General Science Supplement for Forest Beat Officer · Chapter 6

Carbon Compounds, Polymers and Soaps

What to remember

Carbon forms millions of compounds because it is tetravalent and shows catenation (it bonds to itself in chains and rings). It makes covalent bonds by sharing electrons.
Hydrocarbons are classified as alkanes (single bonds), alkenes (double bond) and alkynes (triple bond). Their names follow the number of carbon atoms.
Soaps are sodium or potassium salts of long-chain fatty acids; polymers are very large molecules built from repeating units.

Bonding in carbon compounds

Carbon has atomic number 6 and four valence electrons. It cannot easily lose or gain four electrons, so it shares them. Covalent compounds usually have low melting and boiling points and do not conduct electricity because they have no ions.

Catenation: the ability of carbon atoms to join in long chains, branched chains and rings.
Tetravalency: each carbon forms four bonds.
Isomers are compounds with the same molecular formula but different structures. Butane (C₄H₁₀) has two isomers: straight-chain butane and isobutane.
Allotropes of carbon: diamond, graphite, fullerene. Each has a different arrangement of the same atoms.

Hydrocarbons and homologous series

A homologous series is a family of compounds with the same functional group and similar chemical properties. Each member differs from the next by one CH₂ unit (mass 14).

Type	General formula	Bond	First members
Alkane	CₙH₂ₙ₊₂	Single	Methane CH₄, ethane C₂H₆, propane C₃H₈
Alkene	CₙH₂ₙ	One double bond	Ethene C₂H₄, propene C₃H₆
Alkyne	CₙH₂ₙ₋₂	One triple bond	Ethyne C₂H₂, propyne C₃H₄

Prefixes for the carbon count: meth (1), eth (2), prop (3), but (4), pent (5), hex (6), hept (7), oct (8), non (9), dec (10). Alkanes are saturated; alkenes and alkynes are unsaturated.

Functional groups: alcohol (–OH), aldehyde (–CHO), ketone (>C=O), carboxylic acid (–COOH), halo (–Cl, –Br).

Compound	Formula	Group
Methanol	CH₃OH	Alcohol
Ethanol	C₂H₅OH	Alcohol
Ethanal	CH₃CHO	Aldehyde
Propanone (acetone)	CH₃COCH₃	Ketone
Ethanoic acid (acetic acid)	CH₃COOH	Carboxylic acid

Reactions of carbon compounds

Combustion: carbon compounds burn in oxygen to give CO₂, water, heat and light. Saturated hydrocarbons burn with a clean blue flame. Unsaturated ones burn with a yellow, sooty flame. Incomplete combustion gives carbon monoxide, which is poisonous.
Oxidation: ethanol is oxidised to ethanoic acid by alkaline potassium permanganate or acidified potassium dichromate.
Addition reaction: unsaturated hydrocarbons add hydrogen in the presence of a nickel or palladium catalyst. This is hydrogenation. It converts vegetable oils to solid fats (vanaspati).
Substitution reaction: saturated hydrocarbons react with chlorine in sunlight, replacing hydrogen by chlorine. CH₄ + Cl₂ → CH₃Cl + HCl.
Esterification: an acid and an alcohol react with a little concentrated sulphuric acid to form an ester, which has a sweet, fruity smell. CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O.
Saponification: an ester reacts with a base such as NaOH to give the alcohol and the sodium salt of the acid, which is soap.

Ethanol and ethanoic acid

Ethanol is a colourless liquid, soluble in water, used as a solvent, in medicines and as a fuel additive. Sodium reacts with ethanol to give hydrogen and sodium ethoxide. Concentrated sulphuric acid on heating at about 443 K removes water from ethanol to give ethene. Consuming impure (denatured) alcohol is dangerous; methanol is poisonous. Denatured alcohol is ethanol made unfit to drink by adding methanol or a dye.

