Introduction to Organic Chemistry
This article introduces the basics of organic chemistry and is designed for Secondary 4 Chemistry students. By studying this note, you will be able to:
- Define a homologous series.
- Explain what a functional group is.
- Apply rules for naming organic compounds.
Understanding Organic Chemistry
Organic chemistry is the study of compounds that contain carbon atoms. These carbon-based compounds often also include hydrogen, and they may additionally contain oxygen, nitrogen, or halogens.
- Organic compounds: Any substance featuring carbon is generally considered organic, especially when it also includes hydrogen.
- Hydrocarbons are the simplest form, consisting solely of carbon and hydrogen (for example, methane, CH₄).
- It is important to note that not every carbon-containing compound is organic. For instance, carbon dioxide (CO₂), carbon monoxide (CO), and metal carbonates are classified as inorganic because they do not derive from living organisms.
Homologous Series
A homologous series refers to a group of organic compounds that share a common functional group and exhibit similar chemical behavior. In these series:
- Each member contains the same type of functional group.
- They display analogous chemical reactivity.
- Their physical properties (such as melting point, boiling point, density, viscosity, and flammability) change gradually as the number of carbon atoms increases.
Examples of homologous series include:
- Alkanes
- Alkenes
- Alcohols
- Carboxylic acids
Functional Groups
A functional group is a specific atom or group of atoms within a molecule that is responsible for its characteristic reactions. Organic compounds in the same homologous series have similar reactivity because they contain the identical functional group.
The table below summarizes the key functional groups associated with four common homologous series:
| Homologous Series |
Functional Group Description |
| Alkanes |
No special functional group (only C–H and C–C bonds) |
| Alkenes |
Contains a carbon–carbon double bond (C=C) |
| Alcohols |
Contains a hydroxyl group (–OH) |
| Carboxylic Acids |
Contains a carboxyl group (–COOH) |
Naming Organic Compounds
Organic compound names are generally formed by combining a prefix and a suffix:
-
Prefix – This part indicates the number of carbon atoms in the longest continuous chain:
- 1 carbon: meth-
- 2 carbons: eth-
- 3 carbons: prop-
- 4 carbons: but-
- (and so on…)
-
Suffix – This portion identifies the type of homologous series (i.e., the functional group present):
- For alkanes: -ane
- For alkenes: -ene
- For alcohols: -ol (occasionally prefixed with “an” to ease pronunciation)
- For carboxylic acids: -oic acid (sometimes “anoic acid” is used)
Practical Examples
Below are some examples that demonstrate how to name organic compounds using the rules above.
- Single-Carbon Alkane:
- The molecule has one carbon atom and features only C–H bonds, indicating it is an alkane.
- Prefix: meth-
- Suffix: -ane
- Compound Name: methane
- Three-Carbon Alkene:
- The structure contains three carbon atoms and features a carbon–carbon double bond.
- Prefix: prop-
- Suffix: -ene
- Compound Name: propene
- Two-Carbon Carboxylic Acid:
- This molecule contains two carbon atoms and a carboxyl group (–COOH).
- Prefix: eth-
- Suffix: -oic acid
- Compound Name: ethanoic acid
- Four-Carbon Alcohol:
- With four carbon atoms and a hydroxyl group (–OH), this compound belongs to the alcohol series.
- Prefix: but-
- Suffix: -ol
- Compound Name: butanol
Interactive Game Section
Question 1
Identify the following organic compound:
Options:
- Propane
- Butane
- Ethanol
- Butene
Answer: Butane
- Explanation: With four carbon atoms (prefix but-) and lacking any functional groups apart from standard C–H and C–C bonds, it belongs to the alkane series (suffix -ane).
Question 2
Name the organic compound represented below:
Options:
- Methanol
- Butanoic acid
- Ethene
- Propane
Answer: Ethene
- Explanation: The molecule has two carbons (prefix eth-) and a carbon–carbon double bond (suffix -ene), fitting the alkene series.
Question 3
Determine the name of this organic compound:
Options:
- Ethanol
- Propanol
- Ethanoic acid
- Propanoic acid
Answer: Propanoic acid
- Explanation: Featuring three carbon atoms (prefix prop-) and a carboxyl group (suffix -oic acid), the compound is named propanoic acid.
Question 4
Identify the functional groups in the given organic compound:
Answer:
- The compound contains two key functional groups:
- A carbon–carbon double bond (C=C).
- A carboxyl group (–COOH).
Additional Practice
Example 2: Homologous Series Identification
A student asks whether the two depicted compounds belong to the same homologous series.
Answer: Yes, both compounds have a carbon–carbon double bond, meaning they belong to the alkene series.
Try It Yourself
Question 1:
Three organic compounds are shown below. Two of these compounds are in the same homologous series. Identify which two and explain why.
Answer:
The first and third compounds both contain a hydroxyl group (–OH), which categorizes them as alcohols.
Question 2:
Which statements about a homologous series are correct?
- All members exhibit similar chemical reactivity.
- All members contain the same functional group.
- All members share identical physical properties.
Options:
A. Only the first and second statements
B. Only the first and third statements
C. Only the second and third statements
D. All of the above
Answer:
Option A – Organic compounds in a homologous series have the same functional group and exhibit similar chemical reactions. Their physical properties, however, change gradually along the series.
Question 3:
From the options provided below, choose a set that represents a homologous series.
Answer:
The set in option (1) is correct; it includes compounds from the alkane series, identifiable by the common suffix -ane.
Question 4:
Consider the structures of four organic compounds shown below. Which compounds belong to the same homologous series?
Options:
- A and B
- B and C
- A, B, and D
- A, B, C, and D
Answer:
Option 1 – Compounds A and B both contain a hydroxyl group (–OH) and are therefore part of the alcohol series.
Question 5a:
Below are four organic compounds labeled A, B, C, and D. Arrange these compounds in order according to their homologous series (from A to D):
- Options:
- alkane, alkene, alcohol, carboxylic acid
- alkene, alcohol, carboxylic acid, alkane
- alcohol, alkane, alkene, carboxylic acid
- alkene, alcohol, alkane, carboxylic acid
Answer:
Option 4 – The correct sequence is:
- A: Alkene (features a C=C bond)
- B: Alcohol (contains –OH)
- C: Alkane (no functional group apart from C–H and C–C bonds)
- D: Carboxylic acid (has –COOH)
Question 5b:
Using the same set of compounds, what is the name of compound B?
- Options:
- Ethanoic acid
- Methanol
- Methane
- Propene
Answer:
Option 2 – Compound B contains one carbon and a hydroxyl group, giving it the name methanol.
Keep Exploring
For further studies, consider reviewing related topics such as:
- Metals
- Alcohols
- Speed of Reaction
- Electrolysis
- Energy Changes
- Ammonia
- Alkanes
- Carboxylic Acids
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