19.0     INTRODUCTION TO ORGANIC CHEMISTRY

Organic chemistry can be defined as the chemistry of carbon compounds. Other than the main elements, carbon, organic compounds may contain hydrogen, oxygen and sometimes nitrogen, halogens, phosphorus and sulphur.  Carbon’s tetravalency enables it to form diverse structures, including chains and rings, creating many organic compounds.

• Organic compounds are covalent in nature.
• Organic compounds have low melting and boiling points.
• Most organic compounds are insoluble in water and polar solvents.

HOMOLOGOUS SERIES – It is a family of organic compounds with a regular structural pattern, in which each successive member differs in its molecular formula by a -CH₂– group.

FUNCTIONAL GROUP – It is an atom, a radical or a bond common to a homologous series. This functional group determines the main chemical properties of the series.

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            19.1     ISOMERISM

Isomerism can be defined as the existence of two or more compounds with the same molecular formula but different molecular structure. These compounds are called isomers. They could rise from:

• Branching.
• Existence as a member of different homologous series.
• Positioning of carbon-carbon multiple bonds.

There are three types of isomerism – Structural, Geometric and Optical isomerism.

STRUCTURAL ISOMERISM – In this, compounds with the same molecular formula have different structural arrangements. This isomerism is exhibited by the alkanes. The number of possible structures in which different carbon atoms can link to each other increases with the increase in number of carbon atoms in the alkane molecules. Hence, the number of isomers of alkanes increases with the increase in the number of carbon atoms.

GEOMETRIC ISOMERISM – This is the existence of compounds with the same molecular formula but are not identical because of the difference in spacial arrangement of the compound atoms. Geometric isomers have different orientation in space, giving rise to cis- and trans- forms. This type of isomerism is common among alkenes.

OPTICAL ISOMERISM – This is the existence of two or more compounds with the same molecular formula but different configurations, and because of molecular asymmetry they rotate plane polarized light. These isomers are said to be optically active.

            19.2     CLASSIFICATION OF ORGANIC COMPOUNDS

• ALIPHATIC COMPOUNDS – Molecules are composed of chains of carbon atoms. It is divided into:
  • Acyclic compounds – A straight or branched chain aliphatic compound e.g pentane, 2-methlybutane etc.
  • Cyclic compounds – A ring chain aliphatic compound e.g cyclopropane, cyclohexane etc.
• AROMATIC COMPOUNDS – Includes benzene and its derivatives.

            19.3     ALKANES

• These are saturated hydrocarbons with molecular formula, C_nH_{2n + 2}, where n ≥ 1.
• Alkanes consist of single bonds between carbon atoms, -C-C-.
• Alkanes exhibit structural isomerism, where compounds with the same molecular formula have different structural arrangements. For example, butane (C₄H₁₀) has two isomers: n-butane and isobutane.

• Alkanes show gradual changes in physical properties as the chain length increases e.g alkanes with one to four carbon atoms are gaseous, while others are liquid and soft solids.
• They are slightly soluble in water.
• They undergo substitution and combustion reaction. E.g in substitution reaction,

Methane (CH₄) + Cl₂ → Chloromethane (CH₃Cl) + HCl

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THE FISRT TEN ALKANES

            19.4     ALKENES

• These are unsaturated hydrocarbons with molecular formula, C_nH_2n, where n ≥ 2.
• They contain double carbon to carbon bond in their molecular structure, -C=C-.
• Combustion of alkenes give a more luminous and smoky flame compared to combustion of alkanes.
• Alkenes readily undergo addition reactions, due to the existence of double bonds. For example, the reaction between ethene and bromine:

Ethene (C₂H₄) + Br₂ → C₂H₄Br₂ (1,2-dibromoethane)

• Alkenes like ethene (C₂H₄) polymerize to form polymers like polythene, used in plastics.
• Alkenes show structural and geometric (cis-trans) isomerism due to the restricted rotation around the double bond. For example, Butene (C₄H₈) can exist as 1-butene, 2-butene (cis and trans).

HOMOLOGOUS SERIES OF THE ALKENES

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            19.5     ALKYL GROUP

• The general term alkyl group includes all groups derived from the alkanes by the loss of a hydrogen atom.

They have the general formula, C_nH_{2n + 1}, where n ≥ 1; and named after the parent alkanes by replacing the ending -ane by -yl.

