MED-108 Organic Chemistry Alcohols 1 PDF

MED-108 Organic Chemistry Alcohols 1 LOBs covered Describe physical properties of alcohols on the basis of molecular structure Discuss methods of preparation of alcohols Properties of Alcohols Even small alcohols are liquids, not gases Methanol – wood alcohol, b.p. 64.7 oC Ethanol – drinking alcohol...

MED-108 Organic Chemistry Alcohols 1 LOBs covered Describe physical properties of alcohols on the basis of molecular structure Discuss methods of preparation of alcohols Properties of Alcohols Even small alcohols are liquids, not gases Methanol – wood alcohol, b.p. 64.7 oC Ethanol – drinking alcohol, b.p. 78.37 oC Volatile liquids – evaporate easily Flammable – high vapor pressure Lower alcohols are highly soluble in water Why Liquids? Hydrogen bonding – strong intermolecular forces Solubility of Alcohols Lower alcohols are highly soluble in water - Up to pentanol This is due to hydrogen bonding between water and the alcohol. Past pentanol, the hydrophobic (alkane) portion of the molecule takes over Solubility of Alcohols Higher alcohols not so soluble in water Alcohols – Central Position Alcohols Methods of Preparation Addition of Water to Alkenes Reaction is called hydration and forms alcohols Three methods of hydration (1) Brute-force – Markovnikov addition (2) Oxymercuration – Markovnikov addition (3) Hydroboration – anti-Markovnikov addition Brute-force hydration Brute-force hydration Problems Highly acidic conditions High temperatures Reaction is not ideal for research or undergraduate laboratory environments Ideal for industrial settings Oxymercuration Step 1 – Treatment with mercuric acetate Step 2 – Treatment with sodium borohydride THF - Tetrahydrofuran Markovnikov addition Hydroboration Anti-Markovnikov hydration Syn-addition (-H and -OH are added on the same side of the ring) 5-Minute Break 1,2-Diol (Vicinal Diol) Synthesis What is a 1,2-diol? 1,2-Diol Synthesis There are two methods Alkene + OsO4 gives syn (cis) stereochemistry Alkene → Cyclical ether (epoxide) → Opening up the epoxide gives trans stereochemistry Syn (Cis) Addition Osmium tetroxide (OsO4) Trans Addition Epoxide (cyclical ether) intermediate Alcohols from Reduction of Carbonyl Compounds Aldehydes → Primary alcohols Ketones → Secondary alcohols Reducing agents: NaBH4 – sodium borohydride LiAlH4 (LAH) – lithium aluminium hydride – Difficult to handle – Violent reaction with water – Explosive above 120 oC Reduction of Carbonyl Compounds These reducing agents do not reduce alkenes or benzene rings Recall that H2 reduces alkenes but not carbonyl compounds Selective Reduction Examples Reduction of Carbonyl Compounds Examples Reduction of Carbonyl Compounds Examples Reduction of Carboxylic Acids and Esters Carboxylic acids and esters are more resistant to reduction than aldehydes and ketones NaBH4 is too mild – must use LiAlH4 Summary for Revision Even small alcohols are liquids – this is due to the presence of strong hydrogen bonds. Alcohols up to pentanol are fully soluble in water because the hydrophilic (-OH) part of the molecules dominates. Beyond pentanol, the hydrophobic (alkane) part of the molecules takes over. In terms of synthetic organic chemistry, alcohols take a central position. They are excellent starting points for the synthesis of a wide variety of other organic compounds. They can also be formed from a wide variety of other organic compounds. The principal method of forming alcohols is hydration of alkenes. Brute-force hydration gives Markovnikov alcohols, but it is not normally used in teaching laboratories. Oxymercuration leads to Markovnikov alcohols, whereas hydroboration leads to anti-Markovnikov alcohols. Vicinal (1,2) diols can be formed by two methods: syn-diols form by using OsO4, whereas trans-diols form by using peroxyacids, RCO3H. Alcohols can be formed through the reduction of carbonyl compounds such as aldehydes or ketones. Aldehyde reduction gives primary alcohols, and ketone reduction gives secondary alcohols. Milder NaBH4 can be used, or stronger reducing agent LiAlH4 (LAH) can be used. These reducing agents cannot reduce alkene groups or benzene rings. Carboxylic acids and esters can also be reduced to alcohols, similar to aldehydes and ketones. However, since carboxylic acids and esters are more highly oxidized, they require a stronger reducing agent (LAH).