Preparation of alkenes

Dehydration is the removal of water from a compound. Heating most alcohols with a strong acid causes them to lose a molecule of water and form an alkene. This process is called the dehydration of alcohols. 

When alkyl halides are treated with KOH in an alcohol solution at high temperatures, alkenes are produced. Such reactions in which a hydrogen halide is removed from the compound are called dehydrohalogenation reactions. 

At high temperatures (800° C) and with the catalyst AI2O3, alkanes form alkenes by losing two hydrogen atoms from two adjacent carbon atoms 

When long-chained alkanes are heated at high temperature they decompose to produce a simpler alkane and an alkene. 

When all l halides with two bromine atoms on adjacent carbon atoms are reduced with Zn metal in acidic solution, alkenes are formed. 

Three-dimensional structure of four Ci8 fatty acids 

Fats and oils are triacylglycerols with different physical properties. 

Considerable evidence suggests that an elevated cholesterol level is linked to an increased risk of heart disease. Saturated fats stimulate cholesterol synthesis in the liver, thus increasing the cholesterol concentration in the blood. 

The identity of the three fatty acids in the triacylglycerol determines whether it is a fat or an oil. Increasing the number of double bonds in the fatty acid side chains decreases the meltii point of the triacylglycerol. 

Saturated fats are typically obtained from animal sources, whereas unsaturated oils are common in vegetable sources. Thus, butter and lard are high in saturated triacylglycerols, and olive oil and safflower oil are high in unsaturated triacylglycerols. An exception to this generalization is coconut oil, which is largely composed of saturated alkyl side chains. 

Large cycloalkanes have been prepared by C-alkylation of resin-bound 2-alkylmalonic acid esters [18]. 

Canola, soybeans, and flaxseed are excellent dietary sources of linolenic acid, an essential fatty acid. Oils derived from omega-3 fatty acids (Problem 10.12) are currently thought to be especially beneficial for individuals at risk of developing coronary artery disease. 

CoI r 0 CTide e gesls Sa t S cholesterol level is linked to an increased risk 

EtOjC COjEt Iwasawa. N. Maeyama. K. Kusama, H. Org. Lett., 2001, 3, 3871. 

Kunivasu. H. Sugae, T. Ohtaka, A. Takai, Y. Tanaka, A. Machine, C. Kami s  

Vo-Tranh, G. Boucard, V. Sauriat-Dorizon, H. Guihe. F. Synlett. 2001, 37. 

Tanikawa, M. Ishihara, T. Yamanaka, H. Chem. Lett., 2000 1360. 

Tsuchimoto, T. Maeda, T. Shirakawa. F Kawakami, Y. Chem. Commun., 2000, 1573. OH 

Kennewell, P. Teasdale, A. Sridbaran, V. Tetrahedron Lett., 1993, 34,153 

Tetrahedron Lett., 1994, 35, 2827 (Me3Si)2CBr2, C1CI2 

Commun., 1995, 25, 15 Related Methods Section 207 (Alkenes from Ketones). 

Also called p-elimina(ions, these constitute the principal laboratory method whereby two atoms or groups are removed from adjacent bonded C s. 

KOH in ethanol is most often used as the source of the base, B , which then is mainly CjH,0 . 

Cracking (Section 4.4) of petroleum hydrocarbons is the source of commercial alkenes. 

In dehydration and dehydrohalogenation the preferential order for removal of an H is 3° 2° 1 (Saytzeff rule). We can say the poor get poorer. This order obtains because the more R s on the C==C group, the more stable is the alkene. The stability of alkenes in decreasing order of substitution by R is 

These reactions, which each give only one of two possible stereomers, are called stereoselective. In this case they are more specifically called diastereoselective, because the stereomers are dia-stereomers. 

Banert, K. Haagedom, M. Schlott, J. Chem Lett. 2003,32, 360. REVIEW  

Banert. K. Groth, S. Hiickstadt, H. Lehmann, J. Schlott, J. Vrobel, K. Synthesis2002,1423 

5% RhCl(PPh3)3, PPh3, i-PrOH TMSCHN2, dioxane, 50°C 

Firouzabadi, H. Iranpoor.N. Hazarkhani, H. Karimi, B. Synth. Commun.2003,33, 3653. 

Related Methods Section 207 (Alkenes from Ketones). 

Serizawa, H. Miyaura, N. Hara, S. Suzuki, A. Tetrahedron Lett., 1988,29, 1811. 

Barluenoa. J. Fern ndez-Simon, F. Concelldn, J.M. Yus, M. J. Chem. Soc., Perkin Trans. I, 1989. 77, 

Kataoka, Y. Okazoe, T. Utimoto, K. Tetrahedron Lett., 1987, 28, 1443. 

This section contains dehydrogenations to form alkenes and unsaturated ketones, esters and amides. It also includes the conversion of aromatic rings to alkenes. Reduction of aryls to dienes is found in Section 377 (Alkene-Alkene). Hydrogenation of aryls to alkanes and dehydrogenations to form aryls are included in Section 74 (Alkyls from Alkenes). 

3% Cp RuH3(PPh3), DCM Cl2MeSi Kawanami. Y. Sonoda, Y. Mori, T. Yamamoto. K. Org, Lett.2002, 4,2825 

Alvarez-Manzaneda. EJ. Chahboun, R. Torres, E.C. Alvarez, E. Alvarez-Manzaneda, R. Haidour, A. Ramos, J. Tetrahedron Lett. 2004,45,4453. 

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CHAPTER FIVE Structure and Preparation of Alkenes Elimination Reactions... 

When applied to the preparation of alkenes the reaction is carried out m the presence of a strong base such as sodium ethoxide (NaOCH2CH3) m ethyl alcohol as solvent... 

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