Unit 7: Nomenclature & Homologous Series
Learning the systematic language of organic chemistry to name compounds and understand their relationships.
7.7 Naming Unbranched Aliphatic Molecules
The IUPAC system for naming simple organic compounds is based on a three-part structure: a prefix indicating chain length, a part indicating the type of C-C bonds (-an-, -en-, -yn-), and a suffix for the main functional group.
Prefixes for Carbon Chain Length:
| 1C | Meth- | 2C | Eth- | 3C | Prop- | 4C | But- | 5C | Pent- | 6C | Hex- |
|---|
Suffixes for Functional Groups:
- Alkane: -ane (e.g., Propane)
- Alkene: -ene (e.g., Propene)
- Alkyne: -yne (e.g., Propyne)
- Alcohol: -anol (e.g., Propanol)
- Haloalkanes use a prefix (e.g., Chloropropane)
Locants (Numbers):
For chains of 3 or more carbons (for alcohols/haloalkanes) or 4 or more carbons (for alkenes/alkynes), the position of the functional group must be specified. Number the carbon chain from the end that gives the functional group the lowest possible number.
Example: $CH_3CH_2CH(OH)CH_3$ is butan-2-ol, not butan-3-ol.
Solved Examples:
- Name the molecule $CH_3CH_2CH_3$.
Solution: Prefix for 3 carbons is "prop-". It is an alkane, so the suffix is "-ane". Name: Propane. - Name the molecule $CH_3CH=CHCH_3$.
Solution: Longest chain is 4 carbons ("but-"). It is an alkene ("-ene"). The double bond starts at C2 (numbering from either end). Name: But-2-ene. - Draw the structure for pentan-1-ol.
Solution: A 5-carbon chain with an -OH group on the first carbon: $CH_3CH_2CH_2CH_2CH_2OH$. - Name the molecule $CH_3CH_2CH_2CH_2Br$.
Solution: It is a 4-carbon chain ("butane") with a bromine on the first carbon. Name: 1-Bromobutane. - Why is the name "pent-4-ene" incorrect?
Solution: You must number the chain from the end closest to the double bond. Numbering from the other end gives the name pent-1-ene, which is correct as 1 is lower than 4. - What is the name of a 6-carbon alkane?
Solution: Hexane. - Name the molecule $CH≡CCH_2CH_3$.
Solution: 4-carbon chain ("but-"), alkyne ("-yne"). The triple bond starts at C1. Name: But-1-yne. - Draw propan-2-ol.
Solution: A 3-carbon chain with an -OH group on the middle carbon: $CH_3CH(OH)CH_3$. - Is a number needed to name propan-1-ol?
Solution: Yes. The -OH group could also be on the second carbon (propan-2-ol), which is a different molecule (a positional isomer). - What is the name for a 2-carbon alcohol?
Solution: Ethanol. No number is needed as the -OH group would be on C1 regardless of which end you start numbering from.
7.8-7.10 Naming Complex, Alicyclic, and Identical Molecules
Naming Branched Molecules:
- Identify the longest continuous carbon chain (the parent chain).
- Identify the branches (alkyl groups, e.g., -CH₃ is methyl, -CH₂CH₃ is ethyl).
- Number the parent chain from the end that gives the branches the lowest possible numbers.
- List the branches alphabetically, each with its position number. Use prefixes di-, tri-, tetra- if a branch appears more than once.
Example: $CH_3CH(CH_3)CH_2CH_3$ is 2-methylbutane.
Naming Alicyclic Molecules:
For cyclic compounds, add the prefix cyclo- to the alkane name corresponding to the number of carbons in the ring. Numbering starts at the carbon with the highest priority functional group or branch.
Example: A five-membered carbon ring is cyclopentane. A six-membered ring with a double bond is cyclohexene.
Recognising Identical Molecules:
Molecules can be drawn in many different ways. Two structures are identical if they have the same IUPAC name. Remember that single bonds can rotate freely.
Solved Examples:
- Name the molecule $CH_3C(CH_3)_2CH_3$.
Solution: Longest chain is 3 carbons (propane). There are two methyl branches on C2. Name: 2,2-dimethylpropane. - Draw the structure for 3-ethylpentane.
