ALCOHOlS

Alcohol is a compound that has an –OH (a

hydroxyl group) bonded to a saturated, alkane-

like carbon atom.

A. STRUCTURE OF ALCOHOLS

The functional group of an alcohol is an – OH (hydroxyl) group bonded to a tetrahedral carbon atom.

a. Methanol, Lewis structure b. ball-and-stick model

B. NOMENCLATURE

1. Select the longest carbon chain that contains the –OH group as the parent alkane and number it from the end that gives –OH the lower number.

In numbering the parent chain, the location of the –OH group takes

precedence over alkyl groups, aryl groups, and halogens.

2. Change the ending of the parent alkane from –e to –ol, and use a number to show the location of the –OH group.

For cyclic alcohols, numbering begins at the carbon bearing the –OH group; this carbon is automatically carbon -1.

Note: It’s not necessary to use the number “1” to specify the location

of the –OH group.

3. Name and number the substituents and list them in alphabetical order.

Classification of Alcohols

Alcohols are classified as primary, secondary, ortertiary.

Classification is determined by the number of

alkyl groups attached to the carbon bonded to the hydroxyl.

We classify alcohols according to the number of carbon substituents bonded to the hydroxyl-bearing carbon:

PRIMARY (1˚)-alcohols with one substituent.

SECONDARY (2˚) – alcohols with two substituents.

A primary alcohol(one R group on –OH bearing carbon)

A secondary alcohol (two R groups on –OH bearing carbon)

TERTIARY (3˚)- alcohols with three substituents.

A tertiary alcohol (three R groups on –OH bearing carbon)

In the IUPAC system, a compound containing two hydroxyl groups as diol, one containing three hydroxyl groups as triol, and so on.

In IUPAC names for diols, triols and so on, the final –e in the name of the parent alkane is retained.

1,2-ethanediol(ethylene glycol)

1,2,3-propanetriol(gycerol, glycerin)

Some Common Alcohols Methyl Alcohol (CH3OH , Methanol)

the simplest member of the alcohol family. is commonly known as wood alcohol

because it was once prepared by heating wood in the absence of air.

Is used industrially as starting material for preparing formaldehyde (H2C=0) and methyl tert-butyl ether (MTBE), an octane booster added to gasoline.

 

CO(g) + 2H2(g) cu CH3OH (l)

high pressure 250 C

Methyl alcohol is colorless, miscible with water, and toxic to humans when ingested or inhaled. It causes blindness in low doses (about 15 mL for an adult) and death in larger amounts. (100-250 mL).

Ethyl Alcohol (CH3CH2OH , Ethanol) it is one of the oldest known pure organic chemicals. its production by fermentation of grain and sugar

goes back many thousand years. sometimes called grain alcohol. the alcohol present in all table wines (10-13%), beers

(3-5%) and distilled liquors (35-90%).

During fermentation, starches or complex sugars are converted to simple sugars (C6H12O6), which are converted to ethyl alcohol:

yeast enzymes C6H12O6 2CH3CH2OH + 2CO2

Isopropyl Alcohol [(CH3)2CHOH]

often called rubbing alcohol. is used as a 70% mixture with water for

runbdowns because it cools the skin through evaporation and causes pores to close.

It’s also used as a solvent for medicines, as a sterilant for instruments , and as a skin cleanser before drawing the blood or giving injections.

Ethylene Alcohol ( HOCH2CH2OH)

A dialcohol, is a colorless liquid that is miscible with water and insoluble in nonpolar solvents.

It’s two major uses are as antifreezer and as a starting material for making polyester films and fibers, such as Dacron.

In humans, it is a central-nervous system depressant; a ,lethal dose is about 100 mL.

Glycerol  [HOCH2CH(OH)CH2OH]

trialcohol gylcerol is a colorless liquid that is miscible with water.

it is not toxic but has a sweet taste that makes it useful in candy and prepared foods.

often called as glycerin. It is also used in cosmetics and tobacco

as a moisturizer, in plastic manufacture, in antifreezer and shock-absorber fluids and as a solvent.

PROPERTIES OF ALCOHOLS

Alcohols are much more polar than hydrocarbons because of the electronegative oxygen atom that withdraws electrons from the neighboring atoms.

Straight-chain alcohols with up to 12 carbon atoms are liquids and each boils at a considerably higher temperature than related alkane.

Alcohols with two or more –OH groups can form more than one hydrogen bond . They are therefore higher boiling and more water-soluble than similar alcohols with one –OH group.

What are the characteristic reactions of alcohols?

A. Acidity of Alcohols Alcohols have about the same pKa values as

water. Phenols are weak acids and react with aqueous

sodium hydroxide to form water-soluble salts.

H2O

Phenol

(a water-soluble salt)

B. Acidity-Catalyzed Dehydration Alkenes

We can convert an alcohol to an alkene by eliminating a molecule of water from adjacent carbon atoms in a reaction called dehydration.

Dehydration of alcohol is most often brought by heating with either 85% phosphoric acid or concentrated sulfuric acid.

Primary alcohols –the most difficult to dehydrate –generally require heating in concentrated sulfuric acid ate temperatures as high as 180˚C.

Secondary alcohols undergo acid-catalyzed dehydration at somewhat lower temperatures.

Tertiary alcohols generally undergo acid-catalyzed dehydration at temperatures only slightly above room temperature.

H2SO4

180˚C

+ H2O

Cyclohexanol Cyclohexene

1˚alcohols 2˚alcohols 3˚alcohols

When the acid-catalyzed dehydration of an alcohol yields isomeric alkenes, the alkene having the greater number of alkyl groups on the double bond generally predominates.

EASE OF DEHYDRATION ALCOHOLS

Hydration-dehydration reactions are reversible.

Alkene hydration and alcohol dehydration are competing reactions, and the following equilibrium exists:

hydration

dehydration

In accordance with Le Chatelier’s principle:

Large amounts of water (using dilute aqueous acid) favor alcohol formation.

Scarcity of water (using concentrated acid) or experimental conditions where water is removed (heating the reaction mixture above 100˚C)favor alkene formation.

C. Oxidation of Primary and Secondary Alcohols

Oxidation – is either the loss of hydrogen or the gains of oxygen.

A primary alcohol can be oxidized to an aldehyde or to a carboxylic acid, depending on the experimental conditions.

The reagent most commonly used in the laboratory for the oxidation of a primary alcohol to carboxylic acid is potassium dichromate, K2Cr2O7, dissolved in aqueous sulfuric acid.

Secondary alcohols may be oxidized to ketones by using potassium dichromate as the oxidizing agent.

Menthol- a secondary alcohol that is present in peppermint and other mint oils, is used in liqueurs, cigarettes, cough drops, perfumery, and nasal inhalers.

2-isopropyl-5-methylcyclohexanol (methanol)

2-isopropyl-5-methylcyclohexanone(methone)

Tertiary alcohols resist oxidation because the carbon bearing the –OH is bonded to three carbon atoms and, therefore cannot form a carbon-oxygen double bond.

K2Cr2O7

H2SO4

(NO OXIDATION)

1-methylcyclopentanol

..THANK YOU..and God bless.. =)

PHY.SCI 2