Organic I Prof. Dr. Abdellatif M. Salaheldin Chemistry Department

Chapter 5 Alcohols, Phenols, Ethers, Aldehydes, Ketones and Thiols

Alcohols In an alcohol, a hydroxyl group (—OH) is attached to a carbon chain. In a phenol, a hydroxyl group (—OH) is attached to a benzene ring.

Classification of Alcohols Alcohols are classified as primary, secondary, or tertiary. Classification is determined by the number of alkyl groups attached to the carbon bonded to the hydroxyl. Primary (1º) Secondary (2º) Tertiary (3º) 1 group 2 groups 3 groups H CH3 CH3 | | | CH3—C—OH CH3—C—OH CH3—C—OH | | | H H CH3

Naming Alcohols The IUPAC system replaces the -e in the name of the alkane main chain with –ol. Common names for simple alcohols use the alkyl name followed by alcohol. CH4 methane CH3OH methanol (methyl alcohol) CH3CH3ethane CH3CH2OH ethanol (ethyl alcohol)

Naming Alcohols In the IUPAC names for longer chains, the chain is numbered from the end nearest the -OH group. CH3—CH2—CH2—OH 1-propanol OH | CH3—CH—CH2—CH3 2-butanol CH3 OH | | CH3—CH—CH2—CH—CH3 4-methyl-2-pentanol

Some Typical Alcohols OH | “rubbing alcohol” CH3—CH—CH3 2-propanol (isopropyl alcohol) antifreeze HO—CH2—CH2—OH 1,2-ethanediol (ethylene glycol) glycerol HO—CH2—CH—CH2—OH 1,2,3-propanetriol

Naming Phenols A phenol is a benzene ring with a hydroxyl group. For two substituents, assign C-1 to the carbon attached to the –OH. Number the ring to give the lowest numbers. The prefixes o, m, and p are used for common names.

Examples of Phenols

Phenols in Medicine Many phenols are used as antiseptics and disinfectants.

Derivatives of Phenol Compounds of phenol are the active ingredients in the essential oils of cloves, vanilla, nutmeg, and mint.

Thiols Thiols are carbon compounds that contain the –SH group. In the IUPAC name, thiol is added to the alkane name of the longest carbon chain.

Naming Thiols In thiols with long carbon chains, the chain is number to locate the -SH group. CH3—CH2—CH2—SH 1-propanethiol SH | CH3—CH—CH3 2-propanethiol

Thiols in Nature Thiols: Often have strong or disagreeable odors. Are used to detect gas leaks. Are found in onions, oysters, and garlic.

Ethers Ethers contain an -O- between two carbon groups. Simple ethers are named by listing the alkyl names in alphabetical order followed by ether. CH3—O—CH3 dimethyl ether CH3—O—CH2—CH3 ethyl methyl ether CH3—CH2—O—CH2—CH3 diethyl ether

IUPAC Names for Ethers In the IUPAC system, the shorter alkyl group and the oxygen are named as an alkoxy group attached to the longer alkane. methoxy propane CH3—O—CH2—CH2—CH3 Numbering the longer alkane gives 1-methoxypropane.

Boiling Points of Alcohols Alcohols contain a strongly electronegative O in the OH groups. Thus, hydrogen bonds form between alcohol molecules. Hydrogen bonds contribute to higher boiling points for alcohols compared to alkanes and ethers of similar mass.

Boiling Points of Ethers Ethers have an O atom, but there is no H attached. Thus, hydrogen bonds cannot form between ether molecules.

Solubility of Alcohols and Ethers in Water Alcohols and ethers are more soluble in water than alkanes because the oxygen atom can hydrogen bond with water. Alcohols with 1-4 C atoms are soluble, but alcohols with 5 or more C atoms are not.

