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Organic chemistry: Class 10 Chemistry
Introduction to organic compounds, carbon bonding, classification, functional groups, nomenclature, isomerism, homologous series, and the structure, properties and uses of alcohols, aldehydes, ketones, carboxylic acids and esters.
Key points
Organic chemistry is the study of compounds of carbon, except a few simple ones such as carbon monoxide, carbon dioxide, carbonates and cyanides. Earlier it was thought that these compounds came only from living things. In 1828 Wohler made urea from ammonium cyanate in the laboratory, which showed that organic compounds can be made without a living source.
Carbon forms very many compounds for three reasons. It has four valence electrons and forms four covalent bonds. It shows catenation, which is the ability of its atoms to join in long chains, branched chains and rings. It forms single, double and triple bonds with itself and also bonds well with H, O, N and halogens.
General properties of organic compounds:
- They have covalent bonds, so they are mostly non-polar or weakly polar.
- Most have low melting and boiling points.
- Most dissolve in organic solvents such as ether and benzene and not in water.
- Most are flammable and are non-conductors of electricity.
- Their reactions are usually slow and need heat or a catalyst.
- They show isomerism.
Organic compounds are classified by structure. Acyclic (open chain) compounds have no ring. Cyclic compounds have a closed ring. Cyclic compounds are divided into alicyclic, such as cyclohexane, and aromatic, such as benzene. Compounds containing only C and H are hydrocarbons. They are saturated if all C to C bonds are single and unsaturated if there is a double or triple bond.
A functional group is an atom or a group of atoms that gives a compound its characteristic chemical properties. Compounds with the same functional group react in a similar way. Examples: alcohol (−OH), aldehyde (−CHO), ketone (>C=O), carboxylic acid (−COOH), ester (−COO−), amine (−NH₂), and alkyl halide (−X).
A homologous series is a family of compounds with the same functional group, the same general formula and similar chemical properties. Neighbouring members differ by a CH₂ unit. Their physical properties change gradually, for example the boiling point rises with the number of carbon atoms. Alkanes have the general formula CₙH₂ₙ₊₂ and alcohols CₙH₂ₙ₊₁OH.
IUPAC naming follows a fixed pattern:
- Find the longest continuous carbon chain that contains the functional group. This gives the root: meth (1), eth (2), prop (3), but (4), pent (5), hex (6).
- Add the suffix for the group: −ol for alcohol, −al for aldehyde, −one for ketone, −oic acid for carboxylic acid.
- Number the chain so that the functional group gets the lowest number.
- Name side chains as prefixes (methyl, ethyl) with their positions, in alphabetical order.
- Example: CH₃CH₂CH₂OH is propan-1-ol, and CH₃COCH₃ is propanone.
Isomers are compounds with the same molecular formula but different structures. Structural isomerism has three common types. Chain isomerism: different carbon skeletons, for example butane and 2-methylpropane (C₄H₁₀). Position isomerism: the group sits at a different position, for example propan-1-ol and propan-2-ol. Functional group isomerism: different groups, for example ethanol (CH₃CH₂OH) and methoxymethane (CH₃OCH₃), both C₂H₆O.
Alcohols contain the −OH group. Methanol (CH₃OH) is poisonous and is used as a solvent and fuel. Ethanol (C₂H₅OH) is made by fermentation of sugar using yeast: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. It is used in medicines, as a solvent, and as a fuel mixed with petrol. Lower alcohols dissolve in water because of hydrogen bonding. Alcohols react with sodium to give hydrogen and sodium alkoxide, and with carboxylic acids to give esters.
Oxidation of ethanol with acidified potassium dichromate or potassium permanganate gives ethanal and then ethanoic acid. Ethanal (CH₃CHO) is an aldehyde with a −CHO group at the end of the chain. Propanone (CH₃COCH₃) is the simplest ketone and is used as nail polish remover. Aldehydes are easily oxidized and ketones are not. Tollens' reagent (a silver mirror) and Fehling's solution (red precipitate) identify aldehydes.
Carboxylic acids contain the −COOH group. Ethanoic acid (CH₃COOH) is the acid in vinegar, which is about 4 to 8 percent. Carboxylic acids are weak acids. They turn blue litmus red, react with carbonates to give CO₂, and react with alcohols to give esters. Methanoic acid is present in ant and bee stings.
Esterification is the reaction of an alcohol with a carboxylic acid in the presence of concentrated sulphuric acid, giving an ester and water: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. The ester is ethyl ethanoate. Esters have sweet fruity smells and are used in perfumes and food flavours. Hydrolysis of an ester with NaOH is called saponification and gives soap.
Ethers (R−O−R) are made from two alkyl groups joined by oxygen. Diethyl ether was used as an anaesthetic and is a solvent. It is highly flammable. Amines contain −NH₂ and behave as weak bases, as ammonia does.
