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B3.1 · Compare organic classes by names and structural formulas

Learn to compare organic classes by names and structural formulas through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Organic Chemistry

SCH4U study topic B3.1

Two organic compounds may look similar in a container but have different structures. To compare them, chemists examine how their atoms are connected, not only which elements are present. A molecular formula gives the number of each kind of atom. A structural formula also shows how the atoms are connected. In this lesson, you will use structural features and names together to identify and compare organic classes.

What you will learn

  • Identify common organic classes from their characteristic structural features.
  • Connect class names and naming endings to structural formulas.
  • Compare compounds that have the same molecular formula but different structural formulas.

1. Bridge: reading carbon structures

Organic compounds contain carbon. Carbon atoms can connect to one another and to atoms such as hydrogen, oxygen, and nitrogen. A line in a structural formula represents a bond, or connection, between atoms. One, two, or three lines between a pair of atoms represent a single, double, or triple bond.
A condensed structural formula writes connected groups close together. For example, CH3CH2CH3\mathrm{CH_3CH_2CH_3} represents three carbon atoms joined in a chain. The ends and middle of the formula show how the hydrogen atoms are grouped around those carbons.
A functional group is an atom or group of atoms that gives an organic class a characteristic structural feature. To identify a class, look for its defining bond or functional group first. Then check whether the name agrees. The root of a simple chain name also gives the number of carbon atoms: meth- means one, eth- means two, prop- means three, and but- means four.
  • A molecular formula gives atom counts; a structural formula also shows connections.
  • Identify a class by its defining structural feature, not by appearance or formula alone.
  • A carbon chain is a connected sequence of carbon atoms.

2. Classes identified by carbon-to-carbon bonds

Alkanes contain carbon-to-carbon single bonds only. Their names commonly end in -ane. Propane, CH3CH2CH3\mathrm{CH_3CH_2CH_3}, is an example. Each carbon in this simple chain also has enough hydrogen atoms to make four bonds.
Alkenes contain at least one carbon-to-carbon double bond. Their names commonly end in -ene. In but-1-ene, the double bond begins at carbon 1: CH2=CHCH2CH3\mathrm{CH_2{=}CHCH_2CH_3}. The number locates the first carbon in the double bond. This matters because the double bond can occur at different positions in a chain.
Alkynes contain at least one carbon-to-carbon triple bond. Their names commonly end in -yne. In but-1-yne, the triple bond begins at carbon 1: HC≡CCH2CH3\mathrm{HC{\equiv}CCH_2CH_3}. The number gives the position of the triple bond; it does not count all the bonds in the molecule.
These classes are distinguished by the bonds between carbon atoms. A chain with a carbon-to-carbon double bond is an alkene, not an alkane. Check the structure before using the name ending as confirmation.
  • Alkane: carbon-to-carbon single bonds; commonly ends in -ane.
  • Alkene: at least one carbon-to-carbon double bond; commonly ends in -ene.
  • Alkyne: at least one carbon-to-carbon triple bond; commonly ends in -yne.

3. Classes identified by oxygen- or nitrogen-containing groups

An alcohol contains a hydroxyl group, −OH\mathrm{-OH}, attached to a carbon. Its name commonly ends in -ol. Propan-1-ol is CH3CH2CH2OH\mathrm{CH_3CH_2CH_2OH}. The number identifies the carbon bearing the hydroxyl group. Do not confuse an alcohol group with the hydroxyl part of a larger carboxylic acid group.
A carboxylic acid contains a carboxyl group, −COOH\mathrm{-COOH}. Its name commonly ends in -oic acid. Ethanoic acid is CH3COOH\mathrm{CH_3COOH}. In this group, a carbon is double-bonded to oxygen and also joined to a hydroxyl group. Recognize the whole carboxyl group when classifying the compound.
An ester contains the group −COO−\mathrm{-COO-} between carbon-containing parts of the structure. Ester names have two parts; the second commonly ends in -oate. Methyl ethanoate is CH3COOCH3\mathrm{CH_3COOCH_3}. Its oxygen atoms are part of the ester group, rather than an alcohol hydroxyl group.
Aldehydes and ketones both contain a carbonyl group, in which carbon is double-bonded to oxygen, C=O\mathrm{C{=}O}. In an aldehyde, the carbonyl carbon is at the end of the chain and is bonded to hydrogen. The name commonly ends in -al. Propanal is CH3CH2CHO\mathrm{CH_3CH_2CHO}. In a ketone, the carbonyl carbon is within the chain and bonded to carbon atoms on both sides. The name commonly ends in -one. Propanone is CH3COCH3\mathrm{CH_3COCH_3}.
Amines contain nitrogen bonded to carbon-containing groups and, in common simple examples, hydrogen. A simple amine name commonly ends in -amine. Ethanamine is CH3CH2NH2\mathrm{CH_3CH_2NH_2}. An amide contains a carbonyl group directly attached to nitrogen. Ethanamide is CH3CONH2\mathrm{CH_3CONH_2}. The carbonyl next to nitrogen distinguishes an amide from an amine.
  • Alcohol: hydroxyl group attached to carbon; commonly ends in -ol.
  • Carboxylic acid: carboxyl group; commonly ends in -oic acid. Ester: ester group; commonly ends in -oate.
  • Aldehyde: carbonyl at a chain end; commonly ends in -al. Ketone: carbonyl within a chain; commonly ends in -one.
  • An amide has a carbonyl group directly attached to nitrogen; an amine does not have that arrangement.