Reactions of Alcohols Alcohols undergo combustion with O2 to produce CO2 and H2O. 2CH3OH + 3O2 2CO2 + 4H2O + Heat Dehydration removes H- and -OH from adjacent carbon atoms by heating with an acid catalyst. H OH | | H+, heat H—C—C—H H—C=C—H + H2O | | | | H H H H alcohol alkene

Formation of Ethers Ethers form when dehydration takes place at low temperature. H+ CH3—OH + HO—CH3 CH3—O—CH3 + H2O Two Methanol Dimethyl ether

Oxidation and Reduction In organic chemistry, oxidation is a loss of hydrogen atoms or a gain of oxygen. In an oxidation, there is an increase in the number of C-O bonds. Reduction is a gain of hydrogen or a loss of oxygen. The number of C-O bonds decreases.

Oxidation of Primary Alcohols In the oxidation [O] of a primary alcohol, one H is lost from the –OH and another H from the carbon bonded to the OH. [O] Primary alcohol Aldehyde OH O | [O] || CH3—C—H CH3—C—H + H2O | H Ethanol Ethanal (ethyl alcohol) (acetaldehyde)

Oxidation of Secondary Alcohols The oxidation of a secondary alcohol removes one H from –OH and another H from the carbon bonded to the –OH. [O] Secondary alcohol Ketone OH O | [O] || CH3—C—CH3 CH3—C—CH3 + H2O | H 2-Propanol Propanone (Isopropyl alcohol) (Dimethylketone; Acetone)

Oxidation of Tertiary Alcohols Tertiary alcohols are resistant to oxidation. [O] Tertiary alcohols no reaction OH | [O] CH3—C—CH3 no product | CH3 no H on the C-OH to oxidize 2-Methyl-2-propanol

Ethanol CH3CH2OH Ethanol: Acts as a depressant. Kills or disables more people than any other drug. Is metabolized at a rate of 12-15 mg/dl per hour by a social drinker. Is metabolized at a rate of 30 mg/dl per hour by an alcoholic.

Oxidation of Alcohol in the Body Enzymes in the liver oxidize ethanol. The aldehyde produced impairs coordination. A blood alcohol level over 0.4% can be fatal. O || CH3CH2OH CH3CH 2CO2 + H2O Ethyl alcohol acetaldehyde

Oxidation of alcohols in liver

Breathalyzer test K2Cr2O7 (potassium dichromate) This orange colored solution is used in the Breathalyzer test (test for blood alcohol level) Potassium dichromate changes color when it is reduced by alcohol K2Cr2O7 oxidizes the alcohol

Breathalyzer reaction orange-red green 8H++Cr2O72-+3C2H5OH→2Cr3++3C2H4O+7H2O dichromate ethyl chromium (III) acetaldehyde ion alcohol ion (from K2Cr2O7)

Production of ethanol from grain by fermentation Grain seeds are grounded and cooked → mash Malt (the dried sprouts of barley) or special mold is added → source of the enzyme diastase that catalyzes the conversion of starch to malt sugar, maltose diastase (C6H10O5)2x + H2O → x C12H22O11 starch maltose Pure yeast culture is added C12H22O11 + H2O → 2 C6H12O6 maltose glucose C6H12O6 → 2 CH3CH2OH + 2 CO2 glucose ethanol carbon dioxide

Preparation of alcohols Ethanol is made by hydration of ethylene (ethene) in the presence of acid catalyst

Isopropyl is produced by addition of water to propylene (1-propene)

Methanol is made commercially from carbon monoxide and hydrogen CO + 2H2 → CH3OH

Oxidation of Thiols. Mild oxidizing agens remove two hydrogen atoms from two thiol molecules. The remaining pieces of thiols combine to form a new molecule, disulfide, with a covalent bond between two sulfur atoms. R – S – H H – S – R+I2 → RS – SR+2HI 2 RSH + H2O2 → RS – SR + 2 H2O

Aldehydes and Ketones The carbonyl group is found in aldehydes and ketones. In an aldehyde, an H atom is attached to a carbonyl group. In a ketone, two carbon groups are attached to a carbonyl group.

The Polar Carbonyl Group The carbonyl group (C=O): Consists of a sigma and pi bond. Has a partially negative O and a partially positive C. Has polarity that influences the properties of aldehydes and ketones.

Naming Aldehydes In the IUPAC names for aldehydes, the -e in the corresponding alkane is replaced by –al. Common names use a prefix with aldehyde: form (1C), acet (2C), propion (3), and butyr (4C)

Aldehydes in Flavorings Several naturally occurring aldehydes are used as flavorings for foods and fragrances.