19.6     ALKYNES

• These are unsaturated hydrocarbons with the molecular formula, C_n­H_{2n – 2}, where n ≥ 2. They have a higher degree of unsaturation than the alkenes.
• The alkyne molecule contains a carbon-carbon triple bond, i.e. -C ≡ C-. This triple bond allows for addition reaction. For example, ethyne undergoes addition reactions similar to alkenes, reacting with halogens and hydrogen.
• Ethyne (acetylene) is produced by the reaction of calcium carbide with water:

CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂

• Ethyne, C₂H₂, is used to ripen the fruits such as banana, mango etc. It is also used along with oxygen in oxy-acetylene torch which is used for welding purposes.

19.7     AROMATIC COMPOUNDS

• The most well-known aromatic compound is benzene, C₆H₆.
• Benzene has a cyclic structure with alternating double bonds (delocalized π-electrons).
• It is highly stable due to resonance and undergoes substitution rather than addition reactions.
• Benzene can be found in coal tar (a product of the destructive distillation of coal). It can also be prepared by a process known as catalytic reforming or polymerization of ethyne.
• Benzene is a colorless, volatile liquid with a sweet odor, nonpolar, flammable, insoluble in water, and toxic.
• Benzene is used in the production of plastics, dyes, detergents, and as a solvent.

The structure of benzene is best represented as:

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19.8     ALKANOLS

• Alkanols are represented by the molecular formula C_nH_{2n + 1}OH where n ≥ 1.
• Alkanols (alcohols) contain the -OH (hydroxyl) functional group; responsible for chemical properties.
• TYPES OF ALKANOL -There are three types of alkanol. They are:
  • Monohydric alkanol contains only one hydroxyl group in each molecule.
  • Dihydric alkanol contains two hydroxyl group in each molecule e.g. ethane-1,2-diol.
  • Polyhydric alkanol contains three (trihydric alkanol) or more hydroxyl group in each molecule e.g. propane-1,2,3-triol.
• CLASSIFICATIONS OF ALKANOL – There are three classes of alkanols which are:
  • Primary alkanol – Has only one alkyl group (represented by R) attached to the carbon atom that carries the hydroxyl group (R-CH-2OH).
  • Secondary alkanol – Has two alkyl group (both represented by R) attached to the carbon atom that carries the hydroxyl group (R-CR’H-OH).

Tertiary alkanol – Has three or more alkyl group (represented by R) attached to the carbon atom that carries the hydroxyl group (R-CRR”-OH).

• Methanol, CH₃OH, is the first member of the alkanol series.
• Ethanol, C₂H₅OH, the second and most important member of the alkanols, is a colourless volatile liquid which is soluble in water.
• Ethanol can be produced by fermentation of sugars or from petroleum by-products. Local examples include the fermentation of palm wine to produce gin.
• Primary alkanols oxidize to form aldehydes, secondary alkanols oxidize to form ketones, and tertiary alkanols resist oxidation.
• Lucas Test is used to distinguish between primary, secondary, and tertiary alkanols based on their reactivity with Lucas reagent (HCl + ZnCl₂).

• TEST FOR ALKANOLS
  • It liberates hydrogen gas on reaction with sodium metal.
  • It changes to alkanoic acid on exposure to air / reaction with oxidizing agent.

THE ALKANOL SERIES

• Alkanols have higher boiling and melting points due to hydrogen bondings.
• Alkanols can undergo oxidation, esterification and dehydration.

19.9     ALKANALS AND ALKONONES

• Alkanals (aldehydes) have the general molecular formula RCHO, while alkanones (ketones) have the general formula RCOR’.
• They both contain the carbonyl group (-C=O-).
• Alkanals are prepared by oxidizing primary alkanols, using KMnO4 or K2Cr2O7.
• Alkanones are prepared by the oxidation of secondary alkanols, using KMnO4 or K2Cr2O7.
• Alkanals are readily oxidized, while alkanones are not.
• Alkanals and alkanones are reduced to primary and secondary alkanols respectively.
• Alkanals are oxidized by Fehling’s solution while alkanones are not.
• Alkanals react with Tollens’ reagent (silver mirror test), reducing it to metallic silver; while alkanones do not.

