Formulae, Functional Groups & Terminology

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Notes

Organic Formulae

**Organic compounds** contain carbon (except metal carbonates, CO₂, CO).
**Hydrocarbons** contain only hydrogen and carbon atoms.
**Displayed formula** shows the spatial arrangement of all atoms and bonds.
**Structural formula** (Extended tier) shows only important bonds (e.g., C=C); identical groups may be bracketed.
**Molecular formula** gives the actual number of each atom in a compound.
**Structural isomers** have the same molecular formula but different structural formulae (e.g., C₄H₁₀: butane and 2-methylpropane).

Homologous Series

A **homologous series** is a family of organic compounds with the same **functional group** and similar chemical properties.
The **functional group** is a specific arrangement of atoms responsible for characteristic reactions.
Key functional groups: **alkane** (C–C), **alkene** (C=C), **alcohol** (–OH), **carboxylic acid** (–COOH), **ester** (–COO–).
General formulae: alkane CₙH₂ₙ₊₂, alkene CₙH₂ₙ, alcohol CₙH₂ₙ₊₁OH, carboxylic acid CₙH₂ₙ₊₁COOH.
Members differ by **CH₂** and show a **gradation in physical properties** (e.g., boiling point increases with chain length).
Example: ethanol (C₂H₅OH, b.p. 78°C) and propanol (C₃H₇OH, b.p. 97°C) are both alcohols with –OH group.

Saturated & Unsaturated Compounds

**Saturated** compounds have only **single C–C bonds** (e.g., alkanes).
General formula for alkanes: CₙH₂ₙ₊₂; first three: methane (CH₄), ethane (C₂H₆), propane (C₃H₈).
**Unsaturated** compounds contain at least one **C=C double bond** (e.g., alkenes).
General formula for alkenes: CₙH₂ₙ; first three: ethene (C₂H₄), propene (C₃H₆), butene (C₄H₈).
Mnemonic: **S**aturated = **S**ingle bonds; **U**nsaturated = do**U**ble bonds.

Naming Organic Compounds

Name consists of a **stem** (number of C atoms) and **suffix** (functional group).
Stems: 1=meth, 2=eth, 3=prop, 4=but, 5=pent, 6=hex.
Suffixes: -ane (alkane), -ene (alkene), -anol (alcohol), -anoic acid (carboxylic acid), -yl-anoate (ester).
For positional isomers (Extended tier), number the carbon chain to locate the functional group (e.g., propan-1-ol, but-2-ene).
Carboxylic acids are not numbered; the –COOH carbon is always C1.
Ester names: the alcohol part (alkyl) comes first, then the carboxylic acid part (alkanoate) – e.g., methyl ethanoate from methanol + ethanoic acid.

Key Functional Groups Table

**Alkane**: C–C, suffix -ane.
**Alkene**: C=C, suffix -ene.
**Alcohol**: –OH, suffix -anol.
**Carboxylic acid**: –COOH, suffix -anoic acid.
**Ester**: –COO–, suffix -yl-anoate.

Displayed formula of ethene (C₂H₄) showing all atoms and bonds, including the C=C double bond.

Displayed formula of propane (C₃H₈), a saturated alkane with only single C–C bonds.

Practice questions

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1.Which of the following formulae shows an alkane?
Easy
- AC2H4
- BC4H8
- CC4H10
- DC5H10
2.Which of the following is not a feature of a homologous series?
Easy
- ACompounds have the same functional group
- BCompounds have the same physical properties
- CCompounds have the same general formula
- DCompounds have similar chemical properties
3.What is the general formula of the alkenes?
Easy
- ACnH2n+2
- BCnH2n
- CCnH2n-2
- DCnH2n+1OH
4.What is the name of the compound with the formula CH3CH2CH2OH?
Easy
- APropan-1-ol
- BPropan-2-ol
- CButan-1-ol
- DButan-2-ol
5.Which of the following structures shows a compound that belongs to a different homologous series to octane?
Medium
- AA straight-chain hydrocarbon with all single bonds and 8 carbons
- BA hydrocarbon with 8 carbons and one double bond
- CA hydrocarbon with 8 carbons and all single bonds
- DA hydrocarbon with 8 carbons and only single bonds
6.Which of the following compounds is unsaturated?
Medium
- AC2H6
- BC3H8
- CC2H4
- DCH4
7.What is the name of the compound with the structural formula CH3CH2CH2COOH?
Medium
- AButanoic acid
- BPropanoic acid
- CButanol
- DButanone
8.Three statements about propane and propanol are listed below. 1. Propane and propanol belong to the same homologous series. 2. Propanol is a compound that contains three carbon atoms. 3. Propane is an unsaturated hydrocarbon. Which statements are correct?
Medium
- A1 and 2 only
- B1 and 3 only
- C2 only
- D3 only

