Alcohols, Carboxylic Acids and Esters

 


Alcohols, Carboxylic Acids and Esters

 

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Alcohols, Carboxylic Acids and Esters

 

Chemistry

 

Alcohols

 

Introduction

 

Alcohol is a familiar term to chemists and non-chemists alike. Methanol, also known as methyl alcohol, is used as an additive to unleaded petrol. Rubbing alcohol, propan-2-ol, is used to remove bacteria from the skin before surgery, or an injection. It is a bacteriocide and is also used for massages and as a base for perfumes and creams.

 

Ethyl alcohol is the main component in methylated spirits ('metho').

 

All of these alcohols and others, are used in the laboratory and chemical industry as solvents and reagents.

 

Some alcohols you may have heard of include menthol, found in peppermint oil and also made synthetically, is used in sore throat medicines and in creams for the relief of muscle aches; glycerol, also known as glycerine and is used in cosmetics, and cholesterol which can deposit in our arteries so causing heart problems.

 

To a non-chemist the alcohol is ethyl alcohol - the intoxicating part of beer, wines and spirits. It has, however, given its name to a family of compounds all of which have the same functional group. A chemist calls ethyl alcohol, ethanol.

 

Ethanol has the same carbon skeleton as the alkane ethane, and differs from it by having a hydrogen atom removed from one of the carbon atoms, and replaced by an _OH group, called hydroxy.

 

 

 

 

 Therefore, for every member of the alkane series there is a corresponding alcohol. The alcohols have a general formula CnH2n+1OH and their functional group _OH, is called the hydroxy group. It is the functional group in a molecule which is responsible for the chemical reactions of that molecule. A functional group also influences the physical properties of a molecule. Methanol and ethanol can be seen to resemble water in structure. All three have a hydroxy group.


The observant reader will notice that the names of the alcohols given in this introduction end with -ol

 

Th e   fu n cti o n al   g ro u p   -OH , hydroxyl,  has  had  it s  name updated to “hydroxy”.

 

(from Nomenclature of Organic Chemistry 1993, published by IUPAC).


 

 


Naming alcohols

 

Alkane    alkanol. For example:


 

 

 

CH3OH is methanol

 

CH3CH2OH is                              ?


 

For longer chains, a position number of the hydroxy group is given. For example:

 

 

Regardless of branches, number from the end closer to the functional group.

 


 

 

 

Ethanol can be used as a fuel. It was used to extend petrol supplies during World War II. Petrol-alcohol mixes are called 'gasahol'. Brazil cultivates sugar cane on a large scale to produce fuel for trucks, cars and boats. It is cheaper in Brazil to buy a vehicle running on

100%  ethanol  than  one  using pet rol.  T his  reduces  Brazil's dependenc e  on  oil- pr oduc ing coun tries   and   provi des   a commercial outlet for local sugar cane growers.

 

 

The name fermentation comes from the Latin fermentum, to boil. The bubbles of CO2(g) give the mixture an appearance of boiling.

 

 

Can you write an equation for this reaction?


Preparation of ethanol

 

Ethanol is the most important member of the alcohol family. Its preparation by fermentation is so well known it has been referred to as 'the oldest chemical synthesis.' However, it is not important only because it is well known as a beverage. Ethanol is the most widely used industrial solvent.  It is sold as methylated spirits, which is approximately 95% ethanol. The remainder contains poison - usually methanol - to make the ethanol undrinkable. Adding the poison cuts the cost, as alcoholic beverages are taxed by the government.

 

Fermentation

 

Some industrial alcohol is still made by yeast fermentation of sugar. Fermentation is the name given to the reaction that produces ethanol from sugars using yeasts. The yeasts produce the special catalysts, called enzymes, which cause the breakdown of the sugars.

 

The fermentation reaction for the sugar glucose is:

 

yeast

C6H12O6(aq)       →      2C2H5OH(aq) + 2CO2(g)

glucose                     ethanol

 

More about fermentation is given in Biomolecules at the end of this chapter.

 

Addition of water to alkenes

 

Ethene from the catalytic cracking of gas oil can be converted to ethanol.