Ethanoic acid is a weak acid. A 5 to 8 percent solution in water is called vinegar. Pure ethanoic acid freezes at about 290 K, so in cold weather it forms ice-like crystals and is called glacial acetic acid. It reacts with sodium carbonate and sodium hydrogen carbonate to release carbon dioxide, which turns lime water milky. It is a test for carboxylic acids.

Soaps and detergents

Soap is the sodium or potassium salt of a long-chain fatty acid, made from oils or fats and an alkali like NaOH. A soap molecule has a hydrophilic head (water-loving, ionic) and a hydrophobic tail (water-hating, carbon chain). In water, soap molecules form micelles: tails point inward around the oil or dirt, heads point outward into water. The micelle holds the dirt in suspension so it can be rinsed away.

Hard water contains calcium and magnesium salts. Soap reacts with these to form an insoluble, sticky scum, wasting soap.
Detergents are usually sodium salts of sulphonic acids or ammonium salts with long carbon chains. They do not form scum in hard water and work in both hard and soft water.
Detergents with highly branched chains are not easily broken down by microbes. They persist in water bodies and cause foam and water pollution. Straight-chain detergents are biodegradable.

Polymers

A polymer is a very large molecule made by joining many small molecules called monomers. The process is polymerisation.

Addition polymers form from monomers with double bonds without loss of any molecule. Examples: polythene from ethene, PVC from vinyl chloride, polystyrene, PTFE (Teflon), polypropylene.
Condensation polymers form with the loss of a small molecule such as water. Examples: nylon, polyester (terylene), Bakelite.
Natural polymers: starch, cellulose, proteins, natural rubber, DNA.

Polymer	Monomer	Use
Polythene	Ethene	Bags, films, pipes
PVC	Vinyl chloride	Pipes, cable insulation
Teflon (PTFE)	Tetrafluoroethene	Non-stick coating
Nylon	Diamine and diacid	Fibres, ropes, fishing nets
Bakelite	Phenol and formaldehyde	Electrical switches, handles
Natural rubber	Isoprene	Tyres (after vulcanisation)

Thermoplastics soften on heating and can be remoulded (polythene, PVC). Thermosetting plastics harden permanently on first heating and cannot be softened again (Bakelite, melamine). Most plastics are not biodegradable. They harm soil, rivers and animals. Cloth bags, paper and biodegradable plastics are better; follow reduce, reuse, recycle. Plastics that are burned in the open release toxic gases.

More on common carbon compounds

Methane is the simplest hydrocarbon. It is the chief part of natural gas and biogas and is a greenhouse gas. Ethene is a plant hormone that helps fruits to ripen, and it is the monomer of polythene. Ethyne (acetylene) burns with a very hot flame in oxygen and is used in welding and cutting metals.
Naming rule: the name has a root (carbon count) and a suffix. Use "-ane" for single bonds, "-ene" for a double bond, "-yne" for a triple bond, "-ol" for alcohol, "-al" for aldehyde, "-one" for ketone and "-oic acid" for carboxylic acid.
Cycloalkanes have carbon atoms in a ring, for example cyclohexane C₆H₁₂. Benzene C₆H₆ is an aromatic ring compound with alternate double bonds.
Fuels: a good fuel has a high calorific value, burns without smoke, is cheap and easy to store. Charcoal, coal, wood, cow-dung cakes, petrol, diesel, LPG, CNG and biogas are all carbon-based fuels.
Fermentation: sugars are converted into ethanol and carbon dioxide by the enzyme in yeast. This is used for making alcohol and for raising dough.

Safety and the environment

Burning carbon fuels in a closed room can produce carbon monoxide, which is colourless and odourless and can be fatal. Plastic waste blocks drains and harms animals that eat it. Forest officers should know that plastic left in forests can last for many years and should be collected. Cleaner choices are LPG and biogas for cooking, and cloth or jute bags for carrying goods.

