Chapter8 - CHAPTER 8 Hydroxy Functional Group: Alcohols...

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CHAPTER 8Hydroxy Functional Group:AlcoholsProperties, Preparation, andStrategy of Synthesis
Ethanol is the alcohol contained in alcoholic beverages.Alcohols can be thought of as a derivative of water in which ahydrogen atom has been replaced by an alkyl group.Replacement of the 2ndhydrogen on the water molecule leads toan ether.
Naming the Alcohols8-1The systematic nomenclature of alcohols treats them asderivatives of alkanes.The –e is dropped from the alkane name and is replaced by –ol.AlkaneàAlkanolIn complicated, branched alkanes, the name of the alcohol isbased on the longest chain containing the –OH group.Other substituents are then named using the IUPAC rules forhydrocarbons.
The number of the chain is from the end closest to the OH group.Cyclic alcohols are called cycloalkanols and the carbon carryingthe –OH group is the 1 carbon.
Alcohols can be classified as primary, secondary or tertiary:In common notation (non-IUPAC), the word alcohol directlyfollows the name of the alkane.•Methyl alcohol•Isopropyl alcohol•Tert-Butyl alcohol
Structural and Physical Properties of Alcohols8-2The structure of alcohols resembles that of water.In the structures of water, methanol, and methoxymethane, theoxygen atoms are all sp3+hybridized and their bond angles are allnearly tetrahedral.The O-H bond is shorter than the C-H bonds.The bond strength of the O-H bond is greater than that of the C-Hbonds:•DHoO-H= 104 kcal mol-1•DHoC-H= 98 kcal mol-1
Due to the electronegativity difference between oxygen andhydrogen, the O-H bond is polar.
Hydrogen bonding raises the boiling points andwater solubilities of alcohols.Alcohols have unusually high boiling points compared to thecorresponding alkanes and haloalkanes.Hydrogen bonding between alcohol molecules is much strongerthan the London forces and dipole-dipole interactions in alkanesand haloalkanes, although much weaker than O-H covalentbonds.•O···H-ODHo~ 5-6 kcal mol-1•Covalent O-HDHo= 104 kcal mol-1.The extensive network of H-bonds between neighboring alcoholmolecules makes it difficult for a molecule to leave the surface ofthe liquid.An alcohol molecule makes slightly less than 2 hydrogen bonds toother alcohol molecules on the average. A water molecule, on theother hand, forms hydrogen bonds to slightly less than 4 otherwater molecules. Water has an abnormally high boiling point for amolecule of its size due to this hydrogen bonding.
Many alcohols are appreciably soluble in water whereas theirparent alkanes are not.•Alkanes and most alkyl chains are said to behydrophobic(water-hating).In order to dissolve, alkanes must interrupt the stronghydrogen bonding between water molecules which is thenreplaced by weaker dipole-induced dipole forces (ΔH > 0).In addition, long hydrocarbon chains force water moleculesto form a cage-like (or clathrate) structure about the non-polar chain which greatly reduces the entropy of the watermolecules involved (ΔS < 0).

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Term
Spring
Professor
Whiteshell
Tags
Atom, Organometallic Reagents, Structural and Physical Properties of Alcohols

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