Source: Purdue University General Chemistry, Ch. 24.4 Organic Chemistry Worksheet
Difficulty: Introductory | Prerequisites: Ch. 24.2 (Aliphatic Hydrocarbons), basic understanding of covalent bonding and electronegativity.
Once you can recognise and name a hydrocarbon backbone, the next question is: what else is attached to it? Functional groups are specific arrangements of atoms within a molecule that determine its chemical behaviour. This chapter introduces the seven functional group classes you are expected to identify at the general chemistry level. If you are comfortable naming alkanes, alkenes, and alkynes from Ch. 24.2, you are ready for this.
Functional groups are atom clusters attached to a carbon backbone that give a molecule its characteristic chemical properties. At this level, you need to recognise seven classes: alcohol, aldehyde, ketone, carboxylic acid, ester, amine, and amide. Spot the group, name the class.
Functional group
A specific grouping of atoms within a molecule that determines the molecule's chemical reactivity and properties. Think of it as the "active site" of the molecule: the carbon backbone is the frame, and the functional group is what makes the molecule do something interesting.
Alcohol
Class: alcohol. General formula: R–OH. Functional group: hydroxyl group (–OH). In simple terms, a carbon backbone with an OH stuck on it.
Aldehyde
Class: aldehyde. General formula: RCHO. Functional group: carbonyl group (C=O) at the end of a carbon chain, with a hydrogen on the carbonyl carbon. Think of it as a carbon double-bonded to oxygen, sitting at the terminal position.
Ketone
Class: ketone. General formula: RCOR'. Functional group: carbonyl group (C=O) in the interior of the carbon chain, bonded to two carbon groups. In simple terms, the C=O is sandwiched between two carbon-containing groups rather than sitting at the end.
Carboxylic acid
Class: carboxylic acid. General formula: RCOOH. Functional group: carboxyl group (–COOH). Think of it as a carbonyl (C=O) and a hydroxyl (–OH) on the same carbon. This is the group that makes vinegar sour.
Ester
Class: ester. General formula: RCOOR'. Functional group: ester group (–COO–). In simple terms, it looks like a carboxylic acid, but the H on the OH has been replaced by another carbon group (R').
Amine
Class: amine. General formula: RNH2 (primary). Functional group: amino group (–NH2). Think of it as ammonia (NH3) where one hydrogen has been swapped for a carbon chain.
Amide
Class: amide. General formula: RCONH2 (primary). Functional group: amide group (–CONH2). In simple terms, a carbonyl (C=O) bonded directly to a nitrogen. This is the linkage that holds proteins together (the peptide bond is an amide bond).
R and R'
Shorthand for alkyl groups (carbon-containing chains or branches). R and R' may be the same group or different groups. They represent the hydrocarbon part of the molecule that is not the functional group itself.
Look for atoms other than C and H first: oxygen, nitrogen, and combinations of these with carbon are your main targets.
Scan for characteristic patterns:
–OH on a carbon (not part of COOH): alcohol
C=O at the end of a chain with an H on it: aldehyde
C=O in the middle of a chain (bonded to two carbons): ketone
–COOH (C=O and –OH on the same carbon, at the end): carboxylic acid
–COO– (C=O with an oxygen linking to another carbon group): ester
–NH2 (or –NHR, –NR2) without a neighbouring carbonyl: amine
–CONH2 (or –CONHR, –CONR2): amide
A single molecule can contain more than one functional group. When asked to identify all functional groups, check every heteroatom in the structure.
Tetrahydrogestrinone (THG): Contains a hydroxyl group (–OH, alcohol) and a carbonyl group (C=O, ketone). The molecule also has alkene character (C=C double bonds) in its steroid ring system.
Ibuprofen: Contains a carboxyl group (–COOH, carboxylic acid). The rest of the molecule is a hydrocarbon framework.
Acetaminophen (paracetamol): Contains a hydroxyl group (–OH, alcohol) and an amide group (–NHCOCH3, amide). Note: the NH is bonded to a carbonyl carbon, making it an amide rather than a simple amine.
Vanillin: Contains a hydroxyl group (–OH, alcohol), an aldehyde group (–CHO, aldehyde), and an ether linkage (–OCH3). The aldehyde is the carbonyl at the end of the chain bonded to H.
