipole-dipole
• electrostatic (d/d-)
• Bp suggests dipole-dipole attraction weaker than
  cation-anion

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H2Obp 100 oC (liquid at room temperature)

• What attractive force is operating?
• What is the nature of association between H and O?

DEN 3.5 - 2.1 1.4 not
ionic polar covalent
2.1
3.5
d-
Attractive force electrostatic
dipole-dipole
hydrogen bonding
d
d
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Hydrogen Bonding
d-
d
d
In general...

• Hydrogen bond acceptor
• Need electron density to attract d
• Bond dipole or single atom
• Concentrated d- (small atoms O, N, or F)
• Negative charge (any anion)
• Must have lone pair(s)

Must be large X high EN F, O, N (rarely
anything else)
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Hydrogen Bonding

• Common attractive force
• Not always dipole-dipole
• Example F- in CH3OH

• Important in biology
• Many O-H, N-H, H2O in organisms
• Example DNA base pairs

• Also influences protein structure
• Hydrogen bond strongest when linear
• Dynamic H-O-H----OH2 3 x 10-12 s lifetime

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Arbp -186 oC (gas at room temperature)

• What attractive force is operating?
• Ionic? No electronegativity difference ? no ions

• Dipole-dipole? No covalent bonds ? no bond
  dipoles

• Hydrogen bonding? No hydrogens

• No attractive force? no energy required for
  vaporization?
• -186 oC gt -273 oC (absolute zero)
• Therefore some attractive force must be present
• Bp is very low so attractive force must be weak
• Student homework figure out what attractive
  force exists between two Ar atoms

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Noncovalent Molecular Forces - Part 2

• Lecture Supplement
• Take one handout from the stage

14
Summary of Part 1

• Physical properties such as boiling point and
  solubility controlled by noncovalent association
• Stronger attractive force more energy required
  for vaporization higher boiling point
• Noncovalent attractive forces caused by
  electrostatic attractions
• Examples NaCl bp 1413 oC Noncovalent attractive
  force cation-anion
• BrF bp 20 oC Noncovalent attractive force
  dipole-dipole
• H2O bp 100 oC Noncovalent attractive
  force hydrogen bonding
• Hydrogen bond donor usual O-H or N-H bond
• Hydrogen bond acceptor neutral atom with lone
  pair and high d- O or N
• or
  any anion with lone pair
• Hydrogen bonds of wide biological importance
  protein and DNA structure etc.
• Ar bp -186 oC Noncovalent attractive force ?

15
Arbp -186 oC (gas at room temperature)

• What attractive force is operating?
• Ionic? Dipole-dipole? Hydrogen bonding?

• No attractive force? no energy required for
  vaporization?
• bp -186 oC gt -273 oC (absolute zero) so some weak
  attractive force must be present

d
d-

• Induced charges
• Momentary electrostatic attraction

• called London force or van der Waals force
• All molecules have electrons so all molecules
  influenced by this force

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Strength of van der Waals Forces
What influences strength of van der Waals forces?
Boiling point -269 oC -246 oC -186 oC -152
oC -107 oC -62 oC
Atomic radius 0.32 Å 0.69 Å 0.97 Å 1.10 Å 1.30
Å 1.45 Å

• Polarizability
• Ability to distort electron cloud
• Distortion easy soft Example Rn
• Distortion difficult hard Example He
• Polarizability influenced by...
• Larger atomic radius softer
• Larger electronegativity harder
• Surface area effect?

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Strength of van der Waals Forces
Surface area effect?

• Compare molecules with same polarizability but
  different surface areas
• Hydrogens small hard

Boiling point -183 oC -88 oC
-42 oC -0.5
oC Surface area 56.6 Å2 80.1 Å2
102.7 Å2 125.2
Å2
Maybe this is just a molecular weight effect?
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Strength of van der Waals Forces
Molecular weight effect?

• Compare molecules with same polarizability and
  different surface areas, but same molecular
  weight (isomers of C5H12)

Boiling point 36 oC
30 oC
9.5 oC Shape Most
elongated
Most spherical
Conclusion higher surface area stronger van
der Waals attraction
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Other Noncovalent Interactions

• Caused by electrostatic attractions

• Ion-dipole
• Bond dipole attracted to anion or cation
• Example Na and Cl- in water

• Cation-pi
• Cation attracted to pi electron cloud

• Explains water solubility of NaCl

• Important in some enzyme-substrate binding

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Other Noncovalent Interactions

• Aromatic stacking
• Overlap of p orbitals of two aromatic rings
• Also called pi stacking

• Important in DNA

• Noncovalent interactions may have more than one
  label
• Example Cl- ----- H-OH is hydrogen bonding and
  ion-dipole

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Relative Strength of Noncovalent Forces

• Approximate ranking of noncovalent force
  strengths useful

Dipole-dipole Hydrogen bonding Ion-dipole Cation-p
i Aromatic stacking
Cation-anion gt Covalent bonds gt
gt Van der Waals
Strongest force
Weakest force

• When more than one force operates, strongest
  force dominates
• Example
• Evaporation of CH3OH hydrogen bonding harder to
  overcome than van der Waals forces

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Application of Noncovalent Interactions
Solubility
oil layer
vinegar layer (water acetic acid)

• Questions
• Why acetic acid dissolves in water?
• Why oil does not dissolve in water?

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Application of Noncovalent Interactions
Solubility

• What causes one substance to dissolve in another?
• Solubility is an equilibrium issue...

ABABABA ABABABA BABABAB ABABABA
Two layers A and B immiscible
Homogeneous A and B dissolve

• Dissolving A interrupts attractive forces in B
• Soluble A/B attractions better than A/A and B/B
  attractions
• Insoluble A/B attractions not better than A/A
  and B/B attractions
• Better stronger attractions and/or more
  attractions

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Application of Noncovalent Interactions
Solubility

• Many attractive interactions
• Water CH3COOH soluble

• Poor attraction between oil and water
• Strong attraction between water and water
• Water oil insoluble

Like dissolves like