 

The alcohol produced this way is used as a solvent. Propene (propylene) is used to produce propan-2-ol (rubbing alcohol).


Physical properties of the alcohols

 

The first three alcohols are completely soluble in water. After this, solubility in water declines. Alcohols are soluble in other liquid alcohols and in other organic solvents.

Their boiling points and melting points are higher than those of the corresponding alkanes, for example there are no gaseous alcohols at room temperature. Methanol, the simplest alcohol, has a boiling point of 64.5°C. Methane boils at -161.5°C.

 

Chemical properties of the alcohols

 

Combustion

 

Alcohols burn with a clean blue flame to give carbon dioxide and water.

 

Hydrogen bonding is used to explain both the solubility of alcohols in water, and their much higher boiling points than the corresponding alkanes.


 

CH3OH(l) + 3_ O

2      2(g)


→  CO


 

 

 2(g) + 2H2O(g)


 

Methanol and ethanol have a high octane number and can be used with petrol.

 

Oxidation

 

With air

 

Air bubbled through poor-quality wine produces vinegar. The reaction is speeded up by enzymes that are produced by certain bacteria.

enzyme

CH3CH2OH(aq)    +  O2    

in wine                                                                 vinegar

(about 5 to                                                      dilute aqueous acetic

10% alcohol)                                                      acid (5 to 10%)

 

With acidified potassium dichromate

 

When ethanol is warmed with acidified potassium dichromate, the colour of the oxidant changes from orange to green. The half-equation for acidified potassium dichromate acting as an oxidant is:

 

Cr2O72— + 14H+ + 6e   2Cr3+ + 7H2O

orange                                green

 

 

 


Ethanol produces acetic acid again as it does with oxygen. All alcohols with no branch (i.e. methyl or ethyl group) at the point of attachment of the _OH group are oxidised by acidified potassium dichromate to the corresponding carboxylic acid.

 

The equation can also be written like this:

 

Cr2O7 2—/H+

  


2-     +


 

 

The breathalyser

The simplest form of a breath analyser is used as a screening device. It contains potassium dichromate-sulfuric acid reagent absorbed in particles of silica gel


CH3CH2OH


Cr2O7   /H

 

 

 

 heat


CH3COOH


in a sealed glass ampoule.


 

The equation above, which focuses on organic reactants and products, is called an annotated equation. Organic chemists frequently use annotated equations, which are like one-step flow charts, as a useful simplification.

 

Reduction of alcohols with sodium

 

Alcohols can be seen to resemble water in structure. All three have a hydroxy group.

 

 

When alcohols react with sodium, hydrogen is generated, but less vigorously than with water. This is a test for alcohols.


Breath alcohol screening device

 

Alcoholic air blown through the tube causes a colour change, which gives an indication of blood alcohol content. More sophisticated meters are now used.


With water:


 

HOH + Na ½ H2 + NaOH

 

 

 

HOCH3 + Na ½ H2 + NaOCH3

                                            sodium

                                         methoxide

  

 



         

 

With methanol:

 


 


                                       

 

 

An ester is a compound formed between an alcohol and an organic acid.

 

 

 

Conc. sulfuric acid is a catalyst for making esters.

 

 

Malic acid (the most common carboxylic acid) occurs in apples, citric acid occurs in citrus fruit,


 

 

 

 

Alcohols react with acids to produce sweet smelling esters

 

Many esters are responsible for the odour and flavour in wines and fruit e.g. banana, peach, apple, pineapple and apricot. Some of these esters are made artificially and sold as fruit essences used in cooking. A word equation for the formation of esters is:

 

For example:

 

 

The reaction is called esterification.

 

 

Carboxylic acids

  


 


lactic acid occurs in sour milk, and                                                                                                                                                               


oxalic acid occurs in rhubarb.

 

The functional group  - COOH, carboxyl, has had its named updat ed   t o   carboxy   (f rom Nomenclature of Organic Chemistry 1993, published by IUPAC. )


The chemists call the organic acids, carboxylic acids. The name comes from the functional group that contains a carboxy group.

