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B2.2 · Name and draw structures of the specified organic compound classes

Learn to name and draw structures of the specified organic compound classes through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Organic Chemistry

Recognize the carbon framework, identify the functional group, and connect each structure to its name.

Many familiar materials contain carbon compounds, but their structures are not all alike. A small change in the atoms or bonds attached to a carbon chain can place a compound in a different class. Naming and drawing organic compounds is a way to describe those structures clearly. This lesson focuses on the common classes named in Grade 12 organic chemistry: alkanes, alkenes, alkynes, aromatic compounds, haloalkanes, alcohols, carboxylic acids, esters, amines, and amides.

What you will learn

  • Recognize the main organic compound classes by their bonds or functional groups.
  • Choose and number a parent carbon chain, then apply class endings and substituent prefixes.
  • Read and draw condensed structural formulas while preserving the correct connections between atoms.

1. Start with the carbon framework

Before naming an organic compound, recall two bonding facts. Carbon normally forms four bonds. Hydrogen normally forms one. A line in a structural formula represents a bond between atoms; a double or triple line represents two or three bonds between the same atoms. These rules help you check whether a drawing is possible.
A carbon framework is the connected arrangement of carbon atoms in a molecule. A straight chain has no carbon branch. A branched chain has one or more carbon groups extending from the main chain. A ring is a chain whose ends connect. In a skeletal drawing, each line end and corner represents a carbon atom unless another element is written there. Hydrogens attached to carbon are usually left out of that drawing, but must still be counted when checking the structure.
An observable property is not enough to identify a compound’s class. Instead, examine its particle-level structure: look for carbon–carbon multiple bonds, a ring, or a group of atoms that includes an element such as oxygen or nitrogen. That structural feature is called a functional group when it gives a family of compounds its characteristic pattern.
  • Carbon has four bonds in the structures used here; hydrogen has one.
  • A functional group is a recognizable group of atoms or bond pattern used to classify an organic compound.
  • Condensed formulas show connections in sequence, but you must read the bonds and group boundaries carefully.

2. Identify the class from its structure

Hydrocarbons contain only carbon and hydrogen. Alkanes have only single carbon–carbon bonds. Alkenes have at least one carbon–carbon double bond. Alkynes have at least one carbon–carbon triple bond. A benzene ring is a common aromatic structure; aromatic compounds include this ring pattern. In Grade 12 structural drawings, benzene is often shown as a six-carbon ring with alternating double bonds.
Other classes are identified by the atoms or groups attached to a carbon framework. A haloalkane has a halogen atom, such as chlorine or bromine, bonded to carbon. An alcohol has a hydroxyl group, written ext–OH ext{–OH}, bonded to carbon. A carboxylic acid has the carboxyl group, written ext–COOH ext{–COOH}. An ester has the pattern ext–COO– ext{–COO–} between carbon groups. An amine has nitrogen bonded to carbon and/or hydrogen, without a carbonyl group directly bonded to that nitrogen. An amide has a carbonyl group directly bonded to nitrogen; its central pattern is ext–CONH– ext{–CONH–}, with the number of hydrogens varying by structure.
A carbonyl group is a carbon atom double-bonded to oxygen, written extC=O ext{C=O}. It appears in both esters and amides, so do not classify a compound from the carbonyl alone. Check which atom is attached next to the carbonyl carbon. In an ester, an oxygen connects that carbon to another carbon group. In an amide, nitrogen is directly attached to the carbonyl carbon.
ester: RCOOR′amide: RCONH2\text{ester: }\mathrm{RCOOR'}\qquad \text{amide: }\mathrm{RCONH_2}
  • Single, double, and triple carbon–carbon bonds distinguish alkanes, alkenes, and alkynes.
  • Check the atom attached to the carbonyl carbon to distinguish an ester from an amide.
  • An aromatic ring is a recognizable ring pattern; it is not simply any carbon ring.

3. Build a systematic name

A systematic name describes the parent structure and the features attached to it. First choose the parent chain: the longest continuous carbon chain that includes the principal multiple bond or the carbon bearing the main functional group. The parent name begins with a root showing the number of carbons: meth- for one, eth- for two, prop- for three, but- for four, and pent- for five. Add a suffix that identifies the main class or bond pattern.
Number the parent chain from the end that gives the main functional group, multiple bond, or substituent the lowest suitable number, following the naming priority taught for the compound. A locant is a number that tells where a feature is attached. For example, a double bond beginning at carbon 1 is indicated by the locant 1. A methyl group is a one-carbon branch; a chloro group indicates a chlorine substituent. If the same substituent appears more than once, use a prefix such as di- and give each position.
Common suffixes include -ane for alkanes, -ene for alkenes, -yne for alkynes, -ol for alcohols, and -oic acid for carboxylic acids. The number of a multiple bond or functional group is included when its position can vary. A haloalkane uses the halogen name as a prefix, such as bromo- or chloro-. Esters are named with an alkyl name for the group attached to the single-bonded oxygen, followed by the acid-derived name ending in -oate. For amines and amides, use the class name and position information as needed to show the structure unambiguously.
Drawing reverses the process. Interpret each part of the name: make the parent chain, number its carbons, place the multiple bond or functional group at its stated position, and add branches. Complete the remaining carbon bonds with hydrogen atoms. Then check that every carbon has four bonds, every oxygen has two, and each nitrogen has the bonds shown in its class structure.
  • The parent chain and its root identify the main carbon framework.
  • Locants state positions; prefixes identify branches or halogens; suffixes identify the main class or bond pattern.
  • A finished name and drawing must describe the same atom-to-atom connections.

