Unit 7: Isomerism
Exploring molecules with the same formula but different structures.
7.12 Structural Isomerism
Isomers are molecules that have the same molecular formula but a different arrangement of atoms. Structural isomers are isomers that differ in the connectivity of their atoms, meaning they have a different structural formula. There are three main types:
1. Chain Isomerism
Chain isomers have the same molecular formula and functional group but a different arrangement of the carbon skeleton (e.g., branched vs. unbranched).
Example: $C_4H_{10}$ can be butane (unbranched) or methylpropane (branched).
2. Positional Isomerism
Positional isomers have the same molecular formula and carbon skeleton but differ in the position of the functional group on the chain.
Example: $C_3H_7Cl$ can be 1-chloropropane or 2-chloropropane.
3. Functional Group Isomerism
Functional group isomers have the same molecular formula but different functional groups, placing them in different homologous series. They have very different chemical and physical properties.
Example: $C_3H_6O$ can be propanal (an aldehyde) or propanone (a ketone). Another common example is an alkene and a cycloalkane, such as but-1-ene and cyclobutane ($C_4H_8$).
Solved Examples:
- What is the definition of structural
isomers?
Solution: Molecules with the same molecular formula but a different structural formula (different atom connectivity). - Draw and name the two chain isomers of
$C_4H_{10}$.
Solution: Butane ($CH_3CH_2CH_2CH_3$) and methylpropane ($CH_3CH(CH_3)CH_3$). - What type of isomerism exists between butan-1-ol and
butan-2-ol?
Solution: Positional isomerism. - Give an example of a functional group isomer for propene
($C_3H_6$).
Solution: Cyclopropane. - How many chain isomers does pentane ($C_5H_{12}$)
have?
Solution: Three: pentane, methylbutane, and 2,2-dimethylpropane. - What is the relationship between ethanol ($CH_3CH_2OH$) and methoxymethane
($CH_3OCH_3$)? Both are $C_2H_6O$.
Solution: They are functional group isomers (alcohol and ether). - Draw the two positional isomers of dichlorobenzene
($C_6H_4Cl_2$).
Solution: 1,2-dichlorobenzene, 1,3-dichlorobenzene, and 1,4-dichlorobenzene are the three possible positional isomers. - What type of isomerism is not possible for
propane?
Solution: Positional isomerism (for haloalkanes/alcohols) and chain isomerism are not possible as there is only one way to arrange three carbons and one place to substitute on the end vs. the middle. - Identify the type of isomerism between pent-1-ene and
2-methylbut-1-ene.
Solution: Chain isomerism. Both are alkenes with the double bond on the first carbon, but the carbon skeleton is different. - Can alkanes exhibit functional group
isomerism?
Solution: Yes, with cycloalkanes. For example, hexane ($C_6H_{14}$) and cyclohexane ($C_6H_{12}$) are not isomers, but hexene ($C_6H_{12}$) and cyclohexane ($C_6H_{12}$) are functional group isomers.
7.13 Stereoisomerism (Geometrical Isomerism)
Stereoisomers are molecules with the same molecular formula and the same structural formula, but with a different spatial arrangement of atoms. The type of stereoisomerism we will focus on is geometrical isomerism.
Geometrical (Cis-Trans) Isomerism
This type of isomerism occurs due to the restricted rotation around a carbon-carbon double bond (C=C). A single bond can rotate freely, but a double bond cannot without breaking the pi-bond component.
Two conditions must be met for geometrical isomerism to exist:
- There must be a C=C double bond.
- Each carbon atom in the double bond must be attached to two different groups.
The isomers are named as:
- cis-isomer: The high-priority or identical groups are on the same side of the double bond.
- trans-isomer: The high-priority or identical groups are on opposite sides of the double bond.
Example: But-2-ene ($CH_3CH=CHCH_3$)
Solved Examples:
- What is the key structural feature that leads to geometrical
isomerism?
Solution: Restricted rotation around a C=C double bond. - Does but-1-ene exhibit geometrical isomerism?
Explain.
Solution: No. The first carbon of the double bond ($CH_2=CHCH_2CH_3$) is attached to two identical hydrogen atoms. - Draw the structure of
trans-1,2-dichloroethene.
Solution: A C=C double bond with one H and one Cl on each carbon, with the two Cl atoms on opposite sides of the double bond. - What are the two conditions for a molecule to show cis-trans
isomerism?
Solution: 1. A C=C double bond. 2. Each carbon of the double bond must be bonded to two different groups. - What is the difference between a structural isomer and a
stereoisomer?
Solution: Structural isomers have different atom connectivity (different structural formulae). Stereoisomers have the same connectivity but a different 3D arrangement of atoms. - Is cis-but-2-ene a stereoisomer or a structural isomer of
trans-but-2-ene?
Solution: They are stereoisomers (specifically, geometrical isomers). - Why is there no rotation around a C=C double
bond?
Solution: Because it consists of a sigma bond and a pi bond. Rotating the bond would require breaking the pi bond, which requires a large amount of energy. - Does 2-methylbut-2-ene ($CH_3C(CH_3)=CHCH_3$) show geometrical
isomerism?
Solution: No. The second carbon of the double bond is attached to two identical methyl groups. - Name the two geometrical isomers of
pent-2-ene.
Solution: cis-pent-2-ene and trans-pent-2-ene. - What does the prefix "cis-" signify?
Solution: That the specified groups are on the same side of the double bond.