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19.10   ALKANOIC ACIDS

Alkanoic acids have carboxylic group as their functional group i.e –COOH.They are also known as organic/carboxylic acid. The general molecular formular is C_nH_{2n + 1}COOH. They occur in fats and oils. They are pepared in the laboratory by the oxidation of primary alkanols with excess acidified potassium tetraoxomanganate (vii). The name of any alkanoic acid ends with –ioc.

THE STRUCTURE OF ALKANOIC ACIDS

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19.11   PROPERTIES OF ALKANOIC ACIDS

PHYSICAL PROPERTIES

• Alkanoic acids are very soluble in water; the solubility of alkanoic acid in water decreases as the carbon atom increases (i.e. lower members are more soluble in water).
• They have high boiling and melting points; increases as the carbon chain increases.
• As acid, they turn blue litmus paper red.

CHEMICAL PROPERTIES

• Alkanoic acids react with active metals to liberate hydrogen gas.
• Alkanoic acids react with bases to form salt and water.
• Alkanoic acid liberate carbon(iv) oxide from metallic trioxocarbonate (iv) or hydrogentrioxocarbonate (iv).
• Alkanoic acid reacts reversibly with alkanol in the presence of tetraoxosulphate (vi) acid to form alkanoate (ester). On heating the mixture, a fruity of fragrance aroma is observed. Tetraoxosulphate (vi) acid serves as catalyst. This reaction is called esterification.
• Alkanoic acids are reduced to alkanol by lithium tetrahydridoaluminate (iii) (LiAlH₄).

TEST FOR ALKANOIC ACIDS – Addition of the acid to saturated solution of sodium hydrogentrioxocarbonate (iv) produces effervescence. The gas is colourless and odourless and turns lime water milky.
FIRST TEN MEMBERS OF THE ALKANOIC ACID

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19.12   USES OF ALKANOIC ACIDS

• They make up a series of fatty acids which are extremely good for human health.
• In pharmaceutical industry organic acids are used in making drugs such as aspirin, phenacetin e.t.c.
• Longer-chain alkanoic acids like palmitic acid and stearic acid are used in soap and detergent production.
• Acetic acid is used in producing vinyl acetate, a key material in making plastics.
• Butanoic acid gives a buttery flavor and is used in food flavoring and artificial fragrances.
• Some alkanoic acids are used as herbicides to control weeds, particularly in organic farming.

19.13   ALKANOATES / ESTERS

Alkanoates, also known as esters, are found widely in nature. Short carbon chain simple alkanoates exist as liquids and have a characteristic pleasant smell. They occur in essential oils, many fruits and flowers and sometimes called fruit essence because of the pleasant odours. The general molecular formula is C_nH_{2n + 1}COOC_nH_{2n + 1}.

The structural formula is:

PREPARATION – Alkanoates are prepared by esterification reaction between alkanoic acid and alkanols in the presence of concentrated tetraoxosulphate (vi) acid.

For example:

• Ethanoic acid reacts with ethanol to give ethyl ethanoate.
• Ethyl propanoate is prepared by the reaction between ehanol and propanoic acid.
• Propyl ethanoate is prepared by the reaction of propan-1-ol and ethanoic acid.

19.14   PROPERTIES OF ALKANOIC ACIDS

PHYSICAL PROPERTIES

• Short carbon chain alkanoate have fruity smell; those found in waxes do not have fruity smell.
• Lower members are slightly soluble in water; readily dissolves in organic solvents such as ethanol, benzene e.t.c.
• Their boiling points are lower than those of the corresponding alkanoic acid and even alkanols because they do not have hydrogen bonds.

CHEMICAL PROPERTIES

They are hydrolyzed by water to give their component acid and alkanol.

• Alkaline hydrolysis of alkanoate gives salt of the acid and alkanol.
• They react with ammonia to give amide and alkanol in a process known as ammonolysis.

19.15   USES OF ALKANOATES

• They are used as flavor additives in food products (e.g., fruit-flavored sweets).
• Used in nail polish removers, paints, and coatings due to their effectiveness in dissolving other compounds.
• Used in making synthetic polymers and resins for various industrial applications.
• Alkanoates derived from oils are converted to biodiesel for renewable energy.
• Ethyl ethanoate is used as solvent for cellulose nitrate.

