Cambridge AS & A Level9701

Formulas, functional groups and the naming of organic compounds (13)

Chemistry 9701 Chapter Notes

What this chapter covers

Formulas, functional groups and the naming of organic compoundsCharacteristic organic reactionsShapes of organic molecules; σ and π bondsIsomerism: structural isomerism and stereoisomerism
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Formulas, functional groups and the naming of organic compounds (13) notes

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In this chapter

  1. 1Representing Organic Molecules
  2. 2Functional Groups and Homologous Series
  3. 3Systematic Nomenclature (IUPAC Rules)
  4. 4Structural Isomerism
  5. 5Stereoisomerism: 3D Arrangement

1. Representing Organic Molecules

Organic chemistry deals with millions of compounds, so we need clear ways to draw them. We use several types of formula, each giving a different level of detail. The empirical formula is the simplest ratio of atoms (e.g., CH2O). The molecular formula shows the actual number of atoms of each element in a molecule (e.g., C2H4O2). The structural formula shows how atoms are connected, written on one line (e.g., CH3COOH). The displayed formula shows every single atom and every single bond. Finally, the skeletal formula is a shorthand where carbon chains are drawn as zig-zag lines and hydrogen atoms attached to carbons are omitted for clarity; only non-carbon/hydrogen atoms or functional groups are explicitly drawn.

Empirical Formula: Simplest whole-number ratio of atoms (e.g., CH2).

Molecular Formula: Actual number of atoms of each element (e.g., C6H12).

Structural Formula: Shows atom connectivity unambiguously (e.g., CH3CH(OH)CH3).

Displayed Formula: Shows all atoms and all bonds.

Skeletal Formula: Carbon skeleton as lines, H atoms on C are implied.

Key term

Skeletal Formula: A simplified representation of an organic molecule where carbon atoms are the vertices and ends of lines, and hydrogen atoms attached to carbon are not shown.

Examiner insight

Examiners reward clear, unambiguous diagrams. When drawing a displayed formula, ensure every bond is shown, and when drawing a skeletal formula, ensure the 'zig-zag' chain is clear and functional groups are correctly represented.

Common pitfall

When drawing displayed formulae, students often forget to show the bond between the oxygen and hydrogen in an alcohol (-O-H) or carboxylic acid (-O-H), which is incorrect.

Worked example 12 marks

Butan-2-ol has the molecular formula C4H10O. Draw its(i) displayed formula and(ii) skeletal formula.

  1. 1

    Step 1 (Displayed Formula): Draw a chain of 4 carbon atoms. Add an -OH group to the second carbon atom.

  2. 2

    Step 2 (Displayed Formula): Fill in the remaining bonds on each carbon with hydrogen atoms, ensuring each carbon has 4 bonds in total. The structure is CH3-CH(OH)-CH2-CH3, with all bonds shown.

  3. 3

    Step 3 (Skeletal Formula): Draw a zig-zag line with 4 points/ends, representing the 4-carbon chain.

  4. 4

    Step 4 (Skeletal Formula): On the second carbon position, draw a line ending with 'OH' to represent the hydroxyl group. No other atoms need to be drawn.

Recap

  • The molecular formula gives the actual number of atoms; the empirical formula gives the simplest ratio.
  • Structural formulae show the arrangement of atoms in a condensed format.
  • Displayed formulae show every atom and every bond.
  • Skeletal formulae are a quick and clean way to represent larger molecules.
  • Always check that each carbon atom has four bonds in your drawings.

Quick check

  1. Draw the skeletal formula of pentan-3-one (CH3CH2COCH2CH3).1 mark
  2. What is the molecular formula for the compound with the structural formula CH3CH(CH3)CH2COOH?1 mark

2. Functional Groups and Homologous Series

A functional group is a specific atom or group of atoms in a molecule that is responsible for its characteristic chemical reactions. Molecules with the same functional group react in similar ways. A homologous series is a family of organic compounds that have the same functional group and similar chemical properties. Each successive member of a homologous series differs by a -CH2- group. They share a general formula, for example, the general formula for alkanes is CnH2n+2.