 

Carboxylic acids are very common in nature. They are responsible for the sour taste of unripe fruits, and the taste in some ripe fruit such as lemons. Tartaric acid, found in grapes, is so called because its taste is sharp or tart.

 

Replacing a hydrogen atom in an alkane by a COOH group gives an alkanoic acid.

 

The carboxylic acids have a general formula, CnH2n+1COOH, and their functional group, _COOH, is called the carboxy group, or carboxylic acid group.

 

Naming carboxylic acids


 

 

 Alkane/


→   alkanoic acid. No position number for the carboxy group is needed as it is


Since the functional groups are the onl y  p arts  of  the  mol ecu les undergoing chemical change, it can be tedious to keep writing down the rest of the molecule which does not react.

 

Organic chemists frequently just write R as an abbreviation for the rest  of  the  molecule.  Hence, R-OH, R-COOH etc.

 

What is the systematic name of butyric acid, CH3CH2CH2COOH?


always at the beginning of the parent chain (and forms part of the parent chain).


Physical Properties of the carboxylic acids

 

They have a sharp taste and characteristic odour.

 

These acids have a characteristic, sour taste. The volatile acids also have a strong repugnant smell, like Parmesan cheese, unwashed gym socks, vomit or even worse!

 

They form an homologous series.

 

The first four members are infinitely soluble in water, but as the chain length increases solubility decreases.

 

point of 16.6°C, freezes on cold days and is sometimes called 'glacial acetic acid'.

 

 

Again the solubility in water of the lower members and their much higher boiling points than the corresponding alkanes is explained by hydrogen bonding.

 

 

As in other homologous series as the chain length increases so melting points and boiling points increase.

 

 

Chemical properties of the carboxylic acids

 

They are weak monoprotic acids.

 

The one transferable proton is in the functional group and attached to an oxygen atom. They ionise incompletely in water (as indicated by the dotted line). Few H+ ions are formed. They are weak acids.

At room temperature, only three to four acetic acid molecules in every 1000 ionise.

 

They change the colour of indicators.

 

As weak acids they have excess hydronium ions and will turn blue litmus red. (provided that they are soluble in water).


 

 

Cleopatra was Queen of Egypt and one of the earliest women chemists. She demonstrated that acids release carbon dioxide from carbonates when she dissolved her pearls (CaCO3) in vinegar. This was one of the most expensive chemical reactions ever carried out by one person.


They are weak electrolytes.

 

Partial ionisation gives rise to few ions in solution, and hence they are poor conductors of electricity.

 

They neutralise alkalis and other bases.

 

Dilute sodium hydroxide solution is neutralised by organic acids.

 

CH3COOH(aq) + NaOH(aq)    →   CH3COONa(aq)  + H2O

sodium acetate

 

They release carbon dioxide from aqueous carbonates and bicarbonates.

 

The hydrogen ions released by organic acids in water react with carbonates and bicarbonates to form carbon dioxide and water, leaving behind a carboxylate salt.

 

RCOOH + H2O  →  RCOO-    + H3O+

carboxylate ion

 

2H3O+ + CO32—CO2 + 3H2O

  


2-


2H3O+ + CO3


→  CO2 + 3H2O


 

For example, acetic acid will form acetate ions and citric acid will form citrate ions. Sodium acetate has the formula CH3COONa.

The release of carbon dioxide on addition of an aqueous carbonate or bicarbonate solution to an acid is used as a test for the presence of an acid.

 

The reaction equations can be written like this:

 

CH3COOH(aq) + NaHCO3    →  CH3COONa + H2O + CO2

 

But it is not the acetic acid molecule that reacts - rather it is the H3O+ or H+ ions.

 

 

Esters

 

Many fruity odours in nature are volatile esters and many artificial flavourings - banana, peach, apricot, pear and rum, to name a few - are esters which are used in cooking, and for ice-cream and milk flavourings.

 

It may surprise you to know that all animal fats and many oils are esters too and without them, many of our hot foods would lack flavour.