More questions

9.The displayed formula of an organic compound is shown. It has a chain of four carbon atoms with an -OH group on the second carbon. What is the name of this compound?
Hard
- AButan-1-ol
- BButan-2-ol
- CButan-3-ol
- DButanol
10.Which group of compounds is from a homologous series?
Hard
- ACH3OH, C2H5OH, C3H7OH
- BC3H6, C3H8, C3H7OH
- CCH4, C2H4, C3H8
- DCH3CO2H, CH3CH2OH, HCO2H
11.W, X, Y and Z are the first four members of the carboxylic acid homologous series, but not necessarily in order. Their relative molecular masses are: W=60, X=88, Y=46, Z=74. Which sequence shows the substances in order of decreasing boiling point?
Hard
- AX → Z → W → Y
- BY → W → Z → X
- CW → X → Y → Z
- DZ → X → W → Y
12.The energy released when equal amounts of the first five alkanes were burned is measured. The results are plotted on a grid. Which of the following best describes the trend shown by the line of best fit?
Medium
- AEnergy released increases linearly with the number of carbon atoms.
- BEnergy released decreases linearly with the number of carbon atoms.
- CEnergy released increases, but the increase slows as the number of carbon atoms increases.
- DEnergy released is constant regardless of the number of carbon atoms.
13.A student predicted that the energy released by burning butane would be two times the energy produced by burning ethane. Which of the following best explains why this prediction might be incorrect?
Medium
- AButane has twice the number of carbon atoms as ethane, so the energy released should be about twice.
- BThe relationship between energy released and number of carbon atoms is not exactly proportional due to experimental error.
- CButane has a higher boiling point than ethane, so it releases less energy.
- DThe energy released depends only on the number of hydrogen atoms, not carbon atoms.
14.The table shows the relative molecular masses of W, X, Y and Z, which are the first four members of the carboxylic acid homologous series (not in order). W=60, X=88, Y=46, Z=74. Which sequence shows the substances in order of decreasing boiling point?
Hard
- AX → Z → W → Y
- BY → W → Z → X
- CW → X → Y → Z
- DZ → X → W → Y
15.Which of the following is the general formula for the carboxylic acid homologous series?
Easy
- AC_nH_2n+1COOH
- BC_nH_2nCOOH
- CC_nH_2n-1COOH
- DC_nH_2n+2COOH
16.Using the general formula for alkenes, what is the molecular formula of the alkene with 7 carbon atoms?
Medium
- AC₇H₁₄
- BC₇H₁₆
- CC₇H₁₂
- DC₇H₁₀
17.The displayed formula of a compound shows a chain of 4 carbon atoms with a double bond between the second and third carbon, and an -OH group on the first carbon. What is the name of this compound?
Hard
- Abut-2-en-1-ol
- Bbut-1-en-2-ol
- Cbut-2-en-3-ol
- Dbut-3-en-1-ol
18.A student tests an unknown hydrocarbon with bromine water. The bromine water decolourises. What can be concluded about the hydrocarbon?
Medium
- AIt is saturated.
- BIt is unsaturated.
- CIt is an alkane.
- DIt is a carboxylic acid.
19.Which of the following compounds is an unsaturated hydrocarbon?
Easy
- AC₃H₈
- BC₄H₁₀
- CC₂H₄
- DC₅H₁₂
20.The table shows the boiling points of four alkanes: methane (-162°C), ethane (-89°C), propane (-42°C), butane (-0.5°C). Which statement best explains the trend?
Hard
- ABoiling point decreases as the number of carbon atoms increases.
- BBoiling point increases as the number of carbon atoms increases due to stronger intermolecular forces.
- CBoiling point is not related to the number of carbon atoms.
- DBoiling point increases because the molecules become more polar.
21.Which of the following pairs of compounds are structural isomers?
Medium
- AButane and but-1-ene
- BPropan-1-ol and propan-2-ol
- CEthane and ethene
- DMethane and methanol
22.What is the functional group of an alcohol?
Easy
- A-COOH
- B-OH
- C-C=C-
- D-COO-
23.A compound has the molecular formula C₄H₈O₂. It reacts with sodium carbonate to produce carbon dioxide gas. To which homologous series does it belong?
Medium
- AAlcohol
- BCarboxylic acid
- CEster
- DAlkene
