Chapter 2: Organic Compounds: A First Look

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Chapter 2 Organic Compounds A First Look
Common Bonding Situations
Hydrogen
1 bond
Carbon
4 bonds (neutral and 8 electrons)
Reactive Carbon Species
2
Nitrogen
3 bonds and one unshared pair of electrons
Other relatively stable species
3
Oxygen
2 bonds and 2 unshared electron pairs
Other relatively stable species
hydronium ion
4
Halogens
1 bond and 3 unshared electron pairs
also 4 unshared pairs and negative charge
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Phosphorus
three bonds and an unshared pair of electrons
(like N)
also can have more than 8 electrons in its
valence shell
Sulfur
two bonds and two unshared electron pairs (like O)
also can have more than 8 electrons in its
valence shell
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Bond Strengths and Bond Lengths
Bond Dissociation Energy
energy that must be added to break a bond in a
homolytic fashion (one electron with each atom)
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1. Stronger bonds are shorter.
2. C-H, N-H, O-H 90-110 kcal/mol 1
(10-10 m, 100 pm)
3. C-C, C-N, C-O 65-80 kcal/mol 1.5
4. Bonds become weaker and longer down a
column
5. CC is stronger and shorter than C-C (but
not two times) CO is very strong and very
common
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Table 2-3, p. 35
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Physical Properties and Molecular Structure
melting point, boiling point, solubility
Intermolecular Forces
charge interactions
larger charges stronger interaction
Type Example
very strong ionic bond 188 kcal/mol
ion-ion
moderately strong
ion-dipole
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dipole-dipole
weak
dipole-induced dipole
weaker
instantaneous dipole- induced dipole (London
Force)
weakest
Together these last three are called van der
Waals forces (0.5 5 kcal/mol).
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Hydrogen Bonding
Hydrogen bonds are stronger than other
dipole-dipole attractions because the small size
of H allows less distance between the charges (3
8 Kcal/mol).
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Melting Points
increase with stronger intermolecular forces
increase with more symmetrical shape (molecules
pack better into crystal lattice)
compound melting point
ionic compound
NaCl
801oC
CH3CH2CH3
-190oC
nonpolar
CH3(CH2)3CH3
-130oC
larger, more London forces
more symmetrical
-71oC
has polar bond
-99oC
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Boiling Points
increase with stronger intermolecular forces
increase with more surface area (more London
forces)
(rod shaped)
increase with hydrogen bonding
compound boiling point
ionic compound
NaCl
1413oC
CH3CH2CH3
-42oC
nonpolar
CH3(CH2)3CH3
36oC
larger, more London forces
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CH3(CH2)3CH3
36oC
larger, more London forces
less surface area
10oC
more polar
76oC
117oC
hydrogen bonding
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Table 2-6a, p. 51
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Table 2-6b, p. 51