Alcohols, Ethers and Thiols

Alcohols (R-OH) Alcohols are organic compounds that contain the –OH (hydroxyl) functional group attached to the hydrocarbon skeleton (alkyl or aryl) Simplest alcohol is methanol (CH3OH) Chemistry, Raymond Chang, 10th edition, McGraw-Hill

Alcohols – Hydroxyl Functional Group Many important biological molecules have one or more hydroxyl groups (sugars, fats, proteins) Hydroxyl group is involved in many reactions in biological systems such as oxidation, reduction, dehydration glucose Biological production of ethanol – fermentation of sugar serine

Naming Alcohols IUPAC nomenclature rules for alcohols: 1) Find the parent compound name, which is the longest continuous carbon chain containing the hydroxyl group 2) Replace “-e” with “-ol” For example; ethane ethan(e) + ol 3) Number the parent chain so that the carbon attached to the hydroxyl has the lowest possible number 4) Add substituent names as prefixes to the main name 5) Alcohols with two hydroxyls are named “-diols” Common names are used for some alcohols, with the name of the alkyl group: isopropyl alcohol, ethylene glycol, ethyl alcohol) http://www.ck12.org/book/CK-12-Chemistry-Second-Edition/r13/section/25.4/

Physical Properties of Alcohols Polar molecules (presence of highly electronegative oxygen atom) Form hydrogen bonds between themselves and with water molecules They have high melting and boiling points especially due to hydrogen binding CH3CH2CH2CH3 (butane) MW = 58, b.p. = -0.5 °C CH3CH2CH2OH (1-propanol) MW = 60, b.p. = 97.2 °C Small alcohols (1-4 carbons) are very soluble in water As the hydrocarbon part (nonpolar) becomes larger relative to hydroxyl (polar) part, alcohols become less soluble (alcohols with 7 or more carbons are almost insoluble) http://www.chemhume.co.uk/ASCHEM/Unit%202/Ch11%20Alcohols/images/hydrogen_bonding_ethanol_water.jpg

Medically Important Alcohols Methanol (CH3OH): Used mainly as a solvent, also called wood alcohol, toxic, can cause blindness and even death if ingested Ethanol (C2H5OH): Used as solvent, beverage and raw material for synthesis of more complex organic compounds, produced by fermentation of sugar 2-Propanol (isopropyl alcohol): used as disinfectant (before and after injection/blood test), as an astringent (skin-drying agent), as industrial solvent / toxic when ingested 1,2,3-Propanetriol (glycerol): viscous liquid, very soluble in water, used in cosmetics and pharmaceutical industry

Classification of Alcohols Alcohols are classified as primary (1°), secondary (2°) and tertiary (3°) http://www.chem.shef.ac.uk/chm131-2003/cha03kej/alcohols.html

Reactions of Alcohols 1) Preparation of alcohols: 2) Dehydration of alcohols: 3) Oxidation reactions:

Reactions of Alcohols 1) Preparation of alcohols: Hydration reaction: addition of water to alkenes (carbon-carbon double bond) – follows Markovnikov rule: Hydrogenation (reduction) of aldehydes and ketones (by metal catalyst) http://www.chem.latech.edu/~deddy/chem121/Alcohols.htm

Reactions of Alcohols 2) Dehydration of alcohols (Elimination of Water): Alcohols undergo loss of water (dehydration) when they are heated with acid Reverse process of hydration reaction Predicting products of alcohol dehydration - Zaitsev’s rule- more substituted alkene is preferred R-CH2-CH2-OH + H2SO4 ---> R-CH=CH2 http://www.masterorganicchemistry.com/2012/08/31/elimination-reactions-2-zaitsevs-rule/

Reactions of Alcohols 3) Oxidation Reactions: An oxidation reaction involves gain of oxygen or loss of hydrogen Alcohols can be oxidized to aldehydes, ketones and carboxylic acids Most common oxidizing agents are potassium permanganate (KMnO4) and chromic acid (H2CrO4) Tertiary alcohols can’t be oxidized http://www.wikipremed.com/03_organicmechanisms.php?mch_code=030207_050

Ethers (R-O-R) Ethers are organic compounds that contain the –OR (alkoxy) functional group attached to the hydrocarbon skeleton -OH group in alcohols is replaced by –OR (alkyl or aryl group instead of hydrogen) In other words, ethers are compounds in which two alkyl (or aryl) groups are linked by a bridging oxygen

Naming Ethers In IUPAC system, -OR substituent is named as alkoxy group (similar to hydroxy) CH3-O- (methoxy), CH3CH2-O- (ethoxy) Ethers are named by placing the two alkyl group names in front of the word “ether” The two names are placed either alphabetically or by size (smaller first) examples: dimethyl ether, ethyl methyl ether, ethyl propyl ether

Physical Properties of Ethers 1) Ethers are relatively polar molecules (but not as polar as alcohols) 2) Unlike alcohols, they can’t make hydrogen bonds with itself – so lower mp and bp compared to alcohols 3) They can make hydrogen bonds with water – more soluble than alkanes but less than alcohols http://www.masterorganicchemistry.com/2010/10/25/3-trends-that-affect-boiling-points/

Chemical Properties and Preparation Ethers are moderately inert (not very reactive) Extremely volatile and flammable (easily oxidized in the air), requires great care Prepared by a dehydration reaction from two alcohol molecules in the presence of acid and heat: R-OH + R-OH  R-O-R + H2O (heat/acid) (heated with sulfuric acid at 140 °C)

Medical Importance of Ethers Diethyl ether was the first general anesthetic used (in 1800s) – used in surgeries How does it work? Diethyl ether accumulate in the lipid material of the nerve cells in the central nervous system and interfere with nerve impulse transmission This leads to analgesia, lessened perception of pain and unconsciousness “William Morton, a Boston dentist, introduced diethyl ether into surgical practice in 1846. This painting shows an operation in Boston in 1846 in which diethyl ether was used as an anesthetic. Inhalation of ether vapor produces unconsciousness by depressing the activity of the central nervous system” http://2012books.lardbucket.org/books/introduction-to-chemistry-general-organic-and-biological/s17-08-ethers.html

Thiols Thiols contain sulfhydryl group (-SH), similar to alcohols, but sulfur instead of oxygen IUPAC rules for naming thiols: similar to alcohols, except “-thiol” is added at the end of the name of the parent alkane Thiol compounds have strong smells (onion, garlic etc.) http://2012books.lardbucket.org/books/beginning-chemistry/s20-05-other-functional-groups.html

Importance of Thiols in Biology The amino acid “cysteine” is a thiol Cysteine plays an important role in the structure of proteins (disulfide bonds) Coenyzme A (acetyl group carrier in many biochemical reactions) is also a thiol cysteine Coenzyme A