4.1.2Core Organic Chemistry
Functional Groups
5 min read 5 parts Lesson 3 of 29
Section 1 of 5What a Functional Group Does
Knowing the group tells you which reactions to expect and how to name the compound.
Definition
- A functional group is the atom or group of atoms in a molecule that is responsible for its characteristic reactions.
Same series
- Compounds with the same group form a homologous series with similar chemical properties and a gradual change in physical properties.
Saturated
- A saturated compound contains only single carbon-carbon bonds, as in alkanes.
Unsaturated
- An unsaturated compound contains at least one carbon-carbon double or triple bond, as in alkenes.
Section 2 of 5Common Groups and Their Naming
Common Groups and Their Naming
Alkene
- The C=C group is named with the suffix -ene, as in propene.
Alcohol
- The –OH group gives the suffix -ol, or the prefix hydroxy- when a higher priority group is present.
Carbonyl
- Aldehydes (–CHO, suffix -al) and ketones (C=O in the chain, suffix -one) both contain the C=O bond.
Carboxylic acid
- The –COOH group has the suffix -oic acid, as in ethanoic acid.
Haloalkane
- Halogen atoms are named with the prefixes fluoro-, chloro-, bromo- and iodo-, as in 1-bromopropane.
| Group | Formula | Example |
|---|---|---|
| Alcohol | –OH | Ethanol, CH₃CH₂OH |
| Aldehyde | –CHO | Ethanal, CH₃CHO |
| Ketone | C=O | Propanone, CH₃COCH₃ |
| Carboxylic acid | –COOH | Ethanoic acid, CH₃COOH |
| Ester | –COO– | Methyl ethanoate, CH₃COOCH₃ |
| Amine | –NH₂ | Ethylamine, CH₃CH₂NH₂ |
Section 3 of 5Choosing the Name
1
Identify every functional group present in the molecule from the displayed or structural formula.
2
Decide which group has priority; a carboxylic acid outranks an aldehyde, which outranks a ketone and then an alcohol.
3
Use the highest priority group as the suffix, and name the other groups as prefixes.
4
Number the main chain so the suffix group has the lowest possible number.
5
State the position of every prefix and check that the name matches the structure.
Section 4 of 5Linking Group to Properties
Polarity
- Groups containing oxygen or nitrogen make molecules polar and able to form stronger intermolecular forces.
Hydrogen bonding
- Alcohols and carboxylic acids can form hydrogen bonds, which raises boiling points and gives solubility in water.
Reactivity
- Alkenes react by addition at the C=C bond, whereas haloalkanes react at the polar carbon-halogen bond.
Acidity
- Carboxylic acids release H⁺ ions in water, so they behave as weak acids.
Section 5 of 5Exam Checklist & Tips
4.1.2 – Key Points for the Exam
- Functional group The atom or group of atoms that determines the characteristic reactions of a compound.
- Saturated Containing only single bonds between carbon atoms.
- Unsaturated Containing at least one C=C or C≡C bond.
- Ester A compound containing the –COO– group formed from a carboxylic acid and an alcohol.
Common Mistakes
- Confusing aldehydes and ketonesthe aldehyde C=O is at the end of the chain, a ketone C=O is within the chain.
- Writing an ester group in the wrong order and so naming the alcohol and acid parts the wrong way round.
- Forgetting that the suffix is chosen by group priority and not by the first group seen.
- Writing an alcohol as –COH, which wrongly suggests an aldehyde.