Carbon and its Compounds · Lesson 5 of 13
Will You Be My Friend?
“Swap a hydrogen for the right group and the molecule gets a whole new personality.”
• Explain what a heteroatom is in a carbon compound. • Define a functional group and explain why it controls characteristic chemical behaviour. • Recognise halo, alcohol, aldehyde, ketone and carboxylic acid functional groups. • Explain how a functional group replaces hydrogen while carbon's valency remains satisfied. • Distinguish carbon-chain identity from functional-group identity.
Carbon does not bond only with hydrogen. Hydrogen atoms in a hydrocarbon framework can be replaced by atoms such as chlorine or bromine, or by groups containing oxygen. These replacements create compounds with characteristic chemical behaviour.
Heteroatoms
A Heteroatom is an atom other than carbon or hydrogen that replaces hydrogen or becomes part of a group attached to a carbon chain.
The chapter highlights halogens, oxygen, nitrogen and sulphur as elements that can occur with carbon. The carbon skeleton remains, but the new atom or group often changes the compound's behaviour strongly.
Functional Groups
A Functional Group is an atom or group of atoms that gives a carbon compound characteristic chemical properties.
A functional group is attached to a carbon chain through a free valency and replaces one or more hydrogen atoms. The same functional group can appear on carbon chains of different lengths, yet those compounds can still show similar chemical reactions.
| Class | Functional Group | Recognition Clue |
|---|---|---|
| Halo compound | —Cl or —Br | Chlorine or bromine replaces hydrogen |
| Alcohol | —OH | Hydroxyl group attached to carbon chain |
| Aldehyde | —CHO | Terminal carbonyl-containing group |
| Ketone | —C(=O)— | Carbonyl group within carbon chain |
| Carboxylic acid | —COOH | Carboxyl group |
Why Functional Groups Matter
Suppose two compounds have carbon chains of different lengths but both contain the —OH group. Their physical properties can differ because their molecular masses and chain lengths differ, yet their important chemical behaviour is strongly influenced by the shared alcohol functional group.
Functional Group Versus Carbon Chain
The carbon chain tells us the basic skeleton and size of the molecule. The functional group tells us a major part of its characteristic chemical behaviour. These two pieces of information work together when we classify and name carbon compounds.
Problem
A structure ends in —COOH. What class of compound is it?
- 1.Look for the characteristic attached group.
- 2.The group —COOH is the carboxyl functional group.
- 3.Compounds containing this functional group are carboxylic acids.
- 4.The length of the carbon chain may change the name, but the functional-group class remains carboxylic acid.
Quiz
What is a heteroatom?
Which functional group identifies an alcohol?
Which group identifies a carboxylic acid?
Why are functional groups important?
Which pair represents halo substituents listed in the chapter?
Practice Problems
- Define heteroatom and functional group in your own words.
- Identify the functional groups —OH, —CHO, —C(=O)— and —COOH.
- Explain how a hydrogen atom in a hydrocarbon can be replaced while carbon's valency remains satisfied.
- Explain why compounds with the same functional group can have similar chemical properties even when their carbon chains differ.
- Describe the difference between the carbon skeleton and the functional group.
- Classify structures containing —Br, —OH and —COOH into the appropriate classes.
Key Takeaways
• Carbon compounds can contain heteroatoms such as halogens, oxygen, nitrogen and sulphur. • A functional group gives a carbon compound characteristic chemical properties. • Halo, alcohol, aldehyde, ketone and carboxylic acid groups are important functional groups in this chapter. • Functional groups attach to carbon chains by replacing hydrogen atoms while valencies remain satisfied. • Carbon-chain length and functional-group identity describe different aspects of a molecule.