4.1.1, 5.3.1 Isomerism Flashcards
Taken from chapter 11 - basic concepts of organic chemistry as well as chapter 13 - Alkenes
Define structural isomers
structural isomers are compounds with the same molecular formula but different structual formulas
Define isoelectronic
the same number of electrons
define isomer
Isomers are compounds with the same molecular formula
When do isomers exist in alkanes
when n ≥ 4
When do isomers exist in alkenes
when n ≥ 4
What can be present in structural isomers
Different functional groups
what are the two structures of isomers
- straight chained
- branched
What isomers can Alkenes form
Structural isomers and stereoisomers
Define stereoisomers
Stereoisomers are compounds with the same structural formula (+ molecular formula) but different arrangement of atoms in space.
Why can alkenes form stereoisomers
Alkenes can form stereoisomers because their is restricted rotation around the C=C bond.
How can structural isomers be formed
By changing the position of the functional group and/or the branching of the carbon chain.
what is the role of aldehydes and ketones in isomers
Sometimes two molecules containing different functional groups have the same molecular formula.
what are the types of stereoisomerism
E/Z isomerism and as a subcatagory cis and trans isomerism
define E/Z isomerism
E/Z isomerism is a type of stereoisomerism in which different groups attatched to each carbon of a C=C double bond may be arranged differently in space because of the restricted rotation of the C=C bond.
define cis/trans isomerism
Is a special type of E/Z isomerism in which there are two non-hydrogen groups and two hydrogen atoms around the C=C bond.
The cis isomer (Z isomer) has H atoms on each carbon on the same side.
The trans isomer (E isomer) has H atoms on each carbon atoms on different sides.
When do alkenes have E/Z stereoisomers
alkenes can have E/Z stereoisomers if there are two different groups on the C at each end if the alkene (each C of the double bond has two different groups attached)
What conditions need to be met for a structure to be considered to have E/Z isomerism
- A C=C double bond
- different groups attatched to each carbon atom of the double bond
BOTH are required
Explain why but-1-ene and but-2-ene are not both considered E/Z isomers
In But-1-ene, the carbon atom on one side of the molecule is attatched to two hydrogen atoms. This means that its structure does not satisfy the condition of different groups being attatched to each carbon of the C=C.
But-2-ene on the other hand has a methylgroup and a hydrogen atom on each of the carbon atoms in the double bond, so it has E and Z isomers.
What requirements do cis/trans isomers require
In addition to the requirements of a E/Z isomer (as cis/trans is a subcatagory) one of the attached groups on each carbon atom of the double bond must be hydrogen
how are cis/trans isomers and E/Z isomers connected
cis/trans isomerism is a special case of E/Z isomerism.
A cis isomer is of the Z isomer. It has the hydrogen atoms on each side of the C=C on the same side of the molecule.
A trans isomer is of the E isomer. It has the hydrogen atoms diagonally opposite each other.
How are isomers labled E or Z
Through the Cahn-Ingot-Prelog rules:
In this system the atoms attatched to each carbon atom in a double bond are given a priority based upon their atomic number.
If the groups of higher priority are on the same side of the double bond, the compounds is a Z isomer.
If the groups of higher priority are diagonally placed across the double bond, the compound is a E isomer.
Explain how priority is assigned using the Cahn-Ingot-Prelog rules
- examine the atoms attached directly to the carbon atoms of the double bond and decide which of the two atoms has the highest priority. The higher the atomic number the higher the priority
- If the two atoms attached to a carbon atom in the double bond are the same, then you will need to find the first point of difference. The group which has the higher atomic number at the first point of difference is given the higher priority