Alkane: CnH2n+2

Alkene: CnH2n

Alcohol: CnH2n+1OH

Carboxylic Acid: CnH2n+1COOH

Key term

Functional Group: An atom or group of atoms within a molecule that is responsible for the characteristic chemical reactions of that compound.

Examiner insight

Marks are frequently awarded for simply identifying functional groups from a given structure. You must know the names and structures of the key functional groups listed in the syllabus.

Fun fact

The ester functional group is responsible for the pleasant smells of many fruits, like bananas (pentyl ethanoate) and pineapples (ethyl butanoate).

Worked example 13 marks

The structure of the artificial sweetener Aspartame is shown below. Identify and name the four functional groups present. (Image would show Aspartame structure with amine, carboxylic acid, ester, and amide groups). For this example, let's use a simpler molecule: Identify the functional groups in the molecule with the structural formula HO-CH2-CH=CH-CHO.

  1. 1

    Step 1: Systematically scan the molecule from left to right.

  2. 2

    Step 2: Identify the -OH group. This is an alcohol functional group.

  3. 3

    Step 3: Identify the C=C double bond. This is an alkene functional group.

  4. 4

    Step 4: Identify the -CHO group at the end of the chain. This is an aldehyde functional group.

Recap

  • A functional group determines a molecule's chemical properties.
  • A homologous series is a family of compounds with the same functional group.
  • Members of a homologous series differ by a -CH2- group.
  • Common functional groups include alcohol (-OH), alkene (C=C), and carboxylic acid (-COOH).
  • All members of a homologous series can be represented by a general formula.

Quick check

  1. What is the functional group in a ketone?1 mark
  2. Give the general formula for the homologous series of primary amines.1 mark

3. Systematic Nomenclature (IUPAC Rules)

To avoid confusion, chemists use a systematic naming system called IUPAC (International Union of Pure and Applied Chemistry) nomenclature. The process follows a set of rules:

  1. Identify the Parent Chain: Find the longest continuous chain of carbon atoms that contains the principal functional group. This gives the 'stem' of the name (e.g., 'hex-' for 6 carbons).
  2. Identify the Principal Functional Group: This determines the 'suffix' of the name (e.g., '-ol' for an alcohol). A priority order exists if there are multiple functional groups.
  3. Number the Chain: Number the carbon atoms in the parent chain starting from the end that gives the principal functional group the lowest possible number. If there is no principal group, give any substituents the lowest numbers.
  4. Name and Number Side Chains/Substituents: Name any side chains (alkyl groups like methyl, ethyl) or other functional groups (like chloro-, bromo-). Use prefixes like 'di-', 'tri-' if there are multiples of the same group.
  5. Assemble the Name: List substituents alphabetically (ignoring di-, tri-), followed by the parent chain name and the principal group suffix. Use numbers to locate each group and separate numbers with commas and numbers from letters with hyphens.

Key term

IUPAC Nomenclature: The internationally recognised system of rules for naming chemical compounds, ensuring that any compound has a unique name from which its structure can be determined.

Examiner insight

Examiners look for systematic application of the rules. Even if you make a small error, showing your steps (e.g., identifying the correct parent chain) can earn partial credit.

Common pitfall

The most common mistake is numbering the carbon chain from the wrong end, leading to incorrect locants (numbers) in the name. Always prioritise the principal functional group.

Worked example 13 marks

Give the systematic IUPAC name for the following compound: CH3-CH(Cl)-CH(CH3)-CH2-OH

  1. 1

    Step 1 (Parent Chain): The longest carbon chain containing the principal functional group (-OH) has 4 carbons. The stem is 'but-'.

  2. 2

    Step 2 (Principal Group): The principal functional group is the alcohol (-OH), so the suffix is '-an-1-ol'.

  3. 3

    Step 3 (Numbering): We number from the right to give the -OH group the lowest number (C1). So the structure is numbered HO-CH2(1)-CH(CH3)(2)-CH(Cl)(3)-CH3(4).

  4. 4

    Step 4 (Substituents): There is a 'chloro' group on carbon 3 and a 'methyl' group on carbon 2.

  5. 5

    Step 5 (Assemble): List substituents alphabetically: 'chloro' before 'methyl'. The full name is 3-chloro-2-methylbutan-1-ol.

