CHAPTER 11     Alkenes; Infrared Spectroscopy and Mass Spectrometry

 

11-1 Naming of Alkenes

            Alkenes are name so that both sp2 hybrdized carbon atoms are included in the longest chain. The chain is number from the end that gives the first of these atoms the lowest number. When one or more substituents are present on each carbon of the alkene, stereoisomers are possible. Two groups, one on each carbon, are on the same side, they are said to be cis. Conversely, when they are on opposite sides, they are called trans. A more general system sets priorities for both substituents on each carbon based upon the Cahn, Ingold, Prelog system. When the two higher priority substituents are on the same side, the alkene is called Z. Otherwise, it is called E.

 

11-2 Structure and Bonding in Ethene: The Pi Bond

            The two bonds present in an alkene are different: one is formed from the overlap of sp2 orbitals and is call a sigma bond. The other results from overlap of to p orbitals and is called a pi bond. Whereas there is relatively free rotation about sigma bonds, the pi bond locks the substitutents on the two carbons into one plane that also contains the two carbons of the double bond.

   

11-3 Physical Properties of Alkenes

 The present of the double bond makes only minor changes in the physical properties of alkenes relative to the alkane with the same connectivity of carbon atoms.

 

11-4 Nuclear Magnetic Resonance of Alkenes

            Protons bound to sp2 hybridized carbon atoms of alkenes in the region for 5-7. Coupling between hydrogens that are trans is in the range of 11-18 whereas the range for cis is 6-14. The presence of more electronegative substituents reduces coupling to the low end of the range. For hydrogens on the same carbon (geminal hydrogens) the coupling is small but real and in the range of 0-3.

 

11-5 Catalytic Hydrogenation of Aklenes: Relative Stability of Double Bonds

               Addition of hydrogen to an alkene can be achieve in the presence of a catalyst (Pt, Pd). Using average bond energies, the reactions is exothermic by 31 kcal/mole.

Experimental heats of hydrogenation for the isomeric butenes reveals that trans-2-butene is the most stable, the cis isomer is less stable by 1.0 kcal/mole and 1-butene is the least stable, being 2.7 kcal/mole less stable than is trans-2-butene.

11-6 Preparation of Alkenes from Haloalkanes and Alkyl Sulfonates: Bimolecular Elimination Revisited

E2 elimination reactions generally lead to mixtures as isomers (including cis/trans) are usually possible for the product alkene. While the more substituted alkene is the more stable, there are more hydrogens that can be removed to form the less substituted isomer(s) and therefore energetics works in the opposite direction as statistics.

11-7 Preparation of Alkenes by Dehydration of Alcohols

            Dehydration of alkenes with acid catalysis is a reversible reaction and generally will form the greatest amount of the most stable alkene. Caution needs to be taken as the intermediate carbon cation can undergo rearrangement if in doing so, a secondary cation is converted into a tertiary.

11-8 Infrared Spectroscopy will be covered in chp 17

Most functional groups have characteristic absorptions in the infrared region of the spectrum. Those that result in bond stretching are the most characteristic.

The most useful absorptions are for: carbonyl groups; nitriles; amines; and alcohols. Different types of carbonyl groups give rise to different frequencies.

11-9 Measuring the Molecular Mass of Organic Compounds: Mass Spectrometrywill be covered in chp 17

High resolution mass spectrometry can be used to determine the molecular formula of organic compounds.

11-10 Fragmentation Patterns of Organic Molecules skip

 

11-11 Degree of Unsaturation: Another Aid to Identifying Molecular Structure

            Compounds containing only C, H, and O with no double or triple bonds and no rings will have a formula where the number of carbon and hydrogen atoms will have the relationship: CNH2N+2. For each ring or double bond present, the number of hydrogen atoms decreases by 2. Thus, inspection of the molecular formula reveals the total number of rings and double bonds present (triple bonds count as two double bonds).