1. Alkenes are unsaturated molecules. Their IUPAC names are
derived from alkanes, the longest chain incorporating the double bond serving
as the stem. Double-bond
isomers include terminal, internal, cis, and trans arrangements. Tri- and tetra
substituted alkenes are named according to the E, Z system, in which the R, S
priority rules apply.
2. The double bond is composed of a σ bond and a π bond. The σ bond is obtained by overlap of the two sp2 hybrid lobes on carbon, the π bond by interaction of the two remaining p orbitals. The π bond is weaker (~65 kcal mol-1) than
its σ counterpart (~108 kcal mol-1) but
strong enough to allow for the existence of stable cis and trans isomers.
3. The functional group in the alkenes is flat, sp2 hybridization being responsible for the possibility
of creating dipoles and for the relatively high acidity of the alkenyl hydrogen.
4. Alkenyl hydrogens and carbons appear at low field in 1H
NMR (δ = 4.6– 5.7 ppm) and 13C
NMR (δ = 100– 140 ppm) experiments, respectively. Jtrans is larger than Jcis, Jgeminal is very small, and Jallylic is variable but small.
5. The relative stability of isomeric alkenes can be
established by comparing heats
of hydrogenation. It decreases with decreasing substitution; trans isomers
are more stable than cis.
6. Elimination of haloalkanes (and other alkyl derivatives)
may follow the Saytzev rule (nonbulky base, internal alkene
formation) or the Hofmann rule (bulky base, terminal alkene formation).
Trans alkenes as products predominate over cis alkenes. Elimination is stereospecific, as dictated by
the anti transition state.
7. Dehydration of alcohols in the presence of strong
acid usually leads to a mixture of products, with the most stable alkene being
the major constituent.
8. Infrared spectroscopy measures vibrational
excitation.The energy of the incident radiation ranges from about 1 to 10 kcal
mol-1 (λ ~ 2.5 - 16.7 μm; ʋ ~ 600 – 4000 cm-1). Characteristic peaks are
observed for certain functional groups, a consequence of tretching, bending, and other
modes of vibration, and their combination. Moreover, each molecule exhibits a
characteristic infrared spectral pattern in the fingerprint region below 1500 cm-1.
9. Alkanes show IR bands characteristic of C – H bonds in
the range from 2840 to 3000 cm-1. The C=C stretching absorption for
alkenes is in the range from 1620 to 1680 cm-1, that for the alkenyl
C – H bond is about 3100 cm-1. Bending modes sometimes give useful
peaks below 1500 cm-1. Alcohols are usually characterized by a broad
peak for the O – H stretch between 3200 and 3650 cm-1.
10. Mass spectrometry is a technique for ionizing molecules
and separating the resulting ions magnetically by molecular mass. Because the
ionizing beam has high energy, the ionized molecules also fragment into smaller
particles, all of which are separated and recorded as the mass spectrum of a
compound. High-resolution mass spectral dataallow determination of molecular
formulas from exact mass values. The presence of certain elements (such as Cl,
Br) can be detected by their isotopic patterns. The presence of fragment-ion
signals in mass spectra can be used to deduce the structure of a molecule.
11. Degree of unsaturation (number of rings 1 number of p
bonds) is calculated from the molecular formula by using the equation
Degree of unsaturation = (Hsat - Hactual)/2
where Hsat =
2nC + 2 - nX + nN (disregard oxygen and sulfur).
From "Organic Chemistry" Textbook of VOLLHARDT & SCHORE
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