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Transcript
Ch. 14 Alcohols, Ethers, & Thiols Note: We are skipping Ch 13 HW-14.10, 14.11, 14.13, 14.14, 14.22, 14.33, 14.35,14.37, 14.43, 14.45, 14.54, 14.71 Alcohols, Ethers, & Thiols • Alcohols and Ethers are two classes of oxygen containing organic compounds • Thiols are a class of Sulfur containing organic compounds • Thiols are like alcohols, only they have a -SH functional group instead of an -OH Examples • Alcohol- Ethanol- gas additive, alcohol consumed by OH humans important solvent • Ether- Diethyl ether- 1st anesthetic important solvent O • Thiol- Ethanethiol- additive to natural gas SH Alcohols • Functional group is -OH, the hydroxyl group, bonded to a tetrahedral carbon • Nomenclature Rules – Same as for Alkenes and Alkynes except you only drop the -e, and add -ol! OH propane propanol Nomenclature of Alcohols • 1) Select the longest chain that contains the carbon bonded to the -OH group, and number the chain to give the carbon bonded to the -OH group the lowest number • The -OH group takes precedence over alkyl groups, double bonds, triple bonds, and halogens!!! Nomenclature of Alcohols • 2) Change the suffix by dropping the -e, and adding -ol. Use the number to show location. In cycloalkanes, start numbering from the carbon bonded to the -OH. • 3) Name and number substituents and list them in alphabetical order. Examples OH OH OH OH Br OH Classification • We classify alcohols as 1o, 2o, and 3o, depending on the classification of the carbon they are bonded to. OH OH OH Multiple -OH’s present • Molecules with 2 -OH’s are named as diols • Molecules with 3 -OH’s are named as triols • (Note: you do not drop the -e when using diol, triol, etc) • Compounds with 2 -OH’s are refered to as glycols OH HO OH HO OH Physical Properties of Alcohols • The most important physical property is their polarity • Both the C-O bond and the O-H bond are polar covalent bonds • Thus alcohols are polar molecules • They also have the ability to hydrogen bond. • These factors lead to higher B.P’s, M.P’s. etc Physical Properties of Alcohols • Because of increase london forces between larger molecules, the B.P. of all types of compounds, including alcohols, increase as molecular weight increases • Alcohols are much more soluble in H2O due to their H-bonding capacity. • As MW increases, the water solubility of alcohols decreases • This is because the hydrocarbon portion of the molecule dominates. Reactions of Alcohols A) Acidity of Alcohols -Alcohols are considerably weaker acids than carboxylic acids, but can lose their hydrogen in an acid-base reaction. OH + Base O + Base-H Reactions of Alcohols B) Acid catalyzed dehydration -Converts alcohols to alkenes by eliminating a molecule of water from adjacent carbons When the equivalent of a molecule of water is removed from a compound, it is called a Dehydration Reaction Dehydration of Alcohols • 1o alcohols are more difficult to dehydrate. They require high temperatures in concentrate H2SO4 • 2o alcohols require lower temperatures • 3o alcohols are the easiest to dehydrate and undergo dehydration only slightly above room temperature Examples H 2SO4 OH OH 180oC CH2 + H 2O H 2SO4 140oC H 2SO4 OH H2C 180oC 50oC + H 2O + H 2O Prediction of Product • When the dehydration of an alcohol can yield more than one different double bond, the most substituted double bond will form. H2SO4 + heat OH 80% 20% Recap • Earlier, we learned the acid catalyzed hydration of alkenes OH H2C CH2 + H2O acid H2C H CH2 • Now we are saying alcohols can be dehydrated with acid OH H2C H CH2 acid heat H2C CH2 + H2O Equilibrium Reactions • The fact is that these reactions are reversible • Alkene hydration and alcohol dehydration are competing reactions and the following equilibrium exist: hydration H 2C CH2 + H2O dehydration OH H 2C H CH2 Controlling Equilibriums • Equilibriums are governed by Le Chatelier’s Principle • Le Chatelier’s Principle- when external stress is applied to a system at equilibrium, the system will react to relieve the stress Controlling Equilibriums • We can control the hydration/dehydration equilibrium by: – Using large amounts of water (favors alcohol formation) – Using concentrated acids (favors alkene formation – Removal of water as it forms (favors alkene formation) Reactions of Alcohols C) Oxidation of 1o and 2o alcohols -Remember, oxidation in organic chemistry is defined as the increase in C-O bonds and/or the decrease in C-H bonds O O [O] [O] OH H OH Oxidation of Alcohols • The reagent used for the oxidation of alcohols is potassium dichromate, K2Cr2O7, dissolved in H2SO4 O O K2Cr2 O7 K2Cr2 O7 OH H2SO4 H H2SO4 • The reaction proceeds to form the carboxylic acid unless the aldehyde is removed OH Oxidation of Alcohols • The oxidation of 2o alcohols results in a ketone OH K2Cr2 O7 O H2SO4 • 3o alcohols can not be oxidized because the carbon bonded to the -OH is bonded to 3 other carbons. Ethers • Structure- functional group is a Oxygen bonded to 2 carbons • Simplest ether is dimethyl ether H3 C O CH3 Nomenclature of Ethers • The common naming system is used for simple ethers: – List the alkyl groups bonded to the oxygen in alphabetical order, followed by the work “ether”. H 3C O O CH3 O O O Physical Properties • Ethers are polar compounds • The oxygen has a partial minus charge, the carbons bonded to the oxygen have a partial positive charge • Ether have very weak intermolecular forces which results in low boiling points Reactions of Ethers • Like alkanes, they are resistant to most chemical reactions • Therefore, they are ideal to use as solvents Thiols • The most outstanding property of low molecular weight thiols is their stench!! • They are responsible for the wonderful odors from skunks, rotten eggs, sewage, and paper mills • Structure- functional group is -SH, the sulfhydryl group, bonded to a tetrahedral carbon Naming Thiols • Name just like alcohols, except add -thiol instead of -ol and don’t drop the -e. • Find longest chain containing -SH, number to give -SH lowest number. SH SH SH Physical Properties • Very little difference in electronegativity between Sulfur and Carbon, so the bond is actually nonpolar • They DO NOT hydrogen bond • They have low boiling points and very little water solubility Reactions of Thiols • Thiols are weak acids and react with bases to lose a proton. SH + NaOH S Na + H2O