19.16   FAT AND OILS

Fats and oils occur naturally in living things. They are referred to as naturally occurring esters. There are two main sources:

• Animal source – Fish oil, cod-liver oil, butter, land, fallow etc.
• Plant source – Cocoa butter, fat, palm oil, coconut oil, cotton seed oil, olive oil, corn oil, ground nut oil, sunflower seed oil, olive oil, corn oil and soya bean oil. They are called vegetable oils.

PROPERTIES OF FATS AND OIL

• Pure fats and oils are coloured or white and almost odourless.
• Fats and oils do not have sharp boiling or melting points because they are complex mixtures of two or more different triglycerides (esters).
• They are insoluble in water but soluble in organic solvents.
• Fats and oils can undergo hydrolysis, a chemical reaction where they react with water in the presence of an acid or base to break down into glycerol and fatty acids.
• Under certain conditions, fatty acids in fats and oils can undergo polymerization, forming polymers.
• Isomerization reactions can occur in unsaturated fatty acids, leading to the conversion of cis-isomers to trans-isomers.

HYDROGENATION OF OILS

Natural oils are esters of unsaturated fatty acids. They exist as liquids, which are not edible. To make them edible, they are usually hydrogenated so as to make them hard and obtain saturated edible fats. Margarine is obtained from a refine mixture of animal fats and vegetable oils which have been treated with activated charcoal to remove the colour and odour and carefully neutralize the free acid. The mixture is partially hydrogenated by bubbling hydrogen gas through it at about 2000C and within 2-5 atmospheres using finely divided nickel as the catalyst, until the product is fairly hard. The hardened oils are then mixed with salt, vitamins skimmed milk and various fats to form margarine.

ACID HYDROLYSIS OF FATS AND OILS

When natural fats or oils are boiled with water in the presence of concentrated H₂SO₄, the products obtained are propane-1,2,3-triol (a trihydric alkanol, glycerol) and a mixture of saturated and unsaturated fatty acids which can be separated by fractional distillation.

USES OF FATS AND OILS

• Fats and oils are widely distributed in foods; provide nutrients for body building and are oxidized in the body to provide energy for metabolic processes, which is accompanied by the release of CO₂.
• Commercially used in the production of soaps, propanetriol (glycerol) and margarine.
• As solvents for paints and manufacture of candle vanishes etc.
• Fats play a crucial role in the body, including insulation, protection of organs, and absorption of fat-soluble vitamins.
• They are used in cooking for frying, baking, and as ingredients in various dishes.

19.17   SOAPS AND DETERGENTS

SAPONIFICATION

This is the reaction between fats or oils and caustic soda / alkali (a solution of sodium or potassium hydroxide), to yield the corresponding salt of the fatty acids called soap and a by-product propane-1,2,3-triol. This process can also be referred to as alkaline hydrolysis of esters or an alkanoate.

Soaps are saponification products of fats and oils; usually referred to as soapy detergents.

PRODUCTION OF SOAP

• A mixture of vegetable oil (palm kernel oil) and sodium hydroxide solution is boiled and agitated by passing steam until it is homogenous.
• Saturated solution of sodium chloride (brine) is then added to the slightly cooked mixture, in order to salt out or separate the soap and from the liquid glycerol.
• The soap cord obtained is then slightly dried and mixed thoroughly with perfumes and covalent.

Again, the soap is slightly dried, then pressed before being cut into tablets or bar.

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ACTION OF SOAP AS AN EMULSIFIER

When an oil-water mixture is vigorously shaken, oil forms tiny droplets suspended in water, creating an unstable oil-water emulsion. Over time, the droplets will coalesce, reforming the oil layer. However, adding soap stabilizes the emulsion. Soap molecules surround each oil droplet, with their ionic heads extending into the water. This creates electronic repulsion between droplets, preventing them from merging. Thus, soap acts as an emulsifying agent, maintaining a stable oil-in-water emulsion.

CLEANING ACTION OF SOAP

Soap cleans by surrounding grease or dirt with its molecules. The soap’s hydrophobic (water-repelling) ends attach to oils, while the hydrophilic (water-attracting) ends face outward. This forms tiny droplets that can be rinsed away with water, lifting away grease and dirt effectively, leaving surfaces or fabrics clean.

           19.18     DETERGENT

Detergents are substances that have the ability to clean an object. These include soap, soap powders, dish washing liquids as well as water. There are two types of detergent which are:

• Soapy detergents – Saponification products of fats and oil i.e. the sodium salts of fatty acids.
• Soapless detergents – They are synthesized from petroleum products.