Solution: A 5-carbon chain with an ethyl group (-CH₂CH₃) on the third carbon. - What is the IUPAC name for a 4-carbon ring with an -OH group attached?
Solution: Cyclobutanol. - Are 2-methylpentane and 3-methylpentane the same molecule?
Solution: No. They have different IUPAC names and are structural isomers. - Name the molecule with a 6-carbon ring and a methyl branch.
Solution: Methylcyclohexane. - What is the name for a molecule with a 5-carbon chain, a methyl group on C2 and a chlorine on C3?
Solution: List alphabetically: 3-chloro-2-methylpentane. - Why is "2-ethylpropane" an incorrect name?
Solution: If you draw it, the longest continuous carbon chain is actually four carbons long. The correct name is methylbutane. - Name the diol $HOCH_2CH_2OH$.
Solution: Ethane-1,2-diol. - Is the structure drawn as a "T" shape with 4 carbons the same as a straight chain of 4 carbons?
Solution: No. The T-shape is methylpropane, while the straight chain is butane. They are chain isomers. - Name the molecule with a 6-carbon ring, a double bond between C1 and C2, and a methyl group on C3.
Solution: 3-methylcyclohexene.
7.11 Homologous Series
A homologous series is a family of organic compounds that have the same functional group and similar chemical properties. Each successive member of the series differs by a -$CH_2$- group.
Characteristics of a Homologous Series:
- All members have the same general formula (e.g., Alkanes: $C_nH_{2n+2}$).
- Each member differs from the next by a $CH_2$ unit.
- They show a gradual change in physical properties, such as boiling point. As the carbon chain gets longer, the strength of the intermolecular van der Waals forces increases, leading to higher boiling points.
- They have the same functional group and therefore have similar chemical properties.
Example: The first few members of the alcohol homologous series are methanol ($CH_3OH$), ethanol ($CH_3CH_2OH$), propanol ($C_3H_7OH$), etc. Their boiling points steadily increase.
Solved Examples:
- What is a homologous series?
Solution: A series of organic compounds with the same functional group, where each successive member differs by a $CH_2$ group. - What is the general formula for the alkenes?
Solution: $C_nH_{2n}$. - Which has a higher boiling point: propane ($C_3H_8$) or pentane ($C_5H_{12}$)? Why?
Solution: Pentane. It is a larger molecule with more electrons, leading to stronger intermolecular van der Waals forces that require more energy to overcome. - Do members of a homologous series have similar chemical properties?
Solution: Yes, because they all possess the same functional group, which is the site of most chemical reactions. - What is the molecular formula of the next member of the alcohol series after propanol ($C_3H_8O$)?
Solution: Butanol, $C_4H_{10}O$. - What changes in physical properties are observed as a homologous series is ascended?
Solution: There is a gradual increase in size, mass, boiling point, melting point, and density. - Are ethane and ethene in the same homologous series?
Solution: No. Ethane is an alkane and ethene is an alkene; they have different functional groups. - What is the difference in molar mass between two consecutive members of a homologous series?
Solution: The difference is a $CH_2$ group, which has a mass of 14 g/mol. - Give the first three members of the alkane homologous series.
Solution: Methane ($CH_4$), Ethane ($C_2H_6$), Propane ($C_3H_8$). - Do all members of a homologous series share the same empirical formula?
Solution: Not necessarily. For example, in the alkanes, the empirical formula for ethane ($C_2H_6$) is $CH_3$, but for propane ($C_3H_8$) it is $C_3H_8$.
Knowledge Check (20 Questions)
Answer: Butane.
Answer: Same functional group, successive members differ by a $CH_2$ group.
Answer: 2-chloropropane.
Answer: Pent-.
Answer: $C_nH_{2n+2}$.
Answer: Cyclobutane.
Answer: Hexane.
Answer: 3-methylpentane.
Answer: Identical.
Answer: -anol.
Answer: Propene.
Answer: Due to increasing strength of intermolecular van der Waals forces.
Answer: Methylpropane (or 2-methylpropane).
Answer: Ethyl.
Answer: Ethanol.
Answer: Cyclohexene.
Answer: Similar chemical properties.
Answer: 2,2-dimethylbutane.
Answer: Butan-2-ol.
Answer: Eth-.