4. Work from a name to a structure

Use the name 3-bromobutan-1-ol. The root but- indicates a four-carbon parent chain. The suffix -ol identifies an alcohol, and 1 places its hydroxyl group on carbon 1. The prefix 3-bromo places bromine on carbon 3. Build the four-carbon chain, place those groups, and then add hydrogens so each carbon has four bonds.
In the condensed structure, the first carbon is the end carbon bearing the hydroxyl group. The third carbon bears bromine. The remaining bonds are filled by hydrogen. The structure is therefore extHO–CH2ext–CH2ext–CH(Br)–CH3 ext{HO–CH}_2 ext{–CH}_2 ext{–CH(Br)–CH}_3. Reading it from the other end would not change the molecule, but the name uses the numbering that gives the alcohol group the lower position.
  • The suffix identifies the alcohol group; the locant puts it at carbon 1.
  • The prefix locant puts bromine at carbon 3.
  • Counting bonds confirms the groups are attached to the intended carbons.

Worked example

Name a branched alcohol

Give a systematic name for the condensed structure CH3–CH(OH)–CH(CH3)–CH3\text{CH}_3\text{–CH(OH)–CH(CH}_3\text{)–CH}_3.
  1. Identify the class
    The structure contains an –OH group bonded to a carbon, so it is an alcohol. The longest chain that includes the carbon bearing –OH contains four carbons.
  2. Number the chain
    Number from the end nearest the –OH group. This places the alcohol group at carbon 2. The remaining branch is a methyl group on carbon 3.
    C4 parent; OH at 2; methyl at 3\mathrm{C_4\ parent;\ OH\ at\ 2;\ methyl\ at\ 3}
  3. Write the name
    Use butan- for the four-carbon parent, -2-ol to locate the alcohol group, and 3-methyl- for the branch. The name is 3-methylbutan-2-ol.
    3-methylbutan-2-ol\text{3-methylbutan-2-ol}
Answer: 3-methylbutan-2-ol
Check: The name specifies four parent carbons, an –OH group at carbon 2, and a methyl branch at carbon 3, matching the given structure.

Common mistakes and how to avoid them

Choosing the chain that looks longest in a drawing without checking whether it includes the main functional group.
Correction: Choose the longest suitable parent chain that includes the carbon bearing the principal functional group or the multiple bond.
Numbering from the end that gives a branch the lowest number, even when that gives the alcohol group a higher number.
Correction: Apply the numbering priority for the main functional group first, then locate branches.
Treating every structure with C=O as an ester.
Correction: Inspect the atom attached to the carbonyl carbon. An ester has oxygen there; an amide has nitrogen there.
Adding or removing bonds without checking carbon’s usual four-bond pattern.
Correction: Count each single, double, and triple bond using its bond order, then add hydrogens only where needed.

Lesson summary

  • Classify a structure by its carbon–carbon bonds, ring pattern, or functional group.
  • Choose an appropriate parent chain, number it, and use locants, prefixes, and suffixes to show its features.
  • When drawing from a name, place each named feature first, then complete the structure with the correct number of bonds and hydrogens.

Check your understanding

Question 1

Which class contains a carbon–carbon triple bond?
  1. Alkane
  2. Alkene
  3. Alkyne
  4. Alcohol
Show answer and explanation
Alkyne
An alkyne has at least one carbon–carbon triple bond.

Question 2

In an ester, which atom is directly attached to the carbonyl carbon on the single-bond side of the ester group?
  1. Nitrogen
  2. Oxygen
  3. Chlorine
  4. Hydrogen only
Show answer and explanation
Oxygen
An ester has a carbonyl carbon connected through oxygen to another carbon group.

Question 3

What is the correct name for CH3–CH(OH)–CH3\text{CH}_3\text{–CH(OH)–CH}_3?
  1. Propan-1-ol
  2. Propan-2-ol
  3. 2-methylpropan-1-ol
  4. Propanoic acid
Show answer and explanation
Propan-2-ol
The parent chain has three carbons. Numbering from either end places the hydroxyl group at carbon 2, so the name is propan-2-ol.

Key terms

Carbon framework
The connected arrangement of carbon atoms in an organic structure.
Functional group
A recognizable group of atoms or bond pattern used to classify an organic compound.
Carbonyl group
A carbon atom double-bonded to oxygen, written C=O.
Locant
A number in a name that gives the position of a bond, group, or branch.
Condensed structural formula
A compact formula that shows how groups of atoms connect in a molecule.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Chemistry (SCH4U), expectation B2.2. It is a study resource, not an official curriculum publication.

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