A detergent molecule has a hydrophobic tail and a hydrophilic head. The high solubility of soapless detergents in water is due to the SO^-3 Na⁺ group that is present in the molecules. The cleansing action of a detergent is the same as that soap.

STRUCTURE OF DETERGENT

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DIFFERENCES BETWEEN SOAP AND DETERGENT

                              19.19         AMINES AND AMIDES

• An amine is an organic compound relatd to ammonia that contains a nitrogen atom bounded to one or more alkyls group on each molecule.
• An amide is an organic compound that contains carbonyl group bounded to a nitrogen atom.
• Amines have the general molecular formula, RNH₂ and a functional group of NH_2­.
• CLASSIFICATIONS OF AMINE
  • Primary amine – Has one alkyl group.
  • Secondary amine – Has two alkyl groups.
  • Tertiary amine – Has three alkyl groups.
• Amines are gases or liquids with a fishy odour; it is soluble in water.
• Amines are basic in nature; they neutralize acids.
• Amines are used in making nylons and polyamides.
• Amides have the general molecular formula, RCONH₂ and a functional group of CONH₂.
• Amides are prepared by:
  • Reacting acids (e.g ethanoic acids) or esters with ammonia.
  • Removing a molecule of water from ammonium salt of carboxylic x acids.
• Amides have high melting and boiling points.
• Amides exist in solids except methanamide, which is a liquid.
• Amides can be hydrolyzed in the presence of alkali and mineral acids.
• Amides react with aqueous NaOH or bromine to give the corresponding amines, RNH_2­.

Amides are used in making synthetic resins and plastics, fertilizers and preparation of amines.

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         19.20      CARBOHYDRATES

Carbohydrates are organic compounds of carbon, hydrogen and oxygen in which the ratio of atoms of hydrogen and oxygen is 2:1. The general molecular formula of carbohydrate is C_x(H₂O)_y or C_xH_2yO_y.

SIMPLE AND COMPLEX SUGARS

• Simple carbohydrates consist of sugar molecules, which are bound together in long, complex chains. Foods such as peas, beans, whole grains, and vegetables contain complex carbohydrates. Within the body, both simple and complex carbohydrates are converted into glucose (blood sugar) and used as energy.
• Complex carbohydrates contain longer sugar molecular chains than mere carbohydrates. Since complex carbohydrates have longer chains, they take longer than simple carbohydrates to break down and provide more lasting energy in the body.

CLASSIFICATIONS – Carbohydrates can be classified into:

• MONOSACCHARIDES: These are simple sugars. Examples are glucose and fructose. They cannot be split into simple molecules by acid hydrolysis.
• DISACCHARIDES: They are also sugars e.g. sucrose, maltose, and lactose. They can be split into two molecules of monosaccharide by acid hydrolysis.
• POLYSACCAHRIDES: They are also known as complex sugars. They are non-crystalline, insoluble and tasteless substance e.g. starch and cellulose. They can be hydrolyzed to give many molecules of the same or different types of monosaccharide. Other examples are glycogen (animal starch), inulin, dextrin.

STARCH – Starch is composed of thousands of repeating glucose units linked together to form a giant branched molecule. It occurs naturally in most plants as a stored carbohydrates e.g. maize (corn), cassava, yam, cocoyam, potatoes, rice, wheat, barley and other cereals.

CELLULOSE – it is composed of glucose units which are linked in a slightly different way from that in starch. It is found in cotton wool, hemp, jute, flax, sisal and straw.

REDUCING AND NON-REDUCING SUGAR – Sugar can be divided into reducing sugars and non-reducing sugars. Reducing sugars can also be subdivided into monosaccharide e.g. glucose, fructose, galactose, mannose, etc. and disaccharides e.g. maltose, lactose etc. non-reducing sugars are disaccharides of sucrose.

CHEMICAL TEST FOR CARBOHYDRATES

• Benedict’s test – Identifies reducing sugars (like glucose and fructose). When Benedict’s solution is added to the food sample and heated, an orange or red colour is observed; indicating a positive result.
• Molisch’s test – Using Molisch’s reagent, the food sample gives a purple or violet ring confirming the presence of carbohydrate.
• Fehling’s test – Using Fehling’s reagent, the food sample gives a red precipitate indicating the presence of reducing sugars.
• Tollen’s test – Using Tollen’s reagent the food sample gives a silver mirror, confirming the presence of carbohydrates.
• Iodine test – using this chemical test, when a food sample is reacted with iodine sample food; The yellow-orange iodine will turn blue-black as it reacts with starch.

