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Dr. Peter Wipf
Chemistry 0310 - Organic Chemistry 1
Chapter 2. Structure & Reactivity
Common functional groups include alkanes, alkenes, alkynes, arenas (aromatic rings),
haloalkanes, alcohols, thiols, ethers, amines, aldehydes, ketones, carboxylic acids, esters,
anhydrides, nitriles and amides. We also discussed intermolecular forces and some aspects of
conformational analysis. Finally, aspects of reaction kinetics and thermodynamics, and Brønsted
and Lewis acids and bases were reviewed.
- Alkanes : parent compounds: methane, eth ane. Rotation around the C,C-σ-bond is facile. σ-
Bonds have cylindrical symmetry.
C C
H
H
H
H
H
H
sp
3
54 Å
10 Å
C,C-bond energy: 90 kcal/mol
C C
H
H
H
H
H
H
C C
H
H
H
H
H H
C C
H
H
H
H
H
H
eclipsed
staggered staggered
- Alkenes : parent compound: eth ene. Rotation around the C,C-π-bond is impossible without
breaking the bond (activation energy is approx. 60 kcal/mol). Therefore, cis- and trans -
stereoisomers occur. π-Bonds have a nodal plane.
C C
H
H
H
H
CH 3
C 4
H 9
H H
H H 3
C
C 4
H 9
H
cis - 2 - heptene trans - 2 - heptene
C=C-bond energy: 145 kcal/mol
(contains !" and #-bond)
sp
2
34 Å
09 Å
- Alkynes : parent compound: eth yne. The C,C-triple bond contains one σ-bond and two π-bonds.
The two π-bond orbitals are orthogonal.
H-C!C-H
06 Å
20 Å
C!C-bond energy: 200 kcal/mol
sp
- Arenes : parent compound: benzene. All C,C-bonds in benzene are normalized and have the
same length (1.39 Å). The six π-electrons are fully delocalized over the six C-atoms. Benzene
exists as a resonance hybrid of Lewis structures. All C's are sp
- Alkyl halides = Haloalkanes : parent compound: methyl fluoride. Bond lengths: C-F 1.35 Å, C-
Cl 1.77 Å, C-Br 1.94 Å, C-I 2.14 Å. Bond energies: C-F 116, C-Cl 78, C-Br 68, C-I 51 [kcal/mol].
Distinguish between primary, secondary, and tertiary alkyl halides.
- Alcohols : parent compound: methanol. Bond lengths: C-O 1.43 Å, O-H 1.0 Å. Bond energies:
C-O 86, O-H 110 [kcal/mol]. Distinguish between primary, secondary, and tertiary alkyl alcohols.
- Ethers : parent compound: diethyl ether. Chemically relatively inert. - Amines : parent compounds: ammonia, methyl amine. Bond lengths: C-N 1.47 Å, N-H 1.0 Å.
Bond energies: C-N 73, N-H 92 [kcal/mol]. Distinguish between primary, secondary, and tertiary
alkyl amines. Note: classification is based on # of organic groups that are attached to N-atom.
- Aldehydes and Ketones : parent compounds: formaldehyde [methan al ], acetone [2-
propan one ]. Bond length: C=O 1.20 Å. Bond energy: C=O 191 kcal/mol.
- Carboxylic acids : parent compound: acetic acid [ethan oic acid]. - Esters : parent compound: ethyl acetate [ethyl ethanoate]. - Amides : parent compound: acetamide. Distinguish between un-, mono-, and disubstituted
amides. The amide function is planar due to amide resonance. N, C, and O are sp
H 3
C
C
N
O
H
H
H 3
C
C
N
O
H
H
H 3
C
C
N
!+
O
!-
H
H
amide resonance
sp
2
sp
2
24 Å
33 Å
- Intermolecular Forces : • Electrostatic interactions (ionic bonds, dipole-dipole) • Hydrogen
bonds • Van der Waals forces (attraction between temporary dipoles).
- Solubilities : Polar solvents dissolve polar molecules and salts via dipole-dipole interactions and
H-bonding. Apolar solvents dissolve apolar molecules via van der Waals forces (London
dispersion forces). “Like dissolves like”.
- Acids & Bases : A large number of organic reactions can be classified as acid-base reactions.
According to Brønsted , acids are H
- donors , and bases are H
- acceptors. In the Lewis acid-
base definition, acids are electron-pair acceptors , and bases are electron-pair donors. The
Lewis definition has an even broader applicability than Brønsted's. Important Lewis acids are BF 3
and AlCl 3
, which are characterized as aprotic acids. Atoms bearing nonbonding electron pairs,
such as oxygen, nitrogen, and sulfur, are centers of Lewis or Brønsted acidity. Protic acids
transfer an H
to the lone pair(s) of such atoms, and Lewis acids coordinate with them. The
electrons in π-bonds also react with protic or Lewis acids.
The acidity of protic acids is generally expressed as its pK a
. The lower the pK a
, the stronger
the acid and the weaker its conjugate base. In an acid-base reaction, the equilibrium favors the
weaker acid and the weaker base. The log K eq
of an acid-base reaction is equal to the
difference between the pK a
's of the participating acid and base. Δ G° = - 2.303 RT log K eq
. R =
gas constant = 1.987 cal K
mol
; T = absolute temperature in K. A negative standard free
energy change is the driving force of all chemical reactions. ΔG° has an enthalpy and an entropy
component: Δ G° = Δ H° - T Δ S°.