Study tip: Learn the first six roots and the suffix for each functional group, and draw structures before naming. Write balanced equations with conditions for fermentation, oxidation and esterification, as these are asked often.
Definitions and flashcards
Say the meaning first, then open the card to check.
Catenation
The ability of carbon atoms to join with each other in chains and rings.
Functional group
An atom or group that decides the characteristic properties of a compound.
Homologous series
A family of compounds with the same functional group and general formula, each differing by CH₂.
Isomers
Compounds with the same molecular formula but different structural formulas.
Saturated hydrocarbon
A hydrocarbon with only single bonds between carbon atoms.
Esterification
Reaction of an alcohol and a carboxylic acid to form an ester and water.
Fermentation
Breakdown of sugar by enzymes in yeast to give ethanol and carbon dioxide.
Aromatic compound
A cyclic compound with a benzene type ring.
Saponification
Alkaline hydrolysis of an ester to give soap and glycerol.
Formula sheet
| Name | Formula | What it means |
|---|---|---|
| Alkane | CₙH₂ₙ₊₂ | General formula of alkanes |
| Alcohol | CₙH₂ₙ₊₁OH | General formula of saturated alcohols |
| Carboxylic acid | CₙH₂ₙ₊₁COOH | General formula of saturated monocarboxylic acids |
| Molecular formula | Molecular formula = (empirical formula)ₙ, n = molar mass / empirical formula mass | Used to find the molecular formula |
Short questions with model answers
These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.
Why does carbon form a very large number of compounds? (3 marks)
Carbon has four valence electrons and forms four covalent bonds. It shows catenation, forming long chains, branches and rings. It makes single, double and triple bonds and joins with H, O, N and halogens. These features give millions of compounds.
Define homologous series and give two characteristics. (2 marks)
It is a family of compounds with the same functional group and a general formula, in which neighbouring members differ by CH₂. Two characteristics: they show similar chemical properties, and their physical properties such as boiling point change regularly with molar mass.
Write the structural isomers of C₄H₁₀ and name them. (3 marks)
Butane is CH₃−CH₂−CH₂−CH₃, a straight chain. 2-methylpropane is CH₃−CH(CH₃)−CH₃, a branched chain. They have the same formula but different structures and different boiling points, so they are chain isomers.
How is ethanol prepared by fermentation? Give the equation and two uses. (5 marks)
A sugar solution such as molasses is mixed with yeast and kept warm, at about 30 to 35 °C, with air excluded. The enzyme zymase in yeast converts glucose into ethanol: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. The mixture is then distilled to raise the strength of the alcohol. Uses: as a solvent for varnishes and medicines, and as a fuel blended with petrol.
What is esterification? Write the reaction between ethanoic acid and ethanol. (3 marks)
It is the reaction of a carboxylic acid with an alcohol, with concentrated H₂SO₄ as catalyst, to form an ester and water. CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. The ester is ethyl ethanoate, which has a fruity smell.
How can an aldehyde be distinguished from a ketone? (2 marks)
An aldehyde is easily oxidized, so it gives a silver mirror with Tollens' reagent and a red precipitate when heated with Fehling's solution. A ketone gives neither of these results.
Numericals with worked steps
A compound contains 40% C, 6.7% H and 53.3% O. Its molar mass is 180 g mol⁻¹. Find its molecular formula (C = 12, H = 1, O = 16).
- Moles in 100 g: C = 40 / 12 = 3.33, H = 6.7 / 1 = 6.7, O = 53.3 / 16 = 3.33.
- Divide by the smallest (3.33): C = 1, H = 2, O = 1. Empirical formula = CH₂O.
- Empirical formula mass = 12 + 2 + 16 = 30.
- n = 180 / 30 = 6.
- Molecular formula = (CH₂O)₆ = C₆H₁₂O₆.
Answer: C₆H₁₂O₆
Find the percentage of carbon in ethanol, C₂H₅OH.
- Molar mass = 2(12) + 6(1) + 16 = 46 g mol⁻¹.
- Mass of carbon = 24 g.
- Percentage = 24 / 46 × 100 = 52.2.
Answer: 52.2 % carbon
How many moles are there in 23 g of ethanol?
- Molar mass of ethanol = 46 g mol⁻¹.
- Moles = mass / molar mass = 23 / 46 = 0.5.
Answer: 0.5 mol
Common mistakes
- Counting carbon dioxide and carbonates as organic compounds.
- Naming the chain from the wrong end. The functional group must get the lowest number.
- Mixing up isomers with homologues. Isomers share a formula, homologues differ by CH₂.
- Writing esters as the product of an acid with an acid, or forgetting the water produced in esterification.
- Saying that methanol and ethanol are equally safe. Methanol is poisonous.