Exam traps

1. Alkanes have single bonds; alkenes have a double bond; alkynes have a triple bond. Check the general formula, not only the name.
2. Methane CH₄, ethene C₂H₄, ethyne C₂H₂ are the simplest member of each family.
3. Hydrogenation is an addition reaction and needs a catalyst (nickel); halogenation of alkanes is substitution.
4. Soap is a salt; detergents do not make scum in hard water.
5. Esterification is acid plus alcohol; saponification is ester plus base.
6. Nylon is a condensation polymer; polythene is an addition polymer.
7. Bakelite is thermosetting; polythene is thermoplastic.
8. Glacial acetic acid is pure ethanoic acid; vinegar is a dilute solution.

One-liners

1. Carbon has four valence electrons.
2. Methane is the main component of natural gas and biogas.
3. Ethyne burns with a sooty flame in air but a very hot, clean flame in oxygen, which is used for welding.
4. Ethanol with sodium releases hydrogen.
5. Vinegar is about 5 to 8 percent acetic acid.
6. The head of a soap molecule is hydrophilic.
7. Micelle formation helps soap clean oily dirt.
8. Hard water contains calcium and magnesium salts.
9. Teflon is used for non-stick coating.
10. PVC is made from vinyl chloride.
11. Terylene is a polyester.
12. Vulcanisation of rubber uses sulphur.

Worked examples

1. Molecular formula of an alkane with 8 carbons: CₙH₂ₙ₊₂ = C₈H₁₈ (hydrogen = 18).
2. Molar mass of ethanol C₂H₅OH (C = 12, H = 1, O = 16): 24 + 6 + 16 = 46 g/mol.
3. Mass of CO₂ from complete burning of 16 g of methane (CH₄ = 16, CO₂ = 44): CH₄ + 2O₂ → CO₂ + 2H₂O. 16 g gives 44 g CO₂.
4. Number of hydrogens in an alkene with 5 carbons: 2n = 10, so C₅H₁₀.