 

Liquid esters are also good solvents. Ethyl acetate, used in nail polish remover, is an example. Synthetic vinyl leathers contain esters called alkyl phthalates which keep them supple. Such esters are called plasticisers. These esters are responsible for the 'plasticky' smell of plastics.

 

Esters are formed when an alcohol reacts with an acid.

 

 

 

Esterification

 

Making an ester from an acid and an alcohol is called esterification. The acid may be either organic or inorganic. The word equation is:

 

ACID + ALCOHOL →   ESTER + WATER

 


Not a method recommended for

the wine industry where development of esters enhances the wine flavour!


The reaction is slow even when it is heated in the presence of a catalyst. The catalyst used is a trace of a strong mineral acid such as sulfuric acid.


Esterification is also classified as a condensation reaction because water is squeezed out as a condensate when the two reactant functional groups react together. An example is the preparation of ethyl ethanoate (ethyl acetate).

 

 

The general equation for the preparation of esters is:

 

 

Where R represents an alkyl group or benzene ring, RI is an alkyl group or a benzene ring (or H, if the acid is methanoic).


 

 

Naming Esters

 

Esters are named by a combination of the alcohol and carboxylic acid names from which

 

 

 

they are made.  For example, ethanol and ethanoic acid (acetic acid) form the ester ethyl ethanoate (or ethyl acetate). The ester from propan-1-ol and butanoic acid is called

1-propyl butanoate.

 

 

Hydrolysis of esters

Decomposition of esters to form the parent acid and alcohol is called hydrolysis. The simplest hydrolysis (reaction with water) is acid catalysed and is the reverse of esterification.


Esters have two word names (like salts). They are alkyl alkanoates. The alkyl group is named after the parent acid. All esters made from acetic acid (ethanoic acid) have acetate (or ethanoate) as their second name.


 

Sulfuric acid catalyst

ester + water        →        acid + alcohol

heat

 

The reaction works best by boiling the ester with sodium hydroxide solution. The sodium hydroxide neutralises the organic acid formed and the other product, an alcohol is easily separated from the salt by distillation.

 

e.g.           CH3CH2COOCH3  + NaOH(aq)  →  CH3CH2COONa + CH3OH

methyl propanoate                            sodium propanoate methanol

 

If the ester used is an animal fat or vegetable oil the sodium salts formed are soaps. This soap making reaction is widely used on a commercial scale and is discussed in the chapter on biomolecules.


Biomolecules

 

What are the biomolecules?

 

This is the collective name given to the many different molecules found in living things. Many biomolecules are relatively small e.g. glucose, C6H12O6, while some are very large complex molecules e.g. DNA, deoxyribonucleic acid. All are essential for life.

 

Carbohydrates

 

Carbohydrates occur in all living things and are the major constituents of most plants, making up from 50 to 90% of their dry mass. Starch, cellulose and the sugars are classified as carbohydrates. They are used for food, clothing (linen, cotton, rayon) communication (paper) as well as for fuel and shelter (wood). Historically the term carbohydrate was derived from the molecular formula of glucose, which is C6H12Oand thought to be the hydrate of carbon C6(H2O)6.

 

However, there are no water molecules in glucose and this view was quickly abandoned, but the name has stuck.

 

The most common carbohydrate is also the world's most common organic compound, cellulose. Over half the world's combined carbon is found in this compound. The second most common organic compound is another carbohydrate - starch. Both of these are polymers of glucose.

Photosynthesis makes glucose and oxygen in a complex process where carbon dioxide from the air combines with water, using the sun's energy. This is harvested by green leaf solar collectors which contain the pigment chlorophyll.


 

One is reminded of the well known fairy story of Hansel & Gretel in the witch's gingerbread house wher e  t he  c hildr en  c ould  not di sti ng uish  th e  foo d  fro m  th e furniture.

 

 

 

 

 

This means that the carbohydrate m olec ules  ar e  aldehy des  or ketones with many (poly) hydroxy (-OH) groups and can be very large molecules (polymers).

 

 

 

The three main components of honey are glucose, fructose and sucrose.


