Chem 140A Lecture Notes Winter 2011 Ch 8

Chem 140A Lecture Notes Winter 2011 Ch 8 - Chemistry 140A...

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Chemistry 140A Winter 2011 (K. Albizati) 1 Chapter 8 Alcohols – Properties, Preparation and Strategy of Synthesis The hydroxyl group has richer and much more varied chemical reactivity than alkanes or alkyl halides. This is due to the polarizability of the C-O bond and the hydrogen attached to a heteroatom. Further, the functional group can readily undergo reactions involving “oxidation”, leading to other important functional groups. Nomenclature Some common alcohols of industrial importance: Alcohols are named as derivatives of alkanes. The hydroxyl group determines the suffix of the parent and the –e at the end of the alkane parent is replaced by –ol. Example: cyclohexane becomes cyclohexanol; Note that the position of the hydroxyl group on a chain is indicated by a number with the hydroxyl taking the lowest possible number. However, it becomes a little more complicated than this. In naming a more complex alcohol, the parent chain is the longest chain containing the alcohol group. The chain is numbered to give the alcohol the lowest number.
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Chemistry 140A Winter 2011 (K. Albizati) 2 Examples: Classes of alcohols: Physical Properties The properties of alcohols are dominated by the O-H bond. Three-dimensionally, the region around the O-H bond looks like water (bent geometry) but it is actually tetrahedral and sp 3 hybridized:
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Chemistry 140A Winter 2011 (K. Albizati) 3 Being quite electronegative, the bonds involving oxygen are quite polar, but still covalent. The structure of alcohols in the liquid and solid state is dominated by hydrogen bonding: Hydrogen bond strength is about 1- 5 kcal/mole. The extensive hydrogen-bonding network in alcohols is responsible for 2 important properties. Relative to the corresponding alkanes, alcohols are fairly soluble in water and they boil at much higher temperatures. Acidity and Basicity of Alcohols The acidity of alcohols is similar to that of water. We can define an equilibrium constant
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Chemistry 140A Winter 2011 (K. Albizati) 4 Recall that pK a = - log K a so looking at some pK a values will tell us something about the acidity of alcohols. Alcohol pK a H 2 O 15.7 Ethanol 15.9 Isopropyl alcohol 17.1 t-butyl alcohol 18.0 (the lower the pK a , the greater the acidity) This illustrates a general trend: “Inductive effects” contribute to the acidity of alcohols Inductive Effect - the distribution or concentration of electron density due to differences in electronegativity between atoms; a transmissive through bond phenomenon. We refer to certain atoms or groups as inductively electron-releasing or electron withdrawing. Halogens – electron withdrawing (really, any electronegative atom) Alkyl groups – electron releasing Recall what we said about charges ---- the more highly localized a charge is, the less stable is the ion and the more charge density can be spread out by resonance, the more stable is the ion. The same is true for inductive effects.
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Chemistry 140A Winter 2011 (K. Albizati) 5 Example: the pK a of 2,2,2-trifluoroethanol is 12.4 (pretty low for an alcohol). Why?
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