ORGANIC CHEMISTRY - CHAPTER 12: REACTIONS OF ALKENES

Important Concepts

1. The reactivity of the double bond manifests itself in exothermic addition reactions leading to saturated products.

2. The hydrogenation of alkenes is immeasurably slow unless a catalyst capable of splitting the strong H – H bond is used. Possible catalysts are palladium on carbon, platinum (as PtO₂), and Raney nickel. Addition of hydrogen is subject to steric control, the least hindered face of the least substituted double bond frequently being attacked preferentially. 

3. As a Lewis base, the π bond is subject to attack by acid and electrophiles, such as H⁺, X₂, and Hg²⁺. If the initial intermediate is a free carbocation, the more highly substituted carbocation is formed. Alternatively, a cyclic onium ionis generated subject to nucleophilic ring opening at the more substituted carbon. Carbocation formation leads to control of regiochemistry (Markovnikov rule);onium ion formation leads to control of both regio- and stereochemistry.

4. Mechanistically, hydroboration lies between hydrogenation and electrophilic addition. The first step is π complexation to the electron-deficient boron, whereas the second is a concerted transfer of the hydrogen to carbon. Hydroboration – oxidation results in the anti-Markovnikov hydration of alkenes.

5. Carbenes and carbenoids are useful for the synthesis of cyclopropanes from alkenes.

6. Peroxycarboxylic acids may be thought of as containing an electrophilic oxygen atom, transferable to alkenes to give oxacyclopropanes. The process is often called epoxidation.

7. Osmium tetroxide acts as an electrophilic oxidant of alkenes; in the course of the reaction, the oxidation state of the metal is reduced by two units. Addition takes place in a concerted syn manner through cyclic six-electron transition states to give vicinal diols.

8. Ozonolysis followed by reduction yields carbonyl compounds derived by cleavage of the double bond.

9. In radical chain additions to alkenes, the chain carrier adds to the π bond to create the more highly substituted radical. This method allows for the anti-Markovnikov hydrobromination of alkenes, as well as the addition of thiols and some halomethanes.

10. Alkenes react with themselves through initiation by charged species, radicals, or some transition metals to give polymers. The initial attack at the double bond yields a reactive intermediate that perpetuates carbon – carbon bond formation.

From "Organic Chemistry" Textbook of VOLLHARDT & SCHORE

ORGANIC CHEMISTRY - CHAPTER 11: ALKENES; INFRARED SPECTROSCOPY AND MASS SPECTROMETRY

Important Concepts

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. J_trans is larger than J_cis, J_geminal is very small, and J_allylic 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 = (H_sat - H_actual)/2

where H_sat = 2n_C + 2 - n_X + n_N (disregard oxygen and sulfur).

From "Organic Chemistry" Textbook of VOLLHARDT & SCHORE