 

Much of this glucose is chemically linked in chains to form starch for energy storage or the structural material cellulose, which encloses all plant cells and gives plants support and structure.

 

Carbohydrates are compounds of carbon, hydrogen and oxygen and defined as polyhydroxy aldehydes or ketones, or their polymers. There are three broad classifications:

 

1       Simple sugars or monosaccharides such as glucose or fructose, which cannot be broken down into any smaller sugars. Examples are glucose and fructose - both C6H12O6, and ribose C5H10O5. Most simple sugars contain either 5 or 6 carbon atoms.

2       Double sugars are also called disaccharides. These are made of two linked simple sugar units and the best known is sucrose or ordinary table sugar. It is

one of the most abundant pure chemicals in the world. Its formula is C12H22O11, not

C12H24O12, as on linking glucose and fructose a molecule of water is eliminated.

The reverse process to reconstitute the simple sugars is called hydrolysis.


 


 

3       Complex sugars or polysaccharides have tens, hundreds or thousands of simple sugars chemically linked. The most abundant of these are cellulose and starch and both of these are glucose polymers. Starch itself is actually made of two different glucose polymers, one soluble in cold water and branched; the other unbranched and insoluble in cold water.

 

Glycogen is another many branched glucose polymer and is used as an energy store for animals. It can contain up to 100,000 glucose units. The formulae of all three can be represented as (C6H10O5)n and, like double sugars, can be hydrolysed into simple sugars. In living organisms this is carried out by specific catalysts called enzymes.

 

                                               enzyme

(C6H10O5)n + (n-1)H2O                   nC6H12O6

starch                                  hydrolysis      glucose          

  

 

 

 


                                                         

 

Cellulose is indigestible by humans because we lack the right enzymes, which is why we can digest peas and potatoes but not grass, stems and leaves. Starch is present in almost all plants and is an important constituent of many of our foods. Potatoes contain about 20% starch, 77% water, 1% salts and 2% protein. Wheat contains up to 80% starch.

 

A test for the presence of starch in foods uses iodine. If a dilute iodine solution is added to starch a blue colour forms.

Sugars are most commonly found as 5 or 6 membered ring structures. Some structural formulae are shown below:


Starch granules are insoluble in cold water but in hot water swell and form a paste. This is what happens to starchy foods during cooking.

This diagram for sucrose attempts to show the three-dimensional character of one of the carbon rings. The first ring is in the so called ''chair' form; the second ring is planar. The carbon atoms in the ring are not shown.

 

 

 

 

 

Reactions of carbohydrates

 

Hydrolysis and fermentation are two key processes in bread making and beer-making and in Egypt and Iraq they evolved together. In bread-making one of the products of fermentation is carbon dioxide, which makes the bread rise. In beer and wine making the key product is ethanol. The same fermentation process of the simple sugars, glucose or fructose, provides both.

 

enzyme

C6H12O6              →        2CH3CH2OH + 2CO2

 

 

Fermentation

 

Fermentation is the action of micro-organisms on compounds to produce different compounds, without using oxygen. The micro-organisms produce enzymes to speed up the fermentation. These micro-organisms, which are yeasts or bacteria, feed on the compounds, and the new compounds are waste products of their fermentation.

 

•      Penicillin is made by fermentation.

•      Lactose (milk sugar) can be fermented to form lactic acid which is needed to make cheese.

•      The cocoa bean used to make chocolate is fermented to produce the chocolate flavour.

•      Soy beans are fermented to make soy sauce.

•      Sugars are fermented to give ethanol and carbon dioxide.

 

Sugars are carbohydrates, they contain carbon, hydrogen and oxygen, and have the general formula, Cx(H2O)y.

•      Glucose and fructose have the formula C6H12O6.

•      Maltose and sucrose have the formula C12H22O11.

•      Starch is (C6H10O5)n.

 

Notice that each sugar shown has one, two or many C6  units. The value for n in starches can be more than a million.


 

Sugars are sweet and soluble carbohydrates. Strictly, starch is not a sugar.

 

 

Wine almost makes itself. The crushed grape with its coating of yeast ferments on its own.