Practice questions

The number of valence electrons in a carbon atom is
1. 4
2. 2
3. 6
4. 8
Answer
A. 4
Carbon has atomic number 6, configuration 2,4.
The ability of carbon to form long chains by bonding to itself is called
1. polymerisation
2. saponification
3. isomerism
4. catenation
Answer
D. catenation
Catenation is self-linking of carbon atoms.
The general formula of alkanes is
1. CnH2n-2
2. CnH2n
3. CnH2n+2
4. CnHn
Answer
C. CnH2n+2
Alkanes are saturated hydrocarbons with formula CnH2n+2.
Which of these is an alkyne?
1. Ethane
2. Ethene
3. Methane
4. Ethyne
Answer
D. Ethyne
Ethyne C2H2 has a triple bond.
The functional group present in ethanol is
1. -CHO
2. -OH
3. -COOH
4. -CO-
Answer
B. -OH
Alcohols have the hydroxyl group.
Which functional group is present in ethanoic acid?
1. -CHO
2. -OH
3. -Cl
4. -COOH
Answer
D. -COOH
Carboxylic acids contain -COOH.
The reaction of an alcohol with a carboxylic acid in the presence of concentrated sulphuric acid forms
1. an aldehyde
2. an ester
3. a soap
4. an alkene
Answer
B. an ester
This is esterification; esters smell fruity.
Soap is the sodium or potassium salt of
1. sulphuric acid
2. hydrochloric acid
3. acetic acid only
4. a long-chain fatty acid
Answer
D. a long-chain fatty acid
Soap is made by saponification of fats with alkali.
Which polymer is used as a non-stick coating on cookware?
1. Bakelite
2. Nylon
3. Teflon
4. Polystyrene
Answer
C. Teflon
PTFE (Teflon) is non-stick and heat resistant.
The monomer of polythene is
1. propene
2. ethene
3. ethane
4. ethyne
Answer
B. ethene
Polythene is formed by addition polymerisation of ethene.
Vulcanisation of natural rubber uses
1. chlorine
2. carbon
3. sulphur
4. phosphorus
Answer
C. sulphur
Heating rubber with sulphur makes it stronger and elastic.
Which of these is a thermosetting plastic?
1. Polypropylene
2. Bakelite
3. Polythene
4. PVC
Answer
B. Bakelite
Bakelite cannot be softened once set.
Hydrogenation of vegetable oil to form solid fat uses which catalyst?
1. Nickel
2. Iron
3. Platinum oxide only
4. Copper sulphate
Answer
A. Nickel
Hydrogen adds to double bonds in presence of nickel.
The cleaning action of soap is due to the formation of
1. crystals
2. isomers
3. micelles
4. esters
Answer
C. micelles
Micelles trap oily dirt in their hydrophobic core.
Soap forms an insoluble scum with hard water because hard water contains
1. potassium nitrate
2. ammonia
3. sodium chloride
4. calcium and magnesium salts
Answer
D. calcium and magnesium salts
Ca and Mg ions form insoluble salts with soap.
Glacial acetic acid is
1. 5 percent acetic acid
2. pure ethanoic acid
3. ethanol in water
4. a salt of acetic acid
Answer
B. pure ethanoic acid
Pure ethanoic acid freezes into ice-like crystals below about 290 K.
Which gas is released when ethanoic acid reacts with sodium carbonate?
1. Methane
2. Hydrogen
3. Carbon dioxide
4. Oxygen
Answer
C. Carbon dioxide
CO2 turns lime water milky.
The molecular formula of an alkane with 10 carbon atoms is
1. C10H18
2. C10H24
3. C10H20
4. C10H22
Answer
D. C10H22
2n + 2 = 22.
How many hydrogen atoms are in an alkene with 6 carbon atoms?
1. 14
2. 10
3. 12
4. 6
Answer
C. 12
CnH2n gives C6H12.
How many hydrogen atoms are in an alkyne with 4 carbon atoms?
1. 6
2. 8
3. 10
4. 4
Answer
A. 6
CnH2n-2 gives C4H6.
The molar mass of ethanoic acid CH3COOH (C = 12, H = 1, O = 16) is
1. 44 g/mol
2. 74 g/mol
3. 46 g/mol
4. 60 g/mol
Answer
D. 60 g/mol
24 + 4 + 32 = 60.
The molar mass of ethanol C2H5OH (C = 12, H = 1, O = 16) is
1. 32 g/mol
2. 46 g/mol
3. 44 g/mol
4. 60 g/mol
Answer
B. 46 g/mol
24 + 6 + 16 = 46.
Mass of CO2 formed by complete burning of 30 g of ethane C2H6 (C = 12, H = 1, CO2 = 44) in 2C2H6 + 7O2 -> 4CO2 + 6H2O is
1. 44 g
2. 176 g
3. 88 g
4. 60 g
Answer
C. 88 g
60 g ethane gives 176 g CO2; 30 g gives 88 g.
Mass of water formed when 16 g of methane burns completely (CH4 = 16, H2O = 18) is
1. 44 g
2. 32 g
3. 18 g
4. 36 g
Answer
D. 36 g
CH4 + 2O2 gives CO2 + 2H2O; 2 x 18 = 36 g.
Percentage of carbon by mass in methane CH4 (C = 12, H = 1) is
1. 25
2. 75
3. 80
4. 50
Answer
B. 75
12/16 = 75 percent.
Percentage of carbon in ethene C2H4 (C = 12, H = 1) is
1. 92.3
2. 75
3. 85.7
4. 80
Answer
C. 85.7
24/28 = 85.7 percent.
How many carbon atoms are present in one molecule of glucose, C6H12O6?
1. 12
2. 24
3. 3
4. 6
Answer
D. 6
Each glucose molecule has six carbon atoms.