Recap

  • Find the longest carbon chain containing the main functional group.
  • Number the chain to give the main functional group the lowest possible number.
  • Name side chains as prefixes in alphabetical order.
  • Use di-, tri-, tetra- for multiple identical side chains.
  • Separate numbers with commas and numbers from letters with hyphens.

Quick check

  1. What is the IUPAC name for CH3CH2CH(CH3)2?1 mark

4. Structural Isomerism

Structural isomers are molecules that have the same molecular formula but different structural formulae. This means the atoms are connected in a different order. There are three main types:

  1. Chain Isomerism: The carbon skeleton is arranged differently. For example, butane (a straight chain of 4 carbons) and 2-methylpropane (a chain of 3 carbons with a methyl group branch) are chain isomers of C4H10.
  2. Positional Isomerism: The carbon skeleton is the same, but a functional group or substituent is in a different position on the chain. For example, propan-1-ol and propan-2-ol are positional isomers.
  3. Functional Group Isomerism: The isomers have different functional groups. For example, C2H6O can be ethanol (an alcohol, CH3CH2OH) or methoxymethane (an ether, CH3OCH3). These isomers have very different chemical and physical properties.

Key term

Structural Isomers: Compounds with the same molecular formula but a different structural formula, meaning the atoms are bonded in a different order.

Examiner insight

When asked to draw isomers, you must provide structures that are clearly different. Using systematic names helps you check if your structures are truly unique.

Common pitfall

Drawing the same molecule twice in a different orientation and claiming it is an isomer. For example, rotating a molecule or bending a chain does not create a new isomer.

Worked example 13 marks

Draw and name three structural isomers of C5H12.

  1. 1

    Step 1: Draw the straight-chain isomer first. A chain of 5 carbons is pentane.

  2. 2

    Step 2: Draw a shorter chain with a branch. A chain of 4 carbons with a methyl group on the second carbon is 2-methylbutane. (Note: putting the methyl group on the third carbon is the same molecule flipped over).

  3. 3

    Step 3: Draw an even shorter chain with more branches. A chain of 3 carbons with two methyl groups on the central carbon is 2,2-dimethylpropane.

  4. 4

    Step 4: Check that all three molecules (pentane, 2-methylbutane, 2,2-dimethylpropane) have the molecular formula C5H12 and are different from each other.

Worked example 22 marks

Identify the type of structural isomerism shown between propanal (CH3CH2CHO) and propanone (CH3COCH3).

  1. 1

    Step 1: Determine the molecular formula of both compounds. Propanal is C3H6O. Propanone is also C3H6O. They are isomers.

  2. 2

    Step 2: Identify the functional group in each. Propanal is an aldehyde. Propanone is a ketone.

  3. 3

    Step 3: Since the molecular formula is the same but the functional groups are different, this is an example of functional group isomerism.

Recap

  • Structural isomers have the same molecular formula but different atom connectivity.
  • Chain isomers have different arrangements of the carbon skeleton (branching).
  • Positional isomers have the same carbon skeleton but the functional group is in a different place.
  • Functional group isomers have different functional groups.
  • Always check the molecular formula to confirm you have drawn an isomer.

Quick check

  1. Are but-1-ene and but-2-ene chain, positional, or functional group isomers?1 mark

5. Stereoisomerism: 3D Arrangement

Stereoisomers have the same molecular formula and the same structural formula (atoms are connected in the same order), but have a different arrangement of atoms in 3D space. There are two types for A-Level:

  1. E-Z Isomerism (Geometric Isomerism): This occurs in molecules with a C=C double bond (which prevents free rotation) and where each carbon in the double bond is attached to two different groups. The 'Z' isomer (from German 'zusammen' - together) has the highest priority groups on the same side of the double bond. The 'E' isomer (from 'entgegen' - opposite) has them on opposite sides. Priority is assigned using Cahn-Ingold-Prelog (CIP) rules, where higher atomic number gets higher priority.
  2. Optical Isomerism: This occurs in molecules that are chiral. A chiral molecule has a non-superimposable mirror image. This property usually arises from a carbon atom bonded to four different atoms or groups, known as a chiral centre or asymmetric carbon. The two isomers are called enantiomers. A 50:50 mixture of two enantiomers is called a racemic mixture or racemate.