24.The energy released when equal amounts of the first five alkanes were burned is measured. The results are plotted on a graph. Which of the following best describes the trend?
Hard
- AEnergy released increases as the number of carbon atoms increases.
- BEnergy released decreases as the number of carbon atoms increases.
- CEnergy released is the same for all alkanes.
- DEnergy released increases then decreases.
25.A student predicted that the energy released by burning butane would be two times the energy produced by burning ethane. Which of the following best explains why this prediction is incorrect?
Medium
- AButane has more carbon atoms but the energy increase is not linear.
- BButane has fewer carbon atoms than ethane.
- CButane is unsaturated while ethane is saturated.
- DThe energy released depends only on the number of hydrogen atoms.
26.Which of the following is the correct displayed formula for butanoic acid?
Hard
- ACH3CH2CH2COOH
- BCH3CH2COOH
- CCH3CH2CH2CH2OH
- DCH3CH2CH2COOCH3
27.Which of the following is the general formula for alkanes?
Easy
- AC_nH_2n+2
- BC_nH_2n
- CC_nH_2n+1OH
- DC_nH_2n+1COOH
28.Which of the following compounds is a hydrocarbon?
Easy
- AC2H6
- BCH3OH
- CCH3COOH
- DC2H5Cl
29.What suffix is used for the names of carboxylic acids?
Easy
- A-anoic acid
- B-anol
- C-ene
- D-ane
30.How many carbon atoms are in a molecule of propane?
Easy
- A3
- B2
- C4
- D1
31.Which of the following is not a characteristic of a homologous series?
Medium
- ASame functional group
- BSame physical properties
- CSame general formula
- DSimilar chemical properties
32.A compound has the molecular formula C3H6. To which homologous series does it belong?
Medium
- AAlkene
- BAlkane
- CAlcohol
- DCarboxylic acid
33.What is the name of the compound with the structural formula CH3CH2CH2OH?
Medium
- APropan-1-ol
- BPropan-2-ol
- CButan-1-ol
- DPropanol
34.The displayed formula of a compound shows a chain of four carbon atoms with a double bond between the second and third carbon, and an -OH group on the first carbon. What is the name of this compound?
Hard
- ABut-2-en-1-ol
- BBut-1-en-2-ol
- CBut-2-en-2-ol
- DButan-1-ol
35.What is the functional group of a carboxylic acid?
Hard
- A-COOH
- B-OH
- C-COO-
- DC=C
36.The energy released when equal amounts of the first five alkanes were burned was measured. The results are plotted on a grid. Which of the following best describes the relationship between the number of carbon atoms and the energy released?
Medium
- AEnergy released increases as the number of carbon atoms increases.
- BEnergy released decreases as the number of carbon atoms increases.
- CEnergy released is independent of the number of carbon atoms.
- DEnergy released increases then decreases as the number of carbon atoms increases.
37.The displayed formula of an organic compound is shown. It has a chain of four carbon atoms with a double bond between the second and third carbon, and an -OH group on the first carbon. What is the name of this compound?
Hard
- Abut-2-en-1-ol
- Bbut-1-en-2-ol
- Cbut-2-en-4-ol
- Dbut-1-en-1-ol
38.What is the general formula for the carboxylic acid homologous series?
Easy
- AC_nH_2n+1COOH
- BC_nH_2nCOOH
- CC_nH_2n-1COOH
- DC_nH_2n+2COOH
39.The diagram below shows some simple hydrocarbons and the products of two reactions. Which of these structures are named correctly? (Structure 1: CH3CH2CH3, Structure 2: CH2=CHCH3, Structure 3: CH3CH2CH2OH)
Hard
- AStructure 1 is propane, Structure 2 is propene, Structure 3 is propanol
- BStructure 1 is propane, Structure 2 is propene, Structure 3 is propan-1-ol
- CStructure 1 is propene, Structure 2 is propane, Structure 3 is propanol
- DStructure 1 is propene, Structure 2 is propane, Structure 3 is propan-1-ol
40.A student tests an unknown hydrocarbon with bromine water. The bromine water is decolourised. Which statement about the hydrocarbon is correct?
Hard
- AIt is saturated and contains only single bonds.
- BIt is unsaturated and contains at least one double bond.
- CIt is an alkane.
- DIt is a carboxylic acid.