The worksheet includes a reference table listing each class, its general formula, structural drawing, and functional group name. Use this as a lookup during practice, but aim to memorise it before the exam. The seven classes to know:
Alcohol: ROH, hydroxy group
Carboxylic acid: RCOOH, carboxy group
Ester: RCOOR', ester group
Aldehyde: RCHO, carbonyl group
Ketone: RCOR', carbonyl group
Amine: RNH2, amino group (primary, 1°)
Amide: RCONH2, amide group (primary, 1°)
Alcohols are everywhere: ethanol in drinks and hand sanitiser, methanol as industrial solvent. Carboxylic acids include acetic acid (vinegar) and citric acid (citrus fruits). Esters are responsible for many fruit and flower scents. Acetaminophen (paracetamol), one of the most widely used painkillers, is an amide. Vanillin, the compound that gives vanilla its flavour, combines an aldehyde, an alcohol, and an ether in one small molecule.
Students frequently confuse aldehydes and ketones. Both contain a C=O (carbonyl), but an aldehyde has the carbonyl at the end of the chain (bonded to at least one H), while a ketone has it in the middle (bonded to two carbon groups). Position is the distinguishing feature.
Confusing carboxylic acids and esters. Both have a C=O next to an oxygen, but in a carboxylic acid the oxygen carries an H (–COOH), while in an ester it carries a carbon group (–COOR'). If there is an H on that oxygen, it is an acid.
Labelling any –NH group as an amine. If the nitrogen is directly bonded to a carbonyl carbon (C=O), the group is an amide, not an amine. The carbonyl makes the difference.
Forgetting that molecules can contain multiple functional groups. The exam will test this, particularly with pharmaceutical molecules like acetaminophen and vanillin.
⚠️ You will almost certainly be given a molecular structure and asked to identify every functional group in it and name the compound class. Practise with complex molecules that have more than one group.
⚠️ Know the general formula for each of the seven classes cold. The table from the textbook (Table 25.2) is your best revision tool.
⚠️ The aldehyde vs. ketone distinction and the amine vs. amide distinction are classic exam traps. Expect at least one question designed to test whether you can tell them apart.
⚠️ Real molecules from the worksheet (tetrahydrogestrinone, ibuprofen, acetaminophen, vanillin) are fair game. You may see the same structures or similar pharmaceutical/flavour molecules on the exam.
True or False: A ketone and an aldehyde both contain a carbonyl group (C=O).
Fill in the blank: The functional group in ethanol (CH3CH2OH) is a __________ group, and the compound class is __________.
True or False: An amide contains a nitrogen bonded directly to a carbonyl carbon.
Fill in the blank: The general formula for a carboxylic acid is __________.
True or False: Ibuprofen is classified as an ester.
Answers: 1. True. 2. Hydroxyl (or hydroxy); alcohol. 3. True. 4. RCOOH. 5. False (ibuprofen contains a carboxyl group, making it a carboxylic acid).
Q: Acetaminophen contains an –OH group and an –NHCOCH3 group. What functional group classes are present?
A: The –OH group is a hydroxyl group (alcohol). The –NHCOCH3 group is an amide (nitrogen bonded to a carbonyl carbon). Note that it is not an amine because the nitrogen is adjacent to C=O.
Q: What distinguishes an aldehyde from a ketone?
A: Both contain a carbonyl group (C=O). In an aldehyde, the carbonyl carbon is at the end of the chain and bonded to at least one hydrogen (RCHO). In a ketone, the carbonyl carbon is bonded to two carbon-containing groups (RCOR'), placing it in the interior of the chain.
Q: Vanillin contains –OH, –CHO, and –OCH3. Identify the compound classes for each group.
A: –OH is a hydroxyl group (alcohol). –CHO is an aldehyde (carbonyl at the terminal carbon with an H). –OCH3 is an ether (oxygen bonded to two carbon groups). Vanillin therefore contains three different functional groups.
Q: You are given a molecule with the formula RCOOH. What class does it belong to, and what is the functional group called?
A: Carboxylic acid. The functional group is the carboxyl group (sometimes called the carboxy group).
Q: How can you tell the difference between a carboxylic acid and an ester just by looking at the structure?
A: In a carboxylic acid, the pattern is –COOH: the oxygen after the carbonyl carries a hydrogen. In an ester, the pattern is –COOR': that same oxygen carries a carbon group instead of a hydrogen.
This builds directly on Ch. 24.2 (Aliphatic Hydrocarbons). The hydrocarbon backbone you learned to name is the scaffold onto which functional groups attach. Recognising the backbone first makes spotting functional groups much faster.
Functional groups reappear throughout organic chemistry and biochemistry. Amino acids contain both amine and carboxylic acid groups. The peptide bonds holding proteins together are amide bonds. Understanding these seven classes now sets you up for biochemistry topics later in the course.
If your course covers reaction chemistry, functional groups determine which reactions a molecule can undergo. Alcohols, for example, can be oxidised to aldehydes or ketones, and aldehydes can be further oxidised to carboxylic acids. This reactivity chain starts with knowing what group you are looking at.
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