Key term

Chiral Centre: A carbon atom that is attached to four different atoms or groups of atoms, giving rise to optical isomerism.

Common pitfall

Assuming any molecule with a C=C bond shows E-Z isomerism. You must check that *each* carbon in the double bond is attached to two *different* groups.

Fun fact

The tragic story of the drug Thalidomide in the 1960s was due to optical isomerism. One enantiomer was an effective morning sickness treatment, while its mirror image caused severe birth defects.

Worked example 12 marks

But-2-ene exhibits E-Z isomerism. Draw the structures of E-but-2-ene and Z-but-2-ene.

  1. 1

    Step 1: Draw the C=C double bond. The molecule is but-2-ene, so the double bond is between C2 and C3.

  2. 2

    Step 2: Identify the groups on each carbon of the double bond. Both C2 and C3 are attached to a H atom and a CH3 group.

  3. 3

    Step 3 (Z-isomer): Draw the two higher priority groups (the two CH3 groups) on the same side of the double bond. This is Z-but-2-ene.

  4. 4

    Step 4 (E-isomer): Draw the two higher priority groups (the two CH3 groups) on opposite sides of the double bond. This is E-but-2-ene.

Worked example 23 marks

Identify the chiral centre in 2-bromobutane and draw the two optical isomers (enantiomers).

  1. 1

    Step 1: Draw the structure of 2-bromobutane: CH3-CH(Br)-CH2-CH3.

  2. 2

    Step 2: Identify the chiral centre. Carbon 2 is bonded to four different groups: a H atom, a Br atom, a methyl group (CH3), and an ethyl group (CH2CH3). This is the chiral centre.

  3. 3

    Step 3: Draw the first enantiomer using a 3D representation. Place the chiral carbon at the centre. Draw two bonds in the plane (e.g., to CH3 and CH2CH3), one bond coming out of the page (wedge, e.g., to Br), and one bond going into the page (dash, e.g., to H).

  4. 4

    Step 4: Draw a mirror plane next to it. Draw the reflection of the first molecule. This second molecule is the other enantiomer. It will be non-superimposable on the first.

Recap

  • Stereoisomers have the same structural formula but a different 3D arrangement of atoms.
  • E-Z isomerism requires a C=C bond and two different groups on each carbon of the double bond.
  • The 'Z' isomer has high-priority groups on the same side; 'E' has them on opposite sides.
  • Optical isomerism requires a chiral centre (a carbon with four different groups).
  • Optical isomers are non-superimposable mirror images called enantiomers.
  • A racemic mixture contains equal amounts of both enantiomers.

Quick check

  1. Explain why propene does not show E-Z isomerism.1 mark
  2. How many different groups must be attached to a carbon atom for it to be a chiral centre?1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Define the term 'homologous series' and give two characteristics of the compounds within such a series.3 marks
  2. Draw the skeletal formula for 4-ethyl-2,2-dimethylheptane.2 marks
  3. A compound has the molecular formula C4H8O. Draw the displayed formula for an aldehyde, a ketone, and a cyclic alcohol with this formula.3 marks
  4. Give the systematic IUPAC name for the molecule with the structural formula (CH3)2CHCH(OH)CH3.2 marks
  5. Explain why 1,2-dibromopropane is a chiral molecule but 1,3-dibromopropane is not.3 marks
  6. Draw the structure of the Z-isomer of 1,2-dichloroethene (CHCl=CHCl).1 mark
  7. A straight-chain alcohol, X, has the molecular formula C5H12O. It can be oxidised to a ketone, Y. Deduce the structural formulae of X and Y, and give their systematic names.4 marks
  8. Identify all the functional groups in the molecule of amoxicillin (a diagram would be provided showing ester, phenol, amine, and amide groups).4 marks
  9. Draw and name the three structural isomers of C5H12 that are alkanes.3 marks
  10. Differentiate between a primary, secondary, and tertiary alcohol, using a C4 alcohol as an example for each class.3 marks

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