Slides

Organic Formulae

Organic compounds contain carbon (except metal carbonates, CO2, CO).
A hydrocarbon contains only hydrogen and carbon atoms.
The displayed formula shows all atoms and bonds in a molecule.

Which of the following is a hydrocarbon?

C2H5OH
CH4
CO2
Na2CO3

Homologous Series

A homologous series is a family of organic compounds with the same functional group.
Members have the same general formula and similar chemical properties.
Physical properties (e.g., boiling point) change gradually as chain length increases.

Which of these are features of a homologous series?

Same functional group
Same physical properties
Same general formula
Similar chemical properties

General Formulae

Alkane: CnH2n+2
Alkene: CnH2n
Alcohol: CnH2n+1OH
Carboxylic acid: CnH2n+1COOH

The general formula of an alkane with 5 carbon atoms is ____.

Saturated & Unsaturated Compounds

Saturated compounds have only single C-C bonds (e.g., alkanes).
Unsaturated compounds have at least one C=C double bond (e.g., alkenes).
Alkanes: CnH2n+2; Alkenes: CnH2n.

Which of the following is an unsaturated compound?

C2H6
C3H8
C2H4
CH4

Naming Organic Compounds: Stem

The stem indicates the number of carbon atoms in the longest chain.
1C = meth-, 2C = eth-, 3C = prop-, 4C = but-, 5C = pent-, 6C = hex-.

A compound with 4 carbon atoms in its longest chain has the stem ____.

Naming: Suffixes

Alkane: -ane (C-C only)
Alkene: -ene (C=C)
Alcohol: -anol (OH)
Carboxylic acid: -anoic acid (COOH)

What is the name of a 3-carbon compound with a C=C bond?

Propane
Propene
Propanol
Propanoic acid

Naming with Position (Extended)

For alcohols and alkenes with >3 carbons, number the carbon where the functional group is attached.
Keep numbers as low as possible.
Example: propan-1-ol has -OH on carbon 1; propan-2-ol on carbon 2.

Which is the correct name for CH3CH(OH)CH3?

Propan-1-ol
Propan-2-ol
Butan-2-ol
Propanol

Carboxylic Acids & Esters (Extended)

Carboxylic acids end with -anoic acid; the -COOH carbon is always carbon 1.
Esters are named from the alcohol (first part) and carboxylic acid (second part) with -yl -anoate.
Example: methyl ethanoate from methanol and ethanoic acid.

Which statements about naming are correct?

Butanoic acid has 4 carbons including the COOH carbon.
Ethyl ethanoate is made from ethanol and ethanoic acid.
The carbon in COOH is not counted in the chain.

Structural Isomers (Extended)

Structural isomers have the same molecular formula but different structural formulae.
Example: C4H10 can be butane or 2-methylpropane.
Example: C4H8 can be but-1-ene, but-2-ene, or methylpropene.

Which pair are structural isomers?

C2H6 and C3H8
C2H4 and C2H6
C4H10 and C4H10
CH4 and C2H6

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