Naming Ketones In the IUPAC name, the -e in the alkane name is replaced with –one. In the common name, add the word ketone after naming the alkyl groups attached to the carbonyl group. O O || || CH3—-C—CH3 CH3—C—CH2—CH3 Propanone 2-Butanone (Dimethyl ketone; (Ethyl methyl ketone) Acetone)

Physical Properties The polar carbonyl group provides dipole-dipole interactions. + - + - C=O C=O Without an H on the oxygen atom, aldehydes and ketones cannot form hydrogen bond. Aldehydes and ketones can form hydrogen bonds with the water molecules.

Comparison of Boiling Points Aldehydes and ketones have higher boiling points than alkanes and ethers of similar mass, but lower boiling points than alcohols.

Solubility in Water The electronegative O atom of the carbonyl group of aldehydes and ketones forms hydrogen bonds with water.

Comparison of Physical Properties

Types of Isomers

Chiral Objects Chiral compounds have the same number of atoms arranged differently in space. A chiral carbon atom is bonded to four different groups. Your hands are chiral. Try to superimpose your thumbs, palms, back of hands, and little fingers.

Mirror Images The mirror images of chiral compounds cannot be superimposed. When the H and I atoms are aligned, the Cl and Br atoms are on opposite sides.

Achiral Structures are Superimposable When the mirror image of an achiral structure is rotated, the structure can be aligned with the initial structure. Thus this mirror image is superimposable.

Some Everyday Chiral and Achiral Objects

Fischer Projections A Fischer projection: Is a 2-dimensional representation of a 3-dimensional molecule. Places the most oxidized group at the top. Uses vertical lines in place of dashes for bonds that go back. Uses horizontal lines in place of wedges for bonds that come forward.

Drawing Fischer Projections

D and L Notations By convention, the letter L is assigned to the structure with the —OH on the left. The letter D is assigned to the structure with the —OH on the right.

Oxidation Aldehydes are easily oxidized to carboxylic acids. O O CH3—C—H CH3—C—OH Acetaldehyde Acetic acid

Hydrate formation Hydrate formation is the reason that aldehydes are oxidized ([O]=-2H=oxidation) to carboxylic acids in aqueous media. O OH O || H2O | [O] || RCH ↔ RC – OH → RCOH | H

Tollens’ Test Tollens’ reagent, which contains Ag+, oxidizes aldehydes, but not ketones. Ag+ is reduced to metallic Ag, which appears as a “mirror” in the test tube.

Tollens’ reagent detects an aldehyde Tollens’ reagent (a solution of Ag+ in aqueous NH3) can be used to detect the presence of the aldehyde group in the unknown sample. As the oxidation of the aldehyde proceeds, silver metal is deposited on the walls of the reaction flask as a shiny mirrow. If upon addition of the Tollens’ reagent to the unknown, the test tube becomes silvery, the unknown is an aldehyde. RCHO + 2Ag(NH3)2+ +2OH- → RCOO- + NH4+ + 2Ag + 3NH3 + H2O an Tollens’ salt of a silver aldehyde reagent carboxylic acid mirrow

Benedict’s Test Benedict’s reagent, which contains Cu2+, reacts with aldehydes that have an adjacent OH group. When an aldehyde is oxidized to a carboxylic acid, Cu2+ is reduced to give Cu2O(s).

Test for glucose (Benedict’s test)

Addition Reactions Polar molecules can add to the carbonyl groups of aldehydes and ketones. The negative part of the added molecule bonds to the positive carbonyl carbon. The positive part of the added molecule bonds to the negative carbonyl oxygen. | + - + - | —C=O + X—Y — C—O—X | Y

Acetal Formation Alcohols add to the carbonyl group of aldehydes and ketones. The addition of two alcohols forms acetals.

Hemiacetal Formation The addition of one alcohol to an aldehyde or ketone forms an intermediate called a hemiacetal. Usually, hemiacetals are unstable and difficult to isolate.

Cyclic Hemiacetals Stable hemiacetals form when the C=O group and the —OH are both part of five- or six-atom carbon compounds.