The molecular mass of consecutive members of a homologous series differs by
1. 12 u
2. 14 u
3. 16 u
4. 28 u
Answer
B. 14 u
Each member differs by CH2 = 12 + 2 = 14.
The mass of PVC monomer unit CH2=CHCl (C = 12, H = 1, Cl = 35.5) is
1. 28
2. 62.5
3. 64
4. 58.5
Answer
B. 62.5
24 + 3 + 35.5 = 62.5.
How many isomers does butane C4H10 have, including butane itself?
1. 1
2. 3
3. 2
4. 4
Answer
C. 2
Straight-chain butane and isobutane.
Which flame is produced by burning an unsaturated hydrocarbon like ethyne in air?
1. No flame
2. Clean blue flame
3. Green flame
4. Yellow sooty flame
Answer
D. Yellow sooty flame
High carbon content gives incomplete combustion and soot.
Which substance is formed when ethanol is heated with excess concentrated sulphuric acid at about 443 K?
1. Ethyl ethanoate
2. Ethane
3. Ethanal
4. Ethene
Answer
D. Ethene
Dehydration of ethanol gives ethene.
The head of a soap molecule is
1. non-polar
2. hydrophilic
3. hydrophobic
4. a hydrocarbon chain
Answer
B. hydrophilic
The ionic head dissolves in water; the tail dissolves in oil.
Detergents are preferred to soap in hard water because they
1. have no cleaning action
2. are acids
3. do not form scum with calcium and magnesium ions
4. are plastics
Answer
C. do not form scum with calcium and magnesium ions
Detergent salts of sulphonic acids stay soluble.
Which of the following polymers is natural?
1. Cellulose
2. Teflon
3. PVC
4. Nylon
Answer
A. Cellulose
Cellulose is a natural polymer of glucose.
Consider the statements. 1. Alkenes contain at least one carbon-carbon double bond. 2. Alkanes are unsaturated hydrocarbons. Which is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Alkanes are saturated.
Consider the statements. 1. Hydrogenation is an addition reaction. 2. Chlorination of methane in sunlight is a substitution reaction. Which is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both statements are correct.
Consider the statements. 1. Polythene is a thermoplastic. 2. Bakelite can be melted and remoulded repeatedly. Which is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Bakelite is thermosetting and does not soften again.
Consider the statements. 1. Covalent compounds generally have low melting points. 2. Covalent compounds conduct electricity in the molten state because of ions. Which is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Covalent compounds have no ions, so they are poor conductors.
Consider the statements. 1. Vinegar is a dilute solution of ethanoic acid. 2. Ethanoic acid is a strong acid like hydrochloric acid. Which is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Ethanoic acid is a weak acid.
Consider the statements. 1. Saponification is the reaction of an ester with a base. 2. Soap molecules form micelles in water. Which is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both are standard statements.
Consider the statements. 1. Detergents with branched chains are easily biodegradable. 2. Straight-chain detergents are more biodegradable. Which is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Branched chains resist microbial breakdown.
Consider the statements. 1. Diamond and graphite are allotropes of carbon. 2. Both conduct electricity equally well. Which is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Only graphite conducts.
Consider the statements. 1. Nylon is formed by addition polymerisation of ethene. 2. Polythene is formed from ethene. Which is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Nylon is a condensation polymer.

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1.Soaps are sodium or potassium salts of long chain _________.
2.The total number of isomers of butane and pentane, respectively, are:
3.The unsaturated hydrocarbons which contain one or more double bonds are called-
4.The members of homologous series differ by __________.
5.The IUPAC name for CH 3 COCH 3 is-
6.The process of fermentation involves the conversion of sugars into which of the following compounds?
7.The prefix added to the name of the hydrocarbon with 4 carbon atoms is.
8.Propanone belongs to which homologous series?
9.On heating ethanol at 443K with more concentrated H 2 SO 4, in the dehydration of ethanol _______ is released.
10.________ reacts in the presence of an acid or base to give back alcohol and carboxylic acid-

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