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DEHYDRATION REACTION OF SIMPLE SUGARS – When concentrated H₂SO₄ is added to simple sugars like glucose or sucrose, it acts as a strong dehydrating agent, removing water molecules and forming carbon. This reaction causes the sugar to turn black, releasing heat and producing a charring effect. This reaction is exothermic and needs careful handling as it releases sulfur dioxide (SO₂) gas.

HYDROLYSIS OF COMPLEX SUGAR

• Starch: Starch (from sources like cassava) can be hydrolyzed by heating with acid or using enzymes like amylase, producing glucose or maltose.
• Cellulose: Cellulose (e.g., from cotton) is tougher to hydrolyze and requires strong acid treatment or enzymatic breakdown to yield glucose.

USES OF SUGAR AND STARCH

• Both simple sugars and complex carbohydrates provide energy to the body, though complex carbs release it more slowly.
• Starch (a carbohydrate) is used in textiles for stiffening fabric and in paper for improving strength, similar to some industrial uses of sugars.
• Sugars serve as fillers, sweeteners, or binders in pharmaceutical tablets.
• Starches are also used as disintegrants and fillers in the pharmaceutical industry.
• Sugar aids wound healing by drawing moisture away from bacteria, preventing infection.
• They are both used in biofuel production.
• Both sugars and starches (a complex carbohydrate) are widely used in food and beverage industries as sweeteners, thickeners, and texture enhancers.
• Hydrolyzed starches are fermented by yeast to produce ethanol in alcoholic beverages.

19.21   PROTEINS

Proteins are large biomolecules composed of amino acid chains. They play crucial roles in various biological processes, serving as structural components, enzymes, antibodies, and more. Proteins are essential for the structure, function, and regulation of tissues and organs in the human body.

Proteins have their general molecular formula as NH₂-CHR-COOH.

The structure of a protein is hierarchically organized:

• Primary Structure – The sequence of amino acids in a polypeptide chain.
• Secondary Structure – Localized folding patterns, like alpha helices or beta sheets.
• Tertiary Structure – Overall three-dimensional shape of a single polypeptide chain.

Quaternary Structure – Arrangement of multiple polypeptide chains in a functional protein.

PROPERTIES OF PROTEIN

Proteins are large, complex molecules with crucial roles in the body. They have diverse functions and exhibit various properties:

• Proteins have a unique three-dimensional structure determined by their amino acid sequence. This structure is essential for their function.
• Proteins are made up of amino acids linked together by peptide bonds. The sequence of these amino acids dictates the protein’s properties and function.
• Proteins serve numerous functions, such as enzymes catalyzing biochemical reactions, antibodies defending against infections, and structural proteins providing support to cells and tissues.
• Exposure to heat, pH extremes, or certain chemicals can alter a protein’s structure, causing denaturation and loss of function.
• Proteins often exhibit specificity in their interactions, recognizing and binding to specific molecules, like substrates for enzymes or antigens for antibodies.
• Proteins can vary in solubility, with some being water-soluble and others lipid-soluble. This property influences their distribution and function in the body.
• Enzymes, a type of protein, act as biological catalysts, speeding up chemical reactions in living organisms without being consumed in the process.

TEST FOR PROTEIN

• Biuret Test – If copper (II) tetraoxosulphate (VI) is added to a solution of a protein and the resulting solution made alkaline with sodium hydroxide solution, a violet colour develops.
• Million’s Reagent Test – Add a drop or two of million’s reagent to some egg-white solution in a test tube. The formation of a white precipitate which turns brick red on heating indicates the presence of protein.
• Trioxonitrate (V) acid Test – Add three or four drops of concentrated trioxonitrate (V) acid to 2 cm³ of egg-white solution. The formation of an intense yellow colour indicates the presence of proteins.
• Ninhydrin Test – A sample (protein or amino acid solution) is treated with a few drops of Ninhydrin solution and then heated. The reaction produces a blue or purple color, known as Ruhemann’s purple, indicating the presence of free amino acids. However, for proline (an amino acid with a secondary amino group), a yellow color develops instead of purple.
• Xanthoproteic Test – Add concentrated nitric acid to the protein solution and heat it gently. After cooling, a few drops of an alkali (like sodium hydroxide) are added to the solution. The formation of a yellow color (which turns orange upon adding the alkali) indicates the presence of aromatic amino acids. This reaction occurs because nitric acid nitrates the aromatic rings of these amino acids.

