Formulae, Functional Groups & Terminology

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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).

Structural isomers of C4H10

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.

Ethanol (C2H6O) — displayed formula

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: Saturated = Single bonds; Unsaturated = doUble bonds.

Propene (C3H6) — displayed formula

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.

But-2-ene (C4H8) — displayed formula

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.

Functional groups

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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?
Easy
- AC2H6
- BC3H8
- CC2H4
- DCH4
7.What is the name of the compound with the structural formula CH3CH2CH2COOH?
Easy
- 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

Thêm câu hỏi

9.A compound has a chain of four carbon atoms with an -OH group on the second carbon. What is the name of this compound?
Medium
- AButan-1-ol
- BButan-2-ol
- CButan-3-ol
- DButanol
10.Which group of compounds is from a homologous series?
Medium
- 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.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
13.Using the general formula for alkenes, what is the molecular formula of the alkene with 7 carbon atoms?
Easy
- AC₇H₁₄
- BC₇H₁₆
- CC₇H₁₂
- DC₇H₁₀
14.A compound has 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
15.A student tests an unknown hydrocarbon with bromine water. The bromine water decolourises. What can be concluded about the hydrocarbon?
Easy
- AIt is saturated.
- BIt is unsaturated.
- CIt is an alkane.
- DIt is a carboxylic acid.
16.Which of the following compounds is an unsaturated hydrocarbon?
Easy
- AC₃H₈
- BC₄H₁₀
- CC₂H₄
- DC₅H₁₂
17.The boiling points of four alkanes are: methane (-162°C), ethane (-89°C), propane (-42°C), butane (-0.5°C). Which statement best explains the trend?
Medium
- 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.
18.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
19.What is the functional group of an alcohol?
Easy
- A-COOH
- B-OH
- C-C=C-
- D-COO-
20.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
21.Which of the following is the correct displayed formula for butanoic acid?
Easy
- ACH3CH2CH2COOH
- BCH3CH2COOH
- CCH3CH2CH2CH2OH
- DCH3CH2CH2COOCH3
22.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
23.Which of the following compounds is a hydrocarbon?
Easy
- AC2H6
- BCH3OH
- CCH3COOH
- DC2H5Cl
24.What suffix is used for the names of carboxylic acids?
Easy
- A-anoic acid
- B-anol
- C-ene
- D-ane
25.How many carbon atoms are in a molecule of propane?
Easy
- A3
- B2
- C4
- D1
26.A compound has the molecular formula C3H6. To which homologous series does it belong?
Medium
- AAlkene
- BAlkane
- CAlcohol
- DCarboxylic acid
27.What is the functional group of a carboxylic acid?
Easy
- A-COOH
- B-OH
- C-COO-
- DC=C
28.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
29.Three simple hydrocarbons and the products of two reactions are given below. (Structure 1: CH3CH2CH3, Structure 2: CH2=CHCH3, Structure 3: CH3CH2CH2OH) Which of these structures are named correctly?
Medium
- 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
30.A student tests an unknown hydrocarbon with bromine water. The bromine water is decolourised. Which statement about the hydrocarbon is correct?
Medium
- 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.
31.What is the functional group of an alkene?
Easy
- AC-C
- BC=C
- C-OH
- D-COOH
32.A hydrocarbon is a compound that contains only hydrogen and carbon atoms.
Easy
True or false?
33.Which of the following is the correct general formula for carboxylic acids?
Easy
- AC_n H_2n+2
- BC_n H_2n
- CC_n H_2n+1 OH
- DC_n H_2n+1 COOH
34.What is the name of the compound with the structural formula CH3CH2CH2CH2OH?
Easy
- AButane
- BButanol
- CButanoic acid
- DButene
35.Saturated compounds contain at least one carbon-carbon double bond.
Easy
True or false?
36.Which of the following are characteristics of a homologous series? (Select all that apply)
Medium
- ASame functional group
- BSame general formula
- CSame physical properties
- DSimilar chemical properties
- EDifference of CH2 between consecutive members
37.Match each homologous series to its functional group.
Easy
- Alkane
- Alkene
- Alcohol
- Carboxylic acid
- C-C
- C=C
- -OH
- -COOH
38.Arrange the following compounds in order of increasing number of carbon atoms: butane, ethane, methane, propane.
Easy
- butane
- ethane
- methane
- propane
39.The boiling points of four alcohols are: methanol (65°C), ethanol (78°C), propanol (97°C), butanol (117°C). Which statement best explains the trend in boiling points?
Medium
- AThe boiling points increase because the molecules become larger and have stronger intermolecular forces.
- BThe boiling points increase because the alcohols become more unsaturated.
- CThe boiling points increase because the number of carbon-carbon double bonds increases.
- DThe boiling points increase because the functional group changes.
40.Complete the sentence: The general formula for alkanes is ___.
Easy
The general formula for alkanes is ____.
41.What is the name of the functional group in alcohols?
Easy
42.Explain what structural isomers are and give an example for the molecular formula C4H10.
Medium

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