Isomerism: Structural Isomerism & Stereoisomerism (Cambridge (CIE) A Level Chemistry): Flashcards
Exam code: 9701
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- FrontStructural Isomerism
Define structural isomers.
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Define structural isomers.
Structural isomers are compounds that have the same molecular formula but different structural formulae.
What are the three types of structural isomerism?
The three types are chain isomerism, positional isomerism and functional group isomerism.
True or False?
Propene and cyclopropane are structural isomers.
True.
Both have molecular formula C3H6 but different structural formulae. Propene has a C=C double bond while cyclopropane has a ring of three carbon atoms.
Define chain isomerism.
Chain isomerism is a type of structural isomerism in which compounds have the same molecular formula but different lengths of the longest carbon chain, caused by branching.
Butan-1-ol and butan-2-ol are .......... isomers because they have the same molecular formula but differ in the .......... of the hydroxyl functional group.
Butan-1-ol and butan-2-ol are positional isomers because they have the same molecular formula but differ in the position of the hydroxyl functional group.
Why do functional group isomers have very different chemical properties, even though they share the same molecular formula?
Functional group isomers contain different functional groups, which determine the chemical reactivity of the molecule. Different functional groups react by different mechanisms and with different reagents.
True or False?
Pentane and 2,2-dimethylpropane are positional isomers.
False.
Pentane and 2,2-dimethylpropane are chain isomers. They share molecular formula C5H12 but differ in the length of the longest carbon chain, not in the position of a functional group.
Butan-1-ol and ethoxyethane are .......... isomers. They share the molecular formula .......... but contain different functional groups.
Butan-1-ol and ethoxyethane are functional group isomers. They share the molecular formula C4H10O but contain different functional groups.
What causes chain isomerism to arise?
Chain isomerism arises from branching in the carbon skeleton, which changes the length of the longest chain even though the molecular formula remains the same.
Define stereoisomers.
Stereoisomers are compounds with the same atoms connected to each other in the same order but with those atoms differently arranged in space.
What are the two types of stereoisomerism?
The two types are geometrical (cis/trans) isomerism and optical isomerism.
True or False?
In a cis isomer, the two identical functional groups are on opposite sides of the double bond.
False.
In a cis isomer, identical functional groups are on the same side of the double bond. In a trans isomer, they are on opposite sides.
Define enantiomers.
Enantiomers are the two optical isomers of a chiral molecule. They are non-superimposable mirror images of each other.
Geometrical isomerism in alkenes arises because the .......... bond prevents .......... rotation about the C=C bond, fixing the groups in position.
Geometrical isomerism in alkenes arises because the π bond prevents free rotation about the C=C bond, fixing the groups in position.
How do enantiomers differ from each other in their interaction with polarised light?
One enantiomer rotates the plane of polarised light clockwise and the other rotates it anticlockwise. They rotate it by the same angle but in opposite directions.
True or False?
Geometrical isomerism can occur in cyclic compounds as well as in alkenes.
True.
In cyclic compounds, limited rotation about the C–C bonds that make up the ring fixes substituents either above or below the ring, giving rise to cis/trans geometrical isomers.
Geometrical and optical isomers are both types of .......... isomerism. They differ from .......... isomers, which have different structural formulae.
Geometrical and optical isomers are both types of stereo isomerism. They differ from structural isomers, which have different structural formulae.
Why does free rotation about a C=C double bond not occur, but free rotation about a C–C single bond does?
Rotation about a C=C bond would break the π bond (which requires energy to re-form), so it does not occur freely. A C–C single bond has only a σ bond, which allows free rotation.
Define chiral centre.
A chiral centre is a carbon atom bonded to four different atoms or groups of atoms, giving rise to two non-superimposable mirror image optical isomers.
How many optical isomers can a molecule with two chiral centres form?
A molecule with two chiral centres can form up to four optical isomers (22 = 4). The maximum number of stereoisomers is 2n, where n is the number of chiral centres.
True or False?
Enantiomers are superimposable mirror images of each other.
False.
Enantiomers are non-superimposable mirror images. Just as a left hand cannot be placed exactly over a right hand, one enantiomer cannot be superimposed on the other.
Define chiral molecule.
A chiral molecule is a molecule that contains one or more chiral centres and therefore exists as two non-superimposable mirror image forms (enantiomers).
The maximum number of stereoisomers for a molecule with n chiral centres is .......... . A molecule with three chiral centres can have a maximum of .......... stereoisomers.
The maximum number of stereoisomers for a molecule with n chiral centres is 2n. A molecule with three chiral centres can have a maximum of eight stereoisomers.
How do you identify a chiral centre in a cyclic molecule?
Find the carbon atom in the ring that is bonded to four different atoms or groups of atoms. The two ring bonds count as different groups if the atoms on each side of the ring differ.
True or False?
A molecule with one chiral centre always produces exactly two optical isomers.
True.
One chiral centre gives 21 = two optical isomers (enantiomers), which are mirror images of each other.
To identify a chiral centre, look for a carbon bonded to .......... different atoms or groups. Such a molecule is described as .......... and has no plane of symmetry.
To identify a chiral centre, look for a carbon bonded to four different atoms or groups. Such a molecule is described as chiral and has no plane of symmetry.
What is the key condition for geometrical isomerism to occur in a molecule?
Geometrical isomerism requires restricted rotation about a C–C bond (as in alkenes or cyclic compounds) AND different groups attached to each carbon of the restricted bond.
What is the systematic approach for deducing all possible isomers of an organic compound?
Draw the structural formula.
Decide whether stereoisomerism or structural isomerism applies.
For structural isomers, check for chain, positional and functional group types. For stereoisomers, check for geometrical (cis/trans) and optical types.
True or False?
C4H10 has only one possible structural isomer.
False.
C4H10 has two structural isomers: butane (straight chain) and 2-methylpropane (branched chain). These are chain isomers.
Define positional isomer.
A positional isomer is a structural isomer in which the same functional group is located on a different carbon of the same parent chain.
How many structural isomers does C3H6Br2 (dibromopropane) have and what type of isomerism do they show?
Dibromopropane has four structural isomers: 1,1-dibromopropane, 2,2-dibromopropane, 1,2-dibromopropane and 1,3-dibromopropane. These are all positional isomers.
1,1-dichloroethene is a .......... isomer of 1,2-dichloroethene. However, only .......... exhibits geometrical isomerism because it has restricted rotation and two different groups on each C=C carbon.
1,1-dichloroethene is a structural isomer of 1,2-dichloroethene. However, only 1,2-dichloroethene exhibits geometrical isomerism because it has restricted rotation and two different groups on each C=C carbon.
True or False?
A compound with no restricted bond rotation and no chiral centre can exhibit stereoisomerism.
False.
Stereoisomerism requires either restricted rotation (for geometrical isomers) or a chiral centre (for optical isomers). Without either, only structural isomers are possible.
To deduce if geometrical isomerism is possible, first check for .......... rotation about a C=C or ring bond, then check that each carbon of the double bond has .......... groups attached.
To deduce if geometrical isomerism is possible, first check for restricted rotation about a C=C or ring bond, then check that each carbon of the double bond has two different groups attached.
What two conditions must both be met for a molecule to show geometrical isomerism?
There must be restricted rotation about a C=C bond or within a ring.
Each carbon involved in the restricted bond must have two different groups attached.
True or False?
Chain isomerism arises when the same molecular formula can give molecules with different numbers of carbons in the longest chain.
True.
Chain isomers have the same molecular formula but different carbon chain lengths due to branching. For example, butane and 2-methylpropane are both C4H10.
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