USES OF PROTEIN

Proteins play crucial roles in the body, serving various functions such as:

• Essential for the growth and repair of tissues, including muscles, skin, and organs.
• Act as catalysts for biochemical reactions, facilitating processes like digestion and metabolism.
• Regulate various physiological processes, including insulin for glucose regulation and growth hormone for development.
• Carry important molecules, such as oxygen by hemoglobin in red blood cells.
• Antibodies, a type of protein, help defend the body against pathogens.
• Contribute to the structure of cells, tissues, and organs, maintaining their integrity.
• Essential for communication between nerve cells, influencing mood, cognition, and other brain functions.

19.22   POLYMERS

Polymers are large molecules composed of repeating structural units called monomers. These macromolecules can be found in various forms, ranging from natural biopolymers like proteins and DNA to synthetic polymers used in plastics, fibres and coatings.

CLASSIFICATIONS OF POLYMERS

• BASED ON SOURCE

• Natural Polymers – These are found in nature and are often derived from biological sources. Examples include proteins, DNA, cellulose, starch, and natural rubber (obtained from the latex of a rubber tree).
  • Synthetic Polymers – These are human-made and can be further divided into two categories – addition polymers (e.g polyethene) and condensation polymers (e.g nylon).

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• BASED ON STRUCTURE

• Linear Polymers – Have a linear chain structure without any branches, such as polyethene.
  • Branched Polymers – Contain side branches along the main chain, which affect their properties and processing behaviour.
  • Crosslinked Polymers – Form a three-dimensional network through covalent bonds, leading to a more rigid and insoluble material, like vulcanised rubber.

19.23   POLYMERIZATION

Polymerization is a process whereby two or more simple molecules (monomers) link together to form a much larger molecule (polymer). It is divided into two:

• Addition Polymerization – In this process, two or more of the same monomers are linked together to form the polymer, without elimination of small molecules. Polyethene, poly(chloroethene) and poly(phenylethene) are examples of such polymers.
• Condensation Polymerization – In this process, two or more of the same monomers are linked together to form the polymer, with the elimination of small molecules such as water or ammonia. Synthetic fibres like nylon and terylene are examples of such polymers.

19.24   THERMOPLASTIC AND THERMOSETS

• Thermoplastics are type of synthetic materials which can be heated and remolded to any shape e.g. nylon, polythene, polypropene, Perspex etc. They have linear or branched polymer chains that do not cross-link when heated. They are used in packaging, containers, household goods, and construction materials.
• Thermosets, on the other hands cannot be softened or melted by heat and remolded once they are formed e.g. urea-methanal, bakelite. They have cross-linked polymer chains that create a rigid structure. They are used in electrical insulation, adhesives, cookware handles, and automotive components.

19.25   PROPERTIES AND APPLICATIONS OF POLYMERS

• Polymers can be flexible, elastic, or rigid, depending on their structure and cross-linking. This makes them suitable for use in various products, such as plastics, rubbers, and fibres.
• Polymers can have varying melting and glass transition temperatures, which affect their processing and application in different temperature conditions.
• Some polymers are excellent insulators, while others can conduct electricity, making them useful in electronic and electrical applications.
• Some polymers, particularly certain biopolymers and certain synthetic polymers, are biodegradable, making them environmentally friendly.
• Polymers are used in countless applications, including packaging materials, textiles, medical devices, automotive components, adhesives, and coatings.

RESINS – This is obtained from the rubber tree. The fluid obtained from the tree can be heated and changed to elastic solid known as rubber. The rubber consists of 2-methyl but-1, 3-diene monomers known as isoprene.

VULCANIZATION – This is the process of heating natural rubber with sulphur to give rubber a greater tensile, strength, durability and elasticity over a wide range of temperature.

SYNTHETIC RUBBER – Examples of synthetic rubbers are poly 2-chlorobuta -1,3diene, styrenebutadiene rubber (SBR), poly bute-1,3-diene and poly 2-methyl propene.