 

 

Fermentation you need to know is:

 

C6H12O6(aq) 2CO2(g) + 2C2H5OH(aq)

 Yeasts


 

The role of enzymes - The sequence of converting starch into ethanol is as follows:

It is like cracking a large alkane into smaller ones. However, thermal cracking, as done with alkanes, is not possible. Melting sugar in a saucepan shows this. More heat gradually darkens the sugar. It is said to 'caramelise'. The caramel flavour is due to the heat forming new compounds. The C12 sucrose is not split into two C6 sugars.

Enzymes are needed to do this - not only to direct the breakdown of starch, maltose and sucrose into C6 sugars, but also to speed up these reactions. Enzymes are biological catalysts and each different reaction needs a different enzyme.

 

 

 

 Starch is like a brick wall. The bricks are glucose units. The reactions are:


Don't forget the carbon dioxide!  It helps bread and yeast buns to rise, and when the moist dough dries out in the baking process, the gas cells remain firm and the bread stays risen. The alcohol evaporates almost completely during the bake. It also provides the bubbles in champagne..

 

Conditions

 

It has been said that you can make wine out of any ingredient - even old boots!  This is true provided you have some sugar, acid and yeast in a dilute aqueous solution.

 

Grapes will not grow everywhere and wines from fruits, flowers and roots have been made for centuries. Tinned fruits, dried fruits, dried flowers, dried herbs and even jam, grass, seaweed or grain can be used.

 

Many fruits contain their own sugar and acid. A yeast nutrient such as diammonium phosphate helps too, and warmth is essential.

 

The steps in wine making

 

Grape picking

 

To pick grapes at the right pH is important.

Too much acid gives an undrinkable, vinegary wine. Too little gives a flat uninteresting taste.

The right amount gives wine a fresh, tart taste.

 

Grape Crushing

 

Remove seeds, stems and skins unless the latter are needed for colour (red wine). Grape juice is called 'must'.

 

Killing wild yeasts with sulfur dioxide

 

Wild yeasts frequently lead to slow fermentations, as they do not convert all the sugar to alcohol. Odd  and/or unpleasant odours and flavours are produced.

 

Fermentation of the 'must'

 

For red wine 24 - 26.5°C.

For white wines 10 - 15.5°C (the lower temperatures improve the yield of fruity esters). Fermentation is exothermic and excess heat must be removed using cooling jackets. At about 40°C, yeasts are killed and it is difficult to restart a fermentation.

 

Allow the 'lees' (sediments) to settle

 

Racking in stainless steel storage tanks

 

Many wines are aged in oak barrels. The tannin from the wood adds flavour.

 

Bottled and corked

 

Fining agents are added to remove insoluble particles or a centrifuge is used.

 

Fractional distillation of aqueous alcohol

 

Both fermentation of simple sugars and hydration of ethene produce very dilute ethanol. Wine is nearly nine-tenths water!

 

Unless wine is carefully stored it can be subjected to bacterial attack by vinegar- forming bacteria. Concentrated alcoholic solutions are used as antiseptics and preservatives.  Nothing can live in them. Fractional distillation separates the more volatile ethanol (b  78.3°C/1atm) from the water.

Distillation has been described as the art of turning a liquid into vapour by heat and condensing this vapour by cooling and collecting the condensate (called the distillate), in a separate part of the apparatus.


Did  you  realise  t hat  in  yeast bakery, fermentation produces ethanol and carbon dioxide? What happens to the alcohol?

 

 

 

 

The science of wine making is called oenology.

 

Louis Pasteur discovered that the presence  of  living  yeast  was necessary to turn grape juice into wine  (1857)  and  that  bacteria cause wine to turn into vinegar (1866).

 

 

Wine is made from apples, apricots, beetroot, celery, seaweed, dates figs, pears, peaches, honey, parsnip, quince, rhubarb, rice, honey, potatoes and of course grapes as well as many other fruits and vegetables. Where the sugar content is low, raisins are often added.

 

 The acids in wine are tartaric, acetic, malic, succinic and lactic.