DONATSciences and Maths
Chemistry · Practice book · Senior 5 · Unit 1 of 15

Introduction to organic chemistry

Senior 5 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 1 Introduction to organic chemistry

Key unit competence: Evaluate the structure, classification and nomenclature of organic compounds in relation to their functional groups, types of formulae and isomerism.

Key ideas

  • Classification: acyclic (open-chain, aliphatic) and cyclic compounds; cyclic are homocyclic (alicyclic like cyclohexane, or aromatic like benzene) or heterocyclic (ring contains N, O or S, e.g. pyridine).
  • Types of formulae: empirical (simplest ratio), molecular (actual atoms), structural (arrangement shown), condensed (CH3CH2OH), displayed (all bonds shown) and skeletal (zig-zag lines; C and H not written).
  • Functional groups: alkene C=C, alkyne C≡C, halogenoalkane –X, alcohol –OH, ether –O–, aldehyde –CHO, ketone C=O, carboxylic acid –COOH, ester –COO–, amide –CONH2, amine –NH2, nitrile –CN, acyl chloride –COCl.
  • IUPAC naming: prefix (substituents, alphabetical, with locants) + root (longest chain containing the main group) + suffix (principal functional group). Number the chain to give the principal group the lowest locant. Example: CH3CH(OH)CH2CH3 is butan-2-ol.
  • Structural isomerism: chain (different carbon skeleton), position (different position of group), functional group (different group, e.g. ethanol and methoxymethane, C2H6O).
  • Stereoisomerism: geometric (cis–trans / E–Z) isomerism in alkenes with two different groups on each C=C carbon (but-2-ene); optical isomerism when a carbon atom is bonded to four different groups (chiral centre), giving non-superimposable mirror images (enantiomers) that rotate plane-polarised light in opposite directions.

Common misconceptions

  • "Cyclohexane is aromatic because it has a ring." No: Aromatic compounds have a delocalised π system like benzene; cyclohexane is alicyclic.
  • "But-1-ene shows cis–trans isomerism." No: One carbon of the C=C carries two identical H atoms, so no geometric isomers exist.
  • "Any carbon in a ring is a chiral centre." No: A chiral carbon must have four DIFFERENT groups attached.

Worked examples

Example 1. Draw (describe) and name all the structural isomers of C4H10O that are alcohols.

Solution

Butan-1-ol: CH3CH2CH2CH2OH.

Butan-2-ol: CH3CH2CH(OH)CH3 (chiral).

2-methylpropan-1-ol: (CH3)2CHCH2OH.

2-methylpropan-2-ol: (CH3)3COH.

Example 2. Explain why but-2-ene has two geometric isomers and draw (describe) them.

Solution

Rotation about the C=C double bond is restricted by the π bond.

Each carbon of the double bond carries two different groups (H and CH3).

cis-but-2-ene: both CH3 groups on the same side; trans-but-2-ene: on opposite sides.

They have different physical properties (e.g. boiling points 4 °C and 1 °C).

Practice questions

  1. Give the IUPAC name of CH3CH2COOH.

    Show answer

    Propanoic acid.

    Common mistake: Writing ethanoic acid.

  2. Write the functional group of ketones.

    Show answer

    C=O between two carbon atoms (–CO–).

    Common mistake: Writing –CHO.

  3. What is a homologous series? (one feature)

    Show answer

    Same general formula / same functional group / differ by CH2 (any).

    Common mistake: Saying same molecular formula.

  4. Name CH3CHClCH3.

    Show answer

    2-chloropropane.

    Common mistake: Writing 1-chloropropane.

  5. Define structural isomers.

    Show answer

    Compounds with the same molecular formula but different structural formulae.

    Common mistake: Saying different molecular formulae.

  6. Give one condition for geometric isomerism.

    Show answer

    Restricted rotation (C=C) and two different groups on each carbon of the double bond.

    Common mistake: Saying any alkene.

  7. Identify the chiral carbon in CH3CH(NH2)COOH (alanine).

    Show answer

    C2, bonded to H, NH2, CH3 and COOH.

    Common mistake: Choosing the COOH carbon.

  8. Write the skeletal description of pentane.

    Show answer

    A zig-zag line with 4 segments (5 carbon positions).

    Common mistake: Drawing 5 segments.

  9. Name CH3OCH2CH3.

    Show answer

    Methoxyethane.

    Common mistake: Writing ethyl methyl alcohol.

  10. How are enantiomers distinguished experimentally?

    Show answer

    They rotate plane-polarised light equally but in opposite directions (polarimeter).

    Common mistake: Saying by boiling point.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. The suffix '-nitrile' indicates the group:a. –C≡N
    2. cis-But-2-ene has the two methyl groups:b. functional group isomers
    3. Which formula shows the simplest whole-number ratio of atoms?c. Empirical formula
    4. Ethanol and methoxymethane are isomers of the type:d. on the same side of the double bond
    Show answer

    1 → a (–C≡N) · 2 → d (on the same side of the double bond) · 3 → c (Empirical formula) · 4 → b (functional group isomers)

    • Cyano group.
    • cis means same side.
    • e.g. CH2O for glucose.
    • Same formula C2H6O, different groups.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    2  ·  propanone  ·  4  ·  propan-1-ol
    1. The number of structural isomers of C4H9Cl is .
    2. The IUPAC name of CH3COCH3 is .
    3. The IUPAC name of CH3CH2CH2OH is .
    Show answer

    1: 4 · 2: propanone · 3: propan-1-ol

    • 1- and 2-chlorobutane, 1-chloro-2-methylpropane, 2-chloro-2-methylpropane.
    • Ketone with 3 carbons.
    • OH on carbon 1 of a 3-carbon chain.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Geometric isomerism is possible in:a. C–O–C
    2. In skeletal formulae, each line end and corner represents:b. but-2-ene
    3. Benzene is an example of:c. a carbon atom
    4. The functional group of ethers is:d. an aromatic compound
    Show answer

    1 → b (but-2-ene) · 2 → c (a carbon atom) · 3 → d (an aromatic compound) · 4 → a (C–O–C)

    • Each C=C carbon has two different groups.
    • H atoms on C are not drawn.
    • Delocalised ring of 6 C.
    • Oxygen between two carbon groups.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    heterocyclic  ·  amides  ·  carboxylic acids  ·  aliphatic
    1. –CONH2 is the functional group of .
    2. A compound whose ring contains an atom other than carbon is called .
    3. The functional group –COOH belongs to .
    Show answer

    1: amides · 2: heterocyclic · 3: carboxylic acids

    • Carbonyl attached to nitrogen.
    • e.g. pyridine contains N.
    • Carboxyl group.

Take the Unit 1 quiz → · Back to contents