4. Compare names and structures together

Use two clues when comparing compounds: the structural feature and the name. First, find the bond or group that defines the class. Then check whether the name ending agrees. If the name includes a position number, use it to locate the multiple bond or group on the chain.
A molecular formula alone cannot show how atoms are connected. Propanal and propanone both have the molecular formula C3H6O\mathrm{C_3H_6O}, but their structures differ. In propanal, the carbonyl carbon is at the end of the chain and is bonded to hydrogen. In propanone, it is between two carbon atoms. They therefore belong to different classes.
A clear comparison states both what the compounds share and how they differ. For example, both compounds contain a carbonyl group, but one is an aldehyde because the carbonyl is at a chain end, while the other is a ketone because the carbonyl is within the chain. This connects the class name to evidence in the structure.
  • Use the structural formula as evidence and the name as a confirming clue.
  • Different structures can share a molecular formula and belong to different classes.
  • State where the defining group or bond occurs when explaining a comparison.

Worked example

Distinguishing two three-carbon carbonyl compounds

Compare propanal, CH3CH2CHO\mathrm{CH_3CH_2CHO}, with propanone, CH3COCH3\mathrm{CH_3COCH_3}. Identify the class of each and explain how their names and structures support your answer.
  1. Find the shared feature
    Each structural formula contains a carbonyl group: a carbon atom double-bonded to oxygen. In propanal, the group appears at the chain end; in propanone, it appears within the chain.
  2. Use the position to identify each class
    In propanal, the carbonyl carbon is at the end of the chain and is bonded to hydrogen. That arrangement identifies an aldehyde. In propanone, the carbonyl carbon is bonded to carbon atoms on both sides, so the compound is a ketone.
  3. Check the names
    The ending -al agrees with the aldehyde structure, and -one agrees with the ketone structure. The root prop- indicates three carbon atoms in each compound. The different endings match their different carbonyl positions.
Answer: Propanal is an aldehyde, and propanone is a ketone. Both have three carbon atoms and a carbonyl group, but the carbonyl is at the chain end in propanal and within the chain in propanone.
Check: The class identification follows from the carbonyl position, not simply from the presence of oxygen.

Common mistakes and how to avoid them

Calling any compound with a hydroxyl group an alcohol.
Correction: Check the whole structure. In a carboxylic acid, the hydroxyl group is part of the larger carboxyl group.
Calling an ester an alcohol because it contains oxygen.
Correction: An ester has its characteristic group between carbon-containing parts. An alcohol has a hydroxyl group attached to carbon.
Treating aldehydes and ketones as the same class because both contain a carbonyl group.
Correction: Check the carbonyl position. It is at the chain end in an aldehyde and within the chain in a ketone.
Using a molecular formula alone to decide the organic class.
Correction: Use the structural formula to see how the atoms are connected. Different structures can share a molecular formula.

Lesson summary

  • Organic classes are identified by characteristic bonds or functional groups.
  • Name endings are useful clues, but confirm each class by inspecting its structural formula.
  • Position can locate a multiple bond or functional group and distinguish aldehydes from ketones.
  • A molecular formula gives atom counts but does not show connections, so it may not identify a compound's class.

Check your understanding

Question 1

Which class matches the structure CH3CH2OH\mathrm{CH_3CH_2OH}?
  1. Alcohol
  2. Carboxylic acid
  3. Ester
  4. Ketone
Show answer and explanation
Alcohol
The structure has a hydroxyl group attached to carbon and does not contain a carboxyl group, so it is an alcohol.

Question 2

What feature distinguishes an amide from an amine in the examples in this lesson?
  1. An amide has a carbonyl group directly attached to nitrogen.
  2. An amide always has a carbon-to-carbon triple bond.
  3. An amine has a carboxyl group.
  4. An amide contains no nitrogen.
Show answer and explanation
An amide has a carbonyl group directly attached to nitrogen.
An amide has a carbonyl group directly attached to nitrogen. An amine does not have that carbonyl-nitrogen arrangement.

Question 3

A three-carbon compound has a carbonyl group between two carbon atoms and is named propanone. What class is it?
  1. Aldehyde
  2. Ketone
  3. Alcohol
  4. Carboxylic acid
Show answer and explanation
Ketone
A carbonyl carbon within a chain, bonded to carbon atoms on both sides, identifies a ketone. The ending -one agrees.

Key terms

Carbonyl group
A carbon atom double-bonded to an oxygen atom, written C=O\mathrm{C{=}O}.
Functional group
An atom or group of atoms that gives an organic class a characteristic structural feature.
Molecular formula
A formula that states how many atoms of each element are present but not how they are connected.
Structural formula
A representation that shows